Diabetes-induced activation of protein kinase C inhibits store-operated Ca2+ uptake in rat retinal microvascular smooth muscle

Aims/Hypothesis. To assess the effects of diabetes-induced activation of protein kinase C (PKC) on voltage-dependent and voltage-independent Ca2+ influx pathways in retinal microvascular smooth muscle cells. Methods. Cytosolic Ca2+ was estimated in freshly isolated rat retinal arterioles from streptozotocin-induced diabetic and non-diabetic rats using fura-2 microfluorimetry. Voltage-dependent Ca2+ influx was tested by measuring rises in [Ca2+](i) with KCl (100 mmol/l) and store-operated Ca2+ influx was assessed by depleting [Ca2+](i) stores with Ca2+ free medium containing 5 mumol/l cyclopiazonic acid over 10 min and subsequently measuring the rate of rise in Ca2+ on adding 2 mmol/l or 10 mmol/l Ca(2+)solution. Results. Ca2+ entry through voltage-dependent L-type Ca2+ channels was unaffected by diabetes. In contrast, store-operated Ca2+ influx was attenuated. In microvessels from non-diabetic rats 20 mmol/l D-mannitol had no effect on store-operated Ca2+ influx. Diabetic rats injected daily with insulin had store-operated Ca2+ influx rates similar to non-diabetic control rats. The reduced Ca2+ entry in diabetic microvessels was reversed by 2-h exposure to 100 nmol/l staurosporine, a non-specific PKC antagonist and was mimicked in microvessels from non-diabetic rats by 10-min exposure to the PKC activator phorbol myristate acetate (100 nmol/l). The specific PKCbeta antagonist LY379196 (100 nmol/l) also reversed the poor Ca2+ influx although its action was less efficacious than staurosporine. Conclusion/interpretation. These results show that store-operated Ca2+ influx is inhibited in retinal arterioles from rats having sustained increased blood glucose and that PKCbeta seems to play a role in mediating this effect.