EFFECTS OF EXPRESSION OF A MOUSE-BRAIN L-TYPE CALCIUM-CHANNEL ALPHA-1SUBUNIT ON SECRETION FROM BOVINE ADRENAL CHROMAFFIN CELLS

Regulated exocytosis from bovine chromaffin cells is stimulated by the influx of Ca2+ through plasma membrane ion channels that are opened b y nicotinic stimulation and/or depolarization. Recently, we developed a novel method that enabled us to investigate the function of a cloned Ca2+ channel type C alpha 1 subunit in forming channels that stimulat e exocytosis, In the present study, we demonstrate by immunocytochemis try that bovine chromaffin cells normally express an epitope specific for the type C alpha 1 subunit. We investigated the effects of express ion of additional class C alpha 1 subunits (mouse brain clone) on vari ous aspects of secretory function in bovine chromaffin cells by measur ing secretion of cotransfected human growth hormone (GH, a reporter fo r the regulated secretory pathway in the transfected cells). New chann els were activated in response to depolarization by both elevated K+ a nd nicotinic cholinergic agonist, The new channels had their greatest effects when secretion was stimulated suboptimally. Secretion was enha nced only after the first 30 sec of stimulation, and the enhancement e xtended beyond 5 min of continuous stimulation. In contrast to the end ogenous L-type Ca2+ channels, the latency was not decreased by the dih ydropyridine L-type Ca2+ channel agonist, Bay K 8644. The findings sug gest that (i) the Ca2+-sensitive mechanism for triggering or maintaini ng exocytosis is capable of being saturated by high levels of Ca2+, (i i) secretion caused by nicotinic agonist stimulation can be significan tly enhanced by activation of voltage-sensitive Ca2+ channels, and (ii i) the effects on secretion of the L-type Ca2+ channels formed on expr ession of the mouse brain class C alpha 1 subunit are distinct from th ose of endogenous L-type Ca2+ channels.