Pharmacological characterization of SB-710411 (Cpa-c[D-Cys-Pal-D-Trp-Lys-Val-Cys]-Cpa-amide), a novel peptidic urotensin-II receptor antagonist

Abstract

1. Human urotensin-II (hU-II), a cyclic undecapeptide, is amongst the most potent mammalian vasoconstrictors identified, suggesting that hU-II and its G-protein-coupled receptor (UT) may regulate cardiovascular homeostasis. Such a hypothesis would benefit greatly from the development of selective UT antagonists. 2. Although the somatostatin (SST) antagonist SB-710411 (Cpa-c[D-Cys-Pal-D-Trp-Lys-Val-Cys]-Cpa-amide) is purported to block U-II-induced contractions in rat isolated aorta, little is known about its specific pharmacological properties. 3. SB-710411 (10 micro M) inhibited hU-II-induced contraction in rat isolated aorta causing a significant, parallel shift in the agonist concentration-response curve (pK(b) 6.28+/-0.11; n=8) with no suppression of the E(max). In contrast, SB-710411 did not alter the contractile actions of angiotensin-II, phenylephrine, or KCl. Paradoxically, however, SB-710411 potentiated the contractile response to endothelin-1 (pEC(50) 8.02+/-0.16 and 8.54+/-0.11, P<0.01; n=8). Rather than being specific toSB-710411, this phenomenon appears to be related to somatostatin receptor affinity and not intrinsic activity since the SST agonist somatostatin-14 and antagonist cyclo-somatostatin also potentiated endothelin-1-induced contraction. 4. SB-710411 (10 micro M) did not inhibit carbachol, sodium nitroprusside, IBMX, isoprenaline, and levcromakalim-induced reversal of tone established with noradrenaline. In contrast, however, SB-710411 significantly inhibited the reversal of tone established with endothelin-1 using the same vasorelaxants. 5. In summary, although SB-710411 inhibits the vasoconstrictor actions of hU-II in a competitive, surmountable manner, it also possesses additional pharmacological actions. Thus, whilst the present study is amongst the first to detail the properties of a functional U-II receptor antagonist, the data suggest caution be used when assessing data generated utilizing this moiety and other SST analogues.

Figure 1

Representative experimental traces illustrating that relative to vehicle-treated vessels (a and c), SB-710411 (10 μM; b and d) inhibits or augments the concentration-dependent contractile actions of hU-II or endothelin-1 (ET-1) in rat isolated aorta, respectively. All experiments were performed in endothelium-denuded vessels in the presence of 10 μM indomethacin. In order to normalize the contractile responses to hU-II and ET-1, vessels were precontracted with 60 mM KCl. Following washing (W), endothelial cell integrity was evaluated by assessing the ability of 10 μM carbachol (CCh) to reverse tone established in the isolated aorta with 1 μM noradrenaline (NA). The vertical and horizontal scale bars represent tension (2 g) and time (10 min), respectively.

Figure 2

SB-710411 (10 μM) does not inhibit (a) angiotensin-II, (b) phenylephrine, or (c) KCl induced contraction in the rat isolated aorta. Paradoxically, (d) SB-710411 potentiates endothelin-1-induced vasoconstriction. In contrast, however, (e) SB-710411 inhibits hU-II-induced contraction causing a parallel shift in the concentration-response curve to the agonist with no suppression of the maximum contractile response. All experiments were performed in endothelium-denuded vessels in the presence of 10 μM indomethacin. Values are mean and vertical bars represent the s.e.mean. Curves were derived by fitting experimental data to a logistic equation (Douglas et al., 1995).

Figure 3

SB-710411 (10 μM) does not inhibit reversal of tone establish with noradrenaline. The Figure illustrates concentration-dependent relaxation response curves to (a) carbachol, (b) IBMX, (c) isoprenaline, (d) levcromakalim and (e) sodium nitroprusside in vehicle- and drug-treated rat isolated aortae following precontraction with 100 nM noradrenaline (NA). Responses are expressed as percent reversal of the original tone established in the tissue by NA. All experiments were performed in endothelium-intact vessels. Values are mean and vertical bars represent the s.e.mean. Curves were derived by fitting experimental data to a logistic equation (Douglas et al., 1995).

Figure 4

Representative experimental traces illustrating that relative to vehicle-treated vessels (a), SB-710411 (10 μM; b) inhibits the carbachol-induced reversal of tone in rat isolated aorta precontracted with 30 nM endothelin-1 (ET-1). All experiments were performed in endothelium-intact vessels. Vessels were precontracted with 60 mM KCl. Following washing (W), endothelial cell integrity was evaluated by adding 1 μM noradrenaline (NA) and observing the response to 10 μM carbachol (CCh). The vertical and horizontal scale bars represent tension (2 g) and time (10 min), respectively.

Figure 5

Relative to vehicle-treated vessels, 10 μM SB-710411 inhibits (a) sodium nitroprusside, (b) isoprenaline, (c) levcromakalim, (d) carbachol and (e) IBMX-induced reversal of tone established with endothelin-1 (ET-1). Responses are expressed as per cent reversal of the original tone established in the tissue by 30 nM ET-1. All experiments were performed in endothelium-intact vessels. Values are mean and vertical bars represent the s.e.mean. Curves were derived by fitting experimental data to a logistic equation (Douglas et al., 1995).