cGMP-independent inotropic effects of nitric oxide and peroxynitrite donors: potential role for nitrosylation.

Nitric oxide (NO) has concentration-dependent biphasic myocardial contractile effects. We tested the hypothesis, in isolated rat hearts, that NO cardiostimulation is primarily non-cGMP dependent. Infusion of 3-morpholinosydnonimine (SIN-1, 10(-5) M), which may participate in S-nitrosylation (S-NO) via peroxynitrite formation, increased the rate of left ventricular pressure rise (+dP/dt; 19 +/- 4%, P < 0.001, n = 11) without increasing effluent cGMP or cAMP. Superoxide dismutase (SOD; 150 U/ml) blocked SIN-1 cardiostimulation and led to cGMP elaboration. Sodium nitroprusside (10(-10)-10(-7) M), an iron nitrosyl compound, did not augment +dP/dt but increased cGMP approximately eightfold (P < 0.001), whereas diethylamine/NO (DEA/NO; 10(-7) M), a spontaneous NO. donor, increased +dP/dt (5 +/- 2%, P < 0.05, n = 6) without augmenting cGMP. SIN-1 and DEA/NO +dP/dt increase persisted despite guanylyl cyclase inhibition with 1H-(1,2,4)oxadiazolo-(4,3,-a)quinoxalin-1-one (10(-5) M, P < 0.05 for both donors), suggesting a cGMP-independent mechanism. Glutathione (5 x 10(-4) M, n = 15) prevented SIN-1 cardiostimulation, suggesting S-NO formation. SIN-1 also produced SOD-inhibitable cardiostimulation in vivo in mice. Thus peroxynitrite and NO donors can stimulate myocardial contractility independently of guanylyl cyclase activation, suggesting a role for S-NO reactions in NO/peroxynitrite-positive inotropic effects in intact hearts.

Relative contribution of preload and afterload to the reduction in cardiac output caused by nitric oxide synthase inhibition with L-N(G)-methylarginine hydrochloride 546C88.

OBJECTIVE: The nitric oxide synthase inhibitor L-N(G)-methylarginine hydrochloride (L-NMMA HC1 546C88) causes reductions in cardiac output (CO), a potential limitation to clinical application. This drop in CO exceeds that from phenylephrine at matched systemic arterial pressure. We tested the hypothesis that the greater fall in CO attributable to L-NMMA primarily reflects a difference in venoconstriction between agents, such that phenylephrine produces larger increases in preload (an independent determinant of CO). DESIGN: Random infusion of phenylephrine or L-NMMA. SETTING: An animal research laboratory. SUBJECTS: Eight healthy, conscious, male dogs. INTERVENTIONS: L-N(G)-methylarginine hydrochloride (20 mg/kg for 1 hr) and phenylephrine (0.5 to 3 microg/kg/min) were administered into eight dogs chronically instrumented to measure left ventricular pressure and dimension. Data were measured at a constant heart rate (140 beats/min) to render CO proportional to stroke dimension. MEASUREMENTS AND MAIN RESULTS: At a matched increase in afterload (effective arterial elastance), L-NMMA increased preload (end-diastolic dimension) to a lesser degree (3.8%+/-1.5%, p < .05) than phenylephrine (9.6%+/-1.6%, p < .05 vs. L-NMMA). Neither L-NMMA nor phenylephrine affected the slope of the end-systolic pressure dimension relationship, although L-NMMA shifted the relationship rightward (1.7+/-0.7 mm, p < .05), consistent with a mild negative inotropic effect. L-NMMA decreased the stroke dimension to a greater extent than phenylephrine (-24.1%+/-6.8% and -10.6%+/-3.4%, respectively, p < .05). CONCLUSIONS: Differential CO responses to phenylephrine and L-NMMA were primarily attributable to changes in preload. Variable venular vs. arteriolar constrictor effects must be considered when evaluating the integrated cardiovascular response to a vasoactive agent.

Intravenous allopurinol decreases myocardial oxygen consumption and increases mechanical efficiency in dogs with pacing-induced heart failure.

Allopurinol, an inhibitor of xanthine oxidase, increases myofilament calcium responsiveness and blunts calcium cycling in isolated cardiac muscle. We sought to extend these observations to conscious dogs with and without pacing-induced heart failure and tested the prediction that allopurinol would have a positive inotropic effect without increasing energy expenditure, thereby increasing mechanical efficiency. In control dogs (n=10), allopurinol (200 mg IV) caused a small positive inotropic effect; (dP/dt)(max) increased from 3103+/-162 to 3373+/-225 mm Hg/s (+8.3+/-3.2%; P=0.01), but preload-recruitable stroke work and ventricular elastance did not change. In heart failure (n=5), this effect was larger; (dP/dt)(max) rose from 1602+/-190 to 1988+/-251 mm Hg/s (+24.4+/-8.7%; P=0.03), preload-recruitable stroke work increased from 55.8+/-9.1 to 84. 9+/-12.2 mm Hg (+28.1+/-5.3%; P=0.02), and ventricular elastance rose from 6.0+/-1.6 to 10.5+/-2.2 mm Hg/mm (P=0.03). Allopurinol did not affect myocardial lusitropic properties either in control or heart failure dogs. In heart failure dogs, but not controls, allopurinol decreased myocardial oxygen consumption (-49+/-4.6%; P=0. 002) and substantially increased mechanical efficiency (stroke work/myocardial oxygen consumption; +122+/-42%; P=0.04). Moreover, xanthine oxidase activity was approximately 4-fold increased in failing versus control dog hearts (387+/-125 versus 78+/-72 pmol/min. mg(-1); P=0.04) but was not detectable in plasma. These data indicate that allopurinol possesses unique inotropic properties, increasing myocardial contractility while simultaneously reducing cardiac energy requirements. The resultant boost in myocardial contractile efficiency may prove beneficial in the treatment of congestive heart failure.