Role of basal extracellular Ca 2+ entry during 5‐HT‐induced vasoconstriction of canine pulmonary arteries

1 Measurements of artery contraction, cytosolic [Ca 2+ ], and Ca 2+ permeability were made to examine contractile and cytosolic [Ca 2+ ] responses of canine pulmonary arteries and isolated cells to 5‐hydroxytryptamine (5‐HT), and to determine the roles of intracellular Ca 2+ release and extracellular Ca 2+ entry in 5‐HT responses. 2 The EC 50 for 5‐HT‐mediated contractions and cytosolic [Ca 2+ ] increases was ∼10 −7   M and responses were inhibited by ketanserin, a 5‐HT 2A ‐receptor antagonist. 3 5‐HT induced cytosolic [Ca 2+ ] increases were blocked by 20  μ M Xestospongin‐C and by 2‐APB (IC 50 =32  μ M ), inhibitors of InsP 3 receptor activation. 4 5‐HT‐mediated contractions were reliant on release of InsP 3 but not ryanodine‐sensitive Ca 2+ stores. 5 5‐HT‐mediated contractions and cytosolic [Ca 2+ ] increases were partially inhibited by 10  μ M nisoldipine, a voltage‐dependent Ca 2+ channel blocker. 6 Extracellular Ca 2+ removal reduced 5‐HT‐mediated contractions further than nisoldipine and ablated cytosolic [Ca 2+ ] increases and [Ca 2+ ] oscillations. Similar to Ca 2+ removal, Ni 2+ reduced cytosolic [Ca 2+ ] and [Ca 2+ ] oscillations. 7 Mn 2+ quench of fura‐2 and voltage‐clamp experiments showed that 5‐HT failed to activate any significant voltage‐independent Ca 2+ entry pathways, including store‐operated and receptor‐activated nonselective cation channels. Ni 2+ but not nisoldipine or Gd 3+ blocked basal Mn 2+ entry. 8 Voltage‐clamp experiments showed that simultaneous depletion of both InsP 3 and ryanodine‐sensitive intracellular Ca 2+ stores activates a current with linear voltage dependence and a reversal potential consistent with it being a nonselective cation channel. 5‐HT did not activate this current. 9 Basal Ca 2+ entry, rather than CCE, is important to maintain 5‐HT‐induced cytosolic [Ca 2+ ] responses and contraction in canine pulmonary artery.British Journal of Pharmacology (2005) 144 , 252–264. doi: 10.1038/sj.bjp.0706077