The evolution of vascular smooth muscle responses to histamine and 5-hydroxytryptamine. 3. Manifestation of dual actions of either amine in reptiles.

Abstract

AbstractStudies on the vascular smooth muscle responses to histamine and 5‐hydroxytryptamine and on the distribution of these amines in different tissues were performed in several species of reptiles. The responses to adrenaline, noradrenaline and acetylcholine were studied for comparison. Vascular actions were recorded both in vivo as changes in systemic arterial blood pressure and in vitro as changes in vascular resistance in perfused preparations. The specificity of the action of each drug was evaluated by means of pharmacological blocking agents. Tissue levels of histamine and 5‐hydroxytryptamine were determined spectrofluorometrically. Fluorescence microscopy was used for histochemical localization of the histamine stores. The results indicate that histamine and 5‐hydroxytryptamine exert dual actions (inhibitory and stimulatory) on reptilian vascular smooth muscles which have thus acquired response patterns towards these amines similar to those found in mammals. Inhibitory as well as stimulatory actions of adrena line, noradrenaline and acetylcholine are also present in reptiles. Levels of 5‐hydroxytrypt amine are apparently low in most tissues, whereas many species show very high tissue levels of histamine. Histamine, except that of the stomach, is mainly located in tissue mast cells and blood basophils. Nutritional‐dependent variations in mast cell number are parallelled by varia tions in tissue histamine content. By comparing the present results with those previously ob tained in jawless vertebrates, fish and amphibians, a general outline of the evolution of vascular actions and tissue stores of histamine and 5‐hydroxytryptamine is presented.