Influence of extracellular potassium on energetics of resting heart muscle

The effects of various extracellular K concentrations ([K]e) on energy expenditure and their relationship to ionic exchange mechanisms under quiescent conditions were investigated in the arterially perfused rat heart. The increase in [K]e (from 6 to 12, 24, or 50 mM K) leads to a rapid increase (results are given per gram dry weight) in resting energy expenditure (+5.9 +/- 0.9, +13.6 +/- 1.1, and +30.0 +/- 2.0 mW/g, respectively) followed by a slow decrease toward a new steady rate of heat production but higher (+2.8 +/- 0.7, +6.3 +/- 0.6, and +10.5 +/- 1.1 mW/g) than that observed under control conditions (21.1 +/- 0.7 mW/g). The increase in [K]e from 6 to 50 mM also induced an increase in K influx (calculated from 86Rb uptake and efflux experiments) of approximately 0.25 mumol.g-1.s-1. If this increased K influx is driven by the Na-K pump, an increase in steady resting heat production of approximately 10 mW/g would be expected. This represents 95% of the increase in steady heat production measured for 50 mM K intervention. The simultaneous increase in the cellular Ca flux (+0.1 mumol.g-1.min-1) can only explain (if driven by the sarcolemmal Ca pump) less than 1% of the steady increase in heat production. The work also shows that the initial, transitory increase in resting heat production induced by increasing [K]e is caffeine sensitive and may be at least partially attributable to a transitory enhanced activity of the sarcoplasmic reticulum.