Depression of voltage-dependent Ca2+ current by noradrenaline in the guinea-pig vas deferens in tetraethylammonium medium.

The actions of noradrenaline (NA) on the smooth muscle of the guinea-pig vas deferens in tetraethylammonium (TEA) medium have been studied under constant-current and voltage-clamp conditions. The marked changes observed during exposure to NA are 1) slowness in the rising phase of the action potential, delay in the falling phase and reduction in the amplitude, and 2) depression of the voltage-dependent Ca2+ current with a negative shift of its reversal potential as well as with a reduction in its conductance. These findings might suggest that NA application stimulates the voltage-independent Ca2+ machinery, but suppresses the voltage-dependent Ca2+ machinery through membrane.