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Functional vascular responses to sigma receptor ligands in canine isolated blood vessels
In vivo studies in rats and dogs have shown functional effects of sigma (σ) receptor agonists that can be attenuated by σ receptor antagonists (Okuyama et al., 1994, Matsumoto et al 1995). The numerous reported actions of σ receptor ligands include effects on the cardiovascular system, such as regulation of cardiac contractility and rhythm, and regulation of vascular tone (Monassier & Bousquet, 2002). However, the effects of σ receptor ligands have not been fully investigated in canine vascular tissue. Therefore, the aim of this study was to assess the functional potency and efficacy of σ receptor ligands in canine isolated blood vessels. Saphenous veins and carotid arteries were removed from beagle dogs of either sex (10-20kg), euthanised by pentobarbitone (0.5ml/kg iv) (animals were used in several pharmacological studies and protocols were ethically reviewed by Pfizer ethics committee). The endothelium was removed (only in saphenous vein) by gentle rubbing. Vascular rings (3-5mm in length) were mounted vertically (1.5g resting tension) between triangular hooks in 15ml baths containing oxygenated (95%O2/5%CO2) Krebs (37°C). After an equilibration period (1h), tissue and endothelial viability were assessed using phenylephrine (1µM) and acetylcholine (1µM) respectively. Following a wash-out, tissues were pre-constricted with 1µM (approximate EC70) phenylephrine before construction of cumulative concentration response curves (0.1nM-100µM) to the σ ligands haloperidol, RS67333, SK&F10047, BD1008, DTG ( Monassier & Bousquet, 2002) the nitric oxide donor SNAP, and equivalent DMSO vehicle (0.000001%-1%) in either the absence or presence of σ receptor antagonist BD1047 (30nM, an approximate IC50 at sigma receptor binding sites, Matsumoto et al 1995) . Maximal relaxation (Emax) is expressed as the percentage decrease in tension from the phenylephrine pre-constrictor response. Table 1: Effects of sigma receptor ligands and SNAP on phenylephrine pre-constricted, endothelium-denuded, canine isolated saphenous vein.
Data are mean pEC50/Emax ± s.e.mean. Data were compared using Student’s t-test, with significance set at P<0.05, n = number of dogs. Within the concentration range tested, the σ ligands had no effect on endothelium intact carotid arteries. In contrast, similar to SNAP, full reversal of the phenylephrine pre-constriction was achieved by all σ ligands tested in endothelium-denuded saphenous veins (Table 1), with a rank order of potency of haloperidol > RS67333 > SK&F10047 ≥ BD1008 ≥ DTG. Interestingly, the functional potencies of these ligands were greater than 500-fold weaker than their binding affinities at σ receptors (Monassier & Bousquet, 2002) and these responses were not antagonised by BD1047. It is therefore possible that these effects are non-specific, and consequently one should apply caution when interpreting vasomotor changes associated with σ receptor ligands in vivo.
Monassier, L. and Bousquet, P. (2002) Fundam. Clin. Pharmacol.16, 1-8. |
