| Abstract
| - Contraction-relaxation coupling was studied in rat and guinea-pig papillary muscles during chronic pressure overload induced by aortic stenosis and during acute hypoxia. Coefficient R1 (ratio between maximum shortening and lengthening velocities of the isotonic twitch loaded with preload only) and coefficient R2 (ratio between the positive and negative peak force derivatives of the isomeric twitch) tested the contraction-relaxation coupling under low and heavy load respectively. Cardiac hypertrophy was similar in guinea-pigs (+43 ± 5%) and rats (+55 ± 7%). In both species, cardiac hypertrophy significantly impaired contraction and relaxation phases. In the rat, neither R1 (−1 ± 4%) nor R2 (−5 ± 4%) varied significantly during cardiac hypertrophy whereas, in the guinea-pig, an increase in R1 (+56 ± 18%, P < 0·001) and in R2 (+26 ± 9%, P < 0·01) was noted. These species-related differences might be linked in part to differences in sarcoplasmic reticulum function and myosin ATPase activtty. Acute hypoxia, which leads to a decrease in cellular ATP levels, was responsible for a marked decrease in myocardial performance, while R1 increased (+66 ± 8%, P < 0·05) and R2 decreased (−14 ± 1%, P < 0·05). These results showed that chronic pressure overload modified the contraction-relaxation coupling in a characteristic manner according to the species studied and these changes differed from those observed during acute hypoxia.
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