The direct effect of insulin on the high-affinity Ca2+-Mg2+-ATPase was studied in kidney proximal tubular basolateral membranes (BLM) obtained from control and streptozocin-induced non-insulin-dependent diabetes mellitus (NIDDM) rats. Plasma glucose of the diabetic animals was only mildly elevated (217 ± 9 vs. 138 ± 3 mg/dl). Both high- and low-affinity calcium-dependent Ca2+-Mg2+-ATPase activities were identified in the BLM. Enzyme activity in BLM from diabetic rats was higher at all Ca2+ concentrations tested due to a higher maximum velocity of the enzyme from NIDDM rats. The high-affinity Ca2+-Mg2+-ATPase activity was inhibited by trifluoroperzine (TFP) in both membranes. No difference in calmodulin content was found in the membranes from the diabetic and control rats.
Insulin (16–200 μU/ml) significantly increased the high-affinity Ca2+-Mg2+-ATPase activity (17–40%) in membranes from control animals but had no effect on the enzyme activity in the membranes from the NIDDM rats. The basal activity of the enzyme at 0.1 μM free Ca2+ was higher in the BLM from the NIDDM animals compared to controls (17.8 ± 0.5 vs. 14.7 ± 0.8 nM Pi mg1 · min−1; P < .02). There was no effect of insulin on the Ca2+-independent ATPase activity of BLM preparations.
These findings demonstrate a defect in the ability of insulin to regulate the high-affinity Ca2+-Mg2+-ATPase activity in BLM from diabetic rats. Such a defect in enzyme activity may play a role in the mechanism of impaired insulin action observed in these NIDDM rats.