Ca2+ Transients in Perfused Hearts Revealed by Gated 19F NMR Spectroscopy

Gated acquisition of "T nuclear magnetic resonance spectra from perfused ferret hearts loaded with the fluorinated CaI+ indicator 5,5'-F2-BAPTA allows direct quantitation of the cyclical changes in the intracellular free Ca2+ concentration ([Ca2+],) that underlie contraction in intact hearts. [Ca2+], increased from approximately 200 nM in diastole to —1 fiM or higher hi early systole. Although the 19F spectra that report [Ca2+], changed dramatically and reproducibly during the cardiac cycle, no changes were detectable in gated phosphorus spectra. We exploited the ability to control the coronary arterial flow of our hearts to investigate the mechanism of the fall in contractility that results from a decrease in perfusion even when the flow suffices to sustain normal high energy phosphate concentrations. Under these conditions, the amplitude of Ca2+ transients falls markedly along with the decline in pressure. This down-regulation of Ca2+ transients constitutes a novel protective mechanism that minimizes energy demand during low-flow ischemia. (Circulation Research 1988;63:673-678) The application of various techniques to mea-sure intracellular free Ca2+ concentration([Ca2+]i) in isolated heart muscle and in single cardiac cells has led to considerable concep-tual advances regarding excitation-contraction cou