Microcalorimetric determination of energy expenditure due to active sodium-potassium transport in the soleus muscle and brown adipose tissue of the rat.

1. The resting heat production rate (E) of soleus muscles from young rats and brown adipose tissue from adult rats was measured by means of a perfusable heat flux microcalorimeter in the absence and presence of ouabain. In the soleus muscle, the acute response of E to ouabain was compared with the ouabain-suppressible components of 22Na-efflux and 42K-influx. 2. In standard Krebs-Ringer bicarbonate buffer, ouabain (10(-3)M) induced an immediate but transient decrease in E of around 5%. Both in muscle and adipose tissue this was followed by a progressive rise in heat production rate. 3. When the medium was enriched with Mg (10 mM), ouabain produced a sustained decrease in E of the same magnitude as in the standard medium and the secondary rise was less marked or abolished. Under these conditions, in the soleus muscle, ouabain inhibited E by 5% (i.e. by 1-76 +/- 0-22 mcal.g wet wt.-1.min-1), 22Na-efflux by 58% (0-187 +/- 0-013 micronmole. g wet wt.-1.min-1) and 42K-influx by 34% (0-132 +/- 0-028 micronmole. g wet wt.-1.min-1). 4. When the muscles were loaded with Na by pre-incubation in K-free Mg-enriched medium, the addition of K (3mM) induced an immediate ouabain-suppressible increase in E of 2-98 +/- 0-33 mcal. g wet wt.-1.min-1 and a concomitant stimulation of 22Na-efflux of 0-388 +/- 0-136 micronmole. g wet wt.-1.min-1. 5. Maximum Na/ATP ratios for the active Na-K transport process were computed, with no assumption as to the in vivo free energy of ATP hydrolysis. These were 2-1, 1-9 and 2-3 under the conditions described in paragraphs (2), (3) and (4) respectively. 6. The calculated reversible thermodynamic work associated with active Na-K transport corresponded to 34% of the measured ouabain-induced decrease in E. On the premise that the maximum efficiency of the cellular energy conservation processes is 65%, this estimate indicates that the minimum energetic efficiency of ATP utilization by the active Na-K transport process in mammalian muscle is 52%.