Calcium transport was studied in isolated S2 segments of rabbit superficial proximal convoluted tubules. 45Ca was added to the perfusate for measurement of lumen-to-bath flux (J(lb)(Ca), to the bath for bath-to-lumen flux (J(bl)(Ca), and to both perfusate and bath for net flux (J(net)(Ca). In these studies, the perfusate consisted of an equilibrium solution that was designed to minimize water flux or electrochemical potential differences (PD). Under these conditions, J(lb)(Ca) (9.1 ± 1.0 peq/mm·min) was not different from J(bl)(Ca) (7.3 ± 1.3 peq/mm·min), and J(net)(Ca) was not different from zero, which suggests that calcium transport in the superficial proximal convoluted tubule is due primarily to passive transport. The efflux coefficient was 9.5 ± 1.2 x 10-5 cm/s, which was not significantly different from the influx coefficient, 7.0 ± 1.3 x 10-5 cm/s. When the PD was made positive or negative with use of different perfusates, net calcium absorption or secretion was demonstrated, respectively, which supports a major role for passive transport. These results indicate that in the superficial proximal convoluted tubule of the rabbit, passive driving forces are the major determinants of calcium transport.
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