Compartmental analysis of Ca2+ kinetics in long-lasting diaphragm fatigue: loss of t-tubular membrane Ca2+.

An analytic method based on simulation and modeling of long-term 45Ca2+ efflux data was used to estimate Ca2+ contents (nmol Ca2+/g tissue wet wt) and exchange fluxes (nmol Ca2+.min-1.g-1) for extracellular and intracellular compartments in in vitro hamster diaphragm. Three physiological states were studied: control (n = 5), acute fatigue (after repeated tetany; n = 5), and long-lasting fatigue (1-h recovery; n = 5). Experimental muscles were loaded with 45Ca2+ for 1 h, and efflux data were collected for 8 h by use of a flow-through tissue chamber. Induction of acute diaphragm fatigue led to a uniform 200% elevation of the 8-h efflux curve (expressed as dpm.min-1.mg-1) relative to control. Conversely, in long-lasting fatigue the early component of the efflux curve was depressed compared with control, whereas the balance of the curve was restored to baseline. Analysis of control efflux data revealed that the early curve (0-2 h) contained data on two rapidly exchanging extracellular Ca2+ compartments, whereas the late curve (2-8 h) 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 increase accounted for the model prediction of a twofold 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-tubular and terminal cisternal compartments to escalate nearly threefold. 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 of the control value.