Effects of DPI (2-(3,4-dihydroxyphenylimino)-imidazolidine) on prejunctional alpha-adrenoceptors and prejunctional dopamine receptors in the rabbit ear artery

Adrenergic-induced hyperglycemia in anaesthetized rats: involvement of peripheral alpha 2-adrenoceptors

The selective alpha 2-adrenoceptor agonist, UK 14.304, induces a time- and dose-dependent hyperglycemic response in the anaesthetized rat. This hyperglycemia seems to be mediated through the pancreas because lesions of the pancreatic beta-cells induced with streptozotocin completely abolished this hyperglycemic response to UK 14.304 while ganglionic blockade by chlorisondamine did not affect the response. The non-selective agonist, adrenaline, induces a similar hyperglycemic response which is antagonized selectivity by the alpha 2-adrenoceptor antagonist, idazoxan. Combined alpha 1- and beta-adrenoceptor blockade resulted in an increase in the alpha 2-adrenoceptor-mediated hyperglycemic response to adrenaline. Since adrenaline is known not to cross the blood-brain barrier it is concluded that alpha 2-adrenoceptor stimulation induces hyperglycemic responses through a peripheral effect that involves postsynaptic alpha 2-adrenoceptors in the pancreatic beta-cell which are linked to the inhibition of insulin release.

Neuronal dopamine receptors of the rabbit ear artery: pharmacological characterization of the receptor

Dopamine and apomorphine were examined in the rabbit isolated perfused ear artery for both direct effects on vascular smooth muscle and effects on the response to field stimulation of sympathetic nerve terminals. The neuroinhibitory effect of both dopamine (EC50 = 37 nM) and apomorphine (EC50 = 44 nM) occurred at concentrations which did not produce vasoconstriction. The neuroinhibitory effect of dopamine was shown to be due to inhibition of noradrenaline release by measurement of 3H-overflow from prelabelled tissues. At relatively high concentrations dopamine produced vasoconstriction. In a superfused segment of ear artery, dopamine was found to be a full agonist at the alpha 1-adrenoreceptor, with an EC50 (15 microM) about 75 fold higher than the EC50 for noradrenaline. At concentrations up to 3 microM, apomorphine had no vasoconstrictor activity in the perfused ear artery. Representative examples of several classes of dopamine antagonists, including the phenothiazines, butyrophenones, diphenylbutylpiperidines and benzamides produced competitive antagonism of dopamine or apomorphine-induced inhibition, with nearly identical Kb values against these two agonists. The pharmacological characteristics of the neuronal dopamine receptor on the rabbit ear artery would indicate this receptor to be typical of the D2 subclass, and this tissue to be a useful model for quantitative studies on dopamine receptor agonists and antagonists.

Interaction between the inhibitory action of acetylcholine and the alpha-adrenoceptor autoinhibitory feedback system on release of [3H]-noradrenaline from rat atria and rabbit ear artery

Stimulation-induced increases in the efflux of radioactivity (S-I efflux) were measured in the bathing medium following labelling of the noradrenergic transmitter pools of rat atria and rabbit artery preparations with [3H]-noradrenaline. In atria stimulated with trains of 16 or 60 pulses at 2 Hz, phentolamine enhanced, whereas acetylcholine inhibited S-I efflux. With trains of 16 pulses phentolamine had a smaller enhancing effect than with trains of 60 pulses, whereas the inhibitory effect of acetylcholine was more pronounced with 16 pulses of stimulation. The inhibitory effect of acetylcholine was markedly enhanced by phentolamine when stimulation was with 60 pulses. With 16 pulses of stimulation the effect of acetylcholine was unaltered by phentolamine and abolished by the alpha 2-adrenoceptor agonist 3,4-dihydroxyphenylimino-2-imidazolidine (DPI). Phentolamine had no effect on the negative inotropic effect of acetylcholine on driven left atrial preparations. In arterial preparations stimulated with trains of 30 pulses at 1 Hz, both acetylcholine and clonidine inhibited S-I efflux, whereas yohimbine and idazoxan enhanced S-I efflux. Combining acetylcholine with clonidine did not alter the inhibitory effect of clonidine but the combination of acetylcholine with yohimbine or idazoxan abolished the marked enhancing effects of yohimbine or idazoxan on S-I efflux. These findings indicate that there may be a reciprocal interaction between prejunctional alpha-adrenoceptors and prejunctional muscarinic cholinoceptors.