Glucose increases cytosolic calcium concentration and inositol lipid metabolism in HIT-T15 cells

We have investigated the effects of glucose on cytosolic free calcium concentration in the insulin-secreting cell line HIT-T15. Addition of glucose (10 mM) caused a 20-75% increase in cytosolic Ca2+] within 5 minutes compared to controls in the absence of glucose. A maximal increase in cytosolic Ca2+] was obtained with 5 mM glucose. The magnitude of the response was markedly dependent upon the concentration of extracellular Ca2+, and the rise in cytosolic Ca2+] was inhibited by verapamil. Cytosolic Ca2+] was greatly increased by depolarization of the cells with KCl (50 mM), whereas carbamylcholine had no apparent effect. Glucose and KCl were also effective in stimulating insulin release from HIT cells, although carbamylcholine was again ineffective. The secretory response to glucose was also found to be directly related to the concentration of extracellular Ca2+]. Glucose and KCl, but not carbamylcholine, were found to slightly enhance the production of 3H]-inositol trisphosphate in HIT cells pre-labelled with myo-3H]-inositol, indicating a modest stimulation of inositol lipid hydrolysis.