Abstract
The effects of glucagon in concentrations of 0.294 × 10−6 mol/l, 1.47 × 10−6 mol/l,2.94 × 10−6 mol/l,5.8 × 10−6 mol/l, and 1.47 × 10−5 mol/l on the simultaneously recorded action potentials and contractions, and microsomal and sarcolemmal Na+-K+-ATPase in the myocardium of the guinea pig, rabbit, dog, and pig were investigated. Glucagon in all the concentrations produced an inhibition of the Na+-K+-ATPase associated with an increase in the contractility and shortening of the duration of action potential in dog myocardium. The increase in contraction was concentration-dependent up to a certain concentration. Inhibition of sarcolemmal ATPase was more than that of microsomal ATPase. In none of the concentrations did glucagon produce any significant changes in the Na+-K+-ATPase, contractility, and action potential duration in the myocardium of guinea pig, rabbit, or pig. These results suggest that glucagon-induced positive inotropic effect might be due to an increase in the Ca+ + influx as a result of inhibition of membrane Na+-K+-ATPase. Shortening of the action potential duration might also be due to an increased efflux of potassium as a result of an inhibition of Na+-K+-ATPase.