Differences between presynaptic and postsynaptic alpha-adrenoceptors in the isolated nictitating membrane of the cat: effects of metanephrine and tolazoline.
The alpha-adrenoceptor blocking agent, tolazoline, and the O-methylated metabolite of adrenaline, metanephrine, produced a concentration-dependent increase of tension in the smooth muscle of the cat isolated nictitating membrane. These effects were considered to be due to the activation of postsynaptic alpha-adrenoceptors. 2 The responses to nerve stimulation of this muscle were neither potentiated nor blocked by tolazoline (0.1 to 10 micrometer) or metanephrine (1 to 10 micrometer). 3 3H-transmitter overflow evoked by electrical stimulation was not modified by tolazoline or metanephrine in concentrations in which these drugs stimulated the postsynaptic alpha-adrenoceptors. 4 Since tolazoline and metanephrine failed to activate the presynaptic alpha-adrenoceptors of the cat nictitating membrane under experimental conditions in which they stimulated the postsynaptic alpha-adrenoceptors, these results further support the view that the presynaptic (alpha2) adrenoceptors differ from the postsynaptic (alpha1) adrenoceptors.
D000818:Animals; D002415:Cats; D004558:Electric Stimulation; D004837:Epinephrine / Q000031:analogs & derivatives*; D005260:Female; D066298:In Vitro Techniques; D008297:Male; D008676:Metanephrine / Q000494:pharmacology*; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects; D018377:Neurotransmitter Agents / Q000378:metabolism; D009541:Nictitating Membrane / Q000187:drug effects; D011941:Receptors, Adrenergic / Q000187:drug effects*; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects*; D014043:Tolazoline / Q000494:pharmacology*
D018377:Neurotransmitter Agents; D011941:Receptors, Adrenergic; D011942:Receptors, Adrenergic, alpha; D008676:Metanephrine; D014043:Tolazoline; D004837:Epinephrine