AJP - Renal Physiology, Vol 251, Issue 4 718-F724, Copyright © 1986 by American Physiological Society
Calcium transport across the pars recta of cortical segment 2 proximal tubules
J. E. Bourdeau
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 permeability (P) was estimated from the bath-to-lumen
movement of 45Ca. The transepithelial voltage (psi) and [Ca2+] difference
were varied simultaneously by changing perfusate composition. Tubules that
were perfused and bathed with an identical artificial ultrafiltrate of
plasma displayed a lumen-negative psi, a collectate [Ca] greater than
perfusate, and net Ca secretion. Tubules 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 increased lumen-positive psi but
reduced net Ca absorption. The latter finding was 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.
