Single-channel activities of the human epithelial Ca2+ transport proteins CaT1 and CaT2

The human epithelial channels., CaT1 and CaT2, were expressed in oocytes, a nd their single-channel characteristics were compared. In the presence of N a+ and K+ as charge carriers in the pipette solutions, channel activities w ere observed only when the the extracellular sides of the patches were expo sed to nominally Ca2+- and Mg2+-free solutions. In patches of both CaT1- an d CaT2-expressing oocytes, multiple channel openings were observed, but the current levels were higher in CaT2-expressing oocytes, particularly at mor e negative voltages. With K+ as a charge carrier in patches of CaT1-express ing oocytes, the channel activity was low at -10 to -60 mV, but increased d ramatically at more negative potentials. This voltage dependence was observ ed in the presence of both Na+ and K+. The channel activity with Na+, howev er, was higher at all potentials. Differences between the voltage dependenc ies for the two cations were also observed in CaT2-expressing oocytes, but the channel activities were higher than those in CaT1-expressing oocytes, p articularly in the presence of Na+. We also found that low concentrations o f extracellular Mg2+ (5-50 muM) elicited a strong inhibitory action on the CaT channels. Activation of the CaT1 and CaT2 channels by hyperpolarization and other factors may promote increased Ca2+ entry that participates in st imulation of intestinal absorption and renal reabsorption and/or other Ca2 transport mechanisms in epithelial cells.