Cyclic GMP regulation of the L‐type Ca2+ channel current in human atrial myocytes

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Abstract
The regulation of the L-type Ca2+ current (ICa) by intracellular cGMP was investigated in human atrial myocytes using the whole-cell patch-clamp technique. Intracellular application of 0.5 μm cGMP produced a strong stimulation of basal ICa (+64 ± 5%, n = 60), whereas a 10-fold higher cGMP concentration induced a 2-fold smaller increase (+36 ± 8%, n = 35). The biphasic response of ICa to cGMP was not mimicked by the cGMP-dependent protein kinase (PKG) activator 8-bromoguanosine 3′,5′ cyclic monophosphate (8-bromo-cGMP, 0.5 or 5 μm), and was not affected by the PKG inhibitor KT 5823 (100 nm). In contrast, cGMP stimulation of ICa was abolished by intracellular perfusion with PKI (10 μm), a selective inhibitor of the cAMP-dependent protein kinase (PKA). Selective inhibition of the cGMP-inhibited phosphodiesterase (PDE3) by extracellular cilostamide (100 nm) strongly enhanced basal ICa in control conditions (+78 ± 13%, n = 7) but had only a marginal effect in the presence of intracellular cGMP (+22 ± 7% in addition to 0.5 μm cGMP, n = 11; +20 ± 22% in addition to 5 μm cGMP, n = 7). Application of erythro-9-[2-hydroxy-3-nonyl]adenine (EHNA, 30 μm), a selective inhibitor of the cGMP-stimulated phosphodiesterase (PDE2), fully reversed the secondary inhibitory effect of 5 μm cGMP on ICa (+99 ± 16% stimulation, n = 7). Altogether, these data indicate that intracellular cGMP regulates basal ICa in human atrial myocytes in a similar manner to NO donors. The effect of cGMP involves modulation of the cAMP level and PKA activity via opposite actions of the nucleotide on PDE2 and PDE3.
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