CONTRIBUTION OF SK AND BK CHANNELS IN THE CONTROL OF CATECHOLAMINE RELEASE BY ELECTRICAL-STIMULATION OF THE CAT ADRENAL-GLAND

1. Transmural electrical stimulation (10 Hz, 1 ms, 40 V for 10 a) of c at adrenal glands perfused at room temperature with Krebs-Hepes soluti on produced catecholamine secretory responses which were reproducible when stimulations were applied at 5 min intervals. Such responses were inhibited about 20% by atropine (1 mu M) and 80% by hexamethonium (30 mu M). Apamin (100 nM) increased the secretory response 2.5-fold in t he presence of atropine and 8-fold in the presence of hexamethonium. 2 . Potentiation by apamin of secretory responses evoked by 100-pulse tr ains was similar at 5, 10 and 20 Hz (about 2-fold). When glands were c ontinuously stimulated at 3 Hz, apamin increased 4-fold the initial se cretion plateau. Continuous stimulation at a higher frequency (20 Hz) produced a sharp secretory peak followed by a small, sustained plateau ; apamin did not alter this plateau. Apamin also enhanced the secretor y responses obtained with sustained stimulation with acetylcholine (10 or 200 mu M). 3. Secretion peaks induced by brief acetylcholine pulse s (10 mu M for 10 s) applied to isolated and superfused cat adrenal ch romaffin cells were enhanced more than 3-fold by 100 nM apamin. Charyb dotoxin (10 nM) did not enhance these secretory peaks. 4. In perfused cat adrenal glands, charybdotoxin (10 nM) affected neither the secreti on evoked by trains of electrical stimulation applied at different fre quencies nor the secretion evoked by acetylcholine pulses. 5. In 0.5 m M [Ca2+](o), apamin enhanced 3-fold the secretion evoked by electrical stimulation trains of 100 pulses (10 Hz, 10 s) and almost 6-fold the acetylcholine (10 mu M for 10 s)-induced secretion. In 5 mM Ca2+, apam in enhanced the secretory responses to electrical stimulation and acet ylcholine 2- and 10-fold, respectively. Charybdotoxin enhanced 2.5-fol d the secretory response to electrical stimulation in 0.5 mM Ca2+, alt hough this effect was not statistically significant. A synergistic int eraction between the two toxins on catecholamine release induced by el ectrical stimulation was observed at low but not at high [Ca2+](o). 6. Simultaneous release of acetylcholine and catecholamines upon electri cal stimulation was achieved in glands in which the endogenous acetylc holine stores in the splanchnic nerve terminals had been prelabelled b y perfusion with [H-3]choline. While apamin enhanced more than 2-fold the postsynaptic release of catecholamines, the presynaptic release of acetylcholine remained unaffected. 7. The results are compatible with the hypothesis that, under physiological conditions, Ca2+-activated X K channels present in chromaffin cells control the firing patterns of action potentials induced by the acetylcholine released from splanchni c nerves during stress. Since the activation of L-type Ca2+ channels, which control secretion in cat chromaffin cells, depends on such elect rical activity, it seems that, indirectly, SK channels exert a profoun d regulatory effect on the catecholamine secretory process. BK channel s seem to have, if anything, a minor role at a presynaptic level.