[Effects of isoprenaline and carbachol on delayed after-depolarization in myocardium]

The effects of isoprenaline and carbachol on the delayed after-depolarization (DAD) induced by acetyl strophanthidin of 2.0 x 10(-7) mol/L were observed in sheep cardiac Purkinje fibers. The amplitude of DAD was enhanced by isoprenaline (1.0--3.0 x 10(-8) mol/L) in a dose-dependent manner and the triggered arrhythmia was induced. Carbachol of 2.0 x 10(-6) mol/L alone had no effect on the amplitude of DAD. When the DAD was enhanced by isoprenaline or the triggered arrhythmia was induced, the same concentration of carbachol could decrease the amplitude of DAD significantly and abolish the triggered arrhythmia. However, carbachol did not attenuate the amplitude of the DAD enhanced by high concentration of calcium, aminophylline or histamine. The results suggest that carbachol can antagonize the enhancement of the DAD caused by isoprenaline. This effect is possibly mediated by activation of the muscarinic receptor, which in turn causes a decrease in the adenyl cyclase activity elevated by the activation of the beta-receptor in the membrane.