MgATP-dependent, glucose 6-phosphate-stimulated liver microsomal Ca2+ accumulation: difference between rough and smooth microsomes

Some features of the MgATP-dependent Ca2+-accumulating capacity of rough as compared to smooth liver microsomal fraction were studied. Smooth microsomes accumulate somewhat higher amounts of Ca2+ than rough ones in the presence of MgATP. In the presence of glucose 6-phosphate, which markedly stimulates MgATP-dependent Ca2+ accumulation in both fractions, smooth microsomes exhibit a much higher Ca2+-accumulating capacity than rough ones. Possible reasons of the differences observed between the two fractions were investigated. Smooth microsomes exhibit a higher Ca2+-dependent ATPase activity, suggesting a higher Ca2+ inward transport into smooth vesicles. Also, following the inhibition of active Ca2+ transport by means of vanadate, smooth microsomes appear to release the Ca2+ previously accumulated—both in the absence (i.e., with MgATP only) and in the presence of glucose 6-phosphate—at a lower rate than rough ones. This indicates a lower passive backflux of Ca2+ accumulated in smooth vesicles. On the basis of these data, differences can be envisaged with respect to cellular Ca2+ handling by different domains of endoplasmic reticulum in the liver.