Prazosin has low potency at functional α 1A -adrenoceptors in rat vas deferens: implications for the α 1L -adrenoceptor

James R Docherty. Royal College of Surgeons in Ireland, Dublin, Ireland

Introduction: α 1-Adrenoceptors have been subdivided into 3 subtypes, α 1A-, α 1B- and α 1D-adrenoceptors based on functional and gene studies (see Docherty, 2010). However, a further functional subtype of α 1-adrenoceptor, the α 1L-adrenoceptor, has been proposed at which prazosin has low potency (see Ohmura et al., 1992). The current view is that the α 1L-adrenoceptor is a functional isoform of the α 1A-adrenoceptor, since no gene has been discovered for this subtype (Ford et al., 1997). The initial classification of α 1L-adrenoceptors was based on the low potency of prazosin in tissues including rat vas deferens (Ohmura et al., 1992), a preparation in which it has also been reported that isometric contractions largely involve α 1A-adrenoceptors, with a smaller proportion of α 1D-adrenoceptors (Cleary et al., 2004). We have investigated α 1-adrenoceptors subtypes mediating contractions to noradrenaline in epididymal portions of rat vas deferens employing prazosin, in the absence or presence of cocaine to investigate whether α 1L-adrenoceptors are additionally present.

Methods: Male Wistar rats (250-350g) were killed by overdose of pentobarbitone (60mg/kg, i.p.) and cervical dislocation, and epididymal portions of vas deferens were obtained. In all studies, following a 60 min exposure to antagonist or vehicle, in the absence or presence of cocaine (3 µM), a single noradrenaline concentration response-curve was obtained per tissue.

Results: In the presence of cocaine (3 µM), noradrenaline potency and maximum contraction (1.45±0.07g, n=34) were significantly increased as compared to in the absence of cocaine (1.04±0.08g, n=21) (analysis of variance and Dunnett test, P<0.05). In particular, cocaine amplified contractions to low concentrations of noradrenaline and responses had a greater phasic component. In the absence of cocaine, prazosin exhibited relatively low potency (significant reduction by 10-8M) as an antagonist against the contractile response to low concentrations of noradrenaline (analysis of variance and Dunnett test, P<0.05). In the presence of cocaine, prazosin exhibited higher potency (marked inhibition by 3x10-9M) against the contraction to low concentrations of noradrenaline. Like prazosin, the α 1D-adrenoceptor selective antagonist BMY7378 was more potent against contractions to noradrenaline in the presence of cocaine, whereas the α 1A-adrenoceptor selective antagonist RS100329 was more potent against contractions to low concentrations of noradrenaline in the absence of cocaine. Prazosin pKB values obtained for 3 antagonist concentrations (n=6-9, each) causing significant shifts in agonist potency at the EC50 level (analysis of variance and Dunnett test, P<0.05) varied from 8.23±0.03 in the absence of cocaine to 8.94±-0.06 in the presence of cocaine, with the lower values indicative of α 1A-adrenoceptor potency, and the higher values indicative of α 1D-adrenoceptor potency.

Conclusion: Prazosin has previously unreported selectivity for α 1D- over α 1A-adrenoceptors in functional studies of rat vas deferens and this may largely explain the phenomenon of α 1L-adrenoceptors. Hence, it is suggested that the α 1L-adrenoceptor reported in rat vas deferens is not a functional isoform of the α 1A-adrenoceptor, it is the native α 1A-adrenoceptor.

Cleary L, et al., (2004). Br J Pharmacol. 143, 745-52.

Docherty JR. (2010). Cell Mol Life Sci. 67, 405-417.

Ford, A.P. et al., (1997). Br. J. Pharmacol. 121, 1127-1135.

Ohmura, T. et al., (1992). Br. J. Pharmacol.107, 697-704.