DIRECT MEASUREMENT OF INCREASED MYOCARDIAL CELLULAR NA-23 NMR SIGNALS IN PERFUSED GUINEA-PIG HEART INDUCED BY DIHYDROOUABAIN AND GRAYANOTOXIN-I

The effects of the cardiac glycoside dihydroouabain (DHO), and the ericaceous toxin grayanotoxin-I (GTX-I) on myocardial cellular sodium (Na-i) concentrations were investigated using sodium-23 nuclear magnetic resonance (Na-23 NMR) spectroscopy at 30 degrees C in isolated perfused guinea-pig hearts. The Na-i NMR signals from perfused Langendorff heart preparations were obtained by the modified inversion recovery (IR) method based on the previous observation that the spin-lattice relaxation time (T-1) of the Na-i (25 or 34 msec at 8.46 Tesla (T)) is much faster than that of extracellular sodium (64 msec at 9.4 T). Na-i was estimated from the calibration curve of the frequency area of the Na-23 NMR FT spectra plotted against the standard Na concentration. The Na-i concentration of the heart increased concomitantly with the positive inotropic effects (PIE) of DHO, GTX-I and monensin (MON). The cumulative sequential addition of DHO (5 x 10(-6) M), GTX-I (7 x 10(-8) M) and MON (5 x 10(-6) M), each of which alone induced no appreciable PIE, produced a 22% elevation in Na-i concentration relative to that of the control (100%) accompanying a PIE of 44%. The mechanism of this Na-i elevation induced by combinational addition of DHO, GTX-I and MON may be mediated as follows: GTX-I increases the net Na-inlflux via Na+ channels; DHO inhibits the pumping out of Na+ from the cell; and MON transports external Na+ into the cell, acting as a sodium ionophore. Consequently, these drugs act synergistically to increase the Na-i, thereby increasing the intracellular Ca2+ concentration via Na+-Ca2+ exchange.