Influence of temperature and frequency on the positive inotropic action of phenylephrine in the isolated rabbit papillary muscle.

On the isolated papillary muscle of the rabbit the maximal developed tension induced by phenylephrine bia alpha-adrenoceptors was not affected by changing the temperature, while the basal developed tension of the muscle was significantly increased or decreased by lowering the temperature from 37 degrees C to 32 degrees C or by raising it to 42 degrees C, respectively. When the temperature was lowered from 37 degrees C to 32 degrees C in the muscle stimulated at a frequency of 0.5 Hz, the dose-response curve for phenylephrine via alpha-adrenoceptors was shifted to the left (delta pD2-0.47), while that for phenylephrine via beta-adrenoceptors was also shifted to the left to a similar extent (delta pD2=0.43). When the temperature was raised from 37 degrees C to 42 degrees C, the dose-response curve for phenylephrine via alpha-adrenoceptors was markedly shifted to the right (delta pD2=1.58), whereas that via beta-adrenoceptors was not affected at all by this elevation of the temperature (delta pD2=0.04). The change of the stimulation frequency affected chiefly the intrinsic activity of phenylephrine but not the affinity of the drug for alpha-adrenoceptors expressed as the pD2-value.