Ammonia movement and distribution after exercise across white
muscle cell membranes in rainbow trout: a perfusion study.
Wang, Yuxiang, George J. F. Heigenhauser, and Chris M. Wood.
Department of Biology and Department of Medicine, McMaster
University, Hamilton, Ontario, Canada, L8S 4K1
APStracts 3:0123R, 1996.
Manipulations of pH and electrical gradients in a perfused preparation
were used to analyze the factors controlling ammonia distribution and
flux in trout white muscle after exercise. Trout were exercised to
exhaustion and then an isolated-perfused white muscle preparation
with discrete arterial inflow and venous outflow was made from the
posterior portion of the tail. The tail-trunks were perfused with low
(7.4), medium (7.9), and high (8.4) pH saline, achieved by varying
[HCO3-] at constant Pco2. Intracellular and extracellular pH,
ammonia, CO2, K+, Na+, and Cl- were measured. Muscle intracellular pH
was not affected by changes in extracellular pH. Increasing
extracellular pH caused a decrease in the transmembrane PNH3 gradient
and a decrease in ammonia efflux. When extracellular [K+] was
increased from 3.5mM to 15mM in the medium pH group, a depolarization
of the muscle cell membrane potential from -92 mV to -60mV and a 0.1
unit depression in intracellular pH occurred. Ammonia efflux
increased despite a marked reduction in the PNH3 gradient. Amiloride
(10-4 M) had no effect, indicating that Na+/H+, NH4+ exchange does
not participate in ammonia transport in this system. A comparison of
observed intracellular to extracellular ammonia distribution ratios
with those modelled according to either pH or Nernst potential
distributions supports a model in which ammonia distribution across
white muscle cell membranes is affected by both pH and electrical
gradients, indicating that the membranes are permeable to both NH3
and NH4+. Membrane potential, acting to retain high levels of NH4+ in
the intracellular compartment, appears to have the dominant influence
during the post-exercise period. However, at rest, the pH gradient
may be more important, resulting in much lower intracellular ammonia
levels and distribution ratios. We speculate that the muscle cell
membrane NH3/NH4+ permeability ratio in trout may change between the
rest and post-exercise condition.
Received 26 September 1995; accepted in final form 15 March 1996.
APS Manuscript Number R597-5.
Article publication pending Am. J. Physiol. (Regulatory Integrative
Comp. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 1 April 96