Hydralazine: effect on the outflow of noradrenaline and mechanical responses evoked by sympathetic nerve stimulation of the rat tail artery.

1 The effects of hydralazine on the vasoconstrictor responses to field stimulation of sympathetic nerves were studied in the isolated proximal segments of the rat tail artery. Vasoconstrictor responses to transmural stimulation were depressed by superfusion of hydrazine (0.3, 3 and 30 muM) in a concentration-dependent manner. The inhibition appeared slowly and was not easily reversed by washing. 2 Hydralazine (30 nM, 0.3 and 3 muM) reduced the stimulation-induced overflow of tritium from proximal and distal segments of the tail artery labelled with [3H]-noradrenaline in a concentration-dependent manner. This phenomenon appeared rapidly and was easily reversed by washing. 3 Theophylline (0.5 mM) did not affect the inhibitory effect of hydralazine on the stimulation-induced tritium efflux from the distal segment of the rat tail artery. 4 The present results indicate that hydralazine has, in addition to its action on vascular smooth muscle, a very marked effect on sympathetic nerve terminals. The mechanism of this presynaptic inhibition appears to be different from the postsynaptic effect, in view of the much shorter delay, the shape of the dose-effect curve, and the lack of interaction with theophylline.