Effects of 5-hydroxytryptamine on pial arteriolar calibre in anaesthetized cats.

1. The responses of individual pial arterioles and small arteries to perivascular injections of 5-hydroxytryptamine were studied in anaesthetized cats.2. Pial arteriolar calibre was measured by a television image-splitting technique.3. At normotension, the average response of pial vessels was dilatatory at injected 5-hydroxytryptamine concentrations between 10(-10) and 10(-4)M.4. These pial vascular changes were, however, dependent upon the resting vessel calibre: those arterioles < 70 mum dilated universally, whereas small arteries >/= 200 mum in resting diameter tended to constrict in response to 5-hydroxytryptamine.5. The 5-hydroxytryptamine-induced dilatation of pial arterioles < 70 mum in resting calibre was directly dependent on mean arterial pressure, although no such relationship was observed with the larger cerebral vessels.6. It is concluded that the specific cerebrovascular actions of 5-hydroxytryptamine are largely tone-dependent. The greater the resting tone of a pial arteriole, then the greater will be the tendency of that vessel to dilate in response to 5-hydroxytryptamine, and vice versa.7. The direct vascular actions of 5-hydroxytryptamine are in contradistinction to the observed changes in cerebral tissue perfusion and it would therefore appear that the effects of this amine on cerebral metabolism and neuronal activity outweigh its cerebrovascular actions under physiological conditions.