The force-frequency relationship: A comparative study between warm- and cold-blooded animals

Summary

In a comparative study, the mechanical and electrical responses of the guinea pig's papillary muscles and strips of the turtle's and frog's ventricles to various stimulation patterns were investigated. Typical forcefrequency relationships were found to be present in all preparations. It is, however, much more pronounced in the guinea pig's heart than in the other preparations. Striking differences exist between the warm-blooded and the cold-blooded animals, as far as “pure frequency potentiation” is concerned, i.e., the frequency dependence of the maximal actively developed force after a certain resting period (test-interval) following a series of conditioning rhythmical stimuli. Whereas in the guinea pig's papillary muscle the amplitude of optimal test contraction increases with the frequency of foregoing stimuli, the amplitude is depressed in the cold-blooded preparations by a rise of frequency. This effect is found to be due to the shortening of the action potential. Thus the mechanical response of cold-blooded preparations seems to depend primarily on the duration of depolarization under different conditions of stimulation. In the guinea pig's papillary muscle, the same changes in the time course of depolarization can be observed, but their effect on the contractile force cannot be revealed in such experiments. A much more predominant role in the force development of a papillary muscle may be attributed to the immediate influence of frequency on the contractile mechanism, i.e. to the pure frequency potentiation which does not exist in the myocardium of cold-blooded animals. These differences may be explained by the different development of Ca⁺⁺ stores of the sarcoplasmic reticulum in heart muscle of cold- and warm-blooded animals.