An electrophysiological study of alpha-adrenoceptor mediated excitation-contraction coupling in the smooth muscle cells of the rat saphenous vein.

The effects of perivascular nerve stimulation and application of exogenous alpha-adrenoceptor agonists on the rat saphenous vein were studied by simultaneous recordings of electrical and mechanical activities. The resting membrane potential of the saphenous vein averaged -65 mV. Perivascular nerve stimulation elicited excitatory junction potentials (e.j.ps) and slow depolarizations. Contraction was observed when either the e.j.p. or the slow depolarization reached a critical threshold of about -55 mV. Exogenously applied noradrenaline, B-HT 920 and clonidine induced depolarization and contraction similar to the slow depolarization. The responses to these agonists and the slow depolarizations were antagonized by yohimbine, but not by prazosin. The selective alpha 1-adrenoceptor agonists phenylephrine and methoxamine had very little effect on the electrical and mechanical activities of the saphenous vein. It was concluded that in the rat saphenous vein, only alpha 2-adrenoceptors are present and that these receptors mediate the slow depolarization and contraction induced by nerve stimulation.