Sodium efflux at 0°C in single barnacle muscle fibers

Abstract

A study has been made of the efflux of radiosodium in single barnacle muscle fibers cooled to 0°C. Cooling from 24 to 0°C results in a rapid fall in the Na efflux, the magnitude of which averages 81%. Rewarming leads to almost complete restoration of the Na efflux. The Arrhenius plot shows no “breaks” and gives anE a value of 14.2 kcal/mol. External application of 10−4 m-ouabain following cooling to 0°C causes a fall in the residual efflux (∼8%). Rewarming results in partial restoration of the Na efflux. Lowering external pH (pH e ) results in a rise in Na efflux at 0°C, which peaks and declines rather slowly. The magnitude of the stimulatory response to acidification is a function of pH e over the pH e 7.00–5.8 range, the theoretical threshold being pH e 7.1. The magnitude of the response to acidification of cooled, ouabain-poisoned fibers suspended in Li-ASW is the same as that of fibers suspended in Na-ASW. Injection of pure protein kinase inhibitor into fibers maintained at 0°C fails to reduce the size of the response to acidification. Benzolamide but not acetazolamide, ethoxzolamide or Cl 13,580 abolishes the response to acidification. It also reverses the response to acidification. SITS is also able to abolish the response to acidification. An additional new observation is that the Na efflux at 0°C is stimulated following the injection of CaCl2 in a concentration-dependent manner. A similar response is not seen with MgCl2. Acidification (pH e 5.8) following peak stimulation by injection of CaCl2 is without effect. These results add up to a refutation of the concept that the Na efflux at 0°C is wholly passive and that the response to acidification involves Na:Na or Na:Ca exchange. The results also weaken the argument that stimulation of the efflux by acidification is the result of activation of carbonic anhydrase.