Different mechanisms underlying the stimulation of myocardial alpha- 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.