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Department of Physiology, Centre Médical Universitaire, Geneva, Switzerland.

1. The rate of energy dissipation associated with Na(+)-H+ exchange in isolated, superfused soleus muscles from young rats was measured with an isothermal microcalorimeter during quasi-stationary states of oxidative metabolism. 2. Under normal physiological conditions, amiloride, an inhibitor of the Na(+)-H+ exchange across plasma membranes, had no measurable effect on the specific rate of muscle heat production (E); the ouabain-suppressible part of E was identical whether amiloride was absent or present. 3. E was increased under hyperosmotic conditions and the difference with respect to control (excess E) was proportional to the degree of hyperosmolarity of the superfusate. It was 48% of basal E during a +100 mosM stress (with no change of extracellular Na+ concentration, Na+o). Inhibition of Ca2+ release into the sarcoplasm with sodium dantrolene (10(-5) M) or tetracaine (5 x 10(-5) M) suppressed a substantial part (65 and 53%, respectively) of the steady-state excess E (1.2 mW (g wet weight)-1) induced by the +100 mosM stress. Practically 100% of excess E was suppressed in the nominal absence of extracellular sodium (Na+o = 0, Li+ substitution) or under 15 mM-Na+o, and excess E was enhanced when Na+o was increased (hyperosomolarity by addition of Na2SO4 instead of sucrose). 4. Under hyperosmotic conditions, amiloride at the 5 x 10(-7) M concentration had no effect on excess E whereas at 10(-4) M it induced a significant decrease of excess E. The absolute effect of 10(-4) M-amiloride was -0.34 mW (g wet weight)-1 (equal to 28% of the excess E due to a +100 mosM-sucrose stress and to 14% of the excess E due to a +100 mosM-Na2SO4 stress). It was left unaltered in the presence of dantrolene and was independent of the way the +100 mosM stress was obtained (i.e. 100 mM-sucrose or 50 mM-Na2SO4). It was suppressed at Na+o = 0-15 mM and could be mimicked by guanochlor, another potent inhibitor of Na(+)-H+ exchange. In the presence of 10(-4) M-amiloride, the ouabain-suppressible E was significantly reduced. In the presence of ouabain, amiloride had no effect. 5. Muscle tissue space available to [3H]inulin was measured in parallel experiments. It was 23.3% under control conditions and 30.6% after a 2 h exposure of the muscle to a +100 mosM-Na2SO4 stress.(ABSTRACT TRUNCATED AT 400 WORDS)