Augmented aftercontractions in papillary muscles from rats with cardiac hypertrophy

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Abstract
Paired-pulse stimulation induced larger aftercontractions in papillary muscles from spontaneously hypertensive rats (SHR) than from normotensive Wistar-Kyoto rats (WKY). To determine whether aftercontraction exaggeration is a general characteristic of hypertrophied cardiac muscle, three models were examined: SHRs, deoxycorticosterone acetate (DOCA)-treated rats, and aorta-constricted rats. Responses of pipillary muscles from hypertrophied hearts tested under conditions conducive to aftercontraction generation were compared to pipillary muscles from WKY or sham-treated controls, respectively. Field-stimulated papillary muscles mounted in an oxygenated temperature-controlled physiologic salt solution were exposed to calcium concentrations of 2.5 and 5.0 mM, and temperatures of 27 degrees C and 17 degrees C. Although testing conditions influenced the contractile responses to single stimuli, there was no difference in active tension, time to peak tension, or one-half relaxation time between the three experimental groups and their respective controls. Paired-pulse stimulation induced aftercontractions that were enhanced in high-calcium and low-temperature solutions. Under these conditions, papillary muscles from hypertrophied hearts developed larger aftercontractions than did their respective controls.