Effects of epinephrine and Ca on contractile strength and Ca45 exchange in rabbit atria

A comparison of the effects of Ca and epinephrine on contractile tension changes and Ca45 exchange in isolated rabbit atria was made. Dose response curves of Ca were determined with and without epinephrine (2.7 x 10–5 m) and dose response curves of epinephrine were determined with three concentrations of Ca in the media (0.3 mm, 2.4 mm and 9.6 mm) at both pH 6.9 and pH 7.75. Epinephrine enhanced the effect of a given concentration of Ca two to three times in a competitive manner. However, Ca acted as a noncompetitive inhibitor on the response of atria to epinephrine. Ca45 studies revealed that epinephrine decreased Ca influx. This effect was enhanced by increasing the extracellular Ca concentration. Ca45 efflux was not affected by epinephrine. Increasing the pH acted similar to epinephrine. These data suggest that epinephrine increases contractile strength of the heart by at least two mechanisms, one involving an increased binding of calcium in the membrane (competitive), the other involving a noncompetitive antagonism, perhaps of another essential component in the reaction, such as Mg++.