The role of extracellular sodium on heart muscle energetics.

A study has been made of changing external sodium concentration [Na]e, over the range 75 to 200 mmol X l-1, on contractile parameters and heat production in isolated, arterially perfused, interventricular rabbit septa.- The observed changes in maximum rate of contraction with [Na]e, either in the presence of a constant external Ca concentration [Ca]e or in the presence of a constant [Na]e2/[Ca]e ratio, paralleled those observed for tension development (T). On the other hand the maximal rate of relaxation (-Tmax) and the ratio -Tmax/T increased. While the ratio between active heat production and developed tension remained unaltered (0.111 +/- 0.003 mJ X mN-1 X g-1 dry weight), resting heat production increased with [Na]e2 with a slope of 95 +/- 18 mW X g-1 X mol-2 X l2. Under resting conditions, a decrease in [Na]e of 50 mmol X l-1 induced a fall in 42K uptake of about 16 nmol X s-1 X g-1 without changes in 42K efflux, suggesting that such an intervention depresses K influx. If the depressed K influx, induced by a decrease in [Na]e of 50 mmol X l-1, is associated with a decrease in Na-K pump activity, a fall in resting heat production of about 0.64 mW X g-1 would be expected. This represent 56% of the calculated change in the resting heat production, 1.14 +/- 0.22 mW X g-1 (mean +/- one confidence interval), suggesting that some process in addition to a depressed Na-K pump activity may be altered by changes in [Na]e.