Effect of temperature on the rise in intracellular sodium caused by calcium depletion in ferret ventricular muscle and the mechanism of the alleviation of the calcium paradox by hypothermia.

The effects of temperature over the range 37-10 degrees C on the responses of isolated ferret ventricular trabeculae to the depletion and repletion of bathing calcium has been investigated. Cooling is found to reduce the rate of rise of intracellular sodium activity (measured with an ion-sensitive microelectrode) induced by depletion of bathing divalent cations, without affecting the prolonged depolarization. The rate of rise of [Na]i (with the sodium pump inhibited) shows a dependence on temperature (energy of activation, 67 kJ.mol-1; Q10, 2.3-2.8) that increases with increasing temperature. This contrasts with previously published data for the sodium pump for which the temperature dependence falls with increasing temperature. These results offer an explanation for the alleviation of the calcium paradox caused by cooling during the period of calcium depletion, which is similar to other procedures that offer protection by limiting the rise in [Na]i by having effects on the L-type calcium channel.