Mechanism of inhibition by alloxan of ATP-driven calcium transport by vascular smooth muscle microsomes.

The direct in vitro effects of alloxan on the Ca2+ handling by microsomal membranes isolated from dog mesenteric arteries were investigated. Preincubation of the vascular muscle microsomal membranes with alloxan showed a suppressive effect on both binding of Ca2+ (in the absence of ATP) and ATP-driven Ca2+ transport. Such an inhibition was time dependent, dose dependent, and temperature dependent. ATP-driven Ca2+ transport was much more susceptible to the inhibitory action of alloxan than Ca2+ binding under all experimental conditions examined. Alloxan inhibited ATP-driven Ca2+ transport at a comparable level over the entire period of Ca2+ uptake, but had no significant effect on the efflux of Ca2+ from preloaded microsomal membranes. This suggests that alloxan exerts its inhibitory effect on the ATP-driven Ca2+ transport via its action on the Ca-pump protein rather than the membrane permeability to Ca2+. Catalase and mannitol but not superoxide dismutase partially protected against such as inhibition by alloxan. The possible involvement of H2O2 mediating the inhibitory action of alloxan was further supported by the finding of a similar in vitro inhibitory effect of H2O2 on the ATP-driven Ca2+ transport by the vascular smooth muscle microsomes.