Sequential development of angiotensin receptors and angiotensin I converting enzyme during angiogenesis in the rat subcutaneous sponge granuloma

1. The vasoconstrictor peptide antiotensin II (AII) can stimulate angiogenesis, an important process in wound healing, tumour growth and chronic inflammation. To elucidate mechanisms underlying AII-enhanced angiogenesis, we have studied a subcutaneous sponge granuloma model in the rat by use of ¹³³Xe clearance, morphometry and quantitative in vitro autoradiography.
2. When injected directly into the sponge, AII (1 nmol day⁻¹) increased ¹³³Xe clearance from, and fibrovascular growth in sponge granulomas, indicating enhanced angiogenesis 6 to 12 days after implantation. This AII-enhanced angiogenesis was inhibited by daily doses (100 nmol/sponge) of the specific but subtype non-selective AII receptor antagonist (Sar¹, Ile⁸)AII, and by the selective non-peptide AT₁ receptor antagonists losartan and DuP 532. In contrast, AII-enhanced neovascularization was not inhibited by the AT₂ receptor antagonist PD123319, nor was it mimicked by the AT₂ receptor agonist CGP42112A (each at 100 nmol/sponge day⁻¹).
3. AI (1 nmol/sponge day⁻¹), the angiotensin converting enzyme (ACE) inhibitors captopril (up to 100 μg/sponge day⁻¹) and lisinopril (40 μg/sponge day⁻¹), or AII receptor antagonists did not affect angiogenesis in the absence of exogenous AII.
4. [¹²⁵I]-(Sar¹, Ile⁸)AII binding sites with characteristics of AT₁ receptors were localized to microvessels and to non-vascular cells within the sponge stroma from 4 days after implantation, and were at higher density than in skin throughout the study.
5. [¹²⁵I]-(Sar¹, Ile⁸)AII binding sites with characteristics of AT₂ receptors were localized to non-vascular stromal cells, were of lower density and appeared later than did AT₁ sites.
6. The ACE inhibitor [¹²⁵I]-351A bound to sites with characteristics of ACE, 14 days after sponge implantation. [¹²⁵I]-351A bound less densely to sponge stroma than to skin.
7. We propose that AII can stimulate angiogenesis, acting via AT₁ receptors within the sponge granuloma. AT₁ and AT₂ receptors and ACE develop sequentially during microvascular maturation, and the role of the endogenous angiotensin system in angiogenesis will depend on the balanced local expression of its various components. Pharmacological modulation of this balance may provide novel therapeutic approaches in angiogenesis-dependent diseases.