Neurogenic dilator and constrictor responses of pial arteries in vitro. Differences between dogs and sheep.

Abstract

Two distinct responses to transmural electrical stimulation of cerebrovascular smooth muscle isolated from dogs and sheep were identified. The responses were blocked with tetrodotoxin and were attributed to stimulation of intramural nerves. A constrictor response to transmural nerve stimulation (TNS) which was blocked with guanethidine was attributed to stimulation of sympathetic nerves. In cerebral arteries of the sheep the constrictor response was blocked by phentolamine but not in dog cerebral vessels. Phentolamine (10-6 M) potentiated the response to TNS. Postsynaptic .alpha.-adrenergic receptors of dog cerebrovascular smooth muscle may not be susceptible to phentolamine. The increased response to TNS may be due to phentolamine presynaptic receptor blockade, causing increased transmitter release. After blockade of the constrictor response to TNS and in the presence of smooth muscle tone, a dilator response, much more prominent in sheep, was seen. The dilator response was not blocked by guanethidine, propranolol or phentolamine, and indicates that cerebral vessels may be innervated by nonsympathetic nerves. The possibility that the nerves are cholinergic was investigated, but the dilator response was not blocked by atropine or potentiated by physostigmine. The nature of the transmitter for this dilator response remains unclear. Some of the marked inconsistencies in studies of neurogenic control of the cerebral circulation may be attributed in part to qualitative and quantitative differences in neuroeffector mechanisms among species.