AJP - Renal Physiology, Vol 245, Issue 5 640-F644, Copyright © 1983 by American Physiological Society
Intracellular chloride activities in the isolated perfused shark rectal gland
M. J. Welsh, P. L. Smith and R. A. Frizzell
The isolated, perfused shark rectal gland secretes Cl when stimulated with
adenosine 3',5'-cyclic monophosphate (cAMP). To investigate the mechanism
of secretion, we used Cl-selective and conventional (KCl-filled)
microelectrodes to measure the intracellular Cl activity (aClc). Under
nonsecreting conditions, the electrical potential difference across the
basolateral membrane (psi b) was -78 m V and aClc was 57 mM, a value seven
times greater than predicted for electrochemical equilibrium across the
basolateral membrane. When theophylline and 8-bromo-cAMP were added to the
perfusate, the transglandular electrical potential difference doubled and
the rate of fluid secretion increased 20-fold; however, neither psi b nor
aClc changed. During both nonsecreting and secreting conditions the
intracellular accumulation of Cl results in an electrochemical potential
difference favoring Cl exit across the apical cell membrane. The constancy
of aClc despite the variation in secretion rate suggests that stimulation
is associated with an equivalent enhancement of net Cl movement across both
the apical and basolateral membranes. When stimulated glands were perfused
with Na-free (choline) Ringer, secretion was abolished and aClc fell toward
the value predicted for electrochemical equilibrium. These findings suggest
that the "uphill" step in Cl secretion lies at the basolateral membrane,
where cellular Cl accumulation probably involves secondary active
transport; i.e., Cl entry is driven by an inwardly directed electrochemical
potential difference for Na.
