Calcium-dependent enhancement of myocardial diastolic tone and energy utilization dissociates systolic work and oxygen consumption during low sodium perfusion

The relationships and correlations among functional, metabolic, and ionic consequences of low sodium perfusion were studied in isovolumic, retrograde-aortic perfused working rat hearts by ³¹P nuclear magnetic resonance, oxygen consumption, and atomic absorption spectrometry. Reduction of perfusate sodium from 144 to 74, 51, 39, and 25 mm in four separate groups of hearts via lithium substitution for 15 minutes decreased cell sodium to mean values of 62, 51, 43 and 36 μmol/g dry weight, respectively (P < 0.001 vs. control of 107). There was a transient rise and then a fall in developed pressure and a decline in phosphocreatine and adenosine triphosphate, all of which were graded and correlated with perfusate sodium (P < 0.01 for all parameters vs. perfusate sodium). This was accompanied by a 2- to 7-fold elevation of diastolic pressure while oxygen consumption remained near control levels. All parameters except adenosine triphosphate returned toward baseline values when normal perfusate sodium was reintroduced. Although cell calcium as measured by atomic absorption spectrometry did not differ among the groups, the functional and metabolic changes did not occur if the sodium steps were performed in reduced perfusate calcium (0.08 mm). In hearts in which systolic function was obliterated by verapamil, exposure to zero sodium caused a 4-fold increase in oxygen consumption, an increase in diastolic pressure, and a reduction of high energy phosphates. In the presence of ryanodine, a specific inhibitor of sarcoplasmic reticulum calcium release, the metabolic changes did not occur, and the excess oxygen consumption in zero sodium was substantially reduced. Thus, the effect of lowered perfusate sodium in beating hearts, i.e., to dissociate oxygen consumption and systolic function, and to increase diastolic pressure and its effect in arrested hearts to increase oxygen consumption, are calcium dependent, energy consuming, and modulated by sarcoplasmic reticulum calcium cycling.