Nitric oxide decreases a calcium-activated potassium current via activation of phosphodiesterase 2 in Helix U-cells.

In the present study, we investigated the underlaying mechanism of nitric oxide (NO) and cGMP on the decline of a Ca2+-activated potassium (KCa) current in U-cells of the right parietal ganglion of the pulmonate snail, Helix pomatia. Using a two-electrode voltage-clamp technique, we activated a KCa-current either by opening of endogenous voltage-gated Ca2+-channels during depolarizing voltage steps or by ionophoretic injection of Ca2+ via a third electrode containing 100 mM Ca2+. KCa-current amplitude in U-cells was sensitive to Ba2+, TEA, iberiotoxin, kaliotoxin and charybdotoxin (ChTX), but not to 4-aminopyridine (4-AP) (up to 30 mM) and apamin (up to 300 nM). Thus, the biophysical and pharmacological profile of the KCa-current in U-cells shares similarities with the large-conductance KCa channel (BKCa). The NO-donor sodium nitroprusside (SNP) or S-nitro-N-acetylpenicillamine (SNAP) as well as NO-gas decreased the KCa-current amplitude and decreased the rate of KCa-current activation elicited by Ca2+-injection. Decline of the current amplitude and decrease of activation of KCa-current were qualitatively mimicked by the membrane-permeable cGMP analogue dibutyryl-cGMP (db-cGMP). NO-induced decrease of KCa-current was blocked by methylene blue (50 microM), an inhibitor of the guanylyl-cyclase, and by erytho-9-(2-hydroxyl-3-nonyl) adenine (EHNA) (100 microM), an inhibitor of the cGMP-stimulated phosphodiesterase 2 (PDE2). These experiments suggest that the NO-mediated decrease of KCa-current in U-cells results from synthesis of cGMP by activation of a guanylyl-cyclase and subsequent activation of PDE2.