An Analysis of the Effect of Adrenaline on Electrogenic Na⁺ Pump of Visceral Nerve Fibers in Bullfrogs

抄録

The effect of adrenaline (Ad) on the electrogenic Na⁺ pump of bullfrog visceral nerve fibers was studied, by testing the effect of this drug on the K-activated hyperpolarization (KAH) of these nerve fibers. KAHs were recorded by adding K (0.2-5 mM) to an extracellular K-free solution with which preparations were previously perfused and the actual KAH amplitude (net KAH) was estimated by taking into account the amplitude of membrane depolarization induced by K in the presence of ouabain (0.001 mM). The dose-response relation between K concentrations and net KAHs follows Michaelis-Menten kinetics; the log dose-response relation showed a sigmoid curve and the Lineweaver-Burk Plot of dose-response relation showed a straight line when n_H=1.3. The net KAH was augmented by Ad (0.03-1 mM); facilitative effects of three kinds of catecholamine on the net KAH were Ad>noradrenaline>> isopreterenol. The log dose-response curve was shifted to the left in parallel in the presence of Ad; the Lineweaver-Burk plot showed a straight line when n_H=1.3 and this line met the control line at the ordinate point where K concentration is infinitive, while the apparent dissociation constant (K_m) decreased to 0.82 from 1.3 mM (control) in the presence of 0.3 mM Ad. Net KAH was augmented by the removal of extracellular Cl or by the action of TEA; the membrane resistance was expected to increase under these experimental conditions. The dose-response relation obtained under these conditions showed an increase of maximum response without changes in K_m value. It was concluded on the basis of the present results that the increase in net KAH by Ad was due to an increase in the electrogenic Na⁺ pump current. The mechanism underlying such an Ad action was discussed, and it was suggested that the rate of the electrogenic Na⁺ pump was increased by the action of Ad.