The nature and mechanism of activation of the hepatocyte receptor-activated Ca2+ inflow system.

Progress in elucidation of the properties of the hepatocyte receptor-activated Ca2+ inflow system (RACIS) has been hampered by difficulties in measuring rates of Ca2+ inflow to hepatocytes. These difficulties have led, for example, to different conclusions about the relationship between the extracellular Ca2+ concentration and the movement of Ca2+ through the RACIS. The hepatocyte RACIS admits Mn2+ and a number of other divalent cations as well as Ca2+. Many of these cations also inhibit the movement of Ca2+ through this system. While the RACIS is inhibited by high concentrations of verapamil and by some other Ca2+ antagonists, it is relatively insensitive to inhibition by organic compounds which inhibit other Ca2+ channels and Ca2+ transporters. There is circumstantial evidence which suggests that the hepatocyte RACIS is an exchange system, possibly one which catalyses Ca(2+)-H+ exchange or the co-transport of Ca2+ and OH-. Other circumstantial evidence suggests that the RACIS is a channel, with some similarities to voltage-operated Ca2+ channels in excitable cells. However, experiments using the patch-clamp technique have not yet detected agonist-stimulated Ca2+ movement across the hepatocyte plasma membrane. The molecular components of the RACIS probably differ from those which facilitate the large inflow of Ca2+ to hepatocytes which occurs in the absence of an agonist. The mechanism by which agonists activate the RACIS has not been elucidated.(ABSTRACT TRUNCATED AT 250 WORDS)