Influence of BAY K-8644 on positive inotropic agents in guinea pig atrial muscle

Inotropic effects of ethanol and dihydropyridines on the guinea pig heart atrial muscle

The effects of ethanol and/or dihydropyridines (DHPs) on force of contraction of atrial muscle were studied. Guinea pig atrial strips superfused with Tyrode's solution (36 degrees C) were driven (1.5 Hz) while recording muscle tension. Bay K 8644 (BAYK) increased, while nimodipine or ethanol reduced, the peak tension developed and the maximum velocity of development of tension. The effects of ethanol were readily reversible, but those of the DHPs were not. The combined actions of ethanol and DHPs were the result of the synergism or antagonism of the drugs tested. The shorter duration of the action of ethanol resulted in the effect of DHPs being still evident well after the exposure to the drugs ended. In summary, ethanol and nimodipine exerted depressant actions on atrial contractile force, while BAYK had opposite effects. The different mechanisms of action may explain the different duration of the effects of ethanol (physical agent) and DHPs (receptor-binding chemicals).

Effects of buffer magnesium on positive inotropic agents in guinea pig cardiac muscle

Experiments examined effects of extracellular Mg2+ concentration (Mgo2+) on dose-dependent actions of strophanthidin, norepinephrine, Bay K-8644 and extracellular Ca2+ (Cao2+) in electrically stimulated atrial and ventricular muscle isolated from guinea pig heart. Mgo2+ itself elicited a concentration-dependent negative inotropic effect. Elevation of Mgo2+ between 0.6 and 12 mM increased the concentration of strophanthidin necessary to produce its toxic effects without affecting the maximum developed tension prior to toxicity. Similarly, Mgo2+ did not alter the maximum contractile force elicited by cumulative addition of norepinephrine, Bay K-8644 or Cao2+, but increased their ED50 values. These data suggest that interactions between Mgo2+ and the four positive inotropic agents were not mediated by effects on receptor binding or Na+,K+-ATPase, but rather by alterations at one or more steps involved in excitation-contraction coupling.

Interaction of BAY K-8644 with effects of digoxin in the dog heart-lung preparation

Concentration-dependent effects of BAY K-8644, a dihydropyridine calcium channel agonist, on heart rate (HR), left atrial pressure (LAP) and maximal rate of left intraventricular pressure rise (+dP/dtmax) were compared with those of isoproterenol and digoxin in dog heart-lung preparations during exposure to 0.8 vol% halothane. All three agents reduced LAP to nearly equal levels at maximally effective concentrations. Maximal increases in +dP/dtmax induced by BAY k-8644 and digoxin were of similar magnitude (35 and 31%, respectively), and both agents increased HR slightly. Isoproterenol elevated both +dP/dtmax and HR more than twice as much as the other agents. At a concentration which produced only small changes by itself, BAY K-8644 markedly increased the effects of digoxin on LAP and +dP/dtmax. Furthermore, the same concentration of BAY K-8644 significantly reduced toxic effects of digoxin (arrhythmogenesis, elevation of LAP, reduction in systemic output). These data indicate that in the isolated blood-perfused heart digoxin's positive inotropic action may be enhanced by an appropriate concentration of BAY K-8644, while adverse effects are diminished.