Alpha 2-adrenoceptor binding sites vary along the length of the male reproductive tract: a possible basis for the regional variation in response to field stimulation.
Field stimulation (60V, 1 ms, 10 Hz, 10 s) produced monophasic contractions of cauda epididymides that were enhanced by the noradrenaline uptake inhibitor, nisoxetine, but unaffected by the alpha 2-adrenoceptor antagonist, idazoxan. Similar stimulation of vas deferens produced biphasic contractions that in the presence of idazoxan were enhanced and became monophasic. Nisoxetine accentuated the separation of the two phases. Radioligand binding studies yielded dissociation constants (KD) for [3H]prazosin binding that were similar (0.1-0.2 nM) in membrane preparations of both halves of the vas deferens and in the cauda epididymis; maximum binding density (Bmax) was slightly lower in cauda epididymis (20 fmol/mg protein) than in vas deferens (approximately 50 fmol/mg protein). KD values for [3H]rauwolscine were similar in the two halves of the vas deferens but Bmax values were higher in the prostatic half of the tissue (39 fmol/mg protein) than in the epididymal half (22 fmol/mg protein). We were unable to detect specific binding of [3H]rauwolscine in cauda epididymis. Absence or masking of alpha 2-adrenoceptors in this tissue would explain our findings that alpha 2-adrenoceptor agonists and antagonists do not modulate the responses of the cauda epididymis to trains of field stimulation.
Ventura
S
S
Department of Pharmacology, Monash University, Clayton, Victoria, Australia.
Pennefather
J N
JN
D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D001667:Binding, Competitive / Q000187:drug effects; D004146:Dioxanes / Q000494:pharmacology; D004558:Electric Stimulation; D004822:Epididymis / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D019329:Idazoxan; D066298:In Vitro Techniques; D008297:Male; D008566:Membranes / Q000187:drug effects / Q000378:metabolism; D009119:Muscle Contraction / Q000187:drug effects; D011224:Prazosin / Q000493:pharmacokinetics; D011869:Radioligand Assay; D051381:Rats; D018340:Receptors, Adrenergic, alpha-1 / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D018341:Receptors, Adrenergic, alpha-2 / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D014649:Vas Deferens / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D015016:Yohimbine / Q000493:pharmacokinetics
D000317:Adrenergic alpha-Antagonists; D004146:Dioxanes; D018340:Receptors, Adrenergic, alpha-1; D018341:Receptors, Adrenergic, alpha-2; D015016:Yohimbine; D011224:Prazosin; D019329:Idazoxan