Effects of calcium channel antagonists on the depolarization-evoked release of norepinephrine in the rabbit iris-ciliary body.

The effects of several representative calcium channel antagonists on depolarization-evoked release of [3H]-norepinephrine were investigated in isolated, superfused rabbit iris-ciliary bodies. Potassium (50 mM)-evoked neurosecretion was blocked by 5 mM CoCl2 and partially inhibited by 10(-6) M nitrendipine or verapamil. Electrically-evoked neurosecretion was similarly blocked by CoCl2, but was unaffected by nitrendipine or verapamil. It is concluded from these results that sympathetic terminals in the rabbit iris-ciliary body contain dihydropyridine- and verapamil-sensitive calcium channels which contribute, under conditions of prolonged depolarization, to neurotransmitter release.