Receptor interactions of a series of imidazolines: comparison of the alpha 2-adrenoceptors between the rabbit vas deferens and guinea pig ileum

Inhibition by clonidine of the carbachol-induced tension development and nonselective cationic current in guinea pig ileal myocytes

Effects of clonidine, an imidazoline derivative as well as alpha2-adrenoceptor agonist, on carbachol (CCh)-evoked contraction in guinea pig ileal smooth muscle were studied using isometric tension recording. To investigate the cellular mechanisms of the inhibitory effect of clonidine, its effects on CCh-evoked nonselective cationic current (I(CCh)), voltage-dependent Ca2+ current (I(Ca)) and voltage-dependent K+ current (I(K)) was also studied using patch-clamp recording techniques in single ileal cells. Clonidine inhibited the contraction evoked by CCh (1 microM) in a concentration-dependent manner with an IC50 valve of 61.7 +/- 2.5 microM. High K+ (40 mM)-evoked contraction was only slightly inhibited even when clonidine was used at 300 microM. Externally applied clonidine inhibited I(CCh) dose-dependently with an IC50 of 42.0 +/- 2.6 microM. When applied internally via patch pipettes, clonidine was without effect. An I(CCh)-like current induced by GTPgammaS was also inhibited by bath application of clonidine. None of KU14R and BU224, both imidazoline receptor blockers, and yohimbine, an alpha2-adrenergic blocker, significantly affects the inhibitory effect of clonidine on I(CCh). Clonidine (300 microM) only slightly decreased membrane currents flowing through voltage-gated Ca2+ channels or K+ channels. These data indicate that clonidine relaxes smooth muscle contraction produced by muscarinic receptor activation and suggest that the effect of clonidine seems due mainly to inhibition of I(CCh) via acting directly on the involved cationic channel.

Presynaptic alpha 2-autoreceptors in brain cortex: alpha 2D in the rat and alpha 2A in the rabbit

Presynaptic alpha 2-autoreceptors in rat and rabbit brain cortex were compared by means of antagonists and agonists. Brain cortex slices were preincubated with [3H]-noradrenaline and then superfused and stimulated by 3 (rat) or 4 (rabbit) pulses at a frequency of 100 Hz. The alpha 2-adrenoceptor agonist bromoxidine (UK 14304) reduced the electrically evoked overflow of tritium with EC50 values of 4.5 nmol/l in the rat and 0.7 nmol/l in the rabbit. The antagonists phentolamine, 2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidazo le (BRL 44408), rauwolscine, 1,2-dimethyl-2,3,9,13b-tetrahydro-1H-dibenzo(c,f)imidazo(1,5-a)aze pine (BRL 41992), 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane (WB 4101), 6-chloro-9-[(3-methyl-2-butenyl)oxy]-3-methyl-1H-2,3,4, 5-tetrahydro-3-benzazepine (SKF 104078), imiloxan, prazosin and corynanthine did not per se increase the evoked overflow of tritium but shifted the concentration-inhibition curve of bromoxidine to the right in a manner compatible with competitive antagonism. Up to 4 concentrations of each antagonist were used to determine its dissociation constant KD. The KD values correlated only weakly between the rat and the rabbit. Dissociation constants KA of bromoxidine were calculated from equieffective concentrations in unpretreated brain slices and slices in which part of the alpha 2-adrenoceptors had been irreversibly blocked by phenoxybenzamine. The KA value was 123 nmol/l in the rat and 7.2 nmol/l in the rabbit. The results confirm the species difference between rat and rabbit brain presynaptic alpha 2-autoreceptors. Comparison with data from the literature indicates that the rat brain autoreceptors can be equated with the alpha 2D subtype as defined by radioligand binding, whereas the rabbit brain autoreceptors conform to the alpha 2A subtype. For example, the antagonist affinities for the rat autoreceptors correlate with their binding affinities for the gene product of alpha 2-RG20, the putative rat alpha 2D-adrenoceptor gene (r = 0.97; P < 0.01), but not with their binding affinities for the gene product of alpha 2-C10, the putative human alpha 2A-adrenoceptor gene. Conversely, the rabbit autoreceptors correlate with the alpha 2-C10 (r = 0.98; P < 0.001) but not with the alpha 2-RG20 gene product. Since presynaptic alpha 2-autoreceptors are also alpha 2D in rat submaxillary gland and perhaps vas deferens and alpha 2A in rabbit pulmonary artery, the possibility arises that the majority of alpha 2-autoreceptors generally are alpha 2D in the rat and alpha 2A in the rabbit. Moreover, receptors of the alpha 2A/D group generally may be the main mammalian alpha 2-autoreceptors.