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 α 1 ‐adrenoceptor agonist phenylephrine, but not the α 2 ‐adrenoceptor agonist, xylazine (up to 10 μ m ), elicited concentration‐dependent contractions from preparations of cauda epididymis (EC 50 3.4 μ m ). The L‐type Ca 2+ channel antagonist, nifedipine (10 μ m ), reduced the maximal response to phenylephrine (by 77%). Preincubation of tissues with the α 1B ‐adrenoceptoralkylating agent, chloroethylclonidine (50 μ m , 30 min), shifted phenylephrine concentration‐response curves to the right (4 fold) only when the α 2 ‐adrenoceptor antagonist idazoxan (100 nM) was included during the pre‐incubation with chloroethylclonidine. 2 Xylazine (1 μ m ) 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 μ m ) and the pre‐incubation of preparations with chloroethylclonidine (50 μ m , 30 min) attenuated the potentiating effects of xylazine (1 μ m ). Protection of α 2 ‐adrenoceptors with idazoxan (100 nM) during the chloroethylclonidine (50 μ m , 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 μ m )‐mediated potentiation of phenylephrine concentration‐response curves. 3 Following the pre‐incubation of preparations with chloroethylclonidine (50 μ m , 30 min) 5‐methylurapidil (10 nM to 3 μ m ) shifted phenylephrine concentration‐response curves, in parallel, to the right with mean p K B values in the range of 8.27 (at 10 nM 5‐methylurapidil) to 7.76 (at 3 μ m 5‐methylurapidil), the addition of idazoxan (100 nM) to the incubation medium did not significantly affect the 5‐methylurapidil (10 to 300 nM) p K B values (8.41 to 7.64, respectively). In the presence of both idazoxan (100 nM) and nifedipine (10 μ m ), and following the pre‐incubation with chloroethylclonidine (50 μ m , 30 min), 5‐methylurapidil (30 to 300 nM) still shifted phenylephrine concentration‐response curves to the right (p K B values 7.77 to 7.36, respectively). 4 Phenylephrine (1 μ m to 1 mM) increased the accumulation of [ 3 H]‐inositol phosphates (10 fold) in preparations of cauda epididymis (EC 50 12 μ m ). This effect was sensitive to chloroethylclonidine pretreatment (50 μ m , 30 min), antagonized with low affinity by 5‐methylurapidil (— log p K i 7.8), but not potentiated by xylazine (1 μ m ). Xylazine (10 nM‐100 μ m ) reversed the forskolin (10 or 30 μ m ) stimulated accumulation of [ 3 H]‐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 μ m ) to stimulate the accumulation of [ 3 H]‐cyclic AMP in these tissues. Xylazine did not significantly inhibit the forskolin‐stimulated accumulation of [ 3 H]‐cyclic AMP in either vehicle or pertussis toxin treated tissues. 5 These studies indicate that the epididymis of the guinea‐pig contains α 1 ‐ and α 2 ‐adrenoceptors. On the basis of the actions of chloroethylclonidine and 5‐methylurapidil the α 1 ‐adrenoceptors of this tissue may be of the α 1A ‐ and α 1B ‐subtypes and are linked to both the influx of extracellular Ca 2+ and to phospholipase C. The α 2 ‐adrenoceptors of this tissue are negatively coupled to adenylate cyclase, sensitive to pertussis toxin, but do not amplify phenylephrine‐stimulated [ 3 H]‐inositol phosphate accumulation. Stimulation of the α 2 ‐adrenoceptors of this tissue may selectively potentiate the influx of extracellular Ca 2+ .