Effects of N- and L-type calcium channel antagonists and (±)-Bay K8644 on nerve-induced catecholamine secretion from bovine perfused adrenal glands

Abstract

1. The effects of N- and L-type calcium channel antagonists and (±)-Bay K8644 on catecholamine release from chromaffin cells and acetylcholine release from splanchnic nerve terminals was investigated in bovine perfused adrenal glands.

2. Adrenal glands were perfused retrogradely and preloaded with [³H]-choline. Subsequent efflux of ³H-labelled compounds was taken as an index of acetylcholine release from the splanchnic nerve terminals. Noradrenaline and adrenaline release from the glands was measured by h.p.l.c. with electrochemical detection.

3. A maximally effective frequency of field stimulation of the adrenal nerves, 10 Hz, induced release of catecholamines and ³H-labelled compounds. Tetrodotoxin (1 μM) abolished release of both catecholamines and ³H-labelled compounds. A combination of mecamylamine (5 μM) and atropine (1 μM) inhibited nerve-induced catecholamine release by about 75% but did not inhibit release of ³H-labelled compounds. Reducing the concentration of extracellular calcium 5 fold to 0.5 mM inhibited nerve-induced catecholamine release by 80% and release of ³H-labelled compounds by 50%.

4. (±)-Bay K8644 (1 μM), nitrendipine (1 μM), ω-conotoxin-GVIA (10 nM) and the combination of nitrendipine and ω-conotoxin-GVIA each had no effect on nerve-induced release of ³H-labelled compounds.

5. (±)-Bay K8644 (1 μM) potentiated nerve-induced catecholamine release by 75%. Nitrendipine (1 μM) reduced release by 20% but this did not reach statistical significance. ω-Conotoxin-GVIA (10 nM) reduced nerve-induced catecholamine release by 75%, while the combination of ω-conotoxin-GVIA and nitrendipine reduced release to the same extent as ω-conotoxin-GVIA alone.

6. Exogenous acetylcholine perfusion through the glands produced a concentration-dependent increase in catecholamine release. The maximally effective concentration of acetylcholine for catecholamine release was ⩾300 μM, while 30 μM acetylcholine gave comparable catecholamine release to that obtained with 10 Hz field stimulation.

7. (±)-Bay K8644 (1 μM), nitrendipine (1 μM) and ω-conotoxin-GVIA (10 nM) each had no significant effect on catecholamine release evoked by perfusion of the gland with either a near maximally effective concentration of acetylcholine, 100 μM, or with the lower concentration of 30 μM.

8. The results show that the ω-conotoxin-GVIA-sensitive N-type voltage-sensitive calcium channels located on the chromaffin cells are largely responsible for catecholamine release induced by nerve stimulation in bovine adrenal glands. In contrast, N-type calcium channels are not involved in catecholamine release induced by exogenous acetylcholine. L-type voltage sensitive calcium channels do not play a major role in nerve-induced or exogenously applied acetylcholine-induced catecholamine 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 presynaptic release of acetylcholine.

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