Sources of calcium and α<inf>1</inf>-adrenoceptor-mediated contraction in rat tail artery

1. The relative importance of intracellular and extracellular Ca2+ on α1-adrenoceptor mediated contraction by noradrenaline and St-587 has been studied and correlated with the binding characteristics in intact tail artery from Sprague-Dawley rats. 2. Noradrenaline and St-587 behaved as full agonists inducing a concentration dependent vasoconstriction. 3. Nifedipine (1 μM and 10 μM) blocked by 50% (P < 0.001) and 75% (P < 0.001) respectively, the maximum contraction (E(max)) induced by St-587. Nevertheless, to reach 40% inhibition of E(max) on noradrenaline responses (P < 0.01), 10 μM nifedipine was necessary. 4. Both agonists induced a concentration-dependent accumulation of inositol phosphates, Noradrenaline behaved as a full agonist and St-587 as a partial agonist for this response. 5. [3H]-prazosin binding to intact tail artery rings was saturable and of high affinity (K(D) = 4.44 ± 0.46 nM; B(max) = 36.35 ± 4.22 fmol mg-1 tissue). 6. Competition curves for inhibition of specific [3H] prazosin binding by WB 4101 suggest that the rat tail artery contains two α1-adrenoceptor subtypes in an approximate ratio of 60:40. 7. After irreversible alkylation of α(1B)-adrenoceptors with 100 μM chloroethylclonidine (CEC), nifedipine (1 μM) influenced to a greater extent the St-587- than the noradrenaline induced contraction 8. Our results indicate that the degree of participation of intracellular and extracellular Ca2+ sources, on the α1-adrenoceptor mediated contraction, depends on the agonist used. The two α1-adrenoceptor subtypes observed in binding experiments seem to be unrelated to the Ca2+ sources used for contraction.