EFFECTS OF N-TYPE AND L-TYPE CALCIUM-CHANNEL ANTAGONISTS AND (+ -)-BAY K8644 ON NERVE-INDUCED CATECHOLAMINE SECRETION FROM BOVINE PERFUSED ADRENAL-GLANDS/

1 The effects of N- and L-type calcium channel antagonists and (+/-)-B ay K8644 on catecholamine release from chromaffin cells and acetylchol ine release from splanchnic nerve terminals was investigated in bovine perfused adrenal glands. 2 Adrenal glands were perfused retrogradely and preloaded with [H-3]-choline. Subsequent efflux of H-3-labelled co mpounds was taken as an index of acetylcholine release from the splanc hnic nerve terminals. Noradrenaline and adrenaline release from the gl ands was measured by h.p.l.c. with electrochemical detection. 3 A maxi mally effective frequency of field stimulation of the adrenal nerves, 10 Hz, induced release of catecholamines and H-3-labelled compounds. T etrodotoxin (1 mu M) abolished release of both catecholamines and H-3- labelled compounds. A combination of mecamylamine (5 mu M) and atropin e (1 mu M) inhibited nerve-induced catecholamine release by about 75% but did not inhibit release of H-3-labelled compounds. Reducing the co ncentration of extracellular calcium 5 fold to 0.5 mM inhibited nerve- induced catecholamine release by 80% and release of H-3-labelled compo unds by 50%. 4 (+/-)-Bay K8644 (1 mu M), nitrendipine (1 mu M), omega- conotoxin-GVIA (10 nM) and the combination of nitrendipine and omega-c onotoxin-GVIA each had no effect on nerve-induced release of H-3-label led compounds. 5 (+/-)-Bay K8644 (1 mu M) potentiated nerve-induced ca techolamine release by 75%. Nitrendipine (1 mu M) reduced release by 2 0% but this did not reach statistical significance. omega-Conotoxin-GV IA (10 nM) reduced nerve-induced catecholamine release by 75%, while t he combination of omega-conotoxin-GVIA and nitrendipine reduced releas e to the same extent as omega-conotoxin-GVIA alone. 6 Exogenous acetyl choline perfusion through the glands produced a concentration-dependen t increase in catecholamine release. The maximally effective concentra tion of acetylcholine for catecholamine release was greater than or eq ual to 300 mu M, while 30 mu M acetylcholine gave comparable catechola mine release to that obtained with 10 Hz field stimulation. 7 (+/-)-Ba y K8644 (1 mu M), nitrendipine (1 mu M) and omega-conotoxin-GVIA (10 n M) each had no significant effect on catecholamine release evoked by p erfusion of the gland with either a near maximally effective concentra tion of acetylcholine, 100 mu M, or with the lower concentration of 30 mu M. 8 The results show that the omega-conotoxin-GVIA-sensitive N-ty pe voltage-sensitive calcium channels located on the chromaffin cells are largely responsible for catecholamine release induced by nerve sti mulation in bovine adrenal glands. In contrast, N-type calcium channel s are not involved in catecholamine release induced by exogenous acety lcholine. L-type voltage sensitive calcium channels do not play a majo r role in nerve-induced or exogenously applied acetylcholine-induced c atecholamine release. However, the L-type calcium channels do have the potential to augment powerfully nerve-induced catecholamine release. N- and L-type calcium channels do not play a major role in the presyna ptic release of acetylcholine.