Temperature effects on the inotropic and chronotropic responses to adrenaline in the frog heart

Summary The contractile force and frequency of the spontaneously beating auricles ofRana tempeoraria were recorded as a function of temperature. Tracings of the tension development, its integrated and derived functions showed that the isolated auricles of warmadapted winter frogs responded to temperature with changes in inotropy but not in the tension generated in one minute.Adrenaline, previously shown to act via the in the auricles of this frog, increased both the force and the frequency of the contractions between 5 and 25°C. The affinity for adrenaline was highest at 5°C for all the parameters examined. The maximal effect (efficacy) of adrenaline for T_max, T_max and the tension-time integral was highest at 5°C while the efficacy of adrenaline on the heart rate reached its maximum at 25°C. At 5°C the tension generated in one minute was doubled by the maximally effective dose of adrenaline (1.4 × 10^–6 M). These results strongly suggest that adrenaline, being the main sympathetic neurotransmitter in the frog, has an important role in adjusting the heart to maximal performance at the low temperatures.