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This study sets out to characterise calcitonin gene-related peptide (CGRP) receptors in human and porcine isolated proximal and distal coronary arteries using BIBN4096BS. Human (h)-alphaCGRP induced relaxations that were blocked by BIBN4096BS in all arteries studied. In contrast to the other vessels, the Schild plot slope in the human distal coronary artery segments (0.68 +/- 0.07) was significantly less than unity and BIBN4096BS potently blocked these responses (pK(b) (10 nM): 9.29 +/- 0.34, n = 5). In the same preparation, h-alphaCGRP(8-37) behaved as a weak antagonist of h-alphaCGRP-induced relaxations (pK(b) (3 microM): 6.28 +/- 0.17, n = 4), with also a Schild plot slope smaller than unity. The linear agonists, [ethylamide-Cys(2,7)]-h-alphaCGRP ([Cys(Et)(2,7)]-h-alphaCGRP) and [acetimidomethyl-Cys(2,7)]-h-alphaCGRP ([Cys(Acm)(2,7)]-h-alphaCGRP), had a high potency (pEC(50): 8.21 +/- 0.25 and 7.25 +/- 0.14, respectively), suggesting the presence of CGRP(2) receptors, while the potent blockade by BIBN4096BS (pK(b) (10 nM): 10.13 +/- 0.29 and 9.95 +/- 0.11, respectively) points to the presence of CGRP(1) receptors. Using RT-PCR, mRNAs encoding for the essential components for functional CGRP(1) receptors were demonstrated in both human proximal and distal coronary artery. Further, h-alphaCGRP (100 nM) increased cAMP levels, and this was attenuated by BIBN4096BS (1 microM). The above results demonstrate the presence of CGRP(1) receptors in all coronary artery segments investigated, but the human distal coronary artery segments seem to have an additional population of CGRP receptors not complying with the currently classified CGRP(1) or CGRP(2) receptors.

This chapter discusses the blockade of agonist-induced response through interaction with receptors. Antagonism can be classified operationally in terms of the effects of antagonists on agonist dose-response curves and mechanistically in terms of the molecular effects of the antagonist on the receptor protein. The interference of agonist-induced response can take different forms in terms of the effects on agonist dose-response curves. Specifically, concentration-dependent antagonism can be saturable or apparently unsaturable. The antagonism can be surmountable (dextral displacement of the dose-response curve with no diminution of maxima) or insurmountable (depression of the maximal agonist response). Antagonism of receptors can produce many patterns of concentration-response curves for agonists, including concentration-dependent surmountable antagonism, surmountable antagonism that comes to a maximal limit, depression of dose-response curves with no dextral displacement, and dextral displacement before depression of maximal response in systems with a receptor reserve for the agonist. These patterns are recognized as behaviors of antagonists in different systems and not necessarily characteristics of the molecular nature of the antagonism (i.e., more than one molecular mechanism can produce the same behavior of the concentration-response curves). Therefore, it is important to discover the molecular mechanism of the antagonism and not just describe the antagonistic behavior, as the latter can change with experimental conditions. Kinetic factors can cause some antagonists to produce surmountable antagonism in some systems and insurmountable antagonism in others.

This chapter explains allosteric drug antagonism. Allosteric effects are defined by the interaction of an allosteric modulator at a so-called allosteric binding site on the protein to affect the conformation at the probe site of the protein. Since the probe and modulator molecules do not interact directly, their influence on each other must take place through a change in shape of the protein. Allosteric effects have been studied and described for enzymes. A major molecular mechanism of receptor antagonism involves the binding of the antagonist to its own site on the receptor separate from the binding site of the endogenous agonist. When this occurs, the interaction between the agonist and antagonist takes place through the receptor protein. This is referred to as an allosteric interaction. Allosteric modulators affect the interaction of the receptor and probe molecules by binding to separate sites on the receptor. These effects are transmitted through changes in the receptor protein. These modulators possess properties different from orthosteric ligands. Specifically, allosteric effects are saturable and probe dependent. Allosteric effects result in changes in affinity and efficacy of agonists. Sole effects on affinity (with no change in receptor function) result in surmountable antagonism. The dextral displacement reaches a maximal value leading to a curvilinear Schild regression.

Publisher Summary This chapter describes stimulatory pathways in the regulation of gastric acid secretion. In a study described in the chapter, gastric acid secretion in the isolated stomach of the mouse was measured. The mice were anaesthetized with ether, and the stomach was exteriorized. An incision was made at the pyloric sphincter, and a polythene cannula was inserted and tied into the stomach. A second incision was made in the forestomach and then the content washed out with warm mucosal solution. A second cannula was also inserted and tied into the stomach via this incision. The esophagus was ligated. The stomach was rapidly dissected out and placed into an organ bath containing 40 ml serosal solution—modified buffered Krebs–Henseleit—vigorously gassed with 95% O 2 + 5% CO 2 . Dose-ratios were estimated, and Schild plots constructed where values of log(DR-1) were plotted against log antagonist concentration. Least squares linear regression analysis of the Schild plot was used to estimate pA 2 -values and the slope of regression line with confidence limits.