To identify the role of the myocardial beta-adrenergic pathway in congestive heart failure, we examined beta-adrenergic-receptor density, adenylate cyclase and creatine kinase activities, muscle contraction in vitro, and myocardial contractile protein levels in the left ventricles of failing and normally functioning hearts from cardiac-transplant recipients or prospective donors. Eleven failing left ventricles had a 50 to 56 per cent reduction in beta-receptor density, a 45 per cent reduction in maximal isoproterenol-mediated adenylate cyclase stimulation, and a 54 to 73 per cent reduction in maximal isoproterenol-stimulated muscle contraction, as compared with six normally functioning ventricles (P less than 0.05 for each comparison). In contrast, cytoplasmic creatine kinase activity, adenylate cyclase activities stimulated by fluoride ion and by histamine, histamine-stimulated muscle contraction, and levels of contractile protein were not different in the two groups (P less than 0.05). We conclude that in failing human hearts a decrease in beta-receptor density leads to subsensitivity of the beta-adrenergic pathway and decreased beta-agonist-stimulated muscle contraction. Regulation of beta-adrenergic receptors may be an important variable in cardiac failure.

https://www.nejm.org/doi/pdf/10.1056/NEJM198207223070401?listPDF=true

Decreased catecholamine sensitivity and beta-adrenergic-receptor density in failing human hearts.

Various applications of pAx measurements are discussed based on the hypothesis that drugs and drug antagonists compete for receptors according to the mass law. Examples are given illustrating the use of pAx measurements to identify agonists which act on the same receptors and to compare the receptors of different tissues. Tests of competitive and noncompetitive antagonism are considered in relation to the antagonisms acetylcholine-atropine, histamine-atropine and acetylcholine-cinchonidine. A new measure, pAh, is introduced to express the activity of unsurmountable antagonists.

Some quantitative uses of drug antagonists. 1958.

A study of the desensitization produced by acetylcholine at the motor end-plate.

The traditional receptor-stimulus model of agonism began with a description of drug action based on the law of mass action and has developed by a series of modifications, each accounting for new experimental evidence. By contrast, in this paper an approach to modelling agonism is taken that begins with the observation that experimental agonist-concentration effect, E/[A], curves are commonly hyperbolic and develops using the deduction that the relation between occupancy and effect must be hyperbolic if the law of mass action applies at the agonist-receptor level. The result is a general model that explicitly describes agonism by three parameters: an agonist-receptor dissociation constant, KA; the total receptor concentration, [R0]; and a parameter, KE, defining the transduction of agonist-receptor complex, AR, into pharmacological effect. The ratio, [R0]/KE, described here as the 'transducer ratio', tau, is a logical definition for the efficacy of an agonist in a system. The model may be extended to account for non-hyperbolic E/[A] curves with no loss of meaning. Analysis shows that an explicit formulation of the traditional receptor-stimulus model is one particular form of the general model but that it is not the simplest. An alternative model is proposed, representing the cognitive and transducer functions of a receptor, that describes agonist action with one fewer parameter than the traditional model. In addition, this model provides a chemical definition of intrinsic efficacy making this parameter experimentally accessible in principle. The alternative models are compared and contrasted with regard to their practical and conceptual utilities in experimental pharmacology.

Operational models of pharmacological agonism

A wide variety of tissues undergo a change of functional state on exposure to noradrenaline or adrenaline. Those molecular constituents of the effector cells of a tissue with which molecules of these catecholamines must first interact in order to produce a change of state — or response — of the tissue, are the so-called adrenoceptors (also commonly called adrenergic receptors). For convenience, we refer to noradrenaline, adrenaline and other agents which produce responses in tissues by interacting with adrenoceptors, as adrenergic agonists. An agent which specifically inhibits a response produced by an adrenergic agonist is referred to as adrenergic blocking agent or adrenergic antagonist.

The Classification of Adrenoceptors (Adrenergic Receptors). An Evaluation from the Standpoint of Receptor Theory

It is usually supposed that a drug combines with some part of a tissue as a necessary step in producing its action on that tissue. This view was expressed by Langley in 1878 while discussing experiments with pilocarpine and atropine. He later referred to “receptive substance” (Langley, 1905) and subsequently the term “receptor” has been widely used to express the same concept. Clark (1937a) attempted to put the concept on a quantitative basis by applying the adsorption isotherms of Langmuir which were derived from the application of the mass laws to the adsorption of gases on metal surfaces. The derivation of the equations as they apply to drugs in solution, and receptors or active patches in tissue, may be put as follows: Drug molecules combine with receptors at a rate which is proportional to the concentration of molecules in solution and to the number of free receptors. The resulting complex breaks down at a rate proportional only to the number of complexes. Thus the rate of combination is k1 A (l – y) and the rate of dissociation is k2y, where k1 and k2 are constants, A the concentration of drug, and y the proportion of receptors occupied by the drug. When the two rates are equal we have, putting K for 





1



When y = 0.5, then





1a



Equation 1 therefore relates the concentration of drug and the proportion of receptors which are occupied by drug molecules at equilibrium. Clark, however, went a step further than this and, by taking y as the response, used equation 1 to relate the response of the tissue to the concentration of drug. Thus there was in Clark's treatment the implicit assumption that the percentage of receptors occupied is equal to the percentage response of the tissue. When 50% of the receptors are occupied there is 50% of the possible response; when the response is maximal all the receptors are occupied.

Clark sought to show that the relation between concentration of drug and the effect produced did in fact agree with equation 1, and his apparent success was some justification for the assumption. The two situations he investigated were (a) the contraction of the isolated frog rectus abdominis produced by acetylcholine, and (b) the inhibition produced by acetylcholine of the contraction of strips of electrically stimulated frog ventricle. These tests have a common disadvantage in that both tissues contain enzymes which destroy acetylcholine, so that the concentration of drug in equilibrium with the receptors is less than that applied externally, and, though the applied concentration is known, that in the tissue is not. The receptors, indeed, may not all be in equilibrium with the same concentration of acetylcholine; a gradient is presumably established with a high concentration near the surface and a lower concentration at the centre of the tissue—because the further the acetylcholine penetrates the greater is its chance of being destroyed. Such a gradient would produce a log-concentration-effect curve flatter than the true curve. Furthermore, it cannot be assumed that the ratio of the applied concentration to the effective concentration (or concentrations) is constant for different applied concentrations. A block of cholinesterase activity would therefore be expected to alter the slope of log-concentration-effect curves; Webb (1950) has shown that this is so, and that the change of slope is considerable. Calculation from his graphs shows that, when the cholinesterase of isolated rabbit auricles is blocked with physostigmine, the slope of the acetylcholine log-concentration-effect curve is about 10 times steeper than on normal auricles (see Table I). An attempt has been made to determine the relation between log concentration and effect for acetylcholine and the frog rectus after blocking the cholinesterase with TEPP so that the size of the effect could be assessed in one of the systems used by Clark. This proved to be difficult, since the response of the rectus, always slow, became very much slower after TEPP, and the contractions continued to increase for an hour after acetylcholine was applied. The action of carbachol on normal recti was similarly slow. It was therefore impossible to get an accurate estimate of slope, but it was evident that the increase of effect with increased concentration was greater than required by equation 1. In any case it was clear that reliable evidence in support of any particular equation would be difficult to obtain from this tissue.



Table I

Slopes of Concentration-Effect Curves



Clark discussed several other drug-tissue systems, but he was mainly concerned to show that a graded response was produced over a wide range of concentration and, in most of the examples he quoted, the slopes of the log concentration curves are not in accord with equation 1. A simple way of reducing data about concentration-effect curves to a single figure which is independent of any theory of drug action, and which can therefore be used to test any particular theory, is to use, as Clark did, the ratio of concentrations causing specified effects. Clark used the ratio for 84% and 16% of the maximum contraction; but a narrower range is easier to cover, and there is no need to restrict the ratio measured to any particular pair of responses. Evidence from which such ratios can be calculated is rare, because the maximum response is seldom recorded, but a few examples are shown in Table I. Some of these ratios represent new data for acetylcholine and histamine on guinea-pig ileum; details are given in the section on results. Furchgott and Bhadrakom commented on the theoretical implication of the slope, and considered that their results agreed with equation 1; the ratio given in Table I expresses this agreement. The other ratios in Table I have been calculated from published graphs: they are not in agreement with equation 1.

A variety of slopes can be accounted for by modifications of equation 1 obtained by postulating that a molecule of drug combines with either less or more than one receptor. The latter possibility may be true in some instances, but the postulate is unlikely to be generally correct—particularly where fractions are involved. Equation 1 appears to be the most likely relation between concentration of drug and the percentage of receptors occupied, but Table I shows that there is no experimental justification for supposing that it represents a general relation between concentrations of drug and the response of the tissue. It therefore becomes necessary to consider possibilities other than the one assumed by Clark for the relation between the proportion of receptors occupied and the response, and we can broaden the application of the receptor theory by the following hypotheses:


A maximum effect can be produced by an agonist when occupying only a small proportion of the receptors.


The response is not linearly proportional to the number of receptors occupied.


Different drugs may have varying capacities to initiate a response and consequently occupy different proportions of the receptors when producing equal responses. This property will be referred to as the efficacy of the drug.



The concept of efficacy, which may vary from zero to a large positive value, is at variance with Clark's (1937c) view of the action of a drug at a receptor as all or none; an agonist (Reuse, 1948) causes an effect and an antagonist causes no effect so that the activity of a drug of either kind is simply a measure of its affinity for the receptors. According to hypothesis 3, however, this remains true only for antagonists (which have zero efficacy): the activity of agonists is the product of their affinity and their efficacy. A drug with very low efficacy may produce a response which is less than the maximum, even when it is occupying nearly all the receptors—and because it is occupying the receptors, it diminishes the action of a drug with high efficacy added simultaneously. Compounds with such a low efficacy that they possess properties intermediate between agonists and antagonists will be called “partial agonists.” (See p. 384.)

Ariens and Groot (1954) have also observed substances with these intermediate properties, and they ascribe this to a low “intrinsic activity.” Ariens and Groot, however, continue to assume that all normal agonists have the same “intrinsic activity” and their concept is more limited in application than hypothesis 3.

The above hypotheses account for the progressive variation in properties in a series of alkyltrimethylammonium salts.

The lower homologues have acetylcholine-like activity whereas the higher members have an atropine-like blocking action (Raventos, 1937a and b; Clark and Raventos, 1937). It was expected that the members of such a simple homologous series would display some progressive change in their affinity for the receptors, and consequently, if Clark was correct, the reciprocals of the concentrations of the agonists causing a 50% response (see equation 1a) would form part of the same progression as the affinity constants of the antagonists. The demonstration of a sharp discontinuity between the two sets of constants, such that the agonists are effective in lower concentration than the antagonists, would, however, constitute good evidence for hypothesis 1. Raventos showed that the acetylcholine-like activity of the lower homologues varied rather irregularly, but calculation from his data suggested that the expected discontinuity between agonist and antagonist activity did occur. The nature of the evidence did not allow a firm conclusion. It was therefore decided to re-examine the compounds, with the intention of obtaining accurate values for the affinity constants of the first 4 or 5 antagonists, and to compare these with the reciprocals of the concentrations of the agonists which cause a 50% contraction.

https://bpspubs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1476-5381.1956.tb00006.x

A modification of receptor theory

https://bpspubs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1476-5381.1967.tb01989.x

The affinity and efficacy of onium salts on the frog rectus abdominis.

Two sets of series of compounds, RN(+)Me(3), RN(+)Me(2)Et, RN(+)MeEt(2), RN(+)Et(3), and R'N(+)Me(3), R'N(+)Me(2)Et, R'N(+)MeEt(2), R'N(+)Et(3), have been prepared, in which R is a 2-(diphenylacetoxy)ethyl, 2-(benziloyloxy)ethyl, 2-(2,2-diphenylethoxy)ethyl, 3-(diphenylmethoxy)propyl or 3,3-diphenylbutyrylmethyl group, and R' is a 2-acetoxyethyl, 2-ethoxyethyl, 3-methoxypropyl or butyrylmethyl group: compounds of the first set therefore differ from those of the second set in that they contain a diphenylmethyl group (or a benziloyl group) in place of a methyl group. The former compounds are antagonists of acetylcholine whereas most of the latter act like acetylcholine. The affinity constants of the former compounds for the acetylcholine receptors of the guinea-pig ileum have been determined and the equipotent molar ratios relative to acetylcholine have been measured for the latter compounds. The variation of the affinity constant with the constitution of the onium group in the antagonists (the diphenylmethyl compounds) was sufficiently consistent from one series to another for it to seem likely that corresponding changes in affinity with the constitution of the onium group would occur in the agonists. From the relative activity of the agonists and with this knowledge of relative affinity it was possible to assess the effects of their structure on efficacy. Substitution of one methyl in the onium group by an ethyl group in these compounds increased affinity but decreased efficacy. The replacement of a second methyl by a second ethyl group had little effect on affinity but decreased efficacy still further. The replacement of the ester link in acetylcholine by a 4-ether oxygen atom (as in the diphenylmethoxypropyl and methoxypropyl compounds) did not appreciably reduce affinity but markedly reduced efficacy, whereas the replacement of the ester link by a 3-ether oxygen atom (as in the diphenylethoxyethyl and ethoxyethyl compounds) markedly reduced affinity but did not reduce efficacy. The diphenylbutyrylmethyl compounds had low affinity and the butyrylmethyl compounds had low efficacy. We conclude that the action of acetylcholine at the postganglionic parasympathetic receptors in the guinea-pig ileum depends upon the presence of the 4-carbonyl group (and presumably the onium group) for affinity and on the 3-ether oxygen atom and the trimethylammonium group for efficacy.

https://bpspubs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1476-5381.1963.tb02019.x

An attempt to study the effects of chemical structure on the affinity and efficacy of compounds related to acetylcholine

Analgesic action in a series of tv‐substituted ethyl 4‐phenylpiperidine‐4‐carboxylates

The analgesic potency of a series of new compounds related to pethidine has been measured in rats. The replacement of the methyl group on the nitrogen atom of pethidine by groups containing ether linkages produces an increase in potency. Two different positions for the oxygen atom are particularly beneficial, but the effects, in these two positions, appear to be exerted in different ways and are not additive. Simple toxicity tests in mice and rats and tests on respiration in rats of five of the more potent compounds revealed no unexpected adverse action.

R. P. Stephenson

Papers

A modification of receptor theory

The affinity and efficacy of onium salts on the frog rectus abdominis.

An attempt to study the effects of chemical structure on the affinity and efficacy of compounds related to acetylcholine

Analgesic action in a series of tv‐substituted ethyl 4‐phenylpiperidine‐4‐carboxylates

Analgesic action of ethyl 4‐phenylpiperidine‐4‐carboxylates with oxygenated 1‐substituents