Prazosin potency at functional α 1 -adrenoceptors in rat vas deferens is altered by noradrenaline transporter block

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

It has been variously suggested that contractions of rat vas deferens to exogenous noradrenaline or adrenaline are mediated predominantly by α1A-adrenoceptors, α1D- in addition to α1A-adrenoceptors (see 1), or α1L-adrenoceptors (2). A wide range of prazosin pA2/pKB values have been reported in rat vas deferens: varying between 8.32 and 9.35 (see 1). We have found that re-uptake blockade with cocaine reveals two subtypes of α1-adrenoceptor in rat vas deferens: α1A- and α1D-adrenoceptors (1). Studies differ in choice of noradrenaline transporter blocker. We have now systematically investigated the potency of prazosin at antagonizing contractions to noradrenaline in epididymal portions of rat vas deferens, in the absence or presence of cocaine or desipramine as noradrenaline transporter blockers.

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 agonist studies, following a 60 min exposure to antagonist or vehicle, in the absence or presence of cocaine or desipramine; a single noradrenaline concentration response-curve was obtained per tissue (3).

In the presence of cocaine (3 µM), noradrenaline potency was significantly increased (pD2: 6.15±0.09, n=15) as compared to in the absence of cocaine (5.36±0.15, n=10; analysis of variance and Dunnett test, P<0.05). Cocaine (10 µM) further increased noradrenaline potency (6.48±0.16, n=14). Desipramine (0.1-0.3 µM) also significantly increased noradrenaline potency (e.g. desipramine 0.3 µM: 5.96±0.12, n=7). However, in the presence of cocaine (10 µM) to block the noradrenaline transporter, desipramine (0.1-10 µM) produced concentration-dependent inhibition of the response to noradrenaline (e.g. in the presence of cocaine 10 µM and desipramine 0.1 µM, noradrenaline pD2: 5.82±0.09, n=6).

Prazosin pKB values were obtained from the effects of prazosin (10 nM), in terms of significant shifts in NA potency at the EC50 level. Prazosin pKB values varied from 8.32 in the absence of cocaine (3 µM) to 9.05 in the presence of cocaine (analysis of variance and Dunnett test, P<0.05), with the lower value indicative of α1A-adrenoceptor potency, and the higher value indicative of α1D-adrenoceptor potency. However, in the presence of desipramine prazosin potency did not increase: in the presence of desipramine (0.1µM), the prazosin pKB value was 8.50.

Prazosin potency was greatly affected by the absence or presence of blockade of the noradrenaline transporter by cocaine. However, desipramine acts as an α1-adrenoceptor antagonist over the concentration range that it blocks re-uptake, and so does not behave like cocaine and does not increase the potency of prazosin. Differences in reported prazosin potency in rat vas deferens are due to experimental conditions: rat vas deferens contains α1A- and α1D-adrenoceptors, but demonstration of the α1D-adrenoceptor mediated response, at which prazosin has high potency, is dependent on block of the noradrenaline transporter. α1-Adrenoceptor blockade by desipramine also affects prazosin potency.

(1) Docherty JR (2013). J Auton Autacoid Pharmacol 33, 49-57.

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

(3) Honner V & Docherty JR (1999). Br J Pharmacol 128: 1323-1331.