Calcium-stressed erythrocyte membrane structure and function for assessing glipizide effects on transglutaminase activation.
A proposed mechanism of action of hypoglycemic sulfonylureas is the prevention of transglutaminase-mediated endocytosis of insulin receptors. When activated by high levels of intracellular calcium, transglutaminase (TG) catalyzes the cross-linking of intracellular proteins to membrane proteins and modifies membrane structure and function. This study examined the effects of the sulfonylurea glipizide on TG activity in an erythrocyte model by assessing various membrane ATPase activities and high molecular weight protein polymer formation using sodium dodecyl sulfate-polyacrylamide gel electrophoresis. To activate TG, red blood cells were exposed to 1 mM intracellular Ca2+ using 10(-5) M Ca2(+)-ionophore A23187. In Ca2(+)-stressed cells, calmodulin stimulation (0.1 micrograms/ml) of (Ca2+ + Mg2+)-ATPase was decreased to 21.2% of control activity. Increasing concentrations of calmodulin (0.1-3.0 micrograms/ml) could not overcome the inhibitory effects of TG on the (Ca2+ + Mg2+)-ATPase in Ca2(+)-stressed cells with or without glipizide. An increased Ca2+ sensitivity of calmodulin-independent (Ca2+ + Mg2+)-ATPase due to Ca2+ stress was seen in all Ca2(+)-stressed cells even in the presence of 1 mM glipizide. Structural changes were observed in the form of high molecular weight polymer formation. Cells exposed to high Ca2+ and glipizide (3 x 10(-5)-10(-3) M) showed no improvement in ATPase activity or protection from protein cross-linking compared with cells without the drug. We conclude that in this model glipizide fails to inhibit TG induced protein cross-linking and does not prevent the decrease in (Ca2+ + Mg2+)-ATPase activation in Ca2(+)-stressed red blood cells. This finding considerably weakens the proposal that sulfonylureas act by inhibiting TG activity.