The role of extracellular sodium on heart muscle energetics

Abstract

A study has been made of changing external sodium concentration [Na]_e, over the range 75 to 200 mmol · 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] ²_e /[Ca]_e ratio, paralleled those observed for tension development (T). On the other hand the maximal rate of relaxation$\left( { - \dot T_{\max } } \right)$ and the ratio$- \dot T_{\max } /T$ increased. While the ratio between active heat production and developed tension remained unaltered (0.111±0.003 mJ · mN⁻¹ · g⁻¹ dry weight), resting heat production increased with [Na] ²_e with a slope of 95±18 mW · g⁻¹ · mol⁻² · l². Under resting conditions, a decrease in [Na]_e of 50 mmol · l⁻¹ induced a fall in⁴²K uptake of about 16 nmol · s⁻¹ · g⁻¹ without changes in⁴²K efflux, suggesting that such an intervention depresses K influx. If the depressed K influx, induced by a decrease in [Na]_e of 50 mmol · l⁻¹ is associated with a decrease in Na−K pump activity, a fall in resting heat production of about 0.64 mW · g⁻¹ would be expected. This represent 56% of the calculated change in the resting heat production, 1.14±0.22 mW · 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.