Compartmental analysis of ca2+ kinetics in long-lasting diaphragm
fatigue: loss of t-tubular membrane ca2+.
Howell, Sandra.
Departments of Biokinesiology and Physical Therapy, and Biomedical
Engineering, University of Southern California, Los Angeles, CA
APStracts 3:0046A, 1996.
An analytic method based on simulation and modeling of long-term
45Ca2+ efflux data was used to estimate Ca2+ contents (nmol Ca2+/g
wet wt tissue) and exchange fluxes (nmol Ca2+/min/g) for
extracellular and intracellular compartments in in vitro hamster
diaphragm. Three physiologic states were studied: control (n=5),
acute fatigue (following repeated tetany; n=5), and long-lasting
fatigue (1-hr recovery; n=5). Experimental muscles were loaded with
45Ca2+ for 1 hr and efflux data collected for 8 hrs using a flow
-through tissue chamber. Induction of acute diaphragm fatigue lead to
a uniform, 200% elevation of the 8-hr efflux curve (expressed as
DPM/min-mg) relative to control. Conversely, in long-lasting fatigue,
the early component of the efflux curve was depressed compared with
control while the balance of the curve was restored to baseline.
Analysis of control efflux data revealed that the early curve (0-2
hrs) contained data on two rapidly exchanging extracellular Ca2+
compartments while the late curve (2-8 hrs) reflected information on
two slowly exchanging intracellular compartments. Modeling of acute
fatigue efflux data estimated a 239% increase in one extracellular
Ca2+ compartment (putative T-tubular membrane) and a 546% increase in
one intracellular Ca2+ compartment (putative terminal cisternae).
These increases accounted for the model prediction of a 2-fold rise
in total diaphragm Ca2+. The kinetic data were quantitatively
consistent with the hypothesis that diaphragm Ca2+ overload in acute
fatigue required sarcolemmal Ca2+ permeability to double and Ca2+
diffusion into the T-tubule and terminal cisternae compartments to
escalate nearly 3-fold. Fitting of long-lasting fatigue efflux data
was associated with the sole prediction that T-tubular membrane Ca2+
was reduced to less than one-half the value of control.
Received 2 February 1995; accepted in final form 8 January 1996.
APS Manuscript Number A122-5.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 25 January 96