Compartmental analysis of steady-state diaphragm Ca 2+ kinetics in chronic congestive heart failure

Abstract

An analytic method based on simulation and modeling of long-term 45 Ca 2+ efflux data was used to estimate steady-state Ca 2+ contents (nmol Ca 2+ g −1 tissue wet wt.) and exchange fluxes (nmol Ca 2+ min −1 g −1 tissue wet wt.) for extracellular and intracellular compartments in in vitro resting diaphragm from congestive heart failure (CHF, n=12) and sham-operated (SHAM, n=10) rats. Left hemidiaphragms were excised from experimental animals, loaded with 45 Ca 2+ for 1 h, and washed out with 45 Ca 2+ -free perfusate for 8 h. Tissue from the right hemidiaphragm was used to assess single-fiber cross-sectional area (CSA) as well as the relative proteolytic activity of Ca 2+-dependent calpain. Kinetic analysis of 45 Ca 2+ efflux data revealed that CHF was associated with increased Ca 2+ contents of extracellular and intracellular compartments as well as increased Ca 2+ exchange fluxes for all compartments. This accounted for the model prediction of a 250% increase in total diaphragm Ca 2+. Furthermore, single-fiber CSA was decreased 12% and proteolytic activity of calpain was increased twofold in CHF diaphragm relative to SHAM. Conclusions: The kinetic data are consistent with the hypothesis that diaphragm Ca 2+ overload in CHF required all intercompartmental Ca 2+ fluxes to increase. The potential relationships among Ca 2+ overload, increased activity of calpain, and wasting of the diaphragm in CHF are discussed.