POSTGANGLIONIC SYMPATHETIC DELAY IN VASCULAR SMOOTH MUSCLE

¹ Departments of Pharmacology and Pathology, UCLA Center for the Health Sciences, Los Angeles, California

Two aspects of the contractile response of the isolated pulmonary artery of the rabbit to sympathetic nerve stimulation have been investigated: first, the latency between the commencement of repetitive nerve stimulation and the beginning of the contractile response and, second, the minimum number of indirect stimuli which will just evoke a response. These phenomena have been considered in conjunction with light and electron microscopic studies of the preparation which have shown that the transmitter-containing neurons in this tissue are aggregated in the adventitiomedial junction and that there is no close contact or synapse of these with the vascular muscle cells. When the sympathetic fibers were stimulated with supramixamal voltage at a frequency of 25/sec at 39°C, latency was 1.06 ± 0.04 (S.E.) sec and this, after correction, is independent of stimulation frequency between 5 and 50/sec, of changes in resting tension and of small doses of reserpine, bretylium, guanethidine, cocaine, sodium nitrite or iproniazid. Only the adrenergic blocking agents yohimbine and SY-14 significantly prolonged latency. The minimum effective number of stimuli to cause a justdetectable response varied between 4 and 6. A preliminary analysis suggests that both phenomena are probably due to events or processes that occur after the release of the transmitter from its storage site and the completion of its reaction with its receptors.