Stimulant-evoked depolarization and increase in [Ca2+]i in insulin-secreting cells is dependent on external Na+

The patch-clamp technique and measurements of single cell [Ca2+]i have been used to investigate the importance of extracellular Na+ for carbohydrate-induced stimulation of RINm5F insulin-secreting cells. Using patch-clamp whole-cell (current-clamp) recordings the average cellular transmembrane potential was estimated to be -60 +/- 1 mV (n = 83) and the average basal [Ca2+]i 102 +/- 6 nM (n = 37). When challenged with either glucose (2.5-10 mM) or D-glyceraldehyde (10 mM) the cells depolarized, which led to the initiation of Ca2+ spike potentials and a sharp rise in [Ca2+]i. Similar effects were also observed with the sulphonylurea compound tolbutamide (0.01-0.1 mM). Both the generation of the spike potentials and the increase in [Ca2+]i were abolished when Ca2+ was removed from the bathing media. When all external Na+ was replaced with N-methyl-D-glucamine, in the continued presence of either glucose, D-glyceraldehyde or tolbutamide, a membrane repolarization resulted, which terminated Ca2+ spike potentials and attenuated the rise in [Ca2+]i. Tetrodotoxin (TTX) (1-2 microM) was also found to both repolarize the membrane and abolish secretagogue-induced rises in [Ca2+]i.