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Am J Physiol Renal Physiol 271: F158-F168, 1996;
0363-6127/96 $5.00
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AJP - Renal Physiology, Vol 271, Issue 1 158-F168, Copyright © 1996 by American Physiological Society

ARTICLES

Coupling of entry to exit by peritubular K+ permeability in a mathematical model of rat proximal tubule

A. M. Weinstein
Department of Physiology and Biophysics, Cornell University Medical College, New York, New York 10021, USA.

In the proximal tubule in vivo, glomerulotubular balance requires that tubule epithelial cells accommodate a twofold variation in Na+ reabsorption through the Na+/H+ exchanger of the luminal membrane. In a mathematical model of proximal tubule, in which permeability coefficients are fixed, doubling flux through the Na+/H+ antiporter produces a substantial increase in cell volume and cytosolic HCO3-. In this model, it is possible to vary peritubular K+ permeability with changes in luminal Na+ entry, so that cell volume is constrained to be constant. In these calculations, the model predicts that peritubular hyperpolarization and nearly constant cytosolic HCO3- will accompany increases in luminal Na+ entry. Realistic models of variable peritubular K+ permeability might include a functional dependence on flux through the Na(+)-K(+)-adenosinetriphosphatase, cytosolic pH, or cell volume. When K+ permeability is represented as a function of any of these variables, homeostatic control of cell volume and pH can be obtained over a physiological variation of Na+/H+ flux. However, when luminal Na+ entry is via Na(+)-glucose cotransport, volume homeostasis is best when peritubular K+ permeability depends on the rate of active Na+ transport. For any modulator of K+ permeability, realistic constraints on the value of this parameter suggest that peritubular K+ permeability is, by itself, not sufficient to maintain cell volume within narrow limits. Parallel activation of another exit pathway, such as peritubular Na(+)-3 HCO3- cotransport, may be required to achieve the necessary homeostasis.


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A. M. Weinstein, S. Weinbaum, Y. Duan, Z. Du, Q. Yan, and T. Wang
Flow-dependent transport in a mathematical model of rat proximal tubule
Am J Physiol Renal Physiol, April 1, 2007; 292(4): F1164 - F1181.
[Abstract] [Full Text] [PDF]


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