Sensitivity changes of adrenergic -receptors induced by alterations of the metabolic state of isolated organs.

Abstract

Experiments on isolated organs-rabbit ileum, guinea-pig trachea and atrium — with different “β-receptor subtypes” were carried out in order to determine whether changes of the metabolic state or of the extracellular pH were able to alter the responsiveness of these receptors to sympathomimetic amines. 1. Guinea-Pig Atrium. Lowering of the pH of the bath fluid from 7.48 to 6.79 resulted in a significant decrease of the affinity of isoprenaline (IPN) calculated as the pD2-value. Metabolic inhibition induced by iodoacetic acid (IAA) (5×10−5 M) increased the affinity of Th 1165a. 2. Rabbit Ileum. The affinity of IPN was not changed by lowering the pH, even to values of 6.11. At this pH the spontaneous motility was already markedly impaired. IAA (2.4×10−5 M) caused a moderate increase in the affinity of IPN whereas those of Th 1165a and orciprenaline (OPN) were elevated 10-fold. 3. Guinea-Pig Trachea. Lowering of the pH caused a decrease of the pD2-values of IPN and in particular of Th 1165a. The metabolic inhibitor IAA had no influence upon the pD2 value of IPN. 4. The present results favour the existence of only one type of β-receptor which changes its sensitivity depending on the metabolic state. It seems therefore very likely that the affinity of sympathomimetic drugs depends not only on the structure of the β-receptor but also on the metabolic state of the tissue.