Effects of 5‐hydrooxytryptamine on pial arteriolar calibre in anaesthetized cats

Abstract

The responses of individual pial arterioles and small arteries to perivascular injections of 5-hydroxytryptamine [5-HT] were studied in anesthetized cats. Pial arteriolar caliber was measured by a television image-splitting technique. At normotension, the average response of pial vessels was dilatatory at injected 5-HT concentrations between 10-10 and 10-4 M. These pial vascular changes were dependent upon the resting vessel caliber: those arterioles < 70 .mu.m dilated universally, whereas small arteries .gtoreq. 200 .mu.m in resting diameter tended to constrict in response to 5-HT. The 5-HT induced dilatation of pial arterioles < 70 .mu.m in resting caliber was directly dependent on mean arterial pressure, although no such relationship was observed with the larger cerebral vessels. The specific cerebrovascular actions of 5-HT are largely tone-dependent. The greater the resting tone of a pial arteriole, the greater will be the tendency of that vessel to dilate in response to 5-HT, and vice versa. The direct vascular actions of 5-HT are in contradistinction to observed changes in cerebral tissue perfusion; the effects of this amine on cerebral metabolism and neuronal activity seem to outweigh its cerebrovascular actions under physiological conditions.