Effects of fasting on hepatic catecholamine receptors

Time sequence of changes in the responsiveness of glycogen breakdown to adrenergic agonists in perfused liver of rats with insulin-induced hypoglycemia

The time-course changes of the responsiveness of glycogen breakdown to alpha- and ss-adrenergic agonists during insulin-induced hypoglycemia (IIH) were investigated. Blood glucose levels were decreased prior to the alteration in the hepatic responsiveness to adrenergic agonists. The activation of hepatic glucose production and glycogenolysis by phenylephrine (2 microM) and isoproterenol (20 microM) was decreased in IIH. The changes in the responsiveness of glycogen catabolism were first observed for isoproterenol and later for phenylephrine. Hepatic ss-adrenergic receptors showed a higher degree of adrenergic desensitization than did alpha-receptors. Liver glycogen synthase activity, glycogen content and the catabolic effect of dibutyryl cyclic AMP (the ss-receptor second messenger) were not affected by IIH.

Responsiveness of glycogen catabolism to adrenergic agonists during insulin-induced hypoglycemia in rat livers

1. Insulin-induced hypoglycemia (IIH) promoted decreased responsiveness of hepatic glycogen catabolism to phenylephrine and isoproterenol, but not to glucagon and cyanide. 2. In addition, glycogen phosphorylase activity and glycogen levels were not affected by IIH. 3. It was concluded that hypoglycemia promoted changes in hepatic responsiveness to adrenergic agonists. 4. However, the ability of the liver to mobilize glycogen was not influenced by hypoglycemia.

Hepatotoxicity of microcystin-LR in fed and fasted rats

The LD50 (25 hr, i.p.) for microcystin-LR in fed rats (122 micrograms/kg) was significantly higher than that in fasted rats (72 micrograms/kg). At doses of 100, 150 and 200 micrograms of microcystin-LR per kg, the median times to death were 31.9, 18.2 and 11.2 hr for fed rats, and 1.8, 1.7 and 1.5 hr for fasted rats. A sublethal dose of microcystin (50 micrograms/kg) afforded protection to fasted, but not fed, rats against a subsequent lethal dose (200 micrograms/kg) challenge given 72 hr later. Biochemical and ultrastructural changes resulting from microcystin-LR (100 micrograms/kg, i.p.) were compared in fed and fasted rats 1 hr after injection. In both groups, liver weight and serum levels of sorbitol dehydrogenase and glucose significantly increased. Plasma membranes, isolated from livers of fed or fasted rats, exhibited similar toxin-induced changes in associated cytoskeletal elements. Liver mitochondria from toxin-treated, fasted rats exhibited complete inhibition of state 3 respiration, while those from toxin-treated, fed rats had ADP/O ratios and respiratory control indices comparable to control values. The primary event responsible for enhanced microcystin hepatotoxicity in the fasted state has not yet been identified. Depletion of glycogen stores and a decreased respiratory capacity may, however, play significant roles in this degenerative process.

Selective reduction of alpha 2-adrenergic responsiveness in hamster adipose tissue during prolonged starvation

The influence of fasting on the dual adrenergic control of adipose tissue lipolysis was investigated in hamsters because in this species the adipocytes exhibit both beta-stimulatory and alpha 2-inhibitory adrenergic responses. In adipocytes from fed animals, the number of alpha 2-receptors (identified with [3H]clonidine and [3H]RX 821002) was greater than that of beta-receptors. As in humans, the alpha 2-adrenoceptor number was greater in adipocyte membranes from subcutaneous (inguinal and popliteal) than from internal (perirenal and epididymal) adipose tissues. Despite this difference in alpha 2-adrenoceptor number, the antilipolytic responses to the alpha 2-agonists clonidine and UK 14304 were similar in the two tissues. Food deprivation for a period of 1-6 days induced a net depletion of both adipose tissues. In 6-day starved animals the number of adipocyte alpha 2-adrenoceptors and the maximal antilipolytic effect of UK 14304 were less than 50% of those in fed controls. In contrast, the antilipolytic responses to phenylisopropyladenosine or prostaglandin E1 remained unchanged. Starvation induced a decrease in alpha 2-adrenoceptor number and an increase in beta-adrenergic sensitivity that were greater in adipocytes from subcutaneous than from internal fad pads. The data suggest that the adipocyte beta- and alpha 2-adrenoceptors are independently regulated during starvation. In the adipocyte, the alpha 2-antilipolytic responses and the alpha 2-adrenoceptor levels are dependent on the extent of the adipose mass; they are particularly reduced in emaciated hamsters.

Hormonal control of glucose production and pyruvate kinase activity in isolated rat liver cells: influence of hypothyroidism

Hormonal control of glucose production and of L-pyruvate kinase activity has been measured in isolated liver cells from fed control and thyroidectomized rats. In hypothyroid rats, sensitivity to isoproterenol as measured by these parameters was increased: the apparent K0.5 for isoproterenol-induced stimulation of glucose production decreased from 8.0 +/- 3 X 10(-6) M in control rats to 2.0 +/- 0.2 X 10(-8) M in hypothyroid rats (P less than 0.001) and the apparent K0.5 for inhibition of L-pyruvate kinase was 5 +/- 2 X 10(-7) M vs. 7 +/- 2 X 10(-9) M (P less than 0.001) in control and thyroidectomized rats, respectively. Utilisation of specific adrenergic antagonists confirmed increased beta-adrenergic responsiveness in hypothyroid rats. This phenomenon was not reversed by 3 days of T3 treatment (10 micrograms/100 g body weight). Sensitivity to the alpha-agonist was unchanged by thyroid status. Stimulation of glucose production and inhibition of L-pyruvate kinase activity by glucagon and their reversal by insulin were not affected by hypothyroidism. The dose-response curve to vasopressin and its maximal effect measured on stimulation of glucose production were unchanged in thyroidectomized rats. Thus, hypothyroidism produces a specific enhancement of liver beta-adrenergic responsiveness without affecting sensitivity to glucagon, insulin and vasopressin.

Effects of adrenalectomy on binding to and actions of adrenergic receptors

Adrenalectomy results in significant changes in the mechanism of adrenergic activation of hepatic glycogenolysis. In adrenalectomized rats a greater role for the beta-adrenergic receptor is observed, whereas the alpha 1-adrenergic-mediated phosphorylase activation declines. Our present findings document that adrenalectomy causes a significant decrease in the high-affinity population of the alpha 1-adrenergic receptor labelled with [3H]adrenaline. Our data indicate a large increase in the number of beta-adrenergic binding sites after adrenalectomy. This increase was not consistent with the observed modest increase in the beta-adrenergic-mediated activation of cyclic AMP accumulation and glycogen phosphorylase. When alpha-adrenergic antagonists are present along with the catecholamine, a 100% increase in the adrenaline-mediated accumulation of cyclic AMP in hepatocytes from adrenalectomized rats was observed. Adrenalectomy was also shown to cause a significant increase in the hepatic alpha 2-adrenergic binding sites. These data are consistent with an inhibitory role on the beta-adrenergic-mediated activation of glycogenolysis by the hepatic alpha 2-adrenergic receptor in adrenalectomy.

Impaired insulin receptor phosphorylation in skeletal muscle membranes of db/db mice: the use of a novel skeletal muscle plasma membrane preparation to compare insulin binding and stimulation of receptor phosphorylation

A method has been developed to isolate skeletal muscle plasma membranes from mice in good yield without harsh extraction procedures. The method involves perfusion of mouse hindquarters with a calcium-deficient buffer containing collagenase and hyaluronidase. This is followed by gentle disruption, filtration, and differential centrifugations. The entire procedure takes about six hours and the yield is approximately 4 mg. protein from 10 g. equivalent of hindquarter muscle. The preparation contained predominantly plasma membranes based on specific activities of marker enzymes, electron microscopic data, and specific binding sites for insulin and a -adrenergic ligand. Studies using such preparations from lean, 4-5 week old and 12-20 week old db/db mice showed marked reduction in the phosphorylation of the 95 kDa subunit of the insulin receptor of the obese mice with no change in insulin binding. In addition, there was a progressive reduction in insulin sensitivity in stimulating receptor phosphorylation in the db/db mice.

An arachidonate metabolite is involved in the conversion from alpha 1- to beta-adrenergic glycogenolysis in isolated rat liver cells

In vitro incubation of isolated rat liver cells in a serum-free buffer leads to the suppression of the glycogenolytic effect of phenylephrine and the simultaneous emergence of a glycogenolytic response to isoproterenol within 4 hr. This time-dependent conversion of the adrenergic receptor response from alpha 1 to beta type is prevented by the presence in the incubation medium of 0.5% fatty-acid-free, but not regular, bovine serum albumin. A 20-min exposure of freshly isolated liver cells to arachidonic acid (10 micrograms/ml), but not to stearic or palmitic acid, causes an acute shift in the receptor response from alpha 1 to mixed alpha 1/beta type, similar in direction to that seen after prolonged incubation of the cells. This effect of arachidonic acid is prevented by 0.2 microM ibuprofen but not by the same concentration of nordihydroguaiaretic acid. Ibuprofen (1 microM) or indomethacin (1 microM) also inhibits the time-dependent shift in the receptor response. Actinomycin D inhibits the change in receptor response that is caused by prolonged incubation but not the change that is caused by exogenous arachidonic acid. It is proposed that the time-dependent conversion from alpha 1- to beta-adrenergic receptor-mediated glycogenolysis in isolated rat liver cells is related to a parallel increase in the phospholipase-mediated release of arachidonic acid and the subsequent formation of a key cyclooxygenase metabolite. A protein factor appears to be involved in the regulation of the release of arachidonic acid but not in the action of its metabolite. A possible mechanism by which this metabolite may regulate inverse changes in the coupling of alpha 1- and beta-receptors to postreceptor pathways is discussed.

Evidence for a decrease in the efficiency of beta-receptor coupling to adenylate cyclase in liver membranes from sucrose-fed rats

Sucrose feeding has been shown previously to alter the plasma concentration of several factors which may regulate beta-adrenergic receptors, including corticosteroids and insulin as well as altered sympathetic nervous system (SNS) tone. For this reason we initiated a study of the effects of sucrose feeding on the beta-adrenergic receptor-adenylate cyclase system in rat liver plasma membranes. Beta-Adrenergic responsiveness was monitored by measuring isoproterenol stimulation of adenylate cyclase activity, while beta-adrenergic receptor characteristics were evaluated by analyzing [125I]iodocyanopindolol [( 125I]CYP) binding. Rats fed rat chow ad lib. supplemented by drinking water containing 10% sucrose solution exhibited a 50-75% reduction in hepatic isoproterenol-sensitive adenylate cyclase activity. This effect of sucrose was also observed in adrenalectomized (ADX) and 6-hydroxydopamine-pretreated animals, ruling out a causal role for corticosteroids or the sympathetic nervous system respectively. No effect was observed on basal, glucagon-, fluoride- or GTP-stimulated adenylate cyclase. A small but significant decrease in [125I]CYP specific binding capacity was observed in liver membranes prepared from sucrose-fed ADX rats, whereas no change in [125I]CYP binding capacity was observed in in sucrose-fed normal rats. These observations suggest that beta-receptor to adenylate cyclase coupling efficiency is decreased by the sucrose diet. The activities of two membrane-associated phospholipid methyltransferases and the content of endogenous S-adenosylmethionine in liver were reduced by sucrose feeding, implying a defect in the methylation pathway for phosphatidylcholine synthesis. The possible relationship between this latter finding and the observed decrease in beta-adrenergic receptor to adenylate cyclase coupling efficiency is discussed.

Evidence for heterogeneous distribution of alpha 1, alpha 2- and beta-adrenergic binding sites on rat-liver cell surface

Fractionation of preparations of rat-liver membranes on linear sucrose gradients revealed different profiles for the binding of alpha 1-, alpha 2- and beta-adrenergic radioligands. The peaks of binding activities of [3H]prazosin and [3H]epinephrine were clearly separated from those of [3H]yohimbine and [125I]iodocyanopindolol which appeared at lower sucrose densities. Enzyme marker activities in the sucrose subfractions indicated the presence of plasma membranes in all of the subfractions. Furthermore, the binding peaks of the various adrenergic radioligands cannot be correlated with the presence of membranes derived from microsomes, lysosomes or Golgi apparatus. Pretreatment of rat livers with concanavalin A, in order to prevent the fragmentation of the plasma membranes during isolation, resulted in the shift of the binding of [3H]yohimbine and [125I]iodocyanopindolol to sucrose-gradient subfractions of higher densities, clearly separate from fractions containing microsomes and Golgi apparatus. There was no distinct separation of the binding peaks of prazosin, yohimbine, and cyanopindolol in sucrose-gradient subfractions from concanavalin A-pretreated livers. These results are consistent with the hypothesis that alpha 1-, alpha 2-, and beta-adrenergic binding sites are associated with plasma membranes, and are heterogeneously distributed on the rat-liver cell surface.

Effect of sucrose feeding on alpha 1-adrenergic responses in rat liver

A sustained increase in sympathetic nervous system (SNS) activity was induced by substituting a 10% sucrose solution for the drinking water of rats fed laboratory chow ad libitum. The effects of increased SNS activity on alpha 1-adrenergic processes in liver were examined by evaluating three alpha 1-responses, namely, phenylephrine-stimulated ouabain-sensitive 86Rb+ uptake, 45Ca2+ efflux, and glucose release. Sucrose feeding abolished phenylephrine stimulation of ouabain-sensitive 86Rb+ uptake and 45Ca2+ efflux and induced a three- to fourfold reduction in the ability of phenylephrine to stimulate glucose release from liver slices. Pretreatment with 6-hydroxydopamine markedly reduced liver norepinephrine content. When 6-hydroxydopamine was used to prevent the sucrose-induced increase in SNS activity, the changes in 86Rb+ uptake, 45Ca2+ efflux, and glucose release that otherwise followed sucrose feeding were not observed. Sucrose feeding did not alter binding of the alpha 1-antagonist [3H]prazosin to liver cell membrane alpha 1-receptors or displacement of [3H]prazosin by the alpha-agonist epinephrine. These observations suggest that sustained increases in SNS activity may have profound effects on liver alpha 1-adrenergic events that occur subsequent to hormone-receptor interaction.

Effects of fasting and aminophylline on norepinephrine-stimulated non-shivering thermogenesis

In an attempt to further elucidate the mechanisms of fasting-depressed maximum thermogenesis and cold tolerance, norepinephrine (NE)-stimulated non-shivering thermogenesis (NST) in cold-acclimated rats was used as a functional index of possible alterations in adrenergic efficacy after fasting. Fasting decreased the magnitude of maximum NE-Stimulated NST by 18.2% [6.87 +/- 0.47 Kcal (Kg X 75 X min)-1 well-fed vs. 5.81 +/- 0.39 Kcal (Kg X 75 X min)-1 fasted], but the apparent adrenergic binding affinity was not affected [Ke = 0.43 micrograms NE min-1 well-fed vs. 0.55 micrograms NE min-1 fasted]. Pretreatment with aminophylline [15 mg Kg-1, i.p.], a phosphodiesterase inhibitor, restored the fasting-depressed NE-stimulated NST to the fed level. The results suggest that the depression of maximum thermogenesis after fasting is not due to changes in adrenergic binding characteristics but to alteration in cAMP production/degradation, resulting in decreased substrate mobilization for thermogenesis.

Binding and uptake of [3H]adrenaline by perfused rat liver

The binding and uptake of [3H]adrenaline by the intact perfused rat liver was investigated. We showed that the administration of [3H]adrenaline to liver resulted in the rapid uptake of the radioligand, and that such uptake was independent of any Ca2+ redistributions induced by the hormone. At low adrenaline concentrations (less than 50 nM) uptake was inhibited by prazosin, whereas at higher hormone concentrations a significant proportion of total [3H]adrenaline uptake could not be inhibited by this antagonist. [3H]Adrenaline uptake could be directly correlated with adrenaline-induced responses such as an increased rate of respiration and glycogenolysis. The partial inhibition (approx. 25%) of [3H]adrenaline uptake by antagonists was sufficient for the total inhibition of hormone-induced responses. The effect of various pharmacological agents on [3H]adrenaline uptake was investigated, and the contribution of tissue-related factors to alpha-adrenergic agonist-antagonist interactions in vivo is discussed.

Characterisation of the alpha 1-adrenergic control of hepatic cAMP in male rats

alpha 1-Adrenergic agonists characteristically elicit a mobilization of intracellular Ca2+ in rat liver. These agents also induced accumulation of cAMP in mature male rats (greater than 300 g body weight) and in Ca2+-depleted hepatocytes from 200 g rats although not in Ca2+-depleted cells from juvenile (less than 100 g) male rats. Comparison of these two responses revealed a similar agonist potency order in both cases, although cAMP accumulation was approximately 5-fold less sensitive to agonists. A variety of alpha-antagonists, including prazosin, phenoxybenzamine and dihydroergocryptine were equipotent as inhibitors of each response, although the alpha 1-adrenergic cAMP response was more sensitive to inhibition by WB-4101 and phentolamine. These data are discussed and a model proposed whereby in mature male rats, the same alpha 1-adrenergic receptor population becomes simultaneously coupled to two separate signal transduction mechanisms, namely Ca2+ mobilization and cAMP generation.

Simultaneous loss and reappearance of alpha 1-adrenergic responses and [3H]prazosin binding sites in rat liver after irreversible blockade by phenoxybenzamine

The relative influences of the in vivo administration of phenoxybenzamine on in vitro binding to alpha 1-adrenergic receptors and alpha 1-receptor-mediated responses were studied. Phenoxybenzamine treatment reduced maximal specific binding of the alpha 1-selective antagonist [3H]prazosin to liver cell membranes. This response was rapid (less than 90 min) and half-maximal following a phenoxybenzamine dose of approx. 10 mg/kg. A similar decrease in the ability of phenylephrine to stimulate glucose release and 45Ca2+ efflux from liver slices was also noted after phenoxybenzamine treatment. During the recovery period following administration of 30 mg/kg phenoxybenzamine, [3H]prazosin specific binding and phenylephrine-stimulated glucose release and 45Ca2+ efflux returned to their respective control levels with t 1/2 values of 42, 49 and 38 h, respectively. At all times studied during the recovery period, alpha 1-binding and both of the alpha 1-responses were similar fractions of their respective control values. These observations indicate that a close relationship exists between the density of [3H]prazosin binding sites and the ability of rat liver to respond to alpha 1-stimulation. We suggest that the binding sites identified in studies using the antagonist [3H]prazosin and those through which the agonist phenylephrine stimulates glucose release and 45Ca2+ efflux are either identical or in equilibrium with each other.

Angiotensin II inhibits hepatic cAMP accumulation induced by glucagon and epinephrine and their metabolic effects

Incubation of isolated hepatocytes containing normal Ca2+ levels with angiotensin II, vasopressin or A23187 caused significant inhibition of the cAMP response to glucagon. Angiotensin II also inhibited cAMP accumulation induced by either glucagon or epinephrine in Ca2+-depleted hepatocytes. When submaximal doses of hormone were employed such that cell cAMP was elevated only 3-4-fold (approximately 2 pmol cAMP/mg wet wt cells) inhibition by angiotensin II was correlated with a decrease in phosphorylase activation. The data demonstrate that inhibition of hepatic cAMP accumulation results in reduced metabolic responses to glucagon and epinephrine and do not support the contention that the hepatic actions of glucagon are independent of cAMP.