We have used the whole cell patch clamp technique on single prolactin-secreting bovine lactotrophs to measure plasma membrane capacitance (C m), an index of membrane surface area, under voltage-clamp during cytosol dialysis with Ca and cAMP. cAMP increased the magnitude and rate of Ca-induced exocytosis (C m increase) without affecting membrane conductance; however, cAMP had no detectable effect on C m when intracellular Ca was low. We thus report new evidence that cAMP can facilitate Ca-induced secretion in a synergistic fashion, by acting directly on the secretory apparatus, independently of membrane conductance activation.
