Positive correlation of responsiveness to catecholamines of the rat liver glycogenolytic receptor with other alpha-receptor responses

The hepatic adrenergic receptors

The presence of both alpha- and beta-adrenergic receptors in liver designated the hepatic plasma membrane as a useful tool for the elucidation of the mechanisms by which the hormonal signal is transferred through the membrane via a coupling system to an amplifying entity. This process is well documented for the beta-adrenergic receptor which is linked to adenylate cyclase, whereby it modulates the cyclic AMP level. Much less is known about the alpha-adrenergic receptor. Recently, two factors have contributed to a renewed interest in alpha- and beta-adrenergic receptors in liver: i) The fact that activation of glycogenolysis in isolated liver parenchymal cells by epinephrine may be mediated by either alpha- or beta-adrenergic receptors, depending on the species or on the state of nutrition, and not only by beta-adrenergic receptors as previously thought. ii) The existence of specific adrenergic agonists and antagonists radiolabeled to a high specific activity which has permitted the characterization of adrenergic receptors in terms of nature, number, affinity and regulation. The present review will be devoted to the recent progress made in the physiological, pharmacological and biochemical characterization of alpha- and beta-adrenergic receptors in the liver.

Rat gastric mucosal cAMP and cGMP after adrenergic stimulation and blockade

The effect of adrenergic stimulation and blockade on the concentration of cAMP and cGMP was studied in the rat gastric mucosa. Adrenaline (0.5-2.0 mg/kg) elevated the gastric mucosal cAMP and cGMP levels up to 100% in a dose-dependent manner. Blockade of the beta-adrenoceptors with 4 mg/kg propranolol suppressed the adrenaline effect on the cAMP level and increased the effect on the cGMP concentration up to 500%. The opposite effect was found under alpha-adrenoceptor blockade. Phenoxybenzamine, 4 mg/kg, prevented the adrenaline effect on the cGMP level and increased the effect on the cAMP concentration up to 300%. Predominant stimulation of the beta-adrenoceptors by isoproterenol or of the alpha-adrenoceptors by phenylephrine caused smaller changes in the cyclic nucleotide concentration than did adrenaline. Their effect was more pronounced when the non-stimulated receptor was blocked. The data indicate that the rat gastric mucosa contains alpha- and beta-adrenoceptors. The effect of adrenaline on the gastric mucosal cAMP level is mediated by beta-adrenoceptors and that on the cGMP level by alpha-adrenoceptors.

Metabolic actions of some sympathomimetic amines and their acetyl derivatives in the rabbit

To study how acetylation affects the activity of sympathomimetic amines the effects of tyramine, amphetamine, ephedrine, phenylephrine, orciprenaline and salbutamol and of their O- and N-acetyl derivatives on blood glucose and free fatty acid concentrations were studied in the rabbit. Hyperglycemia was induced by all parent compounds except amphetamine which tended to have a weak hypoglycaemic action. Hyperlipaemia in the doses used was induced by ephedrine and orciprenaline but not by the other parent compounds. Usually acetylation decreased the metabolic effects of the compounds but O-acetylation of tyramine and salbutamol caused hyperlipaemia and O-acetylation of ephedrine increased its fatty acid-mobilizing action, perhaps as a consequence of increased lipid solubility of the compounds. The ultimate effects of the O-acetyl derivatives were probably at least partly due to deacetylation at their sites of action. However O-acetylation of sympathomimetics could perhaps be used to induce drug latentiation.

Cyclic nucleotides and gluconeogenesis by rat liver cells

Gluconeogenesis from lactate, pyruvate, fructose, alanine, and other substrates was accelerated by glucagon or epinephrine in hepatocytes isolated from rat liver. Glucagon and epinephrine also increased cyclic AMP accumulation by rat hepatocytes. Isoproterenol increased cyclic AMP but not gluconeogenesis, while phenylephrine accelerated gluconeogenesis. The activation of gluconeogenesis by epinephrine was unaffected by propranolol but blocked by dihydroergotamine. Dibutyryl cyclic AMP added to hepatocytes stimulated gluconeogenesis at concentrations as low as 1 muM. Exogenous cyclic GMP (0.1- muM) inhibited gluconeogenesis due to either glucagon or epinephrine without affecting basal gluconeogenesis. However, carbamylcholine did not affect gluconeogenesis by hepatocytes. Basal gluconeogenesis and the increases due to all agents were inhibited by removal of extracellular calcium or the presence of A-23187, D-600, or tetracaine. In contrast, added 0.1 muM cyclic GMP, 2 mM NH-4-Cl, and 10 muM phenethylbiguanide inhibited glucagon- or epinephrine-stimulated gluconeogenesis without affecting basal values. Studies with hepatocytes indicate that the hormonal activation of gluconeogenesis is not limited to substrates entering prior to triose phosphate formation. Glucagon may act by increasing cyclic AMP which acts via unknown mechanisms to increase gluconeogenesis. In contrast, epinephrine acts via a cyclic AMP-independent mechamism which does not appear to involve cyclic GMP, Ca-2+ flux, of K+ flux.