Effects of reduced calcium ion concentration and of diltiazem on vasoconstrictor responses to noradrenaline and sympathetic nerve stimulation in rat isolated tail artery

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Abstract
1 In isolated, perfused proximal segments of Sprague-Dawley rat tail artery, idazoxan (100 nmoll1) displaced the concentration-response curve to noradrenaline (NA) to the right. The log shift of the NA concentration-response curve was greater at lower concentrations than at higher concentrations of NA. Idazoxan (100 nmoll−1) had no effect on responses to electrical stimulation. 2 Prazosin (10 nmoll−1) displaced the concentration-response curve to NA to the right as well as markedly reducing responses to sympathetic nerve stimulation. 3 The concentration-response curve to NA, obtained after reducing the concentration of calcium ions in the Krebs solution from 2.5 to 0.6 mmoll−1, was significantly displaced to the right. Responses to sympathetic nerve stimulation were not affected by this reduction in the concentration of calcium ions. 4 Diltiazem (1 and 10 μmoll−1) significantly displaced the concentration-response curve to NA to the right but had no effect on sympathetic nerve stimulation. 5 These in vitro results in peripheral arterial smooth muscle confirm the findings of previous in vivo studies which suggest that α2-adrenoceptors contribute to the vasoconstrictor responses elicited by α-adrenoceptor agonists and that these responses but not those mediated by α1-adrenoceptors are dependent on extracellular calcium.