EFFECTS OF CGMP ON L-TYPE CALCIUM CURRENT OF ADULT AND NEWBORN RABBITVENTRICULAR CELLS

Objective: Cyclic GMP has been shown to be in some respects an inhibit ory modulator of heart function. Various studies on the modulation of cardiac L-type calcium current (I-Ca) by cGMP in different species sho w inconsistency and the role of cGMP remains unclear and controversial . The present study was focused on the differences in the modulation o f basal I-Ca by cGMP in adult and newborn rabbit ventricular cells. Me thods: Enzymatically isolated adult and newborn (1-4-day-old) rabbit v entricular myocytes were used to measure I-Ca under whole-cell voltage clamp conditions with internal perfusion of isolated cells. Results: We have shown that in adult ventricular cells, the intracellular perfu sion of 8BrcGMP did not produce any effect on basal I-Ca, while intrac ellular perfusion of 8BrcGMP or 8CPT-cGMP in newborn ventricular cells significantly and reversibly increased basal I-Ca without changing th e voltage dependence for activation of I-Ca. Both methylene blue and L Y-83583 (which inhibit guanylyl cyclase and thus lower cGMP levels), i n adult ventricular cells, failed to produce any significant effect on basal I-Ca, while in newborn ventricular cells the application of met hylene blue or LY-83583 produced irreversible inhibition of basal I-Ca . Similarly, KT-5823, an inhibitor of cGMP-dependent protein kinase, a lso inhibited basal I-Ca in newborn ventricular cells but not in adult ventricular cells. However, extracellular application of methylene bl ue during the intracellular perfusion of 8BrcGMP was unable to inhibit I-Ca. Extracellular application of nitrosoglutathione which releases nitric oxide produced a significant increase in I-Ca in newborn but no t in adult ventricular cells. Intracellular application of a cAMP-depe ndent protein kinase inhibitor peptide blocked the stimulatory effect of cAMP but not of 8CPT-cGMP, while the stimulatory effect of nitrosog lutathione on I-Ca was not blocked by the presence of a phosphodiester ase inhibitor (isobutylmethyl-xanthine). Conclusions: We propose that, for newborn rabbit ventricular cells, cGMP plays a crucial role in ma intaining basal I-Ca by a mechanism mediated via protein-kinase-G-depe ndent phosphorylation of calcium channels or some associated protein.