Electrical current-induced contraction in the smooth muscle of the rabbit aorta.

1. In the smooth muscle of rabbit aorta, the relationship between the change in membrane potential and the concentration evoked by current application was studied. 2. In normal Krebs solution, outward current produced contraction when the membrane was depolarized to about -45 mV and the membrane resistance was decreased. Further increase in the outward current intensity produced an increase in tension development with very small additional depolarization but with a marked decrease in membrane resistance. 3. With arteries depolarized to about -20 mV, in excess K solution of concentration more than 60 mM, outward current failed to produce further contraction. However, contraction was produced when the inward current was switched off. This may be due to removal of an inactivation process by hyperpolarization of the membrane. 4. In excess K solution, no relaxation was observed with inward current application. However, when artificial stretch was applied to the preparation immersed in 98 mM-K solution, a prolonged hyperpolarizing current of more than 3 sec could cause relaxation. 5. Ca-free solution and Mn ion (0.5 mM) blocked the concentration induced by outward current. Phentolamine and tetrodotoxin had not effect. 6. It is suggested that increased membrane conductance associated with depolarization by outward current increased the Ca influx which causes contraction.