The effect of temperature upon the positive inotropic and chronotropic responses of isolated cardiac preparations to isoprenaline and its modification by experimental variables.

The effects of various experimental variables upon the optimum temperatures for positive inotropic and chronotropic responses to isoprenaline in isolated cardiac preparations were examined. Perfusion rate and animal (heart) size affected the satisfactory determination of rate and tension optima of guinea-pig and rabbit perfused hearts. Isolated spontaneous atria were assumed to have the same temperature optima as perfused hearts and these were independent of the animal size/age. Paced atria, in which spontaneous rate changes were absent, revealed a sharper tension optimum at an unaltered temperature. The inotropic optimum in all preparations varied with the method of expressing the response; however, since the increase tension remained independent of resting developed tension in pacing experiments, this measure was adopted. The resulting inotropic and chronotropic optima, having minimized these variables, were 25 and 35, and 30 and 37.5 degrees C for guinea-pigs and rabbits respectively. This confirms the separation of these responses by temperature.