Role of Na+/Ca2+ exchange and the plasma membrane Ca2+ pump in hormone-mediated Ca2+ efflux from pancreatic acini.

The relative contributions of the Na+/Ca2+ exchange and the plasma membrane Ca2+ pump to active Ca2+ efflux from stimulated rat pancreatic acini were studied. Na+ gradients across the plasma membrane were manipulated by loading the cells with Na+ or suspending the cells in Na+-free media. The rates of Ca2+ efflux were estimated from measurements of [Ca2+]i using the Ca2+-sensitive fluorescent dye Fura 2 and 45Ca efflux. During the first 3 min of cell stimulation, the pattern of Ca2+ efflux is described by a single exponential function under control, Na+-loaded, and Na+-depleted conditions. Manipulation of Na+ gradients across the plasma membrane had minimal effects on resting [Ca2+]i, the rate constant of Ca2+ efflux, and [Ca2+]i levels attained by the cells after 5 min of stimulation. Changing Na+ gradients had no effect on the hormone-induced increase in [Ca2+]i. The results indicate that Ca2+ efflux from stimulated pancreatic acinar cells is mediated by the plasma membrane Ca2+ pump. The effects of several cations, which were used to substitute for Na+, on cellular activity were also studied. Choline+ and tetramethylammonium+ (TMA+) released Ca2+ from intracellular stores of pancreatic acinar, gastric parietal and peptic cells. These cations also stimulated enzyme and acid secretion from the cells. All effects of these cations were blocked by atropine. Measurements of cholecystokinin-octapeptide (CCK-OP)-stimulated amylase release from pancreatic acini, suspended in Na+, TMA+, choline+, or N-methyl-D-glucamine+ (NMG+) media containing atropine, were used to evaluate the effect of the cations on cellular function. NMG+, choline+, and TMA+ inhibited amylase release by 55, 40 and 14%, respectively. NMG+ also increased the Ca2+ permeability of the plasma membrane. Thus, to study Na+ dependency of cellular function, TMA+ is the preferred cation to substitute for Na+. The stimulatory effect of TMA+ can be blocked by atropine.