The determinants of transverse tubular volume in resting skeletal muscle.

The Journal of physiology

PubMedID: 25384782

Sim J, Fraser JA. The determinants of transverse tubular volume in resting skeletal muscle. J Physiol (Lond). 2014;.
The transverse tubular (t-) system of skeletal muscle couples sarcolemmal electrical excitation with contraction deep within the fibre. Exercise, pathology, and the composition of the extracellular fluid (ECF) can alter t-system volume (t-volume). T-volume changes likely contribute to fatigue, rhabdomyolysis, and disruption of excitation-contraction coupling. However, mechanisms that underlie t-volume changes are poorly understood. A multi-compartment, history-independent computer model of rat skeletal muscle was developed to define the minimum conditions for t-volume stability. It was found that the t-system tends to swell due to net ionic fluxes from the ECF across the access resistance. However, a stable t-volume is possible when this is offset by a net efflux from the t-system to the cell and thence to the ECF, forming a net ion cycle ECF?t-system ?sarcoplasm?ECF that ultimately depends on Na(+)/K(+)-ATPase activity. Membrane properties that maximise this circuit flux decrease t-volume, including PNa(t) > PNa(s), PK(t) < PK(s) and N(t) < N(s) (P, permeability; N, Na(+)/K(+)-ATPase density; (t), t-system membrane; (s), sarcolemma). Hydrostatic pressures, fixed charges and/or osmoles in the t-system can influence the magnitude of t-volume changes that result from alterations in this circuit flux. Using a parameter set derived from literature values where possible, this novel theory of t-volume was tested against data from previous experiments where t-volume was measured during manipulations of ECF composition. Predicted t-volume changes correlated satisfactorily. This present work provides a robust, unifying theoretical framework for understanding the determinants of t-volume. This article is protected by copyright. All rights reserved.