Antagonistic action of .ALPHA.- and .BETA.-agonists on the bullfrog atrium.

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Abstract
Action of .alpha.- and .beta.-agonists on the membrane potential, current and tension components on the bullfrog atrial muscle were studied using single or double sucrose-gap technique. Isoproterenol (10-9-10-5 M) and adrenaline [epinephrine, A3] (10-7-10-4 M) produced a dose-dependent positive inotropic effect; methoxamine (10-8-10-4 M) and phenylephrine (10-8-10-3 M) with propranolol (10-6 M) elicited a negative inotropic effect. The positive inotropic effect was accompanied by slight hyperpolarization and marked increase of overshoot and plateau level of action potential; the negative inotropic effect, by opposite changes. Under voltage clamp, isoproterenol and adrenaline produced enhancement of slow inward current (Is), delayed outward current (Ix) and Is-dependent phasic tension, while Is-independent tonic tension was inhibited. Contrarily, methoxamine and phenylephrine depressed these currents and elicited .beta.-antagonistic inotropic actions, especially when isoproterenol was present. Specific feature of .alpha.-agonist action in the presence of isoproterenol was an inward shift of the semi-steady I-V [current-voltage] relationship. In the bullfrog atrium, .alpha.-agonists apparently produce a strong .beta.-receptor blocking action and an inward shift of the background current. A possible relationship between the latter effect and inhibition of the Na-K pump is suggested.