1. The subtypes of a-adrenoceptor mediating the contractile responses of the cauda epididymis of the guinea-pig were investigated. The α1-adrenoceptor agonist phenylephrine, but not the α2-adrenoceptor agonist, xylazine (up to 10 μM), elicited concentration-dependent contractions from preparations of cauda epididymis (EC50 3.4 μM). The L-type Ca2+ channel antagonist, nifedipine (10 μM), reduced the maximal response to phenylephrine (by 77%). Preincubation of tissues with the α(1B)-adrenoceptor-alkylating agent, chloroethylclonidine (50 μM, 30 min), shifted phenylephrine concentration-response curves to the right (4 fold) only when the α2-adrenoceptor antagonist idazoxan (100 nM) was included during the pre-incubation with chloroethylclonidine. 2. Xylazine (1 μM) significantly shifted phenylephrine concentration-response curves to the left (3 fold); this effect was attenuated by idazoxan (100 nM). Both the incubation of preparations with nifedipine (10 μM) and the pre-incubation of preparations with chloroethylclonidine (50 μM, 30 min) attenuated the potentiating effects of xylazine (1 μM). Protection of α2-adrenoceptors with idazoxan (100 nM) during the chloroethylclonidine (50 μM, 30 min) incubation restored the xylazine-mediated enhancement of phenylephrine concentration-response curves. Pertussis toxin (200 ng ml-1, 24 h) attenuated the xylazine (1 μM)-mediated potentiation of phenylephrine concentration-response curves. 3. Following the pre-incubation of preparations with chloroethylclonidine (50 μM, 30 min) 5-methylurapidil (10 nM to 3 μM) shifted phenylephrine concentration-response curves, in parallel, to the right with mean pK(B) values in the range of 8.27 (at 10 nM 5-methylurapidil) to 7.76 (at 3 μM 5-methylurapidil), the addition of idazoxan (100 nM) to the incubation medium did not significantly affect the 5-methylurapidil (10 to 300 nM) pK(B) values (8.41 to 7.64, respectively). In the presence of both idazoxan (100 nM) and nifedipine (10 μM), and following the pre-incubation with chloroethylclonidine (50 μM, 30 min), 5-methylurapidil (30 to 300 nM) still shifted phenylephrine concentration-response curves to the right (pK(B) values 7.77 to 7.36, respectively). 4. Phenylephrine (1 μM to 1 mM) increased the accumulation of [3H]-inositol phosphates (10 fold) in preparations of cauda epididymis (EC50 12 μM). This effect was sensitive to chloroethylclonidine pretreatment (50 μM, 30 min), antagonized with low affinity by 5-methylurapidil (- log pK(i) 7.8), but not potentiated by xylazine (1 μM). Xylazine (10 nM-100 μM) reversed the forskolin (10 or 30 μM) stimulated accumulation of [3H]-adenosine 3':5'-cylic monophosphate (cyclic AMP) in preparations of cauda epididymis (by approximately 45%). Incubation of tissues with both pertussis toxin (200 ng ml-1, 24 h) and pertussis toxin vehicle increased the basal activity of adenylate cyclase (3 fold) but did not increase the capacity of forskolin (30 μM) to stimulate the accumulation of [3H]-cyclic AMP in these tissues. Xylazine did not significantly inhibit the forskolin-stimulated accumulation of [3H]-cyclic AMP in either vehicle or pertussis toxin treated tissues. 5. These studies indicate that the epididymis of the guinea-pig contains α1- and α2-adrenoceptors. On the basis of the actions of chloroethylclonidine and 5-methylurapidil the α1-adrenoceptors of this tissue may be of the α(1A)- and α(1B)-subtypes and are linked to both the influx of extracellular Ca2+ and to phospholipase C. The α2-adrenoceptors of this tissue are negatively coupled to adenylate cyclase, sensitive to pertussis toxin, but do not amplify phenylephrine-stimulated [3H]-inositol phosphate accumulation. Stimulation of the α2-adrenoceptors of this tissue may selectively potentiate the influx of extracellular Ca2+.
|Number of pages||8|
|Journal||British Journal of Pharmacology|
|Publication status||Published - 1 Jan 1996|
- Guinea-pig epididymis