A test of the null equation for functional antagonism

A comparison of effects measured with isotonic and isometric recording: II. Concentration-effect curves for physiological antagonists

If one drug, B, antagonizes another, A, by producing the opposite physiological effect, the antagonist concentration-effect curves should be affected by the recording system, which limits the range of agonist responses. With pieces of isolated guinea-pig ileum taken from adjacent parts of the same animal, one recorded isotonically, the other isometrically with the same load, the isotonic IC(50) values for (-)isoprenaline opposing carbachol or histamine were lower than the isometric values (P<0.01) but there was a significant correlation between them (P<0.01): the isotonic curves were steeper (P<0.01) and there were wider shifts in IC(50) before increasing the agonist reduced the maximum relaxation. In similar experiments with pieces of rat uterus in oestrus from the same animal, the concentration-effect curves for carbachol opposed by increasing concentrations of (-)isoprenaline or (-)adrenaline had slightly lower EC(50) values with isometric recording but there was a significant correlation (P<0.01) with isotonic values. The antagonist effect (ratio of the EC(50) relative to that for the control) was higher with isotonic recording (P<0.01 for (-)isoprenaline, P<0.025 for (-)adrenaline) and all (27) curves were steeper than the corresponding isometric curve (P<0.001). The influence of the method of recording on the results is expected from the narrower operational window and smaller upper limit to relaxation with isotonic recording. A way of obtaining measurements of IC(50) against a standard agonist effect is suggested in an Appendix.

Extended concentration-response curves used to reflect full agonist efficacies and receptor occupancy-response coupling ranges

1. An approach is described for generating extended agonist concentration-response curves where the responses are unconstrained by the normal tissue maximum response. Functional antagonism is employed to hold the tissue state in the range where any change in stimulus can be translated into a measurable response. 2. The maximum response of these extended concentration-response curves provides an index of intrinsic activity reflecting the agonist efficacy and the receptor occupancy-response coupling range. 3. The use of this approach is illustrated with extended concentration-response curves for noradrenaline (NA), vasopressin, acetylcholine (ACh), and 5-methylfurmethide in the small mesenteric and tail arteries of the rat. Both NA and vasopressin can maximally activate the arteries, but the new protocol shows that NA can produce more cellular activation than vasopressin in the tail artery. Both ACh and 5-methylfurmethide are full agonists but ACh has a higher intrinsic activity than 5-methylfurmethide. The ACh muscarinic receptors in the mesenteric artery have a larger occupancy-response range than the ACh muscarinic-receptors in the tail artery, and the alpha-adrenoceptors in the tail artery appear to have a larger occupancy-response coupling range than those in the mesenteric artery. 4. This approach extends our ability to compare the efficacies of full agonists, and to compare the occupancy-response coupling ranges of receptors that can normally maximally activate the assay tissue. This is achieved without the use of an irreversible antagonist and should be applicable to many receptors and pharmacological assay systems where responses are stable and functional antagonists are available.

Interactions between responses mediated by activation of adenosine A2 receptors and alpha 1-adrenoceptors in the rabbit isolated aorta

1. This paper describes aspects of the functional antagonism between the responses mediated by activated alpha 1-adrenoceptors and adenosine A2 receptors in the adventitia- and endothelium-denuded aorta of the rabbit. 2. Adenosine A2 receptor agonists relaxed aortic rings pre-contracted with phenylephrine. The relaxation response was agonist concentration-dependent and saturable. The respective contractile and relaxation responses were stable, reproducible, and reversible. 3. Increasing the phenylephrine concentration caused a progressive attenuation of the action of adenosine A2 receptor agonists, consisting of a decreased maximal response and a dextral shift of the adenosine agonist concentration-response curve. This functional antagonism could be completely reversed upon removal of adenosine by either the addition of adenosine deaminase or by wash-out of the adenosine agonist from the tissue. The relaxation response to the adenosine A2 receptor partial agonists, N6-cyclohexyladenosine and R-(-)-N6-(2-phenylisopropyl)adenosine, was abolished at higher phenylephrine concentrations (e.g. 30 EC50). 4. A 1000 fold increase in the adenosine concentration was required to shift the value of the EC50 of phenylephrine six fold, while a similar increase in the value of the EC50 of adenosine could be elicited by only a 32 fold increase in the phenylephrine concentration. A 30 fold increase in the phenylephrine concentration shifted the value of the EC50 of 5'-N-ethylcarboxamidoadenosine four fold. 5. Analysis of the functional antagonism between the responses mediated by these receptors using the Black & Leff (1983) operational model of agonism allowed for the estimation of the agonist dissociation constant, KA, and the apparent efficacy, tau, for both phenylephrine and adenosine A2 receptor agonists. Increasing the concentration of phenylephrine reduced the value of tau for adenosine agonists in a concentration-dependent and saturable manner. Similarly, increasing the concentration of adenosine reduced the value of tau for phenylephrine in a concentration-dependent and saturable manner. The phenylephrine KA value obtained by the method of functional antagonism (1.9 microM) was similar to that obtained by the receptor inactivation method (2.1 microM). 6. Partial occlusion of the alpha 1-adrenoceptor by the alkylating agent, dibenamine, demonstrated that the magnitude of the adenosine A2 receptor-mediated relaxation was inversely proportional to the number of functional alpha 1-adrenoceptors. 7. It is concluded that the magnitude of functional antagonism is proportional to the stimulus elicited through either receptor. We propose that this tissue preparation and pair of receptors is a good model to study quantitative aspects of functional antagonism between activated receptors.

Dissociation constants and relative efficacies estimated from the functional antagonism of beta-adrenoceptor agonists on transmural stimulation in rat vas deferens

In rat vas deferens, the beta-adrenoceptor agonists, isoprenaline, salbutamol, terbutaline and fenoterol, inhibited the contractions elicited by transmural electrical stimuli to an extent which depended on voltage magnitude. The dose-response curves for these agonists were shifted slightly to the right when the amplitude of the twitch was increased up to twice its original size. Consequently, the values of pD2 for the respective agonists obtained with a '50% twitch' and a '100% twitch' were: 7.6 and 7.5, 6.8 and 6.6, 6.2 and 6.0, 7.6 and 7.4. In addition, the maximal effects of the agonists were significantly reduced (by 13-18%). The values of other parameters for the same agonists, calculated according to a model for functional antagonism, were: negative log of dissociation constants (pKa): 7.2, 6.3, 6.2 and 7.2; percent receptors occupied to induce a half-maximal effect (y50): 42, 46, 63, and 53; relative intrinsic efficacy (er): 1.0, 0.9, 0.7, and 0.8. The usefulness of this type of functional antagonism as an alternative method for the estimation of drug-receptor parameters is discussed in the light of receptor theory.

Experimental testing of Mackay's model for functional antagonism in the isolated costo-uterus of the rat

Several key predictions of a recently developed model for functional antagonism (Mackay, 1981) were experimentally tested using the rat isolated costo-uterine preparation. In the presence of the functional antagonist fenoterol (Fen), the functional constants (KAF) for carbachol and oxotremorine (Oxo) were respectively 9.9 and 3.4 fold greater than their corresponding affinity constants (KA). According to Mackay's model for functional antagonism, the higher KAF/KA ratio for carbachol indicates that this cholinoceptor agonist has a greater efficacy than Oxo. This was confirmed by using conventional pharmacological methods. As predicted from the model of functional antagonism, the plot of KAF/KA-1 against the fraction of cholinoceptors not irreversibly blocked by phenoxybenzamine (Pbz) was linear for both carbachol and Oxo and the lines of best fit crossed the axes at a point not significantly different from the origin. The value of 4.6 for the relative efficacy of carbachol to Oxo estimated from functional antagonism studies was comparable to the value of 5.6 calculated using the method of irreversible antagonism proposed by Furchgott (1966).

The quantification of relative efficacy of agonists

While much pharmacological effort has been expended in the measurement, quantification, and comparison of agonist affinity for the classification of drugs and drug receptors, inordinately less emphasis has been placed on the quantification of the other property of agonists, namely intrinsic efficacy. This is unfortunate as the existing studies of the relative intrinsic efficacy of agonists show this to be a most useful scale for the classification of agonists and the prediction of tissue responses. This paper will review some of the theories that describe efficacy on a molecular level, the methods of measuring relative efficacy, and the factors in these procedures which can lead to artifacts and misleading information for the classification of drug receptors. Lastly, the value of the quantification of efficacy will be discussed in terms of the design of agonists for therapeutic advantage.

Disadvantages of cocaine as a neuronal uptake blocking agent: comparison with desipramine in guinea-pig right atrium

Cocaine and desipramine (DMI) are widely used as neuronal uptake blocking agents in studies of cardiac sympathetic transmission in isolated tissue preparations. It is generally assumed that these pharmacological tools do not alter transmitter release or postjunctional effector response. To test this assumption, we have compared the effects of cocaine and DMI on rate responses to sympathetic nerve stimulation and exogenous noradrenaline in guinea-pig isolated right atria. Right atria were equilibrated with the irreversible alpha-adrenoreceptor antagonist benextramine to prevent any effect of presynaptic alpha-adrenoreceptors. Cumulative (-) noradrenaline concentration-response curves were shifted to the left by DMI (0.01-1 microM) without significant change in the resting or maximum rates. Cocaine (1-100 microM) also caused sensitisation to noradrenaline but caused a biphasic change in resting atrial rate. In addition there was a small but significant depression of the maximum rate at cocaine 10 and 100 microM. Sympathetic nerve stimulation was achieved by applying trains of 1, 2 and 4 electrical field pulses delivered during one atrial refractory period. DMI caused a concentration dependent potentiation of responses to field stimulation. Cocaine (1 microM) caused significant enhancement of peak responses to field stimulation but no further enhancement and indeed depressed peak responses were observed at cocaine 10 and 100 microM respectively. The time for atrial period to return halfway to baseline after field stimulation (t 1/2) was enhanced by cocaine in a concentration dependent manner as was observed with DMI. We conclude that cocaine (but not DMI) decreases the maximum response to exogenous noradrenaline (postjunctional depression). The reduction of the peak response to sympathetic nerve stimulation in the presence of cocaine to below control responses suggests that cocaine also depresses the release of transmitter. These additional depressant properties of cocaine, which occur in a concentration range of neuronal uptake block, are important disadvantages and should discourage its use in experiments on sympathetic transmission.

Determination of agonist affinity for cardiac beta-adrenoceptors during reserpine-induced supersensitivity

The positive inotropic responses to orciprenaline of paced left atria and papillary muscles and the positive chronotropic responses of spontaneously beating right atria of guinea-pigs were recorded. Tissues from guinea-pigs pretreated with reserpine exhibited supersensitivity to beta-adrenoceptor stimulation as demonstrated by the significantly lower EC50 values to orciprenaline. The antagonism of these responses by Ro 03-7894 was characteristic of irreversible beta-adrenoceptor blockade--depression of maximum response and resistant to washout. The dissociation constants (KA) of orciprenaline were determined from the concentration-response curves obtained before and after Ro 03-7894, and no difference was found between tissues from untreated and reserpine-pretreated animals. KA values were also determined from the functional antagonism of the responses by carbachol. The degree of antagonism was less after reserpine pretreatment, but there was no increase in KA value. The reserpine-induced supersensitivity does not therefore appear to be due to an increase in agonist affinity for the cardiac beta-adrenoceptor.