Differential effects of strychnine on two types of vascular muscle

Strychnine (10(-5)-3 x 10(-4) M) increased the amplitude and duration of the spontaneous electrical and mechanical activity of the rat isolated portal vein. Similar effects were seen with tetraethylammonium and 4-aminopyridine. This stimulant action of strychnine was unaffected by tetrodotoxin (3 x 10(-7) M) or prazosin (5 x 10(-8) M) but was significantly reduced by verapamil (3 x 10(-8) M). On the isolated aorta, only inhibitory actions of strychnine were observed, yet tetraethylammonium and 4-aminopyridine had excitatory actions. It is suggested that the stimulant actions of strychnine on the portal vein are likely to be due to a reduction in potassium conductance and/or an increase in calcium conductance of the smooth muscle cell membrane.