Myocardial mechanics in ppillary muscles of the rat and cat.

HENDERSON, A. H., D. L. BRUTSAERT, Mr. Mr. PARMLEY, AND E. H. SONNENBLICK. Myocardial mechanics in papillary muscles of the rat and cat. Am. J. Physiol. 217(5): 1273-1279. 1969.-Species differences in myocardial mechanics were studied in rat and cat papillary muscles. The intrinsic velocity of contractile element shortening (V,,,) was 3 times higher in the rat at comparable low frequencies. Resting and developed tension at the top of the length-tension curves was similar, but time to peak tension was 3 times shorter in the rat while peak rate of tension development (dT/dt) was correspondingly higher. Increased rates of contraction in the rat were associated with decreases in V,,, and dT/dt as well as in time to peak tension. In the rat at low frequencies contractility was not further augmented by paired stimulation, whereas at higher frequencies this increased contractility but, in contrast with the cat, did not shorten time to peak tension. Isoproterenol significantly shortened time to peak tension in both species and increased contractility but only slightly at low frequencies in the rat. These species differences imply basic differences in the control of intracellular calcium movement or in rates of the contractile process.