Effective gibbs free energy change of atp hydrolysis and metabolic
correlates of intracellular ph in progressive fatigue of squid
(lolliguncula brevis) mantle muscle.
P[diaeresis]ortner, H. O., E. Finke, P. G. Lee.
Alfred-Wegener-Institut f[umlaut]ur Polar- und Meeresforschung,
Biologie I/[diaeresis]okophysiologie, Columbusstra[beta]e, D-27515
Bremerhaven and Marine Biomedical Institute, University of Texas
Medical Branch, Galveston, Texas
APStracts 3:0166R, 1996.
Squid (Lolliguncula brevis) were exercised at increasing swimming
speeds in order to analyse the correlated changes in intracellular
metabolic, acid-base and energy status of the mantle musculature.
Beyond a critical swimming velocity of 1.5 mantle lengths sec-1, an
intracellular acidosis developed which was caused by an initial base
loss from the cells, the onset of respiratory acidification and,
predominantly, octopine formation. The acidosis was correlated with
decreasing levels of phospho-L-arginine, and, thus, supported ATP
buffering at the expense of the phosphagen. Monohydrogenphosphate,
the actual substrate of glycogen phosphorylase accumulated, enabling
glycogen degradation despite progressive acidosis. In addition to
octopine, succinate and [alpha]-glycerophosphate accumulation (11),
the onset of acidosis characterizes the critical velocity and
indicates the transition to a non-steady state, time-limited
situation. Accordingly, swimming above the critical velocity caused
cellular energy levels (in vivo Gibb_s free energy change of ATP
hydrolysis, dG/dx) to fall. A minimal value was reached at about -45
kJ mol-1. Model calculations demonstrate that changes in free Mg2+
levels only minimally affect ATP free energy, but mimumum levels are
relevant to maintain functional concentrations of Mg2+ complexed
adenylates. Model calculations also reveal that phosphagen breakdown
enabled L. brevis to reach swimming speeds about three times higher
than the critical velocity. A comparison of two offshore squid
species (Loligo pealei, Illex illecebrosus) with the estuarine squid,
L.brevis indicates that the latter uses a strategy to delay the
exploitation of high energy phosphates and protect energy levels at
higher than the minimum levels (-42 kJ mol-1) characterizing fatigue
in the other species. A more economical use of anaerobic resources
and an early reduction of performance may enable L. brevis to
tolerate more extreme environmental conditions in shallow estuarine
waters and even hypoxic environments and to prevent a fatal depletion
of energy stores.
Received 12 December 1995; accepted in final form 20 April 1996.
APS Manuscript Number R787-5.
Article publication pending Am. J. Physiol. (Regulatory Integrative
Comp. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 8 May 96