LOCALIZED L-TYPE CALCIUM CHANNELS CONTROL EXOCYTOSIS IN CAT CHROMAFFIN CELLS

Depolarizing 1-s pulses to 0 mV from a holding potential of -70 mV, in duced whole-cell currents through Ca2+ channels (I-Ca) in patch-clampe d cat adrenal medulla chromaffin cells. The dihydropyridine (DHP) furn idipine (3 mu M) reduced the peak current by 47% and the late current by 80%. omega-Conotoxin GVIA (CgTx, 1 mu M) reduced the peak I-Ca by 4 2% and the late I-Ca by 55%. Pulses (10 s duration) with 70 mM K+/2.5 mM Ca2+ solution (70 K+/2.5 Ca2+), applied to single fura-2-loaded cat chromaffin cells increased the cytosolic Ca2+ concentration ([Ca2+](i )) from 0.1 to 2.21 mu M; this increase was reduced by 43.7% by furnid ipine and by 42.5% by CgTx. In the perfused cat adrenal gland, secreti on evoked by 10-s pulses of 70 K+/2.5 Ca2+ was reduced by 25% by CgTx and by 96% by furnidipine. Similar results were obtained when secretio n from superfused isolated cat adrenal chromaffin cells was studied an d when using a tenfold lower [Ca2+](o). The results are compatible wit h the existence of DHP-sensitive (L-type) as well as CgTx-sensitive (N -type) voltage-dependent Ca2+ channels in cat chromaffin cells. It see ms, howevever, that though extracellular Ca2+ entry through both chann el types leads to similar increments of averaged [Ca2+](i), the contro l of catecholamine release is dominated only by Ca2+ entering through L-type Ca2+ channels. This supports the idea of a preferential segrega tion of L-type Ca2+ channels to localized ''hot spots'' in the plasmal emma of chromaffin cells where exocytosis occurs.