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

Abstract

In the present study, we investigated the underlaying mechanism of nitric oxide (NO) and cGMP on the decline of a Ca 2+-activated potassium (K Ca) 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 K Ca-current either by opening of endogenous voltage-gated Ca 2+-channels during depolarizing voltage steps or by ionophoretic injection of Ca 2+ via a third electrode containing 100 mM Ca 2+. K Ca-current amplitude in U-cells was sensitive to Ba 2+, 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 K Ca-current in U-cells shares similarities with the large-conductance K Ca channel (BK(Ca)). The NO-donor sodium nitroprusside (SNP) or S-nitro- N-acetylpenicillamine (SNAP) as well as NO-gas decreased the K Ca-current amplitude and decreased the rate of K Ca-current activation elicited by Ca 2+-injection. Decline of the current amplitude and decrease of activation of K Ca-current were qualitatively mimicked by the membrane-permeable cGMP analogue dibutyryl-cGMP (db-cGMP). NO-induced decrease of K Ca-current was blocked by methylene blue (50 μM), an inhibitor of the guanylyl-cyclase, and by erytho-9-(2-hydroxyl-3-nonyl) adenine (EHNA) (100 μM), an inhibitor of the cGMP-stimulated phosphodiesterase 2 (PDE2). These experiments suggest that the NO-mediated decrease of K Ca-current in U-cells results from synthesis of cGMP by activation of a guanylyl-cyclase and subsequent activation of PDE2.