Mechanical Abnormalities in the Rat Ischemic Heart Failure Model Which Lie Downstream to cAMP Production

Objective. We previously showed diminished inotropic responses to isoproterenol in uninfarcted posterior papillary muscles of rats with anterior wall infarcts comprising 30% of the left ventricle. We have also shown that this occurs in the absence of β-receptor downregulation. Our objective was to show a mechanical defect in the rat infarct model which is secondary to cellular defects downstream to cAMP production. Methods. Sprague-Dawley rats were infarcted via coronary ligation. After 3 weeks, the uninfarcted papillary muscle was placed in a muscle bath and exposed to increasing concentrations of dibutyryl cAMP while contracting isometrically at 28°C. Results. The large infarct group showed evidence of right ventricular hypertrophy which was manifested by an increased right ventricular mass. No differences were found in baseline measurements of developed tension (DT); rate of tension development (dT/dt); rate of relaxation (-dT/dt); time to peak tension (TPT); and relaxation time (t1/2R). When these muscles were stimulated with dibutyryl cAMP, the large infarct group had a reduced inotropic response as measured by +dT/dt and TPT. No consistent abnormalities were noted in relaxation. The findings are similar to those we noted previously with isoproterenol stimulation. Conclusion. The impaired response to β stimulation in uninfarcted myocardium from rats with large myocardial infarctions is due to cellular defects which lie downstream to cAMP production.