Negative effect of nitric oxide on shorteningfrequency relationship in cardiac myocytes is diminished after simulated ischemia-reperfusion

Increasing stimulation rate increases function in cardiac myocytes and nitric oxide and cyclic GMP inhibit this effect. We tested the hypothesis that myocyte stunning would blunt both the effects of increases in rate and of nitric oxide and cyclic GMP. Ventricular myocytes from 11 rabbits were used to determine maximum rate of shortening (R_max, µm/s) and %shortening during control and after simulated ischemia 15 min 95% N₂- 5% CO₂] and reperfusion reoxygenation]. Measurements were obtained at 1–4 Hz with vehicle, 1H1,2,4]oxadiazolo4,3,alpha] quinoxaline-1-one (ODQ) 10^–6 M, soluble guanylyl cyclase inhibitor, or N^G-nitro-L-arginine methyl ester, nitric oxide synthase inhibitor (L-NAME) 10^–5 M. In control, increases in rate increased R_max from 69 ± 3 to 254±12 and %shortening from 5.3 ± 0.3 to 8.7 ± 0.5. Both ODQ and L-NAME shifted values higher. With stunning, the effects of pacing on Rmax and %shortening were blunted and ODQ and L-NAME failed to alter these values. Cyclic GMP was 322±37 pmol/10⁵ myocytes at baseline and these values were lowered by ODQ (244 ± 31) and LNAME (207 ± 23), and similar changes were observed in stunned myocytes. Increasing frequency increased function, and reducing nitric oxide/cyclic GMP enhanced this relationship. The effect of nitric oxide was diminished by stunning, but this was not related to altered cyclic GMP levels. This suggested changes in effects of cyclic GMP downstream to its production during myocardial stunning.