Calcium dependency of the AT₁-receptor mediated effects in the rat portal vein: influence of calcium antagonists

Abstract

The calcium dependency of AT₁-receptor mediated contractions was studied in isolated rat portal vein preparations.

The spontaneous phasic contractile force of the rat portal vein was increased (ED₅₀ = 1.76 mmol/l) and the frequency of contractions decreased by raising the extracellular calcium concentration. The Ang 11-induced rise in phasic contractile force (mediated by AT₁-receptors, Zhang et al. 1993) proved most pronounced at 0.9 mmol/l of calcium chloride, but it was weaker at either lower or higher calcium concentrations. The maximal increases in the phasic contractile force induced by Ang II were 2.4±0.4, 14.8±0.9 and 5±0.5 mN at calcium concentrations of 0.5, 0.9 and 2.5 mmol/l, respectively. Calcium antagonists reduced at the lower and abolished at the higher concentrations (nifedipine 2×10⁻⁸ or 10⁻⁷ mol/l; verapamil 10⁻⁷ or 5 × 10⁻⁷ mol/l; diltiazem 3 × 10⁻⁷ or 10–6 mol/l) the spontaneous contractile force. All of these calcium antagonists caused a strong inhibition or suppression of the phasic contractions induced by Ang II.The rank order of potency was nifedipine >verapamil > diltiazem. Ang II (10⁻⁶ mol/l) elicited a tonic contraction which was abolished by the AT₁-receptor antagonist losartan 10-6 mol/l but not by the AT₂-receptor antagonist PD 123177 (10–5 mol/l). Very high concentrations of nifedipine (10–6 mol/l), verapamil (5 × 10-6 mol/l) and diltiazem (5 × 10⁻⁶ mol/l) almost suppressed the tonic effect evoked by the activation of AT₁-receptors.

In a nominally Ca²⁺ “free”, EGTA-containing solution, a single supra-maximal concentration of Ang II (10⁻⁶ mol/l) caused a transient contraction, also mediated by AT₁-receptors. This finding suggests the existence of Ang II-sensitive intracellular calcium stores in this preparation. The depletion of such stores proved complete after 4–6 min of perfusion in a Ca²⁺ “free”, EGTA-containing solution.

In conclusion, various types of contractions (a transient contraction in a Ca²⁺-“free” medium, phasic and tonic contractions) induced by Ang II in the rat portal vein proved to be mediated by AT₁-receptors. These contractions were clearly modified by changes in the availability of extra- and possibly intracellular calcium ions. The calcium movements elicited by stimulation of AT₁-receptors in a calcium containing solution were inhibited by the three calcium antagonists investigated.