Evidence by calorimetry for an activation of sodium‐hydrogen exchange of young rat skeletal muscle in hypertonic media.

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Abstract
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 (.ovrhdot.E); the ouabain-suppressible part of .ovrhdot.E was identical whether amiloride was absent or present. 3. .ovrhdot.E was increased under hyperosmotic conditions and the difference with respect to control (excess .ovrhdot.E) was proportional to the degree of hyperosmolarity of the superfusate. It was 48% of basal .ovrhdot.E during a +100 mosM stress (with no change of extracellular Na+ concentration, Nao+). Inhibition of Ca2+ release into the sarcoplasm with sodium dantrolene (10-5 M) or tetracaine (5 .times. 10-5 M) suppressed a substantial part (65 and 53%, respectively) of the steady-state excess .ovrhdot.E (1.2 mW (g wet weight)-1) induced by the +100 mosM stress. Practically 100% of excess .ovrhdot.E was suppressed in the nominal absence of extracellular sodium (Nao+ = 0, Li+ substitution) or under 15 mM-Nao+, and excess .ovrhdot.E was enhanced when Nao+ was increased (hyperosmolarity by addition of Na2SO4 instead of sucrose). 4. Under hyperosmotic conditions, amiloride at the 5 .times. 10-7 M concentration had no effect on excess .ovrhdot.E whereas at 10-4 M it induced a significant decrease of excess .ovrhdot.E. The absolute effect of 10-4 M-amiloride was -0.34 mW (g wet weight)-1 (equal to 28% of the excess .ovrhdot.E due to a +100 mosM-sucrose stress and to 14% of the excess .ovrhdot.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 Nao+ = 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 .ovrhdot.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. The intracellular water space was 55.1 .+-. 0.3 (mean muscle weight, 23.4 .+-. 0.6 mg) under control conditions, and 47.4 .+-. 0.6 (mean muscle weight, 19.5 .+-. 0.7 mg) at the end of the 2 h hyperosmotic stress. In the same hypertonic conditions but in the continuous presence of amiloride, the intracellular space was 45.5 .+-. 0.6 (mean muscle weight, 18.9 .+-. 0.6 mg), slightly but significantly lower than in the absence of amiloride. 6. The results suggest that the Na+-H+ exchange, which is quiescent under normal physiological conditions in mammalian muscle, is activated and influences cell volume during a hyperosmotic stress. The cellular energy expenditure associated with Na+-H+ exchange is entirely ouabain-suppressible under quasi-stationary conditions, confirming that this ion exchange is a secondary-active process.