KRAX is important operational determinant of tissue response. KRAX as parameter for classification

Interpretation of agonist affinity estimations: the question of distributed receptor states

In the receptor-transducer model of pharmacological agonism, rejection of the traditional assumption that receptor molecules are in vast excess of transducer molecules permits the receptors to become distributed among unbound, bound and complexed states. Under these conditions, agonist affinities are liable to be overestimated when the method of irreversible receptor antagonism is used. Graphical tests have been developed to detect distribution, and these were applied to experimental data from the interaction between 5-HT and phenoxybenzamine on aortic tissue. Significant receptor distribution was not detected by the method. However, in the model it was assumed that there was a linear relation between the concentration of ternary complex and pharmacological effect. If this relation was replaced with a saturable one the effect of receptor distribution would be masked. The implications for pharmacologists and medicinal chemists are discussed.

Affinities of full agonists for cardiac beta-adrenoceptors calculated by use of in vitro desensitization

Positive chronotropic and inotropic responses of guinea-pig isolated spontaneously beating right atria and paced left atria to beta-adrenoceptor agonists were recorded. Cumulative concentration-response curves for the rate and tension responses to isoprenaline, orciprenaline, terbutaline and fenoterol were obtained before incubation of the tissues with isoprenaline (10(-6) M), the tissues were then washed during 1 h to remove isoprenaline before constructing a post-incubation curve to the same agonist. Incubation with isoprenaline for 4 h caused parallel rightwards shifts to the curves, but extending the incubation to 8 h caused additional depression of the rate (85.2 +/- 5.4%) and tension (68.9 +/- 2.3%) maxima. Incubation with isoprenaline for 4 h only shifted the curves for orciprenaline to the right but depressed the post-incubation maximum rate and tension responses to terbutaline (74.8 +/- 1.5 and 33.8 +/- 2.5%) and fenoterol (85.6 +/- 5.5 and 76.0 +/- 5.1%). Thus incubation with isoprenaline for 4 h induced desensitization of the cardiac beta-adrenoceptor which was revealed as a loss in sensitivity to both isoprenaline and other beta-adrenoceptor agonists. The reduced maxima were attributed to a loss of beta-adrenoceptors and exhaustion of the receptor reserve. Those experiments displaying the reduced maxima were used for calculation of dissociation constants (KA) values by the method of Furchgott (1966) for irreversible antagonism. The values for the right atria were 57 (19-170)nM, 29 (8.4-100)microM and 13 (1.9-8.4)microM and for the left atria, 89 (15-510)nM, 25 (9.9-64)microM and 18 (7.7-42)microM for isoprenaline (8 h incubation), terbutaline and fenoterol respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Do pharmacological methods for the quantification of agonists work when the ternary complex mechanism operates?

It is well established that many receptors couple to G-proteins in order to subserve their pharmacological or physiological effects. In those systems it is possible that a ternary complex mechanism operates in which initiation of an effect depends on the concentration of agonist-receptor-G-protein complex formed. Such systems may be considered to obey a receptor-transducer model (Black & Leff, 1983, Proc. R. Soc. B220, 141). A theoretical analysis of this model is presented which seeks to determine how the operation of the ternary complex mechanism affects the quantification of agonists by conventional pharmacological methods. Previous analyses have concluded that pharmacological models may or may not accommodate the ternary complex mechanism depending upon the relationship between the relative concentrations of receptor and transducer units, [R0], and [T0] respectively. The present study extends these in two ways. It considers the impact of the ternary complex mechanism on agonist quantification under a more complete range of conditions relating [R0] and [T0], and it does so with regard to the analysis of partial agonists (by the comparative method) as well as of full agonists (by the method of receptor inactivation). The following predictions are made: (i) reliable estimates of affinity and efficacy can be made using the comparative method under the conditions [R0] much greater than [T0] and [R0] much less than [T0] whereas the inactivation method only works under the former condition; (ii) errors occur in the estimation of affinity and efficacy by both methods when [R0] = [T0] although better estimates are produced by the comparative method; (iii) when errors occur in the absolute estimation of affinity and efficacy, the orders of affinity and efficacy determined by the comparative method will generally be correct but this is not the case for the inactivation method; (iv) in general, the comparative method for agonist quantification appears to produce more reliable information for the purposes of receptor classification and medicinal chemistry than does the receptor inactivation method.