Noradrenaline depresses the voltage-dependent Ca2+ current in guinea-pig vas deferens smooth muscle in TEA medium, likely via alpha-adrenoceptor activation and a reduction in maximum Ca2+ conductance.
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Animal findings on noradrenaline's Ca2+ effects should not change practice; leaves open translation to human cardiovascular smooth muscle.
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.
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Inomata et al. (1989) studied this question.
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