cGMP prevents delayed relaxation at reoxygenation after brief hypoxia in isolated cardiac myocytes

Previous studies in isolated cardiac myocytes suggest that impaired relaxation during reoxygenation after brief hypoxia results from abnormal Ca(2+)-myofilament interaction. Recent studies indicate that guanosine 3',5'-cyclic monophosphate (cGMP)-elevating interventions selectively enhance myocardial relaxation. We investigated the effect of 8-bromoguanosine 3',5'-cyclic monophosphate (8-BrcGMP) on posthypoxic relaxation in single rat myocytes, with simultaneous measurement of contraction and intracellular Ca2+ (indo 1 fluorescence). In control myocytes (n = 11), reoxygenation after 10 min of hypoxia markedly prolonged time to peak shortening (+36.5 +/- 4.2%) and half-relaxation time (+75.7 +/- 11.3% cf. normoxic values; both P < 0.001) and reduced diastolic length but did not change cytosolic Ca2+. Under normoxic conditions, 50 microM 8-BrcGMP slightly reduced time to peak shortening and half-relaxation time and increased diastolic length but did not alter cytosolic Ca2+. In the presence of 8-BrcGMP, there was no posthypoxic delay in twitch relaxation nor was there a decrease in diastolic length (half-relaxation time -5.8 +/- 3.3% cf. normoxic values; P < 0.05 cf. control group; n = 11). Cytosolic Ca2+ remained unaltered. Thus, 8-BrcGMP fully prevents impaired posthypoxic relaxation in isolated cardiac myocytes, probably by altering Ca(2+)-myofilament interaction.