Characterization of glucagon receptors in Golgi fractions of rat liver: evidence for receptors that are uncoupled from adenylyl cyclase

Glucagon and glucagon-like peptides in fishes

Glucagon and glucagon-like peptides (GLPs) are coencoded in the vertebrate proglucagon gene. Large differences exist between fishes and other vertebrates in gene structure, peptide expression, peptide chemistry, and function of the hormones produced. Here we review selected aspects of glucagon and glucagon-like peptides in vertebrates with special focus on the contributions made by analysis of piscine systems. Our topics range from the history of discovery to gene structure and expression, through primary structures and regulation of plasma concentrations to physiological effects and message transduction. In fishes, the pancreas synthesizes glucagon and GLP-1, while the intestine may contribute oxyntomodulin, glucagon, GLP-1, and GLP-2. The pancreatic gene is short and lacks the sequence for GLP-2. GLP-1, which is produced exclusively in its biologically active form, is a potent metabolic hormone involved in regulation of liver glycogenolysis and gluconeogenesis. The responsiveness of isolated hepatocytes to glucagon is limited to high concentrations, while physiological concentrations of GLP-1 effectively regulate hepatic metabolism. Plasma concentrations of GLP-1 are higher than those of glucagon, and liver is identified as the major site of removal of both hormones from fish plasma. Ultimately, GLP-1 and glucagon exert effects on glucose metabolism that directly and indirectly oppose several key actions of insulin. Both glucagon and GLP-1 show very weak insulinotropic activity, if any, when tested on fish pancreas. Intracellular message transduction for glucagon, especially at slightly supraphysiological concentrations, involves cAMP and protein kinase A, while pathways for GLP are largely unknown and may involve a multitude of messengers, including cAMP. In spite of fundamental differences in GLP-1 function between fishes and mammals, fish GLP-1 is as powerful an insulinotropin for mammalian B-cells as mammalian GLP-1 is a metabolic hormone if tested on piscine liver.

Multiple hormone actions: the rises in cAMP and Ca++ in MDCK-cells treated with glucagon and prostaglandin E1 are independent processes

Glucagon and prostaglandin E1 stimulate adenylate cyclase in Madin-Darby canine kidney cells with an approximate EC50 of 3*10(-8) and 1*10(-7) M respectively. The rise in cAMP is accompanied by a transient rise in intracellular Ca++ measured with the fluorescent calcium indicator Indo-1. A comparable increase in intracellular Ca2+ without a rise in cAMP occurs with the cholinergic agonist carbamylcholine. Stimulation of adenylate cyclase by the beta-adrenergic agonist isoproterenol or directly by forskolin has no effect on intracellular Ca++. By all criteria studied the rise in intracellular Ca++ and the increase in cAMP are independent from each other.

Metabolic actions of glucagon-family hormones in liver

This review addresses direct and indirect metabolic actions of hormones co-encoded in the preproglucagon gene of fishes. Emphasis is placed on a critical analysis of the effects of glucagon and glucagon-like peptide (GLP) and the current knowledge of the respective modes of action is reviewed. In mammals GLPs exert no direct metabolic actions. In fish liver, GLP and glucagon act on similar targets of intermediary metabolism by enhancing flux through glycogenolysis, lipolysis and gluconeogenesis. Increases in substrate oxidation are not uniform. Hormonal activation of glycogen phosphorylase and triglyceride lipase and inhibition of pyruvate kinase are implicated in these actions. Hormone-dependent hyperglycemia, depletion of hepatic glycogen and increases in free fatty acids are noticeablein vivo. Glucagon also activates hepatic amino acid uptake and ammonia excretion.Glucagon actions are accompanied by large increases in hepatic cAMP and increased phosphorylation of pyruvate kinase. Metabolic effects measured after GLP administration are associated with minor, if any, increases in cAMP and effects on pyruvate kinase are variable. We hypothesize that different hepatic receptors with differing modes of intracellular message transduction are involved in glucagon and GLP actions while targetting identical metabolic routes. Responses of different species of fish cover a wide spectrum, and variation of response with the circannual cycle of experimental animals makes comparisons of results, even within one species, difficult.

Fluorescent glucagon derivatives. II. The use of fluorescent glucagon derivatives for the study of receptor disposition in membranes

When monolayer cultured hepatocytes were incubated with 1 nM [125I]glucagon at 30 degrees C, equilibrium was reached after 10 min, whereas at 4 degrees C, equilibrium was reached after 60 min. At the higher temperature, 11.2% of the bound ligand was broken down after 60 min, at the lower temperature, the amount of degradation was negligible. At 30 degrees C, acid-washing did not remove specifically bound ligand; thus, it was assumed that the ligand was internalised at this temperature, since some of the specifically bound ligand could be washed off at lower temperatures. This was confirmed in experiments when monolayer cultures of hepatocytes were incubated with fluorescein-labelled derivatives of glucagon. The distribution of specific binding on the cell surface was studied at both 30 and 4 degrees C using video intensification microscopic techniques. In keeping with studies using radiolabelled glucagon, more fluorescence was detected following incubation at 4 degrees C than at 30 degrees C and it could be removed by washing the cells. Video intensification microscopy indicated that at the lower temperature, the bound ligand was distributed all over the cell surface. At the higher temperature, ligand-derived fluorescence could only be detected in mobile intracellular vesicles.

Guanine nucleotide regulation of the interconversion of the two-state hepatic glucagon receptor system of rat

To investigate whether guanine nucleotides regulate interconversion of the two-state hepatic glucagon receptor we have utilized kinetic assays of glucagon binding to partially purified rat liver plasma membranes. Dissociation of glucagon at 30 degrees C exhibited biexponential character in either the absence or presence of GTP, indicating that the system previously seen in intact hepatocytes is independent of intracellular modulators. In each case the receptors underwent a time-dependent conversion from a low affinity to a high affinity state. However, GTP decreased the fraction of receptors in the high affinity state. The rank order for stabilizing the low affinity state was Gpp(NH)p greater than GTP greater than GDP much greater than GMP = no nucleotides. Data from competition binding assays with increasing concentrations of GTP allow calculation of equilibrium constants which are 3.32 nM for glucagon and receptor in the absence of GTP, 18.6 nM for glucagon and receptor in the presence of GTP, 1.55 microM for the association of receptor and GTP presumably linked to an N protein, and 8.86 microM for the association of the glucagon-receptor complex and GTP again presumably linked to an N protein, Glucagon binding to receptor is noncooperative in both the absence and presence of GTP, distinguishing this system from the beta-adrenergic system. With GTP, binding to the low affinity state is favored because of the relative affinities reported. Therefore, GTP regulates the activation by slowing the conversion of the receptor from a low affinity to high affinity form.

Characterization of glucagon receptors in Golgi fractions of fetal rat liver

The present study was designed to determine if Golgi fractions from fetal rat liver contain glucagon receptors and to characterize the properties of such receptors. Purification patterns of liver plasma membranes and Golgi fractions from fetal and adult rats were similar, as verified by morphological and biochemical approaches. Glucagon binding was greater in plasma membranes of adult than fetal rats, while in Golgi fractions glucagon binding was similar in both groups. The modifications in in glucagon binding reflect changes in glucagon receptors. Glucagon association and glucagon receptor inactivation by liver membranes were similar in the two groups of animals, while glucagon degradation was lower in fetal than in adult rats.

Glucagon-like peptides activate hepatic gluconeogenesis

Piscine (anglerfish, catfish, coho salmon) glucagon-like peptides (GLPs), applied at 3.5 nM, stimulate (1.1-1.9-fold) flux through gluconeogenesis above control levels in isolated trout and salmon hepatocytes. Human GLP-1 and GLP-2 also activate gluconeogenesis, but to a lesser degree than their piscine counterparts. Minor increases of substrate oxidation are noticed at times of peak gluconeogenic activation through GLPs. These hormones, which are derived from the same precursor peptide as glucagon are more potent activators of gluconeogenesis than glucagon when applied at equimolar concentrations, and do not appear to employ cAMP or cGMP as the intracellular messenger in hepatic tissue.

Subcellular distribution of alpha 1-adrenergic receptors in rat liver

The distribution of alpha 1-adrenergic receptors in rat liver subcellular fractions was studied using the alpha 1-adrenergic receptor ligand [3H]prazosin. The highest number of [3H]prazosin binding sites was found in a plasma membrane fraction followed by 2 Golgi and a residual microsomal fraction, the numbers of binding sites were 1145, 845, 629 and 223 fmol/mg protein, respectively. When the binding in these fractions was compared with the activity of plasma membrane 'marker' enzymes in the same fractions a relative enrichment of [3H]prazosin binding sites was found in the residual microsomes and one of the Golgi fractions. Photoaffinity labelling with 125I-arylazidoprazosin in combination with SDS-polyacrylamide gel electrophoresis revealed the specific binding to 40 and 23 kDa entities in a Golgi fraction, while in plasma membranes the binders had an apparent molecular mass of 36 and 23 kDa. When [3H]prazosin was injected in vivo into rat portal blood followed by subcellular fractionation of liver, a pattern of an initial rapid decline and thereafter a slow decline of radioactivity was noted in all fractions. Additionally, in the two Golgi fractions a transient accumulation of radioactivity occurred between 5 and 10 min after the injection. The ED50 values for displacement of [3H]prazosin with adrenaline was lowest in the plasma membrane fraction, followed by the residual microsomes and Golgi fractions, the values were 10(-6), 10(-5) and 10(-4) mol/l, respectively. On the basis of lack of correlation between distribution of alpha 1-adrenergic antagonist binding and adenylate cyclase activity, differences in the molecular mass of alpha 1-adrenergic antagonist binders, differences in the kinetics of in vivo binding and accumulation of [3H]prazosin and also differences in agonist affinity between plasma membrane and Golgi fractions, it is concluded that alpha 1-adrenergic receptors are localized to low-density intracellular membranes involved in receptor biosynthesis and endocytosis.