The effect of glucose and calcium on Ca²⁺-adenosine triphosphatase in pancreatic islets isolated from a normal and a non—insulin-dependent diabetes mellitus rat model☆

Abstract 

Regulation of calcium balance is important in the secretory function of pancreatic islets. Ca²⁺-adenosine triphosphatase (ATPase) is altered in tissues of non—insulin-dependent diabetes mellitus (NIDDM) rats, and they have an impaired response to glucose, “glucose blindness.” We propose that the glucose blindness of the diabetic islet is the result of defective cellular calcium metabolism. Since Ca²⁺-ATPase activity is important in the regulation of calcium balance, we investigated the effect of glucose and/or calcium on Ca²⁺-ATPase activity in pancreatic islets in vitro and compared it with the effect in freshly isolated islets from controls and from rats with NIDDM induced by streptozotocin neonatally. Islets were isolated using collagenase and were stored fresh or cultured up to 2 days in RPMI 1640 in the presence of different concentrations of glucose and calcium. Membrane Ca²⁺-ATPase activity, insulin secretion, and insulin content were determined. Ca²⁺-ATPase activity was 1.30 ± 0.20 μmol/L Pi/μg membrane protein in normal noncultured islets and 1.02 ± 0.15 in islets cultured in 5.6 mmol/L glucose. Ca²⁺-ATPase activity progressively decreased to 0.56 ± 0.10 and 0.34 ± 0.14 μmol/L Pi/μg membrane protein when glucose was increased in the culture media to 16.6 and 27.7 mmol/L, respectively. Decreasing glucose to 2.8 mmol/L did not alter Ca²⁺-ATPase activity. Increasing or decreasing the Ca²⁺-ATPase content of the media did not significantly change Ca²⁺-ATPase activity. Islets isolated from NIDDM rats had lower basal Ca²⁺-ATPase activity and insulin content compared with normal controls. Incubation of islets from diabetic rats in high glucose further decreased the Ca²⁺-ATPase content, but incubation in low glucose did not reverse it. Insulin secretion was responsive to glucose and calcium in normal islets, but was suppressed in islets from diabetic animals. From these studies, we conclude that high glucose, but not calcium, decreases Ca²⁺-ATPase activity in islets from normal rats. Islets from NIDDM rats with glucose blindness have decreased Ca²⁺-ATPase activity, likely due to the glucose status. We suggest that this decreased Ca²⁺-ATPase activity may contribute to the pancreatic islets' glucose blindness.

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