Calcium transport across the pars recta of cortical segment 2 proximal tubules

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Abstract
Partes rectae of cortical segment 2 proximal tubules were dissected from rabbit kidneys and perfused in vitro. Ca concentrations of perfused and collected fluids were measured by continuous-flow microcolorimetry. Epithelial CA permeabilty (P) was estimated from the bath-to-lumen movement of 45Ca. The transepithelial voltage (.psi.) and [Ca2+] differences were varied simultaneously by change in perfusate composition. Tubules that were perfused and batched with an identical artificial ultrafiltrate of plasma displayed a lumen-negative .psi., a collectate [Ca] greater than perfusate, and net Ca secretion. Tubule perfused with "late" proximal tubule fluid (high [Cl], low [HCO3], low concentrations of Na+-cotransported solutes) demonstrated a lumen-positive .psi., a perfusate [Ca2+] greater than the bath, a collectate [Ca] less than perfusate, and net Ca absorption. Under each of these conditions, net Ca flux was in the direction predicted by the experimentally measured driving forces for diffusional Ca transport. Tubules that were cooled while being perfused with late proximal tubule fluid showed an increasd lumen-positive .psi. but reduced net Ca absorption. The latter finding was as consistent with reduced Ca ion diffusion related to a smaller P at the lower temperature. I conclude that Ca2+ diffusion is an important component of net Ca absorption in this segment of the nephron.