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

In isolated, perfused proximal segments of Sprague-Dawley rat tail artery, idazoxan (100 nmol l-1) 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 nmol l-1) had no effect on responses to electrical stimulation. Prazosin (10 nmol l-1) displaced the concentration-response curve to NA to the right as well as markedly reducing responses to sympathetic nerve stimulation. The concentration-response curve to NA, obtained after reducing the concentration of calcium ions in the Krebs solution from 2.5 to 0.6 mmol l-1, was significantly displaced to the right. Responses to sympathetic nerve stimulation were not affected by this reduction in the concentration of calcium ions. Diltiazem (1 and 10 mumol l-1) significantly displaced the concentration-response curve to NA to the right but had no effect on sympathetic nerve stimulation. These in vitro results in peripheral arterial smooth muscle confirm the findings of previous in vivo studies which suggest that alpha2-adrenoceptors contribute to the vasoconstrictor responses elicited by alpha-adrenoceptor agonists and that these responses but not those mediated by alpha1-adrenoceptors are dependent on extracellular calcium.