Ca(2+)-dependent heat production by rat skeletal muscle in hypertonic media depends on Na(+)-Cl- co-transport stimulation.
1. The rate of energy dissipation (E) in isolated, superfused soleus muscles from young rats was continuously measured under normosmotic and 100-mosM hyperosmotic conditions. The substantial increase of E with respect to basal level in hyperosmolarity (excess E), which is entirely dependent on the presence of extracellular sodium, was largely prevented or inhibited by bumetanide, a potent inhibitor of Na(+)-Cl- co-transport system, or by the removal of chloride from the superfusate (isethionate substitution). Bumetanide or the removal of chloride also acutely decreased basal E, by about 7%. 2. Bumetanide almost entirely suppressed the major, Ca(2+)-dependent part of excess E in hyperosmolarity, as well as the concomitant increase of 45Ca2+ efflux and small increase in resting muscle tension; in contrast, the part of excess E associated with stimulation of Na(+)-H+ exchange in hyperosmolarity was left unmodified. 3. Reduction of 22Na+ influx by bumetanide was more marked in hyperosmolarity than under control conditions, although stimulation of total 22Na+ influx by a 100-mosM stress was not statistically significant. Inhibition of Ca2+ release into the sarcoplasm using dantrolene sodium did not prevent the stimulation of bumetanide-sensitive 22Na+ influx, but rather increased it about fourfold. 4. It is concluded that the largest part of excess E in hyperosmolarity, which is Ca(2+)-dependent energy expenditure, is suppressed when steady-state stimulation of a Na(+)-Cl- co-transport system is inhibited either directly by bumetanide or the removal of extracellular chloride, or indirectly by the blocking of active Na(+)-K+ transport. How the stimulation of Na(+)-Cl- co-transport, by as little as 1 nmol s-1 (g wet muscle weight)-1 during a 100-mosM stress, enhances Ca(2+)-dependent heat by as much as 2.5 mW (g wet muscle weight)-1 remains to be clarified.
Department of Physiology, University of Geneva, Centre Medical Universitaire, Switzerland.
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D000818:Animals; D001833:Body Temperature Regulation / Q000187:drug effects / Q000502:physiology*; D002034:Bumetanide / Q000494:pharmacology; D002118:Calcium / Q000502:physiology*; D002352:Carrier Proteins / Q000502:physiology*; D002712:Chlorides / Q000502:physiology; D003620:Dantrolene / Q000494:pharmacology; D005260:Female; D066298:In Vitro Techniques; D008297:Male; D009132:Muscles / Q000378:metabolism*; D009994:Osmolar Concentration; D051381:Rats; D017207:Rats, Sprague-Dawley; D017208:Rats, Wistar; D012964:Sodium / Q000378:metabolism; D049992:Sodium Chloride Symporters; D017923:Sodium-Hydrogen Exchangers; D027981:Symporters
D002352:Carrier Proteins; D002712:Chlorides; D049992:Sodium Chloride Symporters; D017923:Sodium-Hydrogen Exchangers; D027981:Symporters; D002034:Bumetanide; D012964:Sodium; D003620:Dantrolene; D002118:Calcium