Scatchard analysis of[3H]-5HT binding to intact platelets revealed two binding sites.
Site Ais ofhigh affmity and low capacit y and site B oflow affmity and high capacity.
The con stants for the control group showed no difference s for either
site between luteal and post-menstrual pha ses (Table I) and pooled data indicated for
site A a K, of 1.3 nM and a capacit y of 2.9 fmol 108 platelets-I and for site Ba K, of
12.9 nM with a capacity of92 .2 fmol lü! platelet s" .
Treatment with oral contraceptives produced marked changes in both A and B
receptor sites (Figures 2 and 3). On day 21 there was an increased capacity of both
sites A and B compared with both controls and da y 28 values, the latter being lower
th an controls in the site B analysis (Figure 3). No significant alteration s were seen in
the affinity ofeither site between day 21, control, or day 28 sampIes.
Figure2 Platelet 5HT receptor (site A). Specific binding of [lH]-5HT to human platelets
Th e uptake of[3H]-5HT into intact cells as a funct ion offree concentration is shown
in Figure 4. The values for day 21 are higher than, and significantly different from
(P < 0.001 at all points), day 28 dat a. The luteal and post-menstrual sampies ofth~
control group were not significantl y different from each other and values were pooled
to give the indicated control curve. These were significantly different from both day
21 and day 28 uptake (P < 0.01 at all points). Double reciprocal ana lysis shows no
change in the Km for upt ake from da y 21 to da y 28 nor when these values are
compared with the control (Figure 4 inset). This alteration in uptake is exactly
analogous to that observed in site B receptor capacity (Figure 3), that is, changes in
receptor characteristics and function seem to be parallel event s.
170 J. R. PETE RS, J. M. ELLIOTT & D. G. GRAHAME-SMIT H
Nora drenaline and 5HT induced platelet aggregation
No differen ce was observed in an y of the parameters of platelet aggregation
measured , between control subjects in the luteal and post-menstrual ph ase.
Aggregation rate and t. OD for both NA and 5HT were unchanged. These data were
pooled therefore and are represented by the controI values for each parameter in
~igure 3 Platelet 5HT receptor (site B). Specific binding of [3H]-5HT to human platelets
... J- I :I: 40 V\ '" "'" '" Oay n
Figure 4 Active uptake of [3H]-5HT by platelet as a function of free [3H]-5HT concentration.
• • oral contraceptive group day 21 , 0---0oral contraceptive group day 28, /::, - - --/::,
control group. Inset showsdouble reciprocal plot ofsame data.
percentage of reference ADP response, were inc reased on day 21. These par ameters
of aggregation were significantly different from both contrals and day 28 values
ß-Oestradiol and progesterone assays
Pla sma ß-oestradiol and proge sterone assays were perform ed in the eight control
subjects during luteal and post-menstrual ph ases. ß-oestradiol showed luteal phase
values of 670 ± 89 pg ml-I and post-menstrual levels of 316 ± 67 pg ml!
PLATELET RECEPTORS AND ORAL CON T RACEPT IVES 171
(P < 0.005). Progesterone luteal phase values were 11.4 7 ± 2.34 ng rnl" and in the
post-m enstrual pha se 3.04 ± 1.66 ng mI-I(P < 0.005 ).
No differences were observed between platelet counts of subjects with in groups
(between 21 and 28 days and between luteal and post-menstrual phases), or between
mean values oforal contraceptive treated compared with control groups.
Noradrenali ne 2 IJM 5-Hydr 1Xyl rypla mi ne 20
Rate mV mln -1 600 Rate mV mln - 1 600
Figure5 Platelet aggregation responses to NA and 5HT in wornen taking cornbined oral
contraceptive on day 21 and day 28 (n = 15) cornpared with untreated control population
Th e present stud y shows that tak ing the combined oral contraceptive pill increases
the number of receptors for 5HT and NA on the hum an plateiet, the aggregation
induced by both biogen ic amines, and the acti ve uptake of 5HT .
Alteration of hormonal and neurotransmitter receptors by drugs and exogenous
hormones has been described using rad ioligand binding methods in various tissues
(Tell et al., 1978). In particular, in animals, oestrogens and progestogens act to mod ify
Further, changes in receptor characteristics have been observed in response to
hormones in widely differing tissues, for example, th yroxin e is known to change the
capacity of ß-receptors in both the myocardium (William s, Lefkowitz, Watanabe,
Hathaway & Besch, 1977) and in adipose cells (Ciaraldi & Marinetti, 1978).
The component of the combined oral contraceptive preparation primarily
responsible for changes in receptor capacity and platelet aggregation and uptake is
more likel y to be eth inyloestradiol then the progestogens. In rabbit myometrium,
oestrogens in contrast to progesterone increase a -receptor capacity (Roberts et al.,
1977) and exogenous oestrogen, but not progesterone alone , produces changes in
5HT receptor capacity in rabb it platelets (Elliott et al., 1980).
The mechanism by which oestrogen s and /or progestogens modulate receptor
binding characteristics is unknown, but the y are belie ved to act via the nucleus,
stimulating mRNA production, protein synthesis (Butler & O'Malley, 1976), and
ste rn cell, the megakar yocyte. Th e mean life span of the human platelet is approxi-
172 J. R. PETERS, J. M. ELLIOTT & D. G. GRAHAME-SMITH
mately nine days, and thus by day 21 all extant platelets would have been formed in a
high oestrogen environment. Following metabolism and excretion of the last dose of
ethinyloestradiol (TI/, 6-10 h) on day 21, almost the whole platelet population would
then be renewed in the absence of exogenous oestrogens, prior to the second receptor
assay on day 28. The chronology of events therefore allows ample time for the
observed receptor changes to be manifest according to the accepted theory ofsteroid
hormone action. Similarly in the control population, the timing of sampies in the
luteal and post-menstrual phases was designed to allow at least 6 days of exposure to
high or low endogenous oestrogen levels, to maxi mise as far as possible any potential
The a-adrenergic receptor on platelets has been well characterised (Alexander,
Cooper & Handin, 1978), and demonstration ofa single site binding system has been
related functionally to adrenaline and noradrenaline induced platelet aggregation
(Cooper, Handin, Young & Alexander, 1978). The normal values for affinity and
capacity obtained here are similar to those found in other studies on intact platelets
The presence of two functional binding sites for 5HT on human platelets is in
concordance with studies on rat (Drummond & Gordon, 1975b) and rabbit platelets
(Elliott et al., 1980). Differential effects on inhibitors on these sites indicate the higher
affmity , lower capacity site to be concerned with 5HT induced aggregation and the
lower affmity higher capacity site with the process of active uptake of 5HT
(Drummond & Gordon, 1975a; Peters & Grahame-Smith, 1980).
The rise and fall in both NA and 5HT receptor capacity during the cyclical
administration of the oral contraceptive as shown in this study is indicative of an
alteration in the number of receptor sites on the platelet surface. Since, according to
the receptor occupancy theory, a biological response to a drug or hormone is a
funct ion ofthe number ofreceptor sites occupied, then aggregation produced by NA
and 5HT on day 21 would be expected to result in an increased response as compared
with day 28 or controls. The observed changes in NA and 5HT induced aggregation
and 5HT uptake parallel exactly those oftheir respective receptors, and as such may
be considered to represent a true functional assessment.
The marked contrast between the oral contraceptive treated group, in which all
binding and aggregation parameters altered between days 21 and 28, and the control
group, in which no such variation was observed between sam pies in the luteal and
menstrual phases, might be due to relatively greater oestrogen activity produced by
the dosage of ethinyloestradiol in the oral contraceptive pill. Thus, the excretion of
total endogenous oestrogen products varies from 5-20 daily in the menstrual
phase to 20-80 daily in the luteal phase of a normal population, indicating a 4-5
fold change. Administration of 30 daily of exogenous ethinyloestradiol represents,
since it is estimated to be between 10 and 20 times more effective than endogenous
oestrogens, a change in oestrogen-like activity between days 21 and 28 much greater
than that seen during the normal cycle.
oestrogens and /or progcstogens, and that these changes are not measureable during
the physiological variation of endogenous oestrogen and /or progesterone in the
The incrcascd incidence ofthrombotic events in women taking oral contraceptives
is well recognised (Royal College ofGeneral Practitioners, 1977; Vessey, McPherson
& Johnson, 1977). Whilst these fmdings would seem to providc an excellent
theoretical basis for a mechanism enhancing platelet aggregability and contributing
to a hypercoagulablc statc, we should caution against their direct extrapolation to the
c1inical situation of thrombosis. The technique of platelet aggregation in vitro,
although used widely as an index of platelet function in vivo, is esscntially artificial,
PLATEL ET RECEPTORS AND ORAL CON TRACE PTIVES 173
being performed in unphysiological ion ic concentration s. Further studies are
requ ired before one ma y relate cha nges in NA and 5HT platelet receptor function
and aggregation to the thrombotic propensit y associa ted with th e use of the oral
Receptor sites analogous to those in the platelet ha ve been demonstrated for NA in
rat brain (G reenberg & Snyder , 1977) and for 5HT in human br ain (Shih & Young,
1978) (also a two site binding system). These receptor sites for NA and 5HT (and in
the case of 5HT thi s includes th e function of active uptake) in the brain ma y be
altered in a simi lar way to th e platelets by oestrogen/progesterone, (Biege n, Bercovitz
& Samuel, 1980). It is tempting th erefor e to speculate th at here lies a potential
explanation for atTective disorders in which hormonal changes have been implicated,
such as post-parturn and pill associ ated depression.
normal females taking the oral contraceptive pill.
Pharmacological parameters of function (NA and 5HT induced platelet
aggregation and 5HT uptake) are shown to vary accordingly.
No changes in either receptor site , or in platelet function are seen in a control
group ofuntreated, normall y cyclin g women, between the luteal and post menstrual
lt is concluded that th e ob served cha nges are due to a ph armacological etTect ofthe
oral contraceptive rather than a no rma l ph ysiological varia tion.
We are grateful to Dr M. D . Mitch ell of the N uffield Departm en t of Ob stetrics and
G ynaecology for measurement of ß -oestr ad iol and progesterone levels. J. M. Elliott
is a Medical Research Council scholar.J. R. Peters is a Wellcome Research Fellow.
Alexander, R. W., Cooper, B. & Handin, R. I. (1978). Characterisation of the human platelet
alpha-adrenergic receptor. Correlation of (lH]-dihydroergocryptine binding with
aggregation and adenylate cyclase inhibition. J. clin. Invest.. 61, 1136-1144.
Bar, R. S. & Roth, J. (1977). Insulin receptor status in disease states of man. Arch.Int. Med., 137,
Biegon, A., Bercovitz, H.& Samuel, H. (1980). Serotonin receptör cöncentration duringthe
estrous cycle ofthe rat. Brain Res., 187,221-225 .
Boullin, D. J. & Elliott, J. M. (1979). Binding of(lH]-dihydroergocryptine to a-adrenoceptors
on intact human platelets. Brit. J. Pharrnac.,66, 89p.
Boullin, D. J., GIenton, P. A. M., Molyneux, D., Peters, J. R. & Roach, B. (1 977). Binding of
5-hydroxytryptamine in human blood platelets. Brit. J. Pharmac.,61, 453p.
Boullin, D. J., Green, R. A. & Price, K. S. (1972). The mechanism of adenosine diphosphate
induced platelet aggregation: binding to platelet receptors and inhibition of binding and
aggregation by prostagiandin EI.J. Physiol., 221,415-426.
Buller, R. E. & O'Malley, B. W. (1976). The biology and mechanism of steroid hormone
receptor interaction with the eukaryocytic nucleus. Biochern. Pharrnac., 25, 1-12.
Ciaraldi, T. P. & Marinetti, G. V. (1978). Hormone action at the membrane level; VIII:
Adrenergic receptors in the rat heart and adipocytes and their modulation by thyroxine.
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174 J. R. PETE RS, J. M. ELLIOTT & D. G . G RAHAME -SMITH
Cooper, 8. , Handin, R. 1., Youn g, L. H. & Alexander, R. W. (1978). Agonist regulation of the
human platelet alpha-adrenergic receptor. Nature. 274, 703-706.
Drumrnond , A. H. & Gordon. J. L. (l9 75a). Inhibition of 5-h ydroxy-[!H]-t ryptamine binding to
rat blood platelets by 5HT antagonists and uptak e inhibitors. Brit, J. Pharm ac.. 55, 275p.
Drummond , A. H. & Gordo n. J. L. (l9 75b). Specific bind ing sites for 5-hydroxytryptamine on
rat blood platel ets. Biochem. J.. 151),1 29-132.
Elliotl, J. M., Peters, J . R. & Grahame-Smith, D. G . (1980). T he elTect of oestroge ns and
progesteron e on n-adrenergic and seroti n receptors in rabbit platelets. Eur. J. Pharmac.. in
G reenberg, D. A. & Snyde r, S. H. (1977). Selective labelling of n -adrenergic receptors in rat
brain with [!H]-dihydroergocryptine. Life Sei.. 20, 927- 932.
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th rombocythem ia: decreased ep inephrine responsiveness associated with a deficiency of
platel et a -adrenergic receptors, New Eng. J. Med., 299, 505-509.
Lee, C. Y. & Ryan, R. J. (1974). Estrogen stimulat ion of human chorio nic gonadotrophin
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Sneddon, J. M. (1969). Sod ium-dependent acc umulation of 5-h ydro xytrypt am ine by rat blood
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Tell, G. P., Haour, F. & Saez, J. M. (1978). Hormonal regulation of membra ne rece ptors and cell
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ofrat platelet aggregation : a compa rative study with human arid'rabbit platelets. Throm b.
Vanderbi/t University Medical School,
A hormone, neurotransmitter or drug produces its efTect by combining with a
receptor site and initiating a series ofch anges which culminate in a measurable effect.
Although it is weil known that increasing the concentration of an agonist which
stimulates a receptor will result in an increase in pharmacological efTect it has onl y
function ofthe receptors to which they bind. Thus the magnitude ofa response in an y
individual is dependent on both the concentration ofagonist present and available for
binding to the receptor site and the number of receptor sites on the efTector cel!. An
increase in receptor number will produce an increase in the response to a given
concentration of agonist, th at is an increase in sensitivity, and ma yaiso produce an
increase in the maximal response. Rec entl y, the development of radiolabelIed
agonists and antagonists which ha ve a very high specific activity has allowed the
direct identification of receptors and studies of their function and contro l to be
performed (Lefkowitz, Roth, Pricer & Pastan , 1970; Freychet, Roth & Neville , 1971;
Conolly & Greenacre, 1977; Lefkowitz , Mukherje e, Co verstone & Ca ro n, 1974;
Aurback , Fedak, Woodjard, Palmer, Hau ser & Troxler, 1974; Lefkowitz & Williams,
1977; Will iams, Sn yderman & Lefkowitz, 1976; Galant, Duriseti, Underwood &
Insel , 1978; Newman , Williams, 8ishopric & Lefkowitz, 1978). Both a- and
ß-adrenergic receptors ha ve been identified and studied in a number of tissue s in
various spec ies. Recentl y, ß-adrenoceptors have been identified on circulating
human leucocytes while a -adrenoceptors have been found on human platelets
allowing the detailed study ofadrenergic receptor function in man.
Animal studies ha ve shown that in vitro exposure to high levels of catecholamines
results in a decrease in th e number of ß-adrenergic receptors (Mickey, Tate &
Lefko witz, 1975; Mukherjee, Caron & Lefkowitz, 1976). Conolly & Grenacre (1976)
found that Iymphocytes from asthma tics who had been treat ed for a prolonged period
with high concentratio ns of ß-adrenoceptor agonists produced less cycli c AMP in
response to isoproterenol (iso prena line) than lyrnphocytes fro m non-asthmatics.
That these changes were a fun ction of the treatment which th ey had received rather
than an y primary abnormality associated with asthma was confirrned by showing
that their responsiveness returned to normal wh en therap y with ß-ad;enoceptor
agonists was stopped and by fmding similar changes in normal subjects to whom
ß-adrenergic receptor agonists were given . This suggested that chronic exposure to
high levels of adrenergic receptor agonists might result in reduction or down
regulation of ß-adrenoceptors. This was confirrned by Galant, Underwood, Duriseti
& Indel (1978) who found that chronic administration ofthe ß-adrenoceptor agonist
terbutaline resulted in an 80% reduction in ß-adrenoceptors on human leucocytes.
Thus, it appears that the administration of pharmacological doses of catecholamines
result in down regulation of adrenergic receptor density in man. However, the
possibility that sympathetic tone, acting through circulating catecholamines, might
be responsible for regulating receptör density and hence adrenergic responsiveness in
normal man has not been examined.
A number of factors have been shown to influence ß-adrenoceptor density in
man. For example, Schocken & Roth (1977) found that leucocyte ß-adrenoceptor
density was reduced in the elderly. While Vestal, Wood & Shand (1979) have shown
that ageing is associated with a decrease in the responsiveness to the adrenergic
agonist isoprenaline so that elderly subjects required up to five times more
isoprenaline than young subjects to produce the same rise in heart rate . In addition,
even within the same age group there was considerable interindividual variation in
sensitivity to isoprenaline. Thus, it is likely that some of the interindividual
variation in sensitivity to adrenergic receptor agonists is due to altered receptor
It was postulated that in man the sympathetic nervous system, acting through
circulating catecholamine levels, controls its own responsiveness by altering receptor
density so that high levels ofcatecholamines would be associated with a reduction, or
down regulation, ofreceptor density and decreased responsiveness while reduction in
catecholamine levels would be associated with elevation of receptor density and
increased sensitivity to adrenergic receptor agonists. If down regulation occurs under
normal physiological conditions then it should be possible to block that down
regulation by administering a ß-adrenergic receptor blocking drug which by
preventing down regulation, will allow receptor density to increase. In addition,
perturbations associated with changes in sympathetic tone and catecholamine levels
such as alterations in sodium intake should result in changes in receptor density.
ß-adrenoceptor density on circulating leucocytes was studied as a model for other
less accessible tissues, such as the heart.
A reciprocal correlation between urinary and plasma catecholamines and
ß-adrenoceptor density was found (Wood, Nadeau, Robertson & Fraser, 1979;
density. That is the higher the subjects catecholamines, the lower was the receptor
density. When propranolol was administered (80 mg three times daily) leucocyte
ß-adrenoceptor density increased by 30% (Fraser & Wood, 1979) and most
importantly the rise in receptor density was correlated with the pretreatment
catecholamine levels (Wood et al.. 1979; Fraser et al., 1980a). That is the higher the
catecholamine levels prior to propranolol the greater the degree of down regulation
that existed and hence the greater was the increase in receptors when propranolol was
Subjects were also studied while receiving diets containing either high or low
sodium content. This is known to alter sympathetic tone , and hence, catecholamine
levels . High sodium diets being associated with reduction in catecholamines and low
sodium diets being associated with elevation in catecholamines. It was shown that
high sodium intake (400 mmol daily) resulted in reduction in catecholamine levels
and an increase in leucocyte ß-adrenoceptor density compared ~o low.salt diets (10
mmol daily) (Nadeau, Fraser, Robertson & Wood, 1980). In addition, rt was shown
that this alteration in leucocyte receptor density was associated with changes in the
cardiac sensitivity to isoprenaline in these individuals. A high salt intake resulted in
ASSESSMENT OFß-ADRENOCEPTOR DOWN REGULATION IN MAN 177
an increase in the sensitivity to isoprenaline measured as the dose of isoprenaline
required to raise the heart rate by 25 beats per minute so that lower dos es of
isoprenaline were required on the high salt diet compared to the low salt diet (Nadeau
One of the concerns about all of the studies which have examined leucocyte
ß-adrenoceptor function in man is whether alterations found in this system reflect
changes in the less accessible, but more physiologically relevant receptor sites such as
in the heart. lt has now been shown that leucocyte ß-adrenoceptor density
correlates with the cardiac sensitivity to isoprenaline suggest ing that changes in
leucocyte ß-adrenoceptor density do indeed reflect changes seen in the heart (Fraser
These studies, therefore , show that in man ß-adrenergic receptor function is
controlled by circulating catecholamine levels and hence by the individual's level of
sympathetic tone. Thus, factors such as dietary sodium intake which alter
sympathetic tone have profound effects on both ß-adrenoceptor density, measured
on leucocytes and on the cardiac sensitivity to ß-adrenoceptor agonists such as
Previous studies have shown that pharmacological doses of adrenergic receptor
agonists result in reduction (down regulation) of [J-adrenoceptor density on
circulating leucocytes. A reciprocal correlation was found between plasma and
urinary adrenaline and noradrenaline levels and leucocyte !3-adrenoceptor density.
Propranolol administration (240 mg daily) resulted in a 30% increase in leucocyte
ß-adrenoceptor density. The rise in !3-adrenoceptor density on propranolol
treatment correlated with the level ofcatecholamines prior to propranolol treatment.
These fmdings, therefore, suggest that circulating levels of catecholamines are
responsible for down regulation of ß-adrenergic receptor density in man. High salt
intake (400 mmol daily) resulted in an increase in leucocyte ß-adrenoceptor density
and an increase in the cardiac sensitivity to isoprenaline. Leucocyte ß-adrenoceptor
density correlated with the cardiac sensitivity to isoprenaline demonstrating that
leucocyte ß-adrenoceptors are a useful model of cardiac ß-adrenoceptor function .
These fmdings, therefore, show that the sympathetic nervous systern , acting through
circulating catecholamines can regulate its own responsiveness through down
regulation ofreceptor dens ity.
This work was supported by United States Public Health Service Grants: GM 15431,
Aurback, G. c., Fedak, S. A., Woodard, C. J., Palmer, J. S., Hauser, D. & Troxler, T. (1974).
Beta-adrenergic receptor: stereospecific interaction of iodinated beta-blocking agent with
highafTmity site. Science, 186, 1223-1224.
Conolly, M. E. & Greenacre, J. K. (1976). The lymphocyte ß -adrenoceptor in normal
subjectsand patients with bronchial asthma.1. clin. Invest .. 58, 1307-1316.
Conolly, M. E. & Greenacre, J. K. (1977). The ß-adrenoceptor ofthe human lymphocyte and
human lung parenchyma. Brit. J. Pharmac.. 59, 17-23.
Fraser, J., Nadeau, J. H., Robertson, D. & Wood, A. J. J. (l980a). Down regulation ofhuman
leukocytebeta-receptorsby physiologicallevelsof catecholamines. Clin. Res.. 28, 541A.
Fraser, J., Nadeau, J. H., Robertson, D. & Wood, A. J. J. (l980b). Leukocyte beta-receptor
density (8 m3x) correlatcs with cardiac sensitivity to isoprotercnol in man . Clin . Res., 28,
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biological activity and binding to fat cells and liver membranes. Biochem. Biophys. Res.
Galant, S. P., Duriseti, L., Underwood, S. & Insel, P. A., (1978). Decreased beta-adrenergic
receptors on polymorphonuclear leukocytes after adrenergic therapy. New Eng. J. Med.,
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(-)-alprenolol binding sites , beta-adrenergic receptor and adenylate cyclase . Biochem.
Biophys. R es. Commun., 60, 703-709.
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receptor. Proc. Nat. Acad. Sei . USA , 74, 515-519 .
Mickey , J., Tate, R. & Lefkowitz, J. (1975). Subsensitivity of adenylate cyclase and decreased
beta-adrenergic receptor binding after chronic exposure to (-)isoproterenol in vitro. J. bio/.
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beta-adrenergic receptors by beta-adrenergic catecholamines. Endocrinology, 99, 347-357.
Nadeau, J. H., Fraser, J., Robertson, D. & Wood, A. J. J. (1980). Increased sodium intake
increases sensitivity to catecholamines through alteration in receptor density. Clin. Res..
Newman, K. 0 ., Williams, L. T., Bishopric, N. H. & Lefkowitz, R. J. (1978). ldentification of
n -adrenergic receptors in human platelets by (lH]-dihydroergocryptine binding. 1. c1in.
Schocken, D. D. & Roth, G . S. (1977). Reduced ß-adrenergic-receptor concentrations in
ageing man. Nature, 267,856-859.
Vestal, R. E., Wood, A. J. J. & Shand, D. G. (1979). Reduced beta-ad renoceptor sensitivity in
the e1derly. Clin . Pharmac. Ther., 26, 181-186.
Williams, L. T., Snyderrnan, R. & Lefkowitz, R. J. (1976). Identification of {J-adrenergic
receptors in human leukocytes by (-) (lH]-alprenolol binding. J . c1in.Inv est., 56, 149-155.
Inotropic Agents and Vasodilators
Committee on Clinical Pha rma cology,
Departments 01 Pharmacological and Physiological Seiences and Medicine,
Improvement in myocardial contractility by inotropic agents is a critical com ponent
in the management of heart failure. U nfortunately although man y drugs have been
shown to ha ve positive inotropic efTects on the heart, few of these agents ha ve been
found to be useful in the treatment of heart failure in man (Farah & Alousi, 1978).
This review will concentrate on drugs which appear to have the greatest potential
The cardiac glycosides remain the mo st commonly used po sitive inotropic agents for
the treatment of congesti ve heart failure. Digitalis glyco sides increase the force of
myocardial contraction, both in the failing and normal heart (Braunwald, 1971;
Smith & Haber, 1973), This po siti ve inotropic efTect is not altered by prior treatment
with reserpine nor by ß-adrenergic receptor blocking agents. Although the positive
inotropic action of digitalis results in an increase in cardiac output of the failing
heart, generally no change is seen in the nonfailing myocardium (Mason, Spann &
heart increased impedance to ventricular ejection negates the haemodynamic
ben efits ofthe positive inotropic efTect ofthc drug. On the other hand, in the patient
with heart failure, an elevated vascular resistance is seen prior to digitalis therapy and
represents a compensatory mechanism for the reduction in efTective cardiac output.
With the administration of digitalis, cardiac output increase s, thereby allowing
withdrawal of the compensatory vasoconstriction (Mason, 1974). This reflex
reduction in vascula r tone is greater in magnitude than the direct vasoconstrictor
action of the cardiac glyco sides. Thus, the increase in co ntractility , in consort with a
reduction in afterload, accounts for the improvement in cardiac output.
Dig italis-induced augmentation of myocardial contractility leads to an increase in
myocardial ox ygen consurnption. In the failing heart this is counter-balanced by the
metabolic benefits resulting from a reduction in left ventricular size that follows
182 L. I. GOLDBERG & S. I. RAJFER
treatment with the glycoside (Covell, Braunwald, Ross & Sonnenblick, 1966;
reduction ofthe radius ofthe ventricle.
The major difficulty in the use of cardiac glycosides is that they have a relatively
poor therapeutic index (arrhythmic dose /positive inotropic dose). The use of
diuretics with concomitant loss of potassium complicates the issue even further
because ofthe increased incidence of digitalis toxicity associated with hypokalaemia.
Accordingly, relatively small doses are used, and it is difficult to determine how
much positive inotropic effect is produced in an individual patient. A non-invasive
study in which positive inotropic effect was estimated by measurement of systolic
time intervals demonstrated that the commonly used dose of digoxin (0.25 mg daily)
exerted a statistically significant but relatively minor shortening of the systolic time
intervals. Increasing the dose to 0.5 mg resulted in a more pronounced shortening
(Carliner, Gilbert, Pruitt & Goldberg, 1974).
Despite enthusiastic claims by certain investigators, there is no conclusive
evidence that there is any difference between the therapeutic indices of the various
glycosides. An important advance in the treatment ofheart failure would be made ifa
cardiac glycoside with a better therapeutic index was found.
The principal cellular action of the digitalis glycosides is inhibition of Na-, K+-
ATPase, an enzyme critical to the functioning ofthe 'sodium pump' (Akera & Brody ,
1977). This 'pump' provides an active transport mechanism whereby intracellular
sodium ions are exchanged for extracellular potassium ions across the plasma
membrane (sarcolemma). The mechanism whereby this action of digitalis might lead
Ca++ available to the contractile proteins of the myocardial cell (Allen & Blinks,
1978), a defmite link between this effect and its action on the 'sodium pump' has not
The value of serum digitalis levels is now under intensive reevaluation. Although
most studies demonstrate elevated digoxin levels (greater than 2 ng ml") in toxic
patients, a considerable number ofpatients with apparent digoxin toxicity have levels
in the 'therapeutic range .' The probability of toxicity does increase with higher
serum concentrations, but there is no clear separation oftoxic and therapeutic levels
(Beller, Smith, Abelmann, Haber & Hood, 1971).
However, the availability ofmethods for measuring serum digitalis levels has made
it possible to investigate clinically significant problems. Thus, a substantial elevation
of serum digoxin levels during concomitant quinidine administration is now weil
documented. lt appears that quinidine impairs digoxin clearance (Doering, 1979). A
50% reduction in the dose ofdigoxin is recommended. Ewy , Kapadia, Yao, Lullin &
Marcus (1979) found that a single dose of digoxin injected intravenously into elderly
due to a decrease in renal function account for this difference. Rogers, Willerson,
Goldbatt & Smith, (1972) reported that during pregnancy digoxin crosses the
placenta and, at term, the serum concentration in the neonate is similar to that in
maternal serum. Digoxin is found in breast milk as weil (Chan, Tse & Wong , 1978).
Sympathomimetic amines act on the cardiovascular system by action on a- and
ßI receptors in the heart, and ß2 receptors in blood vessels, Action on ßI
receptors results in an increase in cardiac contractility and heart rate, while action on
ß2 receptors results in vasodilation of blood vessels , primarily in the skeletal
muscle and mesenteric vascular beds. ßr-adrenergic agonists stimulate adenyl
cyclase, resulting in an increase in cyclic AMP (Sobel & Mayer, 1973). The inotropic
effects of these agents appear to be mediated by cycl ic AMP-dependent processes,
Sympathomimetic amines differ qualitatively and quantitatively in their effects on
these adrenergic receptors. Therefore, the use of the positive inotropic actions of a
ßr-adrenoceptor agonist in the treatment of heart failure demands consideration of
ItS peripheral vascular effects, which can considerably modify the haemodynamic
results of its cardiotonic act ion . An increase in peripheral vascular resistance can
counterbalance the potential inc rease in cardiac output to be expected ·from these
inotropic agents. A decrease in peripheral vascular resistance could be beneficial,
provided the vasodilation took place in vascular beds where increased flow is
required. A problem with the use of ßr-adrenergic receptor agonists is that they may
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