DISTINCT FUNCTIONS OF T-TYPE AND L-TYPE CALCIUM CHANNELS DURING ACTIVATION OF BOVINE ADRENAL GLOMERULOSA CELLS

Calcium influx into adrenal glomerulosa cells is a key event during th e stimulation of aldosterone secretion by physiological increases in e xtracellular potassium concentrations. Two types of voltage-operated c alcium channels, T- and L-types, are present on bovine glomerulosa cel ls, but their respective functions are not yet clearly defined. Using the patch-clamp method in the perforated patch configuration combined with microfluorimetry of cytosolic calcium, we demonstrate that L-type channels are exclusively responsible for the sustained elevation of c ytosolic calcium observed upon stimulation with extracellular potassiu m, even at low, physiological concentrations of this agonist. In contr ast, aldosterone secretion appears closely related to T-type channel a ctivity. Moreover, when the activity of each channel type is selective ly modulated by pharmacological agents, such as dihydropyridines or zo nisamide, the cytosolic calcium response can be clearly dissociated fr om the steroidogenic response. Similarly, modulation of T channel acti vation by protein kinase C results in a parallel inhibition of aldoste rone secretion, without any effect on the levels of cytosolic free cal cium. This direct functional link between T-type calcium channel activ ity and steroidogenesis suggests a model in which calcium entering the cell through these channels bypasses the cytosol to activate intramit ochondrial steps of aldosterone biosynthesis.