A Ca²⁺-dependent chloride current and Ca²⁺ influx via Ca(v)1.2 ion channels play major roles in P2Y receptor-mediated pulmonary vasoconstriction.

BACKGROUND AND
PURPOSE: ATP, UTP and UDP act at smooth muscle P2X and P2Y receptors to constrict rat intrapulmonary arteries, but the underlying signalling pathways are poorly understood. Here, we determined the roles of the Ca²⁺ -dependent chloride ion current (I(Cl,Ca)), Ca(v)1.2 ion channels and Ca²⁺ influx. EXPERIMENTAL APPROACH: Isometric tension was recorded from endothelium-denuded rat intrapulmonary artery rings (i.d. 200-500 µm) mounted on a wire myograph. KEY
RESULTS: The I(Cl,Ca) blockers, niflumic acid and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid and the Ca(v)1.2 channel blocker, nifedipine, reduced peak amplitude of contractions evoked by UTP and UDP by ∼45-50% and in a non-additive manner. Ca²⁺-free buffer inhibited responses by ∼70%. Niflumic acid and nifedipine similarly depressed contractions to ATP, but Ca²⁺-free buffer almost abolished the response. After peaking, contractions to UTP and UDP decayed slowly by 50-70% to a sustained plateau, which was rapidly inhibited by niflumic acid and nifedipine. Contractions to ATP, however, reversed rapidly and fully. Tannic acid contracted tissues per se and potentiated nucleotide-evoked contractions. CONCLUSIONS AND IMPLICATIONS: I (Cl,Ca) and Ca²⁺ influx via Ca(v)1.2 ion channels contribute substantially and equally to contractions of rat intrapulmonary arteries evoked by UTP and UDP, via P2Y receptors. ATP also activates these mechanisms via P2Y receptors, but the greater dependence on extracellular Ca²⁺ most likely reflects additional influx through the P2X1 receptor pore. The lack of a sustained response to ATP is probably due to it acting at P2 receptor subtypes that desensitize rapidly. Thus multiple signalling mechanisms contribute to pulmonary artery vasoconstriction mediated by P2 receptors.