The effect of fasting on mobile calcium in the B-cells of rat pancreatic islets was investigated in view of a possible role of calcium in the fasting-induced impairment of insulin secretion. Mobile calcium (GBHA-Ca), an ionized or readily ionizable calcium fraction, was determined histochemically with glyoxal bis (2-hydroxyanil). Fasting (24–72 h) strongly decreased the GBHACa content of islets in situ (55–60%).
Incubation of isolated islets at 2.5 mM glucose in the presence of 2.5 mM Ca2+ resulted after 15 min in stable GBHA-Ca levels, which were 25% lower in fasted than in fed islets. Glucose (15 mM) caused the GBHA-Ca content of fed islets to decrease rapidly and to rise again after 30 min. These changes did not occur after 24 or 72 h of fasting. GBHA-Ca appeared not to be displaceable with La3+.
At 2.5 mM glucose, withdrawal of Ca2+ rapidly reduced GBHA-Ca in fed and fasted islets. Glucose (15 mM) inhibited this rapid fall, and this inhibitory effect was particularly evident in fed islets.
Washing and preincubation in the absence of Ca2+ (2.5 mM glucose) largely depleted fed and fasted islets of GBHA-Ca. Reintroduction of Ca2+ at 2.5 mM glucose only partially restored the GBHA-Ca levels of fed and fasted islets. By contrast, 15 mM glucose restored the characteristic pattern of GBHA-Ca in fed islets as seen in nondepleted islets, but in fasted islets at lower levels as previously seen.
Thus, fasting decreased the GBHA-Ca content and its response to glucose stimulation. It is suggested that GBHA-Ca, which is presumably mainly localized in the secretory granules, plays a role in the initiation of insulin secretion.