Dopamine and gamma-aminobutyric acid transporters: differential regulation by agents that promote phosphorylation

Treatment of striatal synaptosomes with the protein phosphatase inhibitor okadaic acid significantly decreased gamma-aminobutyric acid (GABA) uptake, indicating that the GABA transporter may be regulated by phosphorylation. Forskolin and 8-bromoadenosine-3,5-cyclic monophosphate (8-br-cAMP) inhibited GABA uptake to the same extent as okadaic acid, suggesting the involvement of protein kinase A in GABA transporter regulation. In contrast, the same treatments did not alter dopamine (DA) uptake into striatal synaptosomal preparations. The results suggest that the structurally related GABA and DA transporters may be subject to different post-translational regulation.

Differential interaction of beta 1- and beta 3-adrenergic receptors with Gi in rat adipocytes

The interaction of beta 1- and beta 3-adrenergic receptors and G(i) proteins was examined in rat adipocytes. In intact adipocytes, cyclic AMP accumulation stimulated by the beta 3-selective agonist, BRL 37344 (BRL), was potentiated by pertussis toxin (PTX), as was the beta 1-sensitive component of isoproterenol (ISO)-stimulated cyclic AMP accumulation. These data suggest that beta 1 and beta 3-receptors interact with both Gs and G(i) in intact adipocytes. Further analysis of the activation of adenylyl cyclase by the beta-receptor subtypes was performed in adipocyte membranes in which the activity of G(i) was manipulated by both GTP and PTX. Unlike cyclic AMP accumulation in cells, the activation of membrane adenylyl cyclase by ISO could be clearly resolved into components mediated by beta 1-(high affinity) or beta 3-(low affinity) receptors. The beta 3-receptor-mediated activity was dramatically reduced at 0.1 mM GTP compared to 0.1 microM GTP, but the activity mediated by beta 3-receptors was significantly reduced at concentrations of GTP in which G(i) proteins are active. Adenylyl cyclase activity stimulated by BRL was also inhibited at high concentrations of GTP. PTX abolished the inhibition of beta 3-receptor-stimulated activity by high GTP concentrations. This is the first study to indicate that G(i) proteins can limit beta 3- but not beta 1-stimulated adenylyl cyclase activity and are consistent with the hypothesis that beta 3-receptors interact with both Gs and G(i), whereas beta 1-receptors couple predominantly to Gs.

Characterization of the human beta 3-adrenergic receptor gene

Comparison of the rodent and human beta 3-adrenergic receptor cDNAs with the respective genomic sequences has revealed unexpectedly that these genes contain two protein-coding exons. The rat gene was cloned recently and was found to contain three exons and two introns. In the present report, the human beta 3 receptor gene was characterized and was found to consist of two exons and a single intron. Sequence analysis of the human beta 3 receptor gene identified regions in the intron that were homologous to the second exon and second intron of the rat gene. It appears that both species utilize homologous 5' donor sites in the first intron and 3' acceptor sites of the final exon. However, splicing signals within the human intron that are homologous to the second exon of the rat gene are not used. Nuclease protection assays of tissue RNA and polymerase chain reaction-amplified cDNA demonstrated conclusively that beta 3 receptor mRNA, containing two protein-coding exons, is expressed in human adipose and intestinal tissues. The pharmacological properties of the full length human beta 3 receptor were determined for the first time in Chinese hamster ovary cells, where catecholamine agonists activated adenylyl cyclase with low potency. The beta 3 receptor agonists CGP 12177 and BRL 37344 also activated adenylyl cyclase. CGP 12177 was 10-15 times more potent than either isoproterenol or BRL 37344 in stimulating adenylyl cyclase activity. These pharmacological properties differed somewhat from those reported previously for Chinese hamster ovary cells expressing the truncated receptor. However, direct comparison indicates that it is unlikely that the amino acid sequence derived from the second exon can account for these differences.

Rodent and human beta 3-adrenergic receptor genes contain an intron within the protein-coding block

DNA blot analysis of the cloned rat beta 3-adrenergic receptor gene revealed unexpected restriction enzyme cleavage sites that suggested the presence of one or more introns near the end of the coding block. This region of the rat gene was mapped and sequenced and was found to contain two introns. The first intron occurs 12 amino acids from the end of the coding block, as deduced by comparison with the beta 3 receptor cDNA. Sequence analysis of the first intron indicates that it might contain enhancer elements that could be important in the adipose tissue-specific expression of this gene. The mouse and human beta 3 receptor genes have been assumed to be intronless; however, these genes contain potential splice sites that are homologous to those present in the rat gene. The relevant regions of the mouse and human beta 3 receptor cDNAs were cloned and, by comparing them to the respective genomic sequences, it was concluded that these genes also contain one or more introns. Sequence analysis of the mouse and human beta 3 receptor cDNAs indicates that they code for proteins that are, respectively, 12 and 6 amino acids larger than previously deduced from genomic clones.

Coexisting beta 1- and atypical beta-adrenergic receptors cause redundant increases in cyclic AMP in human neuroblastoma cells

In SK-N-MC human neuroblastoma cells, the cAMP response to 10 nM isoproterenol (ISO) is mediated primarily by beta 1-adrenergic receptors. However, responses to higher concentrations of ISO (100-1000 nM) were only weakly blocked by beta 1- and beta 2-selective antagonists. When beta 1 receptors were blocked with 10 microM CGP 20712A, catecholamines still maximally activated cAMP accumulation, with only small decreases in potency. In the presence of CGP 20712A, beta blockers inhibited the response to ISO stereoselectively but with relatively low potencies. Pindolol derivatives were partial agonists with low potencies, and the atypical agonist BRL 37344 was a partial agonist with an intermediate potency. All binding sites in these cells labeled by 125I-cyanopindolol were of the beta 1 subtype. Nuclease protection assays indicated that SK-N-MC cells contain mRNA for both the human beta 1- and beta 3-adrenergic receptors, with the beta 3 subtype mRNA being expressed 25-50% more abundantly than that for the beta 1 subtype. Northern blot hybridizations showed the presence of two beta 3 mRNA transcripts of 3.1 and 2.4 kilobases. These results suggest that beta 1- and atypical beta-adrenergic receptors coexist in these cells and cause redundant increases in cAMP formation. Although molecular approaches suggest that the atypical subtype is the beta 3, the observed drug specificity differs from that reported for the expressed recombinant human beta 3 receptor.

Adrenergic regulation of neonatal brown fat adenylyl cyclase and Gs alpha activity

Norepinephrine (NE)-stimulated adenylyl cyclase (AC) activity increases during the perinatal period in rat brown adipose tissue (BAT), and this increase is associated with changes in the activities of both the catalytic subunit (C) and Gs alpha, the GTP-binding protein that mediates activation of C. The present study examined the role of the sympathetic nervous system in the postnatal sensitization of AC. The sympathetic innervation of BAT increased 7- to 13-fold after birth, and this increase was temporally correlated with the postnatal enhancement of AC responsiveness. 6-hydroxydopamine (6-HDA) treatment of neonates reduced tyrosine hydroxylase levels by greater than 90%. This treatment greatly reduced the perinatal increase in NE- and NaF-stimulated AC and completely abolished the increase in forskolin-Mn(2+)-stimulated activity. However, sympathectomy did not alter the postnatal increase in Gs alpha-specific activity and did not prevent the postnatal reduction in Gs alpha levels. These results demonstrate that the sympathetic innervation of BAT develops fully after birth and is essential for the postnatal increase in the activity of C but not of Gs alpha.

Effects of agonist exposure on the coupling of beta 1 and beta 3 adrenergic receptors to adenylyl cyclase in isolated adipocytes

The coupling of beta 1 and beta 3 adrenergic receptors to adenylyl cyclase was examined in membranes of isolated white adipocytes. The activation of adenylyl cyclase by isoproterenol (ISO) was biphasic. The high-affinity activation of adenylyl cyclase, which occurred at submicromolar concentrations of ISO, was mediated by beta 1 receptors, whereas low-affinity activation was mediated by beta 3 receptors. The relative activation of adenylyl cyclase by beta 3 receptors was less when ISO was used to stimulate activity, compared to when the beta 3-selective agonist BRL 37344 was used. These data indicate that both receptor subtypes can stimulate the same adenylyl cyclase in membranes of control cells, and that the activation of adenylyl cyclase by beta 1 receptors by low concentrations of catecholamines can obscure the activation by beta 3 receptors by high concentrations of catecholamines. Exposure of adipocytes to ISO at concentrations that either selectively stimulate beta 1 receptors or nonselectively stimulate both beta receptor subtypes greatly decreased the ability of beta 1, but not beta 3, receptors to activate adenylyl cyclase. In contrast, the exposure of cells to the beta 3-selective agonist BRL only slightly desensitized beta 1 receptors and did not affect beta 3 receptor activation of adenylyl cyclase. These data indicate that acute agonist exposure desensitizes beta 1, but not beta 3, receptors.

Perinatal changes in the coupling of beta 1- and beta 3 adrenergic receptors to brown fat adenylyl cyclase

The stimulation of adenylyl cyclase by catecholamines in neonatal brown adipose tissue (BAT) is markedly biphasic, suggesting the existence of receptors that have both high and low affinities for catecholamines. The identities of these receptors were examined by comparing responses in neonatal BAT membranes to those of Chinese hamster ovary cells which had been transfected to express the cloned rat beta 1 and beta 3 receptors. The results from these experiments indicate that high-affinity stimulation of adenylyl cyclase by catecholamines in BAT is mediated by beta 1 receptors, as evidenced by the potencies of norepinephrine and isoproterenol at this receptor and the potent blockade of the receptor by alprenolol. The low-affinity catecholamine receptor appears to be the beta 3 receptor, as indicated by the low potency of catecholamine agonists and the inability of low concentrations of alprenolol to block this activity. Furthermore, this receptor, like the cloned rat beta 3 receptor, was antagonized by (-)-4-(3-t-butylamino-2-hydroxypropoxy)benzimidazol-2-one (CGP 12177) and was stimulated by (R',R')-4-(2-[(2[(3-chlorophenyl)-2- hydroxyethyl]amino)propyl]phenyl)phenoxyacetic acid (BRL 37344). These results indicate that both beta 1 and beta 3 receptors couple to adenylyl cyclase in BAT and that activation of adenylyl cyclase in neonatal BAT is mediated primarily by beta 3 receptors. Beta 3 receptors were also clearly detected in weanling BAT with the beta 3-selective agonist BRL 37344. However, when catecholamines were used to stimulate activity, the activation of adenylyl cyclase by beta 1 receptors, which occurred at low concentrations of catecholamines, obscured the activation of adenylyl cyclase by beta 3 receptors, which occurred only at high concentrations.

Differential adrenergic regulation of beta 1- and beta 3-adrenoreceptor messenger ribonucleic acids in adipose tissues

The levels of beta 1- and beta 3-adrenoreceptor mRNAs in several rat tissues were determined simultaneously with a sensitive nuclease protection assay. The beta 1-receptor gene was expressed to varying degrees in most tissues examined. By contrast, high levels of beta 3-receptor mRNA were only found in brown and white adipose tissues, indicating that beta 3-receptor gene expression is essentially adipose tissue specific. Surgical sympathectomy of interscapular brown adipose tissue increased beta 3-receptor mRNA levels by 2.4-fold, but did not affect beta 1-receptor mRNA levels. Exposure of rats to 4 C, which increases sympathetic nerve stimulation of IBAT, reduced beta 3-receptor mRNA levels in intact tissue but did not affect the denervation-induced increase in beta 3-receptor mRNA. Acute treatment of rats with norepinephrine greatly reduced beta 3 mRNA levels in both white and brown adipose tissues, but did not alter beta 1-receptor mRNA levels. These results indicate that beta 1- and beta 3-receptor mRNAs are differentially regulated and that norepinephrine released from sympathetic nerves is an important inhibitory regulator of beta 3-receptor mRNA levels. Injections of the beta-receptor agonist isoproterenol and the beta 3-receptor agonist BRL 26830 each reduced beta 3-receptor mRNA in brown and white fat, whereas injections of glucagon reduced beta 3-receptor mRNA in brown fat only. These data indicate that while stimulation of beta 3-receptors is sufficient to down-regulate beta 3 mRNA, other receptors that stimulate adenylyl cyclase have the same effect. Finally, the agonist-induced down-regulation of beta 3-receptor mRNA was associated with a reduction in beta 3-receptor activation of adenylyl cyclase in white adipose tissue.

Molecular cloning and expression of the rat beta 3-adrenergic receptor

Rat adipose tissues contain atypical beta receptors that display certain pharmacological sensitivities that are similar to those found in the recently cloned human beta 3 receptor. However, there are also certain pharmacological differences between the human atypical beta 3 receptor and atypical receptors in rodent adipose tissues, which could indicate strong species differences, the existence of multiple atypical receptor subtypes, or both. To help decide among these possibilities, a rat beta 3 receptor clone was obtained and expressed in Chinese hamster ovary cells. The predicted primary structures of the rat and human receptors are greater than 90% similar. Despite this similarity, the pharmacological properties of the rat receptor differed from those reported for the human receptor but were similar to the properties exhibited by atypical receptors in rat adipose tissue. Specifically, the rat beta 3 receptor had a high affinity for BRL 37344 and a relatively low affinity for norepinephrine and was partially activated by the beta 1 and beta 2 receptor antagonist CGP 12177. Northern blot analysis and nuclease protection assays of RNA from rat tissues indicate that the beta 3 receptor is abundantly expressed only in adipose tissues.

Splicing pattern of Gs alpha mRNA in human and rat brain

Gs alpha, the GTP-binding protein that mediates receptor stimulation of adenylyl cyclase, exists in small (Gs alpha-S) and large (Gs alpha-L) molecular forms. Previous work has established that these Gs alpha isoforms are products of alternatively spliced Gs alpha pre-mRNA. The present work examined the splicing pattern of Gs alpha mRNA in rat and human brain using a method based upon the polymerase chain reaction. In rat brain, transcripts encoding Gs alpha-L were most abundant and constituted 92% of total Gs alpha mRNA in hippocampus and cerebral cortex and 84% in striatum. Compared to rat brain, the relative abundance of Gs alpha-L mRNA was less in human brain and constituted 84% of total Gs alpha mRNA in hippocampus and cortex and 66% in striatum. In addition, it was found that approximately 40% of the Gs alpha-S mRNA utilizes an alternative splice site that encodes serine at amino acid 72. No regional variations in this splicing pattern were found.

Characterization of the distribution of G alpha o in rat striatal synaptosomes and its colocalization with tyrosine hydroxylase

Dopaminergic striatal synaptosomes can be detected and isolated with a fluorescence-activated cell sorter (FACS). In the present study, two antigens were detected simultaneously with primary antisera raised in different species and species-specific fluorescent secondary antibodies with different emission spectra. Double-label FACS analysis was used to determine whether tyrosine hydroxylase (TH) and the alpha subunit of Go (G alpha o) are colocalized in striatal synaptosomes. Rabbit antibodies generated against a synthetic fragment of G alpha o (corresponding to amino acids 22-35) combined with fluorescein-conjugated secondary antibodies were used to detect G alpha o-containing striatal synaptosomes. Preadsorption of G alpha o antiserum with the synthetic peptide antigen reduced labeling to the level obtained with preimmune serum. Approximately 65-75% of striatal synaptosomes were specifically labeled by G alpha o antiserum. Tyrosine hydroxylase-containing synaptosomes were detected with a mouse monoclonal antibody to TH and R-phycoerythrin-conjugated secondary antibody. They comprised 15-17% of total striatal synaptosomes. Double-label studies indicated that at least 50% of TH-containing synaptosomes also contained G alpha o. These findings suggest that G alpha o may not be a protein component of all striatal nerve terminals, and provide a basis for a role for G alpha o in signal transduction within subpopulations of intrinsic and afferent nerve terminals, including those of nigrostriatal dopamine neurons.

Developmental changes in adenylyl cyclase and GTP binding proteins in brown fat

During the perinatal period, norepinephrine (NE)-stimulated adenylyl cyclase activity increased in interscapular brown adipose tissue (IBAT) membranes and then declined to adult levels by 23 days postpartum. The developmental patterns of NE- and NaF-stimulated activities were identical, indicating that the developmental increase in transmitter-stimulated activity resulted from the increased interaction of alpha-subunit of guanine nucleotide-binding stimulatory protein of adenylyl cyclase (Gs alpha) with the catalytic subunit (C). This increased Gs alpha-C interaction was the result of an increase in Gs alpha specific activity, as assessed in cyc- reconstitution assays, as well as an increased C activity, as assessed by forskolin-Mn(2+)-stimulated adenylyl cyclase activity. Although adenylyl cyclase activity increased during the perinatal period, total Gs alpha levels significantly declined because of the loss of the small-molecular-mass form of Gs alpha. Thus the ratio of large to small form of Gs alpha increased threefold and might have contributed to the perinatal increase in activity. Gi alpha-like proteins, as assessed by pertussis toxin-catalyzed [32P]ADP ribosylation, declined dramatically after birth. However, this loss of Gi alpha did not contribute to developmental changes in adenylyl cyclase activity because pertussis toxin treatment failed to alter NE-stimulated activity. In contrast to G alpha subunits, there were no changes in membrane levels of G beta subunits.

Subcellular distribution of adenylyl cyclase and Gs alpha in rat brown adipose tissue

The subcellular distribution of Gs alpha (the alpha-subunit of guanine nucleotide-binding stimulatory protein of adenylyl cyclase) was examined in interscapular brown adipose tissue (IBAT) to determine (1) if Gs alpha is completely colocalized with adenylyl cyclase in the plasma membrane, and (2) whether cold exposure, which increases adenylyl cyclase activity, changes the subcellular distribution of Gs alpha. Subcellular fractions were prepared from IBAT by differential centrifugation and analyzed for Gs alpha by immunoblotting. Adenylyl cyclase activity and Gs alpha were detected in all the subcellular fractions except the cytosol. The plasma membrane fraction showed the greatest enrichment of adenylyl cyclase and Gs alpha. However, the enrichment of adenylyl cyclase in the plasma membrane fraction was greater than that for Gs alpha, which was also associated to a large degree with the mitochondrial fraction. Thus, compared with the mitochondrial fraction, both 5' nucleotidase and adenylyl cyclase were enriched by over 200% in the plasma membrane fraction, but Gs alpha was enriched by only 50%. Exposure of rats to 4 degrees C for 3 days increased fluoride-stimulated adenylyl cyclase activity, but did not increase the amount of immunoreactive Gs alpha in any of the subcellular fractions examined. The above results demonstrate that not all Gs alpha in IBAT is colocalized with adenylyl cyclase in the plasma membrane. The finding that cold exposure did not change the subcellular distribution of Gs alpha indicates that the cold-induced increase in adenylyl cyclase activity is not due to translocation of Gs alpha from subcellular compartments to the plasma membrane.

CGP 12177A modulates brown fat adenylate cyclase activity by interacting with two distinct receptor sites

The interaction of [(-)-4-(3-t-butylamino-2-hydroxy-propoxy)benzimidazol-2-one] (CGP 12177) (CGP) with receptors that couple to adenylate cyclase was examined in membrane homogenates from rat interscapular brown adipose tissue (IBAT). Although typically regarded as a beta adrenoceptor antagonist, CGP stimulated adenylate cyclase activity with an activation constant of about 3 microM. Consistent with its classification as an antagonist, CGP inhibited norepinephrine-stimulated cyclase activity and did so at concentrations that had little or no stimulatory effect. CGP also inhibited activity stimulated by the atypical agonist [(R*,R*)-4-[2-[[2[(3-chlorophenyl)-2- hydroxyethyl]amino]propyl]phenyl]phenoxyacetic acid (BRL 37344), but only at CGP concentrations that stimulated activity when tested alone. The beta-1-selective antagonist ICI 89,406 blocked norepinephrine-stimulated adenylate cyclase activity, but did not inhibit the activity stimulated by CGP. Together, these results indicate that CGP modulates IBAT adenylate cyclase by interacting with two receptors. One is the beta-1 receptor of which CGP is a high-affinity antagonist. The second appears to be an atypical receptor of which CGP is a partial agonist.

Relationship between Gs alpha messenger ribonucleic acid splice variants and the molecular forms of Gs alpha protein in rat brown adipose tissue

Recent work indicates that multiple molecular forms of Gs alpha can be produced by alternative splicing of a primary Gs alpha transcript. This study examined the pattern of expression of messenger RNAs (mRNAs) encoding the large (Gs alpha-L) and small (Gs alpha-S) molecular forms of Gs alpha in several rat tissues, including brown adipose tissue. In all tissues examined, Gs alpha-L mRNA was most abundant. This was most pronounced for brain tissue (95% of total), and least for brown adipose tissue (57%). The pattern of Gs alpha isoform expression of a given tissue was strongly correlated with the Gs alpha mRNA splicing pattern in that tissue. Cold exposure and surgical denervation are treatments that have been shown to, respectively, increase or decrease total Gs alpha mRNA in rat brown adipose tissue. Despite producing up to 4-fold differences in total Gs alpha mRNA, these treatments did not affect the proportions of Gs alpha mRNA splice variants in brown fat. In contrast, perinatal stimulation of brown fat was associated with changes in the splicing pattern of Gs alpha mRNA, and these changes were reflected in Gs alpha protein expression. Specifically, during the perinatal period Gs alpha-L mRNA increased significantly but Gs alpha-L protein levels did not change, whereas Gs alpha-S mRNA did not change and Gs alpha-S protein levels significantly declined. These data provide further evidence that the increased Gs alpha mRNA levels that occur in interscapular brown adipose tissue during periods of stimulation serve to maintain membrane levels of Gs alpha protein.

Developmental expression of Go in neuronal cultures from rat mesencephalon and hypothalamus

The developmental expression of the alpha-subunit of Go was examined in neuronal cultures derived from rat mesencephalon (MES) and hypothalamus (HYP). These cultures were essentially free of contaminating glia and were maintained as a stable population for periods up to 3 weeks. Immunoblotting utilizing specific antisera against Go indicated that in neurons from both brain regions, membrane concentrations of Go increased dramatically during the first 2 weeks in vitro. Thereafter, increases in the amount of Go per neuron kept pace with increasing process (axons and dendrites) formation. Multiple forms of immunoreactive Go were detected in MES and HYP neurons, and the proportions of these forms changed between 4 and 14 days in culture. Finally, increasing neuron density significantly increased membrane levels of Go in MES but not HYP cultures.

Neural control of the alpha-subunit of Gs messenger ribonucleic acid in rat brown adipose tissue

Neural stimulation of rat interscapular brown adipose tissue (IBAT) can greatly modify transmembrane signalling in this tissue. As part of an investigation into the molecular mechanisms of such modulation we have examined the effects of surgical denervation of BAT and cold exposure (a treatment that increases neural stimulation of BAT) on the levels of mRNA encoding for the alpha-subunit of Gs, the GTP-binding protein that regulates adenylate cyclase. The G alpha s mRNA content of BAT per unit RNA was 5-10 times greater than that of lung, liver, or caudate-putamen. Surgical denervation of BAT of warm-adapted rats reduced G alpha s mRNA content by 30% within 8 h postsurgery and by 65% by 72 h. Concurrent infusion of norepinephrine completely prevented the denervation-induced decline in G alpha s mRNA at 72 h. Infusion of the beta-adrenergic agonist isoproterenol also prevented the denervation-induced decline in G alpha s mRNA, but infusion of the alpha 1-adrenergic agonist phenylephrine was without effect. Exposing rats to 4 C for 3 days approximately doubled the level of G alpha s mRNA. Interestingly, surgical denervation did not prevent the cold-induced increase in G alpha s mRNA levels, indicating that this effect is independent of local innervation. Despite reducing G alpha s mRNA levels by about 60%, surgical denervation failed to alter the concentration of G alpha s protein in IBAT membranes of warm-adapted rats, as determined by immunoblotting. Previous work indicates that cold exposure does not increase the concentration of G alpha s in IBAT membranes. Taken with the present results, these data indicate that there is no simple relationship between tissue G alpha s mRNA content and membrane G alpha s concentration in IBAT.

Neural modulation of the stimulatory regulatory protein of adenylate cyclase in rat brown adipose tissue

We reported previously that cold exposure increases by 2- to 3-times the maximal responsiveness of interscapular brown adipose tissue (IBAT) adenylate cyclase to stimulation by norepinephrine or fluoride and that the sensitizing effect of cold is the result of an increase in neural transmission to the tissue because it is abolished by prior surgical denervation of IBAT. The present report examined further the molecular basis of the sensitization of IBAT adenylate cyclase. Cold exposure increased the maximal responsiveness of adenylate cyclase to stimulation by guanylyl-5'-imidodiphosphate without altering the apparent affinity of the enzyme for the nucleotide. In addition, cold exposure increased the maximal extent of cholera toxin-mediated ADP-ribosylation of the alpha subunit of the stimulatory regulatory protein of adenylate cyclase (Gs alpha) in proportion to the increase in adenylate cyclase activity, but did not alter pertussis toxin-mediated labeling of the inhibitory regulatory protein. Direct measurement of Gs alpha by immunoblotting, however, revealed that sensitized membranes did not contain more Gs alpha protein. These results indicate that neural stimulation of IBAT produced by cold exposure alters the proportion of existing Gs molecules that are functional. The additional observation that sensitized adenylate cyclase activity did not reconstitute into cyc- S-49 lymphoma membranes suggests that the ability of neural stimulation to increase Gs function in IBAT might depend upon the interaction of Gs with factors that are unique to membranes of cold-exposed rats.

Norepinephrine infusions increase adenylate cyclase responsiveness in brown adipose tissue

Previous work from our laboratory demonstrated that exposing rats to cold increases interscapular brown adipose tissue (IBAT) adenylate cyclase activity through a postreceptor modification of the adenylate cyclase system. The cold-induced sensitization is correlated with an increase in the activity of the sympathetic innervation of IBAT, and is prevented by prior surgical denervation of this tissue. The present experiments were aimed at identifying the neurogenic signal that mediates cold-induced sensitization. We found that, like cold exposure, infusions of norepinephrine increased adenylate cyclase activity and enhanced the ability of cholera toxin to ADP-ribosylate the stimulatory regulatory protein of adenylate cyclase (Gs) in warm-adapted rats whose IBAT had been denervated surgically. Infusions of isoproterenol increased adenylate cyclase responsiveness more potently than norepinephrine; however, the maximal effect achieved by isoproterenol was less than that produced by norepinephrine. Infusions of phenylephrine and clonidine had no effect on adenylate cyclase responsiveness. The effects of low doses of isoproterenol, however, were greatly potentiated by coinfusion of phenylephrine. Furthermore, the sensitizing effects of norepinephrine could be blocked by either propranolol or prazosin, indicating that the effects of norepinephrine require simultaneous stimulation of beta and alpha-1 adrenergic receptors.

Neural control of adenylate cyclase responsiveness in brown adipose tissue

The level of prior adrenergic stimulation is known to affect the responsiveness of tissue to subsequent stimulation. We investigated the effects of cold exposure on sympathetic nerve activity in interscapular brown adipose tissue (IBAT) and the impact of those changes on the responsiveness of the adenylate cyclase system. Exposing rats to 4 degrees C for 3 days increased norepinephrine turnover in IBAT and increased the ability of norepinephrine to stimulate adenylate cyclase activity. This 2- to 3-fold increase in adenylate cyclase activity appeared to result from increased nerve transmission because it was abolished by prior surgical denervation of IBAT. Additional studies indicated that cold exposure did not affect the number of beta adrenergic receptors in IBAT. Moreover, cold exposure increased the maximal response of adenylate cyclase to fluoride and guanosine-5--(beta-gamma-imino)triphosphate stimulation by 2- to 3-fold. We conclude that increased nerve transmission produces supersensitivity of IBAT adenylate cyclase and this effect is mediated by a postreceptor modification of the adenylate cyclase system.

Food intake and gastric emptying in rats with streptozotocin-induced diabetes

Recent studies suggest that the rate of nutrient transit through the upper gastrointestract may provide cues that are important to the control of food intake. We examined gastrointestinal function in rats with streptozotocin-induced diabetes and related these findings to concomitant changes in food intake. Control and diabetic rats were adapted to one of two isocaloric diets either high in carbohydrate or fat. Control rats ate similar amounts of each diet. In contrast, diabetic animals fed high-carbohydrate diet were hyperphagic, whereas those fed low-carbohydrate diet ate normal amounts of food. Gastric emptying, intestinal mass, disaccharidase activity, and glucose absorption were increased in normophagic diabetic rats fed a low-carbohydrate diet. Feeding diabetic rats high-carbohydrate diet potentiated each of these effects, and food intake was highly correlated with rate of gastric emptying. These and other results indicate that diabetes enhances gastric emptying and intestinal carbohydrate digestion and absorption, even in the absence of hyperphagia. Consequently, the hyperphagia of diabetic rats may be in part a behavioral response to a greatly accelerated clearance of nutrients from the upper gastrointestinal tract that occurs when these animals are fed diets rich in carbohydrate.

Effects of sucrose feeding and denervation on lipogenesis in brown adipose tissue

Rats were fed standard laboratory chow or sucrose + chow for 2-3 weeks. Sucrose feeding significantly increased the size and norepinephrine content of interscapular brown adipose tissue (BAT). Sucrose feeding stimulated in vivo lipogenesis in BAT, white adipose tissue, and liver. Unilateral denervation reduced BAT norepinephrine content by 90%, and significantly reduced BAT protein and DNA content. Prior surgical denervation of BAT had no effect on lipogenesis in chow-fed rats, but reduced the sucrose-induced lipogenesis in BAT by 75% and increased lipogenesis in retroperitoneal white adipose tissue of sucrose-fed rats. These data demonstrate that the optimal stimulation of lipogenesis in BAT by sucrose feeding requires innervation of this tissue.

Effect of sucrose overfeeding on brown adipose tissue lipogenesis and lipoprotein lipase activity in rats

During cold-induced nonshivering thermogenesis, interscapular brown adipose tissue (BAT) lipoprotein lipase (LPL) activity and lipogenesis are elevated. Because of the many similarities between cold- and diet-induced thermogenesis, we examined the effect of ad libitum access to a 32% sucrose solution on caloric intake, adiposity, and BAT enzyme activities in male rats. Daily caloric intakes of sucrose-fed animals were elevated by 20%-25%, and 8 wk of sucrose feeding doubled carcass fat content. This sucrose-feeding induced obesity was associated with increases in circulating triglyceride and insulin levels as well as increased retroperitoneal white adipose tissue LPL activity. However, the increased carcass lipid content accounted for less than half of the excess calories ingested by the sucrose-fed rats. Sucrose feeding stimulated in vivo lipogenesis in BAT and elevated BAT fatty acid synthetase and acetyl-CoA carboxylase activities but not LPL activity. These findings suggest that overeating enhances endogenous lipogenesis but not uptake of circulating triglyceride in BAT. Thus, both cold- and diet-induced thermogenesis increase BAT lipogenesis, while only cold-induced thermogenesis is associated with elevated LPL activity in BAT.

Effects of photoperiod and castration on post-fast food intake and body weight gain in golden hamsters

This experiment examined the effects of photoperiod and castration of post-food-restriction food intake and body weight gain in male golden hamsters (Mesocricetus auratus). Hamsters were ether castrated or sham-operated and housed in a 14:10 light-dark (LD) or a 8:16 LD cycle for 8 weeks. All animals were restricted to a 6 g of chow per day for 13 days and then returned to ad lib feeding. Following food restriction the hamsters gained weight rapidly without overeating. The long photoperiod (LD 14:10) increased post-restriction weight gain in castrated hamsters but decreased weight gain in gonadally intact animals. These differences in body weight gain were associated with significant group differences in feed efficiency (grams weight gain per gram eaten). The results suggest that changes in energy utilization may be more important than changes in energy intake in the adaptation of hamsters to food restriction.