Defective sarcolemmal phosphorylation associated with noninsulin-dependent diabetes

Noninsulin-dependent diabetes is associated with a decrease in the activity of sarcolemmal phosphatase 1, but no change in the activities of phosphatase 2A, 2B, or 2C. Also unaffected by diabetes were the activities of protein kinase C, cAMP-dependent protein kinase and calcium-calmodulin protein kinase. Because of the decrease in phosphatase 1 activity, 32P incorporation into sarcolemmal phosphoproteins catalyzed by either intrinsic protein kinases or extrinsic cAMP-dependent protein kinase was elevated in the diabetic. Among the proteins whose phosphorylation was elevated in diabetes was the phospholamban-like protein, which has been implicated in the regulation of ATP-dependent calcium transport. The phosphate-linked increase could be prevented by exposing the membranes to a phosphatase inhibitor and either extrinsic cAMP-dependent protein kinase or alamethicin. In addition to the phosphatase-linked effects, analysis of individual sarcolemmal phosphoproteins by SDS-polyacrylamide gel electrophoresis indicated that diabetes caused a specific elevation in membrane phosphorylation of some proteins (43 kDa and 78 kDa), but a decrease in the phosphorylation state of other phosphoproteins (31 kDa and 49 kDa). The data indicate that membrane phosphorylation is dramatically altered by diabetes. The possibility that this contributes to altered myocardial function is discussed.