Inhibition of cyclic GMP hydrolysis with zaprinast reduces basal and cyclic AMP‐elevated L‐type calcium current in guinea‐pig ventricular myocytes

Cyclic GMP (cGMP) has been shown to be an important modulator of cardiac contractile function. A major component of cGMP regulation of contractility is cGMP‐mediated inhibition of the cardiac calcium current (ICa). An under‐appreciated aspect of cyclic nucleotide signalling is hydrolysis of the cyclic nucleotide (i.e., breakdown by phosphodiesterases (PDEs)). The role of cGMP hydrolysis in regulating ICa has not been studied. Thus the purpose of this study was to investigate if inhibition of cGMP hydrolysis can modulate ICa in isolated guinea‐pig ventricular myocytes. Zaprinast, a selective inhibitor of cGMP‐specific PDE (PDE5), caused a significant increase in cGMP levels in myocytes, but was without affect on basal or β‐adrenergic stimulated cAMP levels (consistent with its actions as a specific inhibitor of PDE5). Zaprinast inhibited ICa that was pre‐stimulated with cAMP elevating agents (isoproterenol, a β‐adrenergic agonist; or forskolin, a direct activator of adenylate cyclase). The effect of zaprinast was greatly reduced by KT5823, an inhibitor of cGMP‐dependent protein kinase (PKG). Zaprinast also significantly inhibited basal ICa when perforated‐patch or whole‐cell recording with physiological pipette calcium concentration (10−7 M) was used. However, this effect was not observed when using standard calcium‐free whole‐cell recording conditions. These results indicate that inhibition of cGMP hydrolysis can decrease both basal and cAMP‐stimulated ICa. Thus, cGMP hydrolysis may likely be an important step for physiological modulation of ICa. This regulation may also be important in disease states in which cGMP production is increased and PDE5 expression is altered, such as heart failure. British Journal of Pharmacology (2003) 138, 986–994. doi:10.1038/sj.bjp.0705112