Cross-Talk Between AT₁ and AT₂Angiotensin Receptors in Rat Anococcygeus Smooth Muscle

Abstract

Schild regressions for the selective AT 1 and AT 2 receptor antagonists, losartan and PD123319 (S-[ϩ]-1-[(4-dimethylamino]-3-methylphenyl)methyl]-5- [diphenylacetyl]-4,5,6,7-tetrahydro-1H-imidazol[4,5-c]pyridine-6-carboxilic acid), respectively, were calculated to analyze the heterogeneity of receptor populations in the rat anococcygeus muscle. For a one-receptor system, the Schild regression has a slope of unity and an intercept of K B for competitive antagonists. However, in a two-receptor system, a deviation from the single-receptor plot will occur. This is predicated on the assumption that the secondary receptor is less sensitive to the antagonist than the primary receptor. Results showed that the Schild regression for losartan did not produce a slope of unity, and PD123319 did not produce any effect. However, tissue incubation with losartan plus PD123319 resulted in a Schild regression that has a slope of unity and a pK B of 9.32. In the presence of prazosin, an ␣ 1 -adrenoceptor antagonist, losartan did not produce any effect. Conversely, PD123319 enhanced the angiotensin II (Ang II)-induced contraction in a concentration-dependent fashion, suggesting an inhibitory AT 2 -mediated effect. This effect was confirmed with assays that showed a relaxant response induced by Ang II on precontracted tissues incubated with prazosin. PD123319 and N G -nitro-L-arginine methyl ester [nitric-oxide (NO) synthase inhibitor)] markedly inhibited the relaxant response of Ang II. In contrast, losartan did not produce any significant effect. Consequently, results show that the mechanism underlying the AT 2 -mediated effect is highly dependent on NO generation. Results indicate the presence of a heterogeneous angiotensin receptor population in the rat anococcygeus muscle following a negative cross-talk relationship between the AT 1 and AT 2 subtypes.

Angiotensin II-Induced Relaxation of Anococcygeus Smooth Muscle via Desensitization of AT1Receptor, and Activation of AT2Receptor Associated with Nitric-Oxide Synthase Pathway

“…Male Sprague-Dawley rats (300 -350 g) were sacrificed by decapitation and the ASM was removed as previously described (Gillespie, 1972;de Godoy and de Oliveira, 2002). The tissues were then transferred to oxygenated (95% O 2 /5% CO 2 ) Krebs' physiological solution of the following composition: 118.1 mM NaCl, 4.7 mM KCl, 2.5 mM CaCl 2 , 1.16 mM MgSO 4 , 1.0 mM NaH 2 PO 4 , 25 mM NaHCO 3 , and 11.1 mM glucose at 37°C.…”

“…In the rat ASM, Ang II has been shown to cause contraction via AT 1 receptor activation, whereas AT 2 receptor leads to inhibition (de Godoy and de Oliveira, 2002).…”

“…Early studies have suggested that activation of the AT 2 subtype counteracts the effects of AT 1 activation in different tissues (Nouet and Nahmias, 2000;de Godoy and de Oliveira, 2002;Rattan et al, 2002) via the release of inhibitory autacoids such as nitric oxide (NO) (Siragy and Carey, 1999;Israel et al, 2000).…”

Translocation of AT1- and AT2-Receptors by Higher Concentrations of Angiotensin II in the Smooth Muscle Cells of Rat Internal Anal Sphincter

“…For several years, most of the effects of Ang II were attributed to the activation of the AT 1 -R subtype. Recently, however, an increasing number of reports also suggested the involvement of AT 2 -R in the actions of Ang II (Siragy et al, 2000;Wu et al, 2001;de Godoy and De Oliveira, 2002;Rattan et al, 2002;de Godoy et al, 2004b).…”

Effect of caveolin-1 scaffolding peptide and 17β-estradiol on intracellular Ca2+ kinetics evoked by angiotensin II in human vascular smooth muscle cells

“…[Ca 2ϩ ]i. A series of experiments was carried out to characterize the short-term effects of PD-123319 (an AT2 antagonist, 1 nM) (10,39) and losartan (an AT1 antagonist, 1 and 10 M) on the [Ca 2ϩ ]i increase induced by 1 M ANG II. We also characterized the short-term effects of losartan (10 M), E 2 (10 nM), and CSP-1 (10 nM) on CGP42112A-induced effects (an AT2 agonist, 10 nM) (31).…”