Contractile effects by intracellular angiotensin II via receptors with a distinct pharmacological profile in rat aorta

1 We studied the effect of intracellular angiotensin II (Ang II) and related peptides on rat aortic contraction, whether this effect is pharmacologically distinguishable from that induced by extracellular stimulation, and determined the Ca2+ source involved.

2 Compounds were delivered into the cytoplasm of de-endothelized aorta rings using multilamellar liposomes. Contractions were normalized to the maximum obtained with phenylephrine (10(-5) M).

3 Intracellular administration of Ang II (incorporation range: 0.01 - 300 nmol mg(-1)) resulted in a dose-dependent contraction, insensitive to extracellular administration (10(-6) M) of the AT(1) receptor antagonist CV11947, the AT(2) receptor antagonist PD 123319, or the non-selective AT receptor antagonist and partial agonist saralasin ([Sar(1),Val(5),Ala(8)]-Ang II (P <0.05).

4 Intracellular administration of CV11947 or PD 123319 right shifted the dose-response curve about 1000 fold or 20 fold, respectively. PD 123319 was only effective if less than 30 nmol mg(-1) Ang II was incorporated.

5 Contraction was partially desensitized to a second intracellular Ang II addition after 45 min (P <0.05).

6 Intracellular administration of Ang I and saralasin also induced contraction (P <0.05). Both responses were sensitive to intracellular CV11947 (P <0.05), but insensitive to PD 123319. The response to Ang I was independent of intracellular captopril.

7 Contraction induced by extracellular application of Ang II and of Ang I was abolished by extracellular pre-treatment with saralasin or CV11947 (P <0.05), but not with PD 123319. Extracellular saralasin induced no contraction.

8 Intracellular Ang II induced contraction was not affected by pre-treatment with heparin filled liposomes, but completely abolished in Ca2+-free external medium.

9 These results support the existence of an intracellular binding site for Ang II in rat aorta. Intracellular stimulation induces contraction dependent on Ca2+-influx but not on Ins(1,4,5)P-3 mediated release from intracellular Ca2+-stores. Intracellular Ang I and saralasin induce contraction, possibly via the same binding site. Pharmacological properties of this putative intracellular receptor are clearly different from extracellular stimulated AT(1) receptors or intracellular angiotensin receptors postulated in other tissue.