Abstract
Abstract
Myocardial mechanics in the male spontaneously hypertensive rat (SHR) and Wistar-Kyoto rat (WKY) at 18 months of age were studied. Left ventricular hypertrophy was documented in the SHR by an increase in left ventricle/body weight and left ventricle/tibial length ratios when compared to the WKY (P < 0.001). Isolated left ventricular papillary muscles were studied at 28°C while contracting 12 times/min at the apex of the length-tension curve. Active and passive length-tension relations were measured at relatively early (65 ± 3 min) and late (200 ± 5 min) times following sacrifice. No significant differences in passive length-tension relations between strains were observed. Between early and late measurements, a significant decrease in passive tension within the length spectrum 89-100% L max occurred in both SHR and WKY, accompanied by a significant increase in passive stiffness (P < 0.01, SHR; P < 0.001, WKY). Isometric performance was measured at relatively early (81 ± 3 min) and later (190 ± 5 min) times following sacrifice. Strain differences in active muscle performance were of a greater electromechanical delay time (EMD) (P < 0.05, early; P < 0.001, late) and time-to-peak tension (TPT) (P < 0.001, late) in SHR compared to WKY. Between early and late measurements, decreases in EMD (P < 0.01, SHR; P < 0.001, WKY), TPT (P < 0.001; P < 0.001), the half-time of relaxation (P < 0.001; P < 0.001), and the resting tension (P < 0.01; P < 0.001) were observed, and the maximum rate of fall of tension increased (P < 0.01; P < 0.01). We conclude that studies must be precisely referenced from the time of sacrifice of the animal in order to accurately evaluate the effects of experimental hypertrophy on isolated muscle performance. No evidence for the depression of papillary muscle isometric performance was seen in the 18-month SHR when compared to the WKY, although prolonged EMD and TPT were observed.
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