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Department of Physiology and Biophysics, Cornell University Medical College, New York, New York 10021
A mathematical model of the inner medullary
collecting duct (IMCD) of the rat has been developed representing
Na+, K+, Cl
,
HCO
3, CO2,
H2CO3, phosphate, ammonia, and urea. Novel
model features include: finite rates of hydration of CO2, a
kinetic representation of the H-K-ATPase within the luminal cell
membrane, cellular osmolytes that are regulated in defense of cell
volume, and the repeated coalescing of IMCD tubule segments to yield
the ducts of Bellini. Model transport is such that when entering
Na+ is 4% of filtered Na+, approximately 75%
of this load is reabsorbed. This requirement renders the area-specific
transport rate for Na+ comparable to that for proximal
tubule. With respect to the luminal membrane, there is experimental
evidence for both NaCl cotransport and an Na+ channel in
parallel. The experimental constraints that transepithelial potential
difference is small and that the fractional apical resistance is
greater than 85% mandate that more than 75% of luminal
Na+ entry be electrically silent. When Na+
delivery is limited, an NaCl cotransporter can be effective at reducing
luminal Na+ concentration to the observed low urinary
values. Given the rate of transcellular Na+ reabsorption,
there is necessarily a high rate of peritubular K+
recycling; also, given the lower bound on luminal membrane
Cl
reabsorption, substantial peritubular
Cl
flux must be present. Thus, if realistic limits on
cell membrane electrical resistance are observed, then this model
predicts a requirement for peritubular electroneutral KCl exit.
epithelial sodium ion transport; sodium chloride cotransport; potassium chloride cotransport
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