Role of basal extracellular Ca2+ entry during 5-HT-induced vasoconstriction of canine pulmonary arteries

1 Measurements of artery contraction, cytosolic [Ca2+], and Ca2+ permeability were made to examine contractile and cytosolic [Ca2+] responses of canine pulmonary arteries and isolated cells to 5-hydroxytryptamine (5-HT), and to determine the roles of intracellular Ca2+ release and extracellular Ca2+ entry in 5-HT responses. 2 The EC50 for 5-HT-mediated contractions and cytosolic [Ca2+] increases was similar to10(-7) M and responses were inhibited by ketanserin, a 5-HT2A-receptor antagonist. 3 5-HT induced cytosolic [Ca2+] increases were blocked by 20 muM Xestospongin-C and by 2-APB (IC50 = 32 muM), inhibitors of InsP(3) receptor activation. 4 5-HT-mediated contractions were reliant on release of InsP(3) but not ryanodine-sensitive Ca2+ stores. 5 5-HT-mediated contractions and cytosolic [Ca2+] increases were partially inhibited by 10 muM nisoldipine, a voltage-dependent Ca2+ channel blocker. 6 Extracellular Ca2+ removal reduced 5-HT-mediated contractions further than nisoldipine and ablated cytosolic [Ca2+] increases and [Ca2+] oscillations. Similar to Ca2+ removal, Ni2+ reduced cytosolic [Ca2+] and [Ca2+] oscillations. 7 Mn2+ quench of fura-2 and voltage-clamp experiments showed that 5-HT failed to activate any significant voltage-independent Ca2+ entry pathways, including store-operated and receptor-activated nonselective cation channels. Ni2+ but not nisoldipine or Gd3+ blocked basal Mn2+ entry. 8 Voltage-clamp experiments showed that simultaneous depletion of both InsP(3) and ryanodine-sensitive intracellular Ca2+ 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 Ca2+ entry, rather than CCE, is important to maintain 5-HT-induced cytosolic [Ca2+] responses and contraction in canine pulmonary artery.