Norepinephrine inhibits calcium action potential through alpha 2-adrenoceptors in rabbit vesical parasympathetic neurons.

Intracellular and voltage-clamp recordings were made from neurons in rabbit vesical parasympathetic ganglia (VPG) maintained in vitro. Norepinephrine (NE, 10 nM-10 microM) reduced the Ca2+ component of the action potential and the afterhyperpolarization. Clonidine and UK14304, the selective alpha 2-adrenoceptor agonists, mimicked the inhibitory effects of NE on the action potential. NE and UK14304 blocked the Ca2+ spike elicited in the presence of tetrodotoxin and tetraethylammonium. UK14304 suppressed the inward Ca2+ current induced by depolarizing step command under the voltage-clamp condition. These inhibitory actions were antagonized by yohimbine and idazoxan but not by prazosin and propranolol. It is suggested that alpha 2-adrenoceptors mediate the inhibition of voltage-dependent Ca2+ entry during the action potential.