AJP - Renal Physiology, Vol 250, Issue 3 516-F519, Copyright © 1986 by American Physiological Society
Stoichiometry of sodium chloride transport by rectal gland of Squalus acanthias
P. Silva and M. A. Myers
The stoichiometry of the sodium chloride cotransport system was examined in
isolated perfused rectal glands of Squalus acanthias by kinetic analysis of
the effect on chloride secretion of progressive substitutions of other ions
for sodium and chloride in the perfusate. Secretion was stimulated by a
constant infusion of dibutyryl cyclic AMP (5 X 10(-5) M) and theophylline
(2.5 X 10(-4) M). Sodium was replaced by N-methyl-D-glucamine, whereas
chloride was replaced by gluconate. The Km values for sodium, obtained
using three different graphic methods, were close to or at the normal
concentration of sodium in the plasma of the shark, suggesting that plasma
sodium concentration regulates transport by the gland. The Km values for
chloride were far below the normal concentration of chloride in the plasma,
indicating that the chloride sites are normally saturated and therefore
plasma chloride concentration cannot control transport by the gland. Hill
plots revealed slopes of 1.06 for sodium and 1.6 for chloride, consistent
with the hypothesis that 2 Cl- and 1 Na+ interact in the cotransport
process. The cotransport linkage of 2 Cl- with 1 Na+ in the initial step of
entry into the cell can be viewed as a device that doubles the energetic
efficiency of salt transport, allowing 2 NaCl to be secreted for every 1
Na+ actively pumped.
