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.
Dunne
M J
MJ
Department of Physiology, University of Liverpool, England.
Yule
D I
DI
Gallacher
D V
DV
Petersen
O H
OH
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002460:Cell Line; D005947:Glucose / Q000494:pharmacology; D005985:Glyceraldehyde / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism*; D008564:Membrane Potentials / Q000187:drug effects; D012964:Sodium / Q000502:physiology*; D014044:Tolbutamide / Q000493:pharmacokinetics
D007328:Insulin; D005985:Glyceraldehyde; D014044:Tolbutamide; D012964:Sodium; D005947:Glucose; D002118:Calcium