AJP - Renal Physiology, Vol 250, Issue 1 169-F175, Copyright © 1986 by American Physiological Society
Can causality be determined from proximal tubular reabsorption and peritubular physical factors?
B. J. Tucker, C. A. Mundy and R. C. Blantz
Many studies in the literature have drawn conclusions regarding the
mechanism of change in absolute proximal tubular reabsorption (APR) based
on steady-state measurements of proximal reabsorptive rates and the
peritubular capillary. The proximal reabsorptive rate, APR, is the product
of the effective reabsorptive pressure (ERP) and the peritubular capillary
reabsorptive coefficient (LpAR) (APR = ERP . LpAR). The ERP is defined by
the net hydrostatic and oncotic pressure gradient acting across the
capillary wall from interstitium to peritubular capillary flow. The
relationship APR = ERP . LpAR is predefined, and steady-state measurements
do not permit determination of causality because primary changes in any
variable obligate a proportional change in a second variable. As an example
of the difficulties in interpretation of this type of analysis, we have
examined the APR and factors contributing to ERP and LpAR before and after
the administration of benzolamide, a carbonic anhydrase inhibitor, to
saline-expanded Munich-Wistar rats. Alterations in peritubular capillary
fluid uptake cannot always be interpreted as casual mechanisms for changes
in absolute fluid reabsorption but may result from primary alterations in
epithelial transport.
