Calcium ions and the cardiotonic action of glucagon (calcium fluxes-myocardial function-asystole-membrane transport-cat)

The positive inotropic action of glucagon on the cat heart is dependent upon the extracellular Ca concentration. The inotropic effect is proportionately greater, the lower the concentration of Ca in the perfusate (above zero, at which glucagon is without significant effect). Nevertheless, Ca influx (as measured with (45)Ca) into the myocardium is not demonstrably influenced by glucagon, except at 0.09 mM Ca, where the interpretation of the effect is complicated by the fact that there is mechanical asystole at that Ca concentration in the absence of glucagon, while excitation-contraction coupling still occurs when the hormone is added. The possibility is suggested that glucagon may influence intracellular movement of Ca, even when its transmembrane movement is affected only slightly.

Influx of calcium into rabbit myocardium in relation to its ionic environment

In the first 150 seconds after contracture of rabbit myocardium has been induced by shifting from perfusion with zero [Ca(2+)] and low [K(+)] to solutions with normal levels of those cations, there is a large influx of Ca(2+) as measured both by isotopic tracer flux and by total tissue [Ca]. Tracer studies indicate that the influx is 90 per cent complete in 90 seconds. Contracture due to substitution of either Li(+) or K(+) for Na(+) in perfusion fluids is also associated with an increased influx, but of lesser magnitude. The latter types of contracture are reversible while the former is not. It seems probable that the irreversible contracture is induced by the large Ca(2+) influx.