Effects of cGMP on L-type calcium current of adult and newborn rabbit ventricular cells

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Abstract
Objective: Cyclic GMP has been shown to be in some respects an inhibitory modulator of heart function. Various studies on the modulation of cardiac L-type calcium current ( ICa ) by cGMP in different species show inconsistency and the role of cGMP remains unclear and controversial. The present study was focused on the differences in the modulation of basal ICa by cGMP in adult and newborn rabbit ventricular cells. Methods: Enzymatically isolated adult and newborn (1–4-day-old) rabbit ventricular myocytes were used to measure ICa under whole-cell voltage clamp conditions with internal perfusion of isolated cells. Results: We have shown that in adult ventricular cells, the intracellular perfusion of 8BrcGMP did not produce any effect on basal ICa , while intracellular perfusion of 8BrcGMP or 8CPT-cGMP in newborn ventricular cells significantly and reversibly increased basal ICa without changing the voltage dependence for activation of ICa . Both methylene blue and LY-83583 (which inhibit guanylyl cyclase and thus lower cGMP levels), in adult ventricular cells, failed to produce any significant effect on basal ICa , while in newborn ventricular cells the application of methylene blue or LY-83583 produced irreversible inhibition of basal ICa . Similarly, KT-5823, an inhibitor of cGMP-dependent protein kinase, also inhibited basal ICa in newborn ventricular cells but not in adult ventricular cells. However, extracellular application of methylene blue during the intracellular perfusion of 8BrcGMP was unable to inhibit ICa . Extracellular application of nitrosoglutathione which releases nitric oxide produced a significant increase in ICa in newborn but not in adult ventricular cells. Intracellular application of a cAMP-dependent protein kinase inhibitor peptide blocked the stimulatory effect of cAMP but not of 8CPT-cGMP, while the stimulatory effect of nitrosoglutathione on ICa was not blocked by the presence of a phosphodiesterase inhibitor (isobutylmethyl-xanthine). Conclusions: We propose that, for newborn rabbit ventricular cells, cGMP plays a crucial role in maintaining basal ICa by a mechanism mediated via protein-kinase-G-dependent phosphorylation of calcium channels or some associated protein.

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