Investigation of the subtypes of alpha1-adrenoceptor mediating contractions of rat vas deferens

Abstract

1 The subtypes of alpha1-adrenoceptor mediating contractions of rat vas deferens to endogenous and exogenous noradrenaline and to the exogenous agonists methoxamine, phenylephrine and A61603 have been examined. 2 The effects of antagonists on the shape of concentration-response curves, both tonic and phasic, to the four agonists were analysed. Prazosin produced parallel shifts in all cases. Particularly for RS 17053 against noradrenaline, there was some evidence for a resistant component of the agonist response. High concentrations of RS 17053 (1-10 microM) virtually abolished tonic contractions but phasic contractions were resistant. 3 A series of nine antagonists (the above and WB4101, benoxathian, phentolamine, BMY 7378, HV 723, spiperone) were investigated against contractions to noradrenaline. The correlation with the potency of the series of alpha1-adrenoceptor antagonists against contractions to noradrenaline was significant only for the alpha1A-adrenoceptor ligand binding site (r=0.88, n=9, P<0.01). 4 In epididymal portions (nifedipine 10 microM), the isometric contraction to a single electrical pulse is alpha1-adrenoceptor mediated. The correlation with ligand binding sites for 11 antagonists (the above plus ARC 239 and (+)-niguldipine) was significant only for the alpha1D-adrenoceptor subtype (r=0.65, n=11, P<0.05). 5 In conclusion, tonic contractions of rat vas deferens produced by exogenous agonists are mediated predominantly by alpha1A-adrenoceptors, although a second subtype of receptor may additionally be involved in phasic contractions. Nerve-stimulation evoked alpha1-adrenoceptor mediated contractions seem to predominantly involve non-alpha1A-adrenoceptors, and the receptor involved resembles the alpha1D-receptor.

Figure 1

Tracing of a typical experiment illustrating the tonic and phasic nature of contractions to agonists in rat vas deferens. This experiment shows the effects of increasing concentrations of noradrenaline (0.1–30 μM). Tonic contractions were taken as the peak sustained contraction, and phasic contractions as the maximum intermittent tension (including sustained) obtained. Time scale: 20  min of recording shown.

Figure 2

Tonic (a) and phasic (b) contractions obtained to noradrenaline in whole vas deferens, following exposure to vehicle (veh), prazosin (10 nM) (praz-8), RS 17053 (100 nM) (RS-7) or 5-methyl-urapidil (30 nM) (5-MU-7.5). Values are expressed as percentage of the maximum for each experiment. Vertical bars represent s.e.mean from at least three experiment. For clarity, vehicle experiments are combined but, since pK_B values were obtained in experiments with paired vehicles, the shift in agonist potency produced by antagonists is not readily obtained from these curves.

Figure 3

Phasic contractions obtained to methoxamine (a), phenylephrine (b) and A 6163 (c) in whole vas deferens, following exposure to vehicle (veh), prazosin (10 or 30 nM) (praz-8 or −7.5), RS 17053 (100 nM) (RS-7) or 5-methyl-urapidil (30 nM) (5-MU-7.5). Values are expressed as percentage of the maximum for each experiment. Vertical bars represent s.e.mean from at least three experiments. For clarity, vehicle experiments are combined but, since pK_B values were obtained in experiments with paired vehicles, the shift in agonist potency produced by antagonists is not readily obtained from these curves.

Figure 4

Tonic (a) and phasic (b) contractions obtained to noradrenaline in whole vas deferens expressed as absolute tension (g), following exposure to vehicle (veh), RS 17053 (100 nM) (RS-7), RS 17053 (1 μM) (RS-6), DMSO or RS 17053 (10 μM) (RS-5). Vertical bars represent s.e.mean from at least four experiments. For clarity, vehicle experiments for the two lower concentrations of RS 17053 are combined but, since DMSO, the vehicle for the highest concentration of RS 17053 (10 μM), also produced significant effects, this is included separately.

Figure 5

Correlation between published antagonist affinity for α_1A-, α_1B- and α_1D-ligand binding sites (data from Perez et al., 1991; Schwinn & Lomasney, 1992; Faure et al., 1994; Forray et al., 1994; Goetz et al., 1995; Knepper et al., 1995; Marshall et al., 1996; Ford et al., 1996; Kenny et al., 1996) and antagonist postjunctional potency (pK_B) against contractions to noradrenaline in whole vas deferens. Numbers indicate compounds as listed in Tables 2 and 3. Vertical and horizontal bars indicate s.e.mean. There was a significant correlation between antagonist postjunctional potency and affinity for α_1A-adrenoceptor ligand binding site.

Figure 6

Effects of antagonists on the contractile response of epididymal portions of rat vas deferens in the presence of nifedipine (10 μM) to eliminate non-adrenergic contractions: prazosin (praz), benoxathian (ben), WB 4101 (WB), 5-methyl-urapidil (5-mu), HV 723 (HV), phentolamine (phen), BMY 7378 (BM). ARC 239 (ARC), RS 17053 (RS), (+)-niguldipine (niguld), spiperone (spip). Vertical bars represent s.e.mean from at least three experiments.

Figure 7

Correlation between antagonist postjunctional potency (pIC₅₀) against stimulation-evoked contractions to a single stimulus in epididymal portions of rat vas deferens in the presence of nifedipine (10 μM) with and without cocaine. There was a very significant correlation between potencies with and without cocaine.

Figure 8

Correlation between published antagonist affinity for α_1A-, α_1B- and α_1D-ligand binding sites (data from Perez et al., 1991; Schwinn & Lomasney, 1992; Faure et al., 1994; Forray et al., 1994; Goetz et al., 1995; Knepper et al., 1995; Marshall et al., 1996; Ford et al., 1996; Kenny et al., 1996) and antagonist postjunctional potency (pIC₅₀) against stimulation-evoked contractions to a single stimulus in epididymal portions of rat vas deferens in the presence of nifedipine (10 μM). Numbers indicate compounds as listed in Tables 2 and 3. Vertical and horizontal bars indicate s.e.mean. There was a significant correlation between antagonist potency and affinity for the α_1D-adrenoceptor ligand binding site