Differential, direct effects of H+ on Ca2+ -activated force of skinned fibers from the soleus, cardiac and adductor magnus muscles of rabbits.

Abstract

The effect of acidosis on Ca2+-activated force generation was studied in rabbit soleus, left ventricular, and adductor magnus muscles. Fibers were skinned (sarcolemma peeled off or mechanico-chemically disrupted) to facilitate direct manipulation and standardization of their intracellular ionic milieus according to bathing solution composition. Skinned single skeletal and small bundles of cardiac fibers were mounted in a photodiode force transducer and activated by immersion in buffered-Ca2+ bathing solutions. The magnitude of steady state isometric force at each [Ca2+] was determined at pH 7.0 and 6.5 (paired data) at both 1 mM and 10 mM Mg2+ in order to detect artifacts of errors in calculated [Ca2+]. All bathing solutions contained: 7 mM total EGTA [ethyleneglycol-bis-(β-amino-ethylether)-N,N′ tetra-acetic acid], 70 mM (Na++K+), 2 mM MgATP2− (Mg adenosine triphosphate), 15 mM CP2− (creatine phosphate), 15 units/ml CPK (creatine phosphokinase), imidazole (adjusted ionic strength to 0.15 M), and propionate anion at 23±1° C. Maximum tensions were similar at both [Mg2+]s but less at pH 6.5 than at pH 7.0, with the following order of mean magnitude of acidotic depression adductor>cardiac>soleus. The proportionately greater acidotic depression of submaximum (relative to maximum) forces that occurred only at 1 mM Mg2+ (cardiac>adductor>soleus) implicates acidotic depression of Ca2+-activated force as a major cause of decreased cardiac contractility.