α‐Adrenoceptor mediated responses of the cauda epididymis of the guinea‐pig

1 The subtypes of α‐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 α1B‐adrenoceptoralkylating 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 pKB 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) pKB 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 (pKB 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 pKi 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 α1A‐ and α1B‐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+.