Effects of Ca2+ channel antagonists on acetylcholine and catecholamine releases in the in vivo rat adrenal medulla

Abstract

To elucidate the types of voltage-dependent Ca2+ channels controlling ACh and catecholamine releases in the in vivo adrenal medulla, we implanted microdialysis probes in the left adrenal medulla of anesthetized rats and investigated the effects of Ca2+ channel antagonists on ACh, norepinephrine, and epinephrine releases induced by nerve stimulation. The dialysis probes were perfused with Ringer solution containing a cholinesterase inhibitor, neostigmine. The left splanchnic nerves were electrically stimulated at 2 and 4 Hz before and after intravenous administration of Ca2+ channel antagonists. ω-Conotoxin GVIA (an N-type Ca2+ channel antagonist, 10 μg/kg) inhibited ACh release at 2 and 4 Hz by ∼40%, norepinephrine release at 4 Hz by ∼50%, and epinephrine release at 2 and 4 Hz by ∼45%. A fivefold higher dose of ω-conotoxin GVIA (50 μg/kg) did not further inhibit these releases. ω-Conotoxin MVIIC (a P/Q-type Ca2+ channel antagonist, 50 μg/kg) inhibited ACh and epinephrine releases at 4 Hz by ∼30%. Combined ω-conotoxin GVIA (50 μg/kg) and MVIIC (250 μg/kg) inhibited ACh release at 2 and 4 Hz by ∼70% and norepinephrine and epinephrine releases at 2 and 4 Hz by ∼80%. Nifedipine (an L-type Ca2+ channel antagonist, 300 and 900 μg/kg) did not change ACh release at 2 and 4 Hz; however, nifedipine (300 μg/kg) inhibited epinephrine release at 4 Hz by 20%, and nifedipine (900 μg/kg) inhibited norepinephrine and epinephrine releases at 4 Hz by 30%. In conclusion, both N- and P/Q-type Ca2+ channels control ACh release on preganglionic splanchnic nerve endings while L-type Ca2+ channels do not. L-type Ca2+ channels are involved in norepinephrine and epinephrine releases on chromaffin cells.