An analysis of the positive inotropic effect of AR-L 115 BS on isolated rat atria.

The effect of AR-L 115 BS on mechanical and electrical properties of isolated rat atria was investigated. AR-L 115 BS (10(-7) M -5 X 10(-4) M) produced a dose-dependent increase in rate, contractile force and df/dtmax and shortened the sinus node recovery time and the effective refractory period. The increase in contractile force occurred concomitantly with an increase in the height of the plateau of the atrial action potential. The positive inotropic effect produced by AR-L 115 BS depended on the external Ca concentration and was diminished in low Na solution. The drug also decreased the amplitude-interval relationship and post-extrasystolic potentiation. The positive inotropic effect of AR-L 115 BS was inhibited by verapamil and caffeine. These results suggest that in isolated rat atria the positive inotropic effect of AR-L 115 BS might be due to an increase in transmembrane Ca influx into atrial fibres as well as to an effect probably related to the inhibition of Ca sequestration by the sarcoplasmic reticulum.