Peroxynitrite contributes to spontaneous loss of cardiac efficiency in isolated working rat hearts

We examined the mechanism of the time- and protein synthesis-dependent decl ine in cardiac mechanical function in isolated working rat hearts. Hearts w ere perfused with Krebs-Henseleit buffer for 120 min in the presence or abs ence of the protein synthesis inhibitor cycloheximide (CX; 10 mu M). Cardia c work remained stable for 60 min and then spontaneously decreased during 6 0-120 min of perfusion. This was accompanied by an increase in myocardial i nducible nitric oxide synthase (iNOS) and xanthine oxidase (XO) activities and enhanced dityrosine formation in the perfusate, an indicator of peroxyn itrite generation. CX markedly attenuated the loss in contractile function and prevented the increase in iNOS and XO activities and dityrosine level. Despite the decline in cardiac work in control hearts, the coupling between tricarboxylic acid (TCA) cycle activity and oxygen consumption remained co nstant in both groups. ATP, creatine phosphate, and glycogen levels were no t different between control and CX groups and did not differ over 120 min o f perfusion. We concluded that the delayed and spontaneous loss in myocardi al mechanical function in isolated working rat hearts is 1) attenuated by C X treatment, 2) accompanied by a concomitant increase in both iNOS and XO a ctivities and peroxynitrite generation in the heart, and 3) not dependent o n a direct impairment in myocardial ATP production, myocardial oxygen consu mption, or TCA cycle acetyl-CoA production but may be due to an inefficienc y of the heart to utilize ATP for contractile work.