Sympathetic transmission in small mesenteric arteries from the rat: highly calcium-dependent at low stimulation rates.

In the present study we examine the calcium requirements of the neurogenic response in vitro of small arteries (150-200 microns diameter) from the mesentery of Wistar rats. Intramural nerves were activated with electrical field stimulation. Responses to single impulses and to low-frequency repeated stimulation were reduced or abolished by reducing the Ca2+ concentration in the bathing solution from 2.5 to 1.0 mM. Responses to higher frequencies (16 Hz) were only slightly affected. Since calcium reduction had markedly less effect on responses to direct activation of the smooth muscle and on responses to any dose of exogenous noradrenaline, the calcium reduction had mainly pre-junctional effects. The data show that part of the neurogenic response is highly calcium-sensitive, perhaps more so than would be expected of a purely noradrenergic transmission.