Congestive heart failure (CHF) is characterized by abnormal vasoconstriction and an impairment in nitric oxide (NO)-mediated vasodilatation. We have previously demonstrated that the decrease in sensitivity to NO lies at least partially at the level of the smooth muscle and is due to a reduction in the relative expression of the leucine zipper positive (LZ(+)) isoform of the myosin targeting subunit (MYPT1) of myosin light chain phosphatase. We hypothesized that since the attenuated vasodilatory response to NO in CHF has been shown to be secondary to an increased activity of the renin-angiotensin system, angiotensin converting enzyme (ACE) inhibition could affect MYPT1 isoform expression. To test this hypothesis, a rat myocardial infarction (MI) model of CHF was used; following left coronary artery ligation, rats were divided into control and captopril-treated groups. A third group of rats was given prazosin for 4 weeks. In the untreated control group, left ventricular function (LVF) was reduced at 2 weeks post-MI and remained at this level. Captopril treatment attenuated the fall in LVF. In the control aorta and iliac artery, the expression of the LZ(+) MYPT1 isoform fell 44-52% between 2 and 4 weeks post-MI, whereas in animals treated with captopril, MYPT1 isoform expression did not change. A decrease in the sensitivity to cGMP-mediated smooth muscle relaxation occurred coincident with the decrease in LZ(+) MYPT1 expression. The change in LZ(+) MYPT1 expression was not due to the decrease in afterload, as prazosin therapy produced an improvement in LVF but did not increase the relative expression of LZ(+) MYPT1 isoform. These data suggest that ACE inhibition, unique from pure afterload reduction, prevents MYPT1 isoform switching, which would preserve normal flow, or NO-mediated vasodilatation.