Background. An agent that improves left ventricular (LV) performance by non-cAMP-mediated mechanisms would be valuable in the treatment of chronic heart failure. We have shown earlier that the thyroid hormone analogue 3,5-diiodothyropropionic acid (DITPA) binds to nuclear receptors, alters transcription of T3-responsive genes, and increases +dP/dt... in hypothyroid rats with substantially less effect on heart rate and metabolism than thyroid hormone, which makes it a selective cardiotonic

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... Methods and Results. To determine whether DITPA might be useful in treating heart failure, we compared chronic treatment with normal saline, captopril (2 g/L), or the combination of DITPA (375 ,ug/100 g) and captopril (2 g/L) in Sprague-Dawley rats beginning 3 weeks after coronary artery ligation. Both DITPA/captopril and captopril treatment decreased LV end-diastolic pressure compared with controls (21±2 and 26±2 mm Hg, respectively, vs 34±3 mm Hg, P<.05 for each). The addition of DITPA to captopril produced a 36% increase in resting cardiac index (P<.05) and shifted the cardiac function curve upward and to the left, indicative of enhanced myocardial performance. Also, DITPA/captopril compared with captopril treatment or control produced an increase in the rate of LV relaxation, as manifested by a decrease in a, the time constant of LV pressure decline (17.5±1.0 vs 22.2±1.7 milliseconds, P<.05) and a larger absolute value for -dP/dtm" (-4561±361 vs -3346±232 mm Hg/s, P<.05). These changes occurred without changes in heart rate, LV mass, LV systolic pressure, or peripheral resistance relative to captopril treatment (P>.05). Conclusions. The combination of DITPA and captopril improved cardiac output, increased -dP/dtm., and increased the rate of LV relaxation to a greater extent than captopril treatment in the rat postinfarction model of heart failure. Use of a cardiotonic analogue of thyroid hormone represents a new approach to improving LV performance and may be a useful adjunct to afterload reduction for the treatment of heart failure.