monensin and dihydroouabain

The effects of the cardiac glycoside dihydroouabain (DHO), and the ericaceous toxin grayanotoxin-I (GTX-I) on myocardial cellular sodium (Nai) concentrations were investigated using sodium-23 nuclear magnetic resonance (23Na NMR) spectroscopy at 30 degrees C in isolated perfused guinea-pig hearts. The Nai 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 (T1) of the Nai (25 or 34 msec at 8.46 Tesla (T)) is much faster than that of extracellular sodium (64 msec at 9.4 T). Nai was estimated from the calibration curve of the frequency area of the 23Na NMR FT spectra plotted against the standard Na concentration. The Nai 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 Nai concentration relative to that of the control (100%) accompanying a PIE of 44%. The mechanism of this Nai elevation induced by combinational addition of DHO, GTX-I and MON may be mediated as follows: GTX-I increases the net Na-influx 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 Nai, thereby increasing the intracellular Ca2+ concentration via Na(+)-Ca2+. exchange.

Topics: Animals; Diterpenes; Edetic Acid; Female; Guinea Pigs; In Vitro Techniques; Ion Transport; Magnetic Resonance Spectroscopy; Male; Membrane Potentials; Monensin; Myocardial Contraction; Myocardium; Ouabain; Perfusion; Potassium; Sodium; Sodium-Potassium-Exchanging ATPase; Ventricular Pressure

The potassium potential EK, of rat brain slices was estimated by determining the uptake of 86Rb+. The ERb was the same for slices prepared from five rostral brain regions, the average value being 66.4 mV. The ERb values in the presence of 20 microM ouabain were only slightly lower than the resting values; increasing concentrations of ouabain above 20 microM resulted in a graded depolarization in all five brain regions. High concentrations (1 mM) of two other inhibitors of Na+,K+-ATPase, dihydro-ouabain and strophanthidin, produced no more depolarization than did 20 microM ouabain. Competitive binding studies indicated that the differential effects were due to the relative binding to brain slices. Erythrosin B, an inhibitor of Na+,K+-ATPase, had no measurable effect on ERb. Intermediate concentrations of the Na+/H+ ionophore monensin slightly hyperpolarized striatal slices, whereas the same monensin concentrations plus 20 microM ouabain, 1 mM strophanthidin or 70 microM erythrosin B resulted in marked depolarization. Measurement of the membrane potential via uptake of methyltriphenylphosphonium cation indicated that ERb was indeed a valid estimation of the membrane potential. EK was measured directly by monitoring 42K+ uptake in striatal slices and was found to be essentially identical to ERb. Uptake of 22Na+ was consistent with the values for ERb or EK. Several conditions that resulted in little or no measurable depolarization of striatal slices did induce efflux of exogenously loaded GABA and dopamine; these conditions included 20 microM ouabain, 1 mM dihydro-ouabain or strophanthidin, and 70 microM erythrosin B. Neurotransmitter efflux in the absence of general cell depolarization was not accompanied by altered rates of respiration or decreased ATP levels.

Topics: Animals; Brain; Dopamine; Erythrosine; Female; gamma-Aminobutyric Acid; In Vitro Techniques; Membrane Potentials; Monensin; Ouabain; Rats; Rubidium; Sodium-Potassium-Exchanging ATPase; Strophanthidin; Synaptic Transmission; Veratridine