Unsurmountable antagonism to 5-hydroxytryptamine in rat kidney results from pseudoirreversible inhibition rather than multiple receptors or allosteric receptor modulation

Experiments were conducted in the isolated perfused rat kidney to determine the mechanism of unsurmountable antagonism by metergoline toward the vasoconstrictor response to 5-hydroxytryptamine (5-HT). Three possible mechanisms were investigated: 1) multiple receptors for 5-HT; 2) pseudoirreversible inhibition; and 3) allosteric modulation of 5-HT receptors. Metergoline and six other 5-HT antagonists acted as potent unsurmountable antagonists, only (-)-propranolol acted competitively (pA2 = 6.5). Multiple receptors for 5-HT were ruled out as the mechanism of unsurmountable antagonism because 5-HT and four 5-HT analogs gave similar pA2 values with (-)-propranolol. In addition, the KA value for 5-HT (185 nM) and the order and relativity of agonist potency suggest the presence of a single 5-HT2 receptor. Pseudoirreversible inhibition, over allosteric modulation of the 5-HT2 receptor, is suggested from experiments in which reversal of metergoline-induced inhibition by (-)-propranolol (studied in the presence of metergoline) was matched, in magnitude, by wash-out of metergoline. (-)-Propranolol is suggested to mimic wash-out by occupying and protecting receptors from which metergoline has dissociated. This is accomplished by virtue of its faster kinetics, on-off the 5-HT receptor, allowing restoration of 5-HT responses via a competitive interaction. Pseudoirreversible antagonism, over allosteric receptor modulation, offers an explanation that does not involve the postulation of allosteric effector sites or changes in the state of the 5-HT2 receptor.