The effect of increasing extracellular potassium concentration on the resting heat rate of the isolated rat papillary muscle

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Abstract
The effect of elevating extracellular K+ concentration on the basal metabolism of the isolated rat left ventricular papillary muscle has been investigated. The preparation was mounted on a thermopile and connected to a force transducer, to allow simultaneous measurement of muscle heat production and force. The resting heat rate (RHR) of the quiescent preparation was measured as an index of basal metabolism. Throughout all of the experiments, the muscles were maintained under a resting force of 10 mN and all measurements of RHR were made at times when there was no active force present above this passive level. Elevating the extracellular K+ concentration from 5.9 to 20, 40, then 80 mM produced graded increases in the RHR. The increase in RHR produced by 40 mM K+ was observed to be time-dependent, its effect being significantly greater at 5–7 h than at 2–4 h after cardiectomy. Averaged over all times, the percentage increases in RHR produced by 20, 40, and 80 mM K+ in the presence of 2 mM Ca2+ were 6.4±2.0%, 28.7±2.3%, and 51.3±8.9% (mean±SEM) respectively. The high K+-induced increase in basal metabolism was also shown to be Ca2+-dependent, the increase in RHR produced by 40 mM K+ being greater the higher the extracellular Ca2+ concentration (0.5–8.0 mM). The addition of verapamil was found to partially inhibit the K+-induced increase in resting metabolism. These results show that elevation of the extracellular K+ concentration produces a graded increase in the RHR that is Ca2+-dependent. It is suggested that the increases in RHR observed in this investigation are in part associated with elevated levels of myoplasmic free Ca2+ which stimulate the internal Ca2+ pumps.