Aims/hypothesis: Postprandial hyperglycaemia carries an increased risk of macrovascular disease even without Type II (non-insulin-dependent) diabetes mellitus. Chronic hyperglycaemia activates protein kinase C (PKC) in vitro and in vivo but it is not known whether PKC is regulated by short-term post-prandial hyperglycaemia in vivo in humans. We investigated whether PKC is regulated in vivo in hyperglycaemic and hyperinsulinaemic infusion tests and correlated the results to stimulations in vitro.
Methods: Protein kinase C regulation was measured in platelets obtained from 8 healthy subjects who were infused with glucose and insulin for 2 h attaining peak concentrations of 16 mmol/l glucose and in platelets from 8 healthy young subjects, 8 older subjects without diabetes, and 10 older subjects with Type II diabetes after incubation in vitro with 16 mmol/l glucose or glucose and insulin. For precise quantification, a shortened PKC beta1 standard protein was generated by bacterial expression and PKC alpha, beta1, beta2 and delta isoenzyme values were measured by immunoblot analyses.
Results: Hyperglycaemic and hyperinsulinaemic in vivo tests increased the amounts of PKC alpha, beta1 and beta2 in the membrane fraction of platelets to 225 +/- 87 %, 164 +/- 22 % and 302 +/- 135 %, respectively, when compared with the baseline values in young healthy volunteers (n = 8, p < 0.05). The expression of PKC delta did not change. In comparison to the recombinant PKC beta1 standard protein, 5 ng PKC beta1/ microg protein was measured before the test and 2 ng/microg were translocated to the membrane fraction after the infusion. No change in the absolute amount of PKC beta1 was detected. In contrast, after incubation in vitro PKC was not regulated by glucose or glucose and insulin in 8 young healthy subjects (age 26 +/- 0.7 years) and in 8 older, healthy subjects (age 64,8 +/- 4 years) although 100 nmol/l 12-O-tetradecanoylphorbol 13-acetate caused maximal activation. In marked contrast, PKC beta1 and PKC beta2, but not PKC alpha or PKC delta, were increased in vitro in the membrane fraction by 292 +/- 61% and 432 +/- 88% (p < 0.05) in 10 subjects with Type II diabetes mellitus matched for age, sex and BMI.
Conclusion/interpretation: We found that short-term hyperglycaemia activates PKC alpha, beta1 and beta2 in platelets of healthy persons making them potential candidates for mediating the increased cardiovascular risk of postprandial hyperglycaemia. Hyperglycaemia and hyperinsulinaemia did not cause short-term activation of PKC in platelets in vitro suggesting the existence of additional stimuli. Subjects with Type II diabetes showed a markedly altered reactivity of platelet PKC beta in vitro indicating some diabetes-related regulation.