It has been reported that platelet aggregation in diabetic patients with microangiopathy is increased compared with healthy subjects. Chronic hyperglycemia is known to cause an increase in diacylglycerol level in various tissues. We examine whether protein kinase C (PKC) isoform content in platelets from diabetic patients is increased compared with healthy subjects, as previously described in the retina, aorta, and heart of diabetic rats. Platelet PKCalpha, beta and zeta immunoreactivity in cytosol, membrane and cytoskeleton (CS) fractions were analyzed by immunoblotting in 20 type 2 diabetic patients (who had been treated with diet alone, sulphonylureas or insulin, and whose condition was complicated with retinopathy, nephropathy, neuropathy and/or macroangiopathy) and in five healthy subjects. PKCalpha, beta and zeta immunoreactivity in cytosol, membrane and CS fractions in platelets from diabetic subjects were not significantly higher than those from healthy subjects. However, platelet PKCbeta immunoreactivity in cytosol fraction was significantly higher in diabetic patients with normal serum creatinine (Cr) level than in diabetic patients with abnormal Cr level (Cr > or =1.5 mg/dl) or in healthy subjects. Moreover, significant negative correlation between PKCbeta immunoreactivity in cytosol fraction of platelets and serum Cr level was found in diabetic patients (P < 0.05). To clarify the effect of treatment for diabetes, PKC isoform immunoreactivity in platelets was measured in type 2 diabetic patients treated with diet alone, sulphonylurea or insulin treatment. Serum creatinine level in diabetic patients with insulin treatment was significantly higher than in diabetic patients with sulphonylurea treatment and diet alone. In addition, PKCbeta immunoreactivity in diabetic patients with insulin treatment was significantly suppressed compared with that in patients treated by sulphonylurea treatment. These results suggest that chronic hyperglycemia may activate platelet PKCbeta isoform, and that insulin treatment may decrease platelet PKCbeta activity. Finally, not only PKCbeta antagonists, but also glycemic control by insulin may prevent development of diabetic microangiopathy.