DIFFERENT MECHANISMS UNDERLYING THE STIMULATION OF MYOCARDIAL ALPHA-ADRENOCEPTORS AND BETA-ADRENOCEPTORS

On isolated electrically driven rat left atria (1 Hz) experiments were undertaken in order to characterize further the mechanism of the positive inotropic effect underlying the stimulation of cardiac α-adrenoceptors.- The increase in extracellular Ca2+ from 1.25 to 2.5 mM increased the pD2-value for the α-mimetic effect of phenylephrine by 0.6 log units. Under the same conditions the pD2-value for its β-mimetic positive inotropic effect increased by only 0.19 log units.-When the amplitude of contraction was diminished by carbachol which is known to inhibit the influx of Ca2+ the pD2-value for the α-mimetic effect of phenylephrine was significantly increased while that for the β-mimetic effect was decreased.-After increasing concentrations of dinitrophenol the pD2-value for the α-effect of phenylephrine was significantly increased by maximal 0.6 log units whereas that for the β-effect was lowered by 0.25 log units.-In conclusion, all measures favouring the conditions for an enhanced influx of Ca2+-elevation of the gradient for Ca2+, shortening of the plateau phase of action potential by carbachol, increased efflux of Ca2+ by dinitrophenol- are able to increase the pD2-value, i.e. the affinity of phenylephrine to cardiac α-adrenoceptors, thus demonstrating the importance of an increased Ca2+-influx induced by stimulation of α-adrenoceptors. © 1979.