Exogenous nitric oxide reduces oxygen consumption of isolated ventricular myocytes less than other forms of guanylate cyclase stimulation

We tested the hypothesis that increasing cyclic GMP with nitric oxide (NO) would reduce cardiac myocyte metabolism less than other forms of guanylate cyclase stimulation. The steady state O₂ consumption (VO₂) of a suspension of ventricular myocytes in 2.0 mM Ca²⁺ isolated from hearts of New Zealand white rabbits was measured in a glass chamber using Clark-type oxygen electrode. The cellular cyclic GMP levels, determined by radioimmunoassay, were increased by (1) adding 3-morpholinosydnonimine (SIN- 1, 10^-8-10^-5 M) and nitroprusside (10^-8-10^-5 M), NO donors-soluble guanylate cyclase stimulators; (2) carbon monoxide (CO, 1.5 x 10^-8-1.5 x 10^-6 M), soluble guanylate cyclase stimulator and (3) guanylin (10^-8- 10^-5 M), particulate gunnylate cyclase stimulator. The baseline myocyte cyclic GMP level was 86 ± 13 fmol/10⁵ myocytes with a corresponding VO₂ of 268 ± 21 nl O₂/min per 10⁵ myocytes. An inverse relationship between cellular cyclic GMP levels and VO₂ existed in these myocytes. The regression equations for the four treatments were: VO₂ = -0.45 x [cyclic GMP] + 294.4, r = 0.94 for SIN-1; VO₂ = -1.46 x [cyclic GMP] + 444.7, r = 0.96 for CO; VO₂ = -1.25 x [cyclic GMP] + 389.1, r = 0.84 for guanylin and VO₂ = -0.55 x [cyclic GMP] + 322.8, r = 0.79 for nitroprusside. The regression lines of the two NO donors were parallel. A similar result was also evident for the regressions of CO and guanylin. However, the slopes of both the SIN-1 and nitroprusside regression line were significantly less steep than that of either the CO or guanylin lines. Therefore, VO₂ is reduced less for a similar increase in cyclic GMP with NO donors compared to direct stimulation with CO or guanylin. These results suggest that NO has metabolic effects on myocytes in addition to its stimulatory effects on cellular cyclic GMP.