Calcium channel subtypes and exocytosis in chromaffin cells: a different view from the intact rat adrenal

In the intact rat adrenal gland perfused with an oxygenated Krebs-bicarbona te solution at 37 degrees C, the electrical field stimulation of splanchnic nerves (100 V, 0.5 ms duration, 10 Hz during 10 s) produced transient cate cholamine release peaks that were reproduced in subsequent stimuli, applied at 8-min intervals. omega-Conotoxin GVIA (0.3 mu M) caused only a modest i nhibition of the secretory response, suggesting that the N-subtype of volta ge-dependent Ca2+ channels are scarcely involved in such a response. Both o mega-conotoxin MVIIC (1 mu M) and furnidipine (1 mu M) halved the secretion , suggesting that the L- and P/Q-types of Ca2+ channels were involved. N-ty pe Ca2+ channels appear to be involved in the maintenance of secretion in r esponse to sustained stimulus since w-conotoxin GVIA (0.3 mu M) reduced the catecholamine output to 28%. When secretion was elicited by acetyl choline (10 mu M), furnidipine reduced the catecholamine release by 50% and omega- conotoxin MVIIC by 40%, whereas omega-conotoxin GVIA did not modify the res ponse. The K+-induced secretory responses (23.6 mM K+, 15 s) were reduced 7 5% by fumidipine and 45% by omega-conotoxin MVIIC, indicating that this typ e of stimulation preferentially recruited L-type channels. These data show that electrical stimulation recruits Ca2+ channel subtypes different from t hose recruited by direct depolarization of chromaffin cells.