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

Publisher Website
Google Scholar
Abstract
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±6nm (n=37). When challenged with either glucose (2.5–10mm) ord-glyceraldehyde (10mm) 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 (0.01–0.1mm). 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 μm) was also found to both repolarize the membrane and abolish secretagogue-induced rises in [Ca2+] i .