Dopamine contracts the rat isolated seminal vesicle by activation of postjunctional alpha-1-adrenoceptors.
The contractile effect of dopamine in the rat isolated seminal vesicle was investigated to determine the nature of the receptors involved. Noradrenaline (NA; 10(-8) to 10(-4) mol/l), phenylephrine (PE; 10(-7) to 10(-4) mol/l) and dopamine (DA; 10(-5) to 10(-2) mol/l) produced concentration-dependent contractions with no sign of tachyphylaxis. The relative potencies of NA:PE:DA derived from their EC50 values were 1:0.23:0.01. However, while NA and PE produced similar maximal contractions, the Emax obtained with DA was significantly less (intrinsic activity = 0.066). Depletion of tissue catecholamine stores by pretreating rats with reserpine did not significantly influence the responses of the seminal vesicle to DA or PE. Moreover, cocaine (10(-5) mol/l) did not significantly affect the course of the concentration-response curve to DA. Prazosin (10(-9) to 10(-8) mol/l), phentolamine (10(-7) to 10(-6) mol/l), haloperidol (10(-7) to 10(-6) mol/l) and yohimbine (10(-6) to 10(-5) mol/l) all produced a rightward displacement of the concentration-response curves for the agonists. The Arunlakshana and Schild plots of the data were linear and had slopes not significantly different from unity. The potency estimates obtained for each antagonist were always similar irrespective of the agonist used. The order of potency of the antagonists was: prazosin > phentolamine > haloperidol > yohimbine. In addition, phenoxybenzamine (10(-8) mol/l) produced a rightward shift of the concentration-response curves for both NA and DA.(ABSTRACT TRUNCATED AT 250 WORDS)
Sharif
S I
SI
Department of Pharmacology, Faculty of Medicine, Al-Arab Medical University, Benghazi, Libya.
D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D004298:Dopamine / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D009130:Muscle, Smooth / Q000502:physiology; D009469:Neuromuscular Junction / Q000378:metabolism*; D009638:Norepinephrine / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D018340:Receptors, Adrenergic, alpha-1 / Q000378:metabolism*; D012669:Seminal Vesicles / Q000187:drug effects* / Q000378:metabolism / Q000502:physiology
D000317:Adrenergic alpha-Antagonists; D018340:Receptors, Adrenergic, alpha-1; D010656:Phenylephrine; D004298:Dopamine; D009638:Norepinephrine