Peroxynitrite-induced cardiac depression: role of myofilament desensitization and cGMP pathway.

The oxidant species peroxynitrite, the reaction product of nitric oxide (NO.) and superoxide, has been implicated in several pathophysiological conditions of the heart. Here, we studied the mechanism of peroxynitrite-induced cardiac depression using specific drugs and simultaneous analyses of myocardial function and intracellular Ca(2+) ([Ca(2+)](i)). Rat hearts were perfused retrogradely and left ventricular function (balloon method) and [Ca(2+)](i) transients were recorded on a beat-to-beat basis using the aequorin bioluminescence method. Peroxynitrite was infused at 10.8+/-0.93 microM via sideline for 10 min, followed by a 15-min recovery period to monitor irreversible effects. Test drugs were infused prior to and during peroxynitrite application. Peroxynitrite depressed left ventricular developed pressure (LVDevP; -40%), yet increased systolic and diastolic [Ca(2+)](i) (1.3- and 2.3-fold, respectively; n=12). When Ca(2+) entry through Ca(2+) channels or Na(+)/Ca(2+) exchange transport was blocked using nicardipine (1 microM, n=3) or dichlorobenzamil (30 microM, n=5), respectively, cells showed lower [Ca(2+)](i) and accentuated negative inotropic action in response to peroxynitrite. Peroxynitrite slowed left ventricular relaxation and [Ca(2+)](i) transients, both of which were not affected by extracellular Ca(2+) restriction. Importantly, the oxidant greatly depressed myofilament responsiveness to Ca(2+), which was partly antagonized by Rp-8-(4-chlorophenylthio)guanosine-3',5'-cyclic monophosphorothioate (Rp-cGMPS), an inhibitor of cGMP-dependent protein kinase. Also, the inhibitor partially restored left ventricular contractility (n=6). Peroxynitrite stimulated cardiac cGMP production and coronary cGMP efflux (n=6). Decomposed peroxynitrite had no effect in any of the tests. Peroxynitrite depresses myocardial contractility by decreasing the ability of Ca(2+) to trigger contraction, and this effect is partly mediated by the cGMP/cGMP-dependent protein kinase pathway.