ALPHA-ADRENOCEPTOR-MEDIATED RESPONSES OF THE CAUDA EPIDIDYMIS OF THE GUINEA-PIG

1 The subtypes of alpha-adrenoceptor mediating the contractile respons es of the cauda epididymis of the guinea-pig were investigated. The al pha(1)-adrenoceptor agonist phenylephrine, but not the alpha(2)-adreno ceptor agonist, xylazine (up to 10 mu M), elicited concentration-depen dent contractions from preparations of cauda epididymis (EC(50) 3.4 mu M). The L-type Ca2+ channel antagonist, nifedipine (10 mu M), reduced the maximal response to phenylephrine (by 77%). Preincubation of tiss ues with the alpha(1B)-adrenoceptor-alkylating agent, chloroethylcloni dine (50 mu M, 30 min), shifted phenylephrine concentration-response c urves to the right (4 fold) only when the alpha(2)-adrenoceptor antago nist idazoxan (100 nM) was included during the pre-incubation with chl oroethylclonidine. 2 Xylazine (1 mu M) significantly shifted phenyleph rine concentration-response curves to the left (3 fold); this effect w as attenuated by idazoxan (100 nM). Both the incubation of preparation s with nifedipine (10 mu M) and the pre-incubation of preparations wit h chloroethylclonidine (50 mu M, 30 min) attenuated the potentiating e ffects of xylazine (1 mu M). Protection of alpha(2)-adrenoceptors with idazoxan (100 nM) during the chloroethylclonidine (50 mu M, 30 min) i ncubation restored the xylazine-mediated enhancement of phenylephrine concentration-response curves. Pertussis toxin (200 ng ml(-1), 24 h) a ttenuated the xylazine (1 mu M)-mediated potentiation of phenylephrine concentration-response curves. 3 Following the pre-incubation of prep arations with chloroethylclonidine (50 mu M, 30 min) 5-methylurapidil (10 nM to 3 mu M) 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 mu M 5-methylurapidil), the addition of idazoxan (100 nM) to the incubation medium did not signif icantly 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 mu M), and following the pre-incubation with chloroethy lclonidine (50 mu M, 30 min), 5-methylurapidil (30 to 300 nM) still sh ifted phenylephrine concentration-response curves to the right (pK(B) values 7.77 to 7.36, respectively). 4 Phenylephrine (1 mu M to 1 mM) i ncreased the accumulation of [H-3]-inositol phosphates (10 fold) in pr eparations of cauda epididymis (EC(50) 12 mu M) This effect was sensit ive to chloroethylclonidine pretreatment (50 mu M, 30 min), antagonize d with low affinity by 5-methylurapidil (- log pK(i) 7.8), but not pot entiated by xylazine (1 mu M). Xylazine (10 nM-100 mu M) reversed the forskolin (10 or 30 mu M) stimulated accumulation of [H-3]-adenosine 3 ':5'-cylic monophosphate (cyclic AMP) in preparations of cauda epididy mis (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 mu M) to stimulate the accumulation of [H-3] -cyclic AMP in these tissues. Xylazine did not significantly inhibit t he forskolin-stimulated accumulation of [H-3]-cyclic AMP in either veh icle or pertussis toxin treated tissues. 5 These studies indicate that the epididymis of the guinea-pig contains alpha(1)- and alpha(2)-adre noceptors, On the basis of the actions of chloroethylclonidine and 5-m ethylurapidil the alpha(1)-adrenoceptors of this tissue may be of the alpha(1A)- and alpha(1B)-subtypes and are linked to both the influx of extracellular Ca2+ and to phospholipase C. The alpha(2)-adrenoceptors of this tissue are negatively coupled to adenylate cyclase, sensitive to pertussis toxin, but do not amplify phenylephrine-stimulated [H-3] -inositol phosphate accumulation. Stimulation of the alpha(2)-adrenoce ptors of this tissue may selectively potentiate the influx of extracel lular Ca2+.