Variations in Adrenergic Innervation and Contractile Responses of the Rabbit Saphenous Artery

Abstract

The Falck fluorescence histochemical method was used to compare the adrenergic innervation in segments from the proximal saphenous artery, the distal saphenous artery and the popliteal artery. Adrenergic neurons penetrated the media of the proximal saphenous artery but not that of the distal saphenous artery or the popliteal artery. Responses of these vessel segments to exogenous l-norepinephrine and to electrical stimulation of their intramural innervation were measured. The median effective dose (ED₅₀) of l-norepinephrine was ten times greater in the proximal saphenous artery than it was in the distal saphenous artery and the popliteal artery. The response of the proximal saphenous artery to neural stimulation was nearly double that of the distal vessel and required one-fifth of the time to achieve maximum tension; the response of the popliteal artery was negligible due to its low density of innervation. The three-dimensional medial distribution of the adrenergic neuronal plexus in the proximal saphenous artery should (1) increase the efficiency of neuronal uptake of l-norepinephrine and thereby account for the increased ED₅₀ of l-norepinephrine, and (2) decrease the mean distance between adrenergic nerves and medial smooth muscle cells. Therefore, transmitter would accumulate more rapidly and reach a higher level in the media during neural activity.