Effect of truncal vagotomy on pancreatic polypeptide response after intravenous glucose administration

Intravenous glucose infusion was performed in six dogs with and without truncal vagotomy, and plasma pancreatic polypeptide (PP) responses were compared before and after truncal vagotomy. Following truncal vagotomy, basal PP levels decreased significantly from 286 ± 64 pg/ml (mean ± S.E.) to 94 ± 14 pg/ml (P < 0.05). Basal plasma insulin and blood glucose levels also tended to be lower, but not significantly. During the influsion of glucose, blood glucose concentrations rose rapidly in both groups and after 15 min reached peak values which were not significantly different from each other. In the vagotomized group the plasma insulin response to intravenous glucose infusion was significantly lower than in the control group. Following intravenous glucose loading, plasma PP concentrations decreased rapidly in both groups, but the PP level in the vagotomized group was suppressed only to 77 ± 4% of the basal level whereas in the control group it decreased to 45 ± 8%, significantly lower than in the vagotomized group (P < 0.01).These results suggest that basal PP is regulated by vagal tonus and that vagus controls, at least in part, suppression by intravenous glucose administration.     

Increase in liver glucose transporter mRNA levels during rat liver regeneration

Gene expression of liver facilitated glucose transporter was rapidly induced during the liver regenerating process in rats. It reached maximum of 2.7 times at 8 hr of the regenerating course and returned to normal by 48 hr. The protein synthesis inhibitor, cycloheximide, did not interfere with the increased gene expression of liver facilitated glucose transporter. By contrast, erythrocyte/brain-type glucose transporter mRNA could not be detected in the livers of partially hepatectomized rats and sham-operated rats. The plasma glucose levels were transiently increased within 2 hr of the regenerative course and then decreased to a nadir at 4 hr. These results suggest that the increased gene expression of liver facilitated glucose transporter contributes to the decrease in plasma glucose levels.     

A K(ATP) channel deficiency affects resting tension, not contractile force, during fatigue in skeletal muscle

Theobjective of this study was to determine how an ATP-sensitiveK⁺ (K_ATP) channel deficiency affects thecontractile and fatigue characteristics of extensor digitorum longus(EDL) and soleus muscle of 2- to 3-mo-old and 1-yr-old mice.K_ATP channel-deficient mice were obtained by disrupting theKir6.2 gene that encodes for the protein forming the pore ofthe channel. At 2-3 mo of age, the force-frequency curve, the twitch,and the tetanic force of EDL and soleus muscle of K_ATPchannel-deficient mice were not significantly different from those inwild-type mice. However, the tetanic force and maximum rate of forcedevelopment decreased with aging to a greater extent in EDL and soleusmuscle of K_ATP channel-deficient mice (24-40%) thanin muscle of wild-type mice (7-17%). During fatigue, theK_ATP channel deficiency had no effect on the decrease intetanic force in EDL and soleus muscle, whereas it caused asignificantly greater increase in resting tension when compared withmuscle of wild-type mice. The recovery of tetanic force after fatiguewas not affected by the deficiency in 2- to 3-mo-old mice, whereas in1-yr-old mice, force recovery was significantly less in muscle ofK_ATP channel-deficient than wild-type mice. It is suggestedthat the major function of the K_ATP channel during fatigueis to reduce the development of a resting tension and not to contributeto the decrease in force. It is also suggested that theK_ATP channel plays an important role in protecting muscle function in older mice.

     

Solution structure of the chitin-binding domain of Bacillus circulans WL-12 chitinase A1

The three-dimensional structure of the chitin-binding domain (ChBD) of chitinase A1 (ChiA1) from a Gram-positive bacterium, Bacillus circulans WL-12, was determined by means of multidimensional heteronuclear NMR methods. ChiA1 is a glycosidase that hydrolyzes chitin and is composed of an N-terminal catalytic domain, two fibronectin type III-like domains, and C-terminal ChBD(ChiA1) (45 residues, Ala(655)-Gln(699)), which binds specifically to insoluble chitin. ChBD(ChiA1) has a compact and globular structure with the topology of a twisted beta-sandwich. This domain contains two antiparallel beta-sheets, one composed of three strands and the other of two strands. The core region formed by the hydrophobic and aromatic residues makes the overall structure rigid and compact. The overall topology of ChBD(ChiA1) is similar to that of the cellulose-binding domain (CBD) of Erwinia chrysanthemi endoglucanase Z (CBD(EGZ)). However, ChBD(ChiA1) lacks the three aromatic residues aligned linearly and exposed to the solvent, which probably interact with cellulose in CBDs. Therefore, the binding mechanism of a group of ChBDs including ChBD(ChiA1) may be different from that proposed for CBDs.     

MTABC3, a novel mitochondrial ATP-binding cassette protein involved in iron homeostasis

Atm1p, a mitochondrial half-type ATP-binding cassette (ABC) protein in Saccharomyces cerevisiae, transports a precursor of the iron-sulfur (Fe/S) cluster from mitochondria to the cytosol. We have identified a novel half-type human ABC protein, designating it MTABC3 (mammalian mitochondrial ABC protein 3). MTABC3 mRNA is ubiquitously expressed in all of the rat and human tissues examined. MTABC3 protein is shown to be present in the mitochondria, as assessed by immunoblot analysis and confocal microscopic analysis of subcellular fractions of Chinese hamster ovary cells stably expressing MTABC3. Accumulation of iron in the mitochondria, mitochondrial DNA damage, and respiratory dysfunction in the yeast ATM1 mutant strain (atm1-1 mutant cells) were almost fully reversed by expressing MTABC3 in these mutant cells. These results indicate that MTABC3 is a novel ortholog of the yeast and suggest an important role in mitochondrial function. Interestingly, the human MTABC3 gene has been mapped to chromosome 2q36, a region within the candidate locus for lethal neonatal metabolic syndrome, a disorder of the mitochondrial function associated with iron metabolism, indicating that MTABC3 is a candidate gene for this disorder.     

Insulin secretion and differential gene expression in glucose-responsive and -unresponsive MIN6 sublines

We have established two sublines derived from the insulin-secreting mouse pancreatic beta-cell line MIN6, designated m9 and m14. m9 Cells exhibit glucose-induced insulin secretion in a concentration-dependent manner, whereas m14 cells respond poorly to glucose. In m14 cells, glucose consumption and lactate production are enhanced, and ATP production is largely through nonoxidative pathways. Moreover, lactate dehydrogenase activity is increased, and hexokinase replaces glucokinase as a glucose-phosphorylating enzyme. The ATP-sensitive K(+) channel activity and voltage-dependent calcium channel activity in m14 cells are reduced, and the resting membrane potential is significantly higher than in m9 cells. Thus, in contrast to m9, a model for beta-cells with normal insulin response, m14 is a model for beta-cells with impaired glucose-induced insulin secretion. By mRNA differential display of these sublines, we found 10 genes to be expressed at markedly different levels. These newly established MIN6 cell sublines should be useful tools in the analysis of the genetic and molecular basis of impaired glucose-induced insulin secretion.     

Role of ATP-sensitive K+ channels in electrophysiological alterations during myocardial ischemia: a study using Kir6.2-null mice

The role of cardiac ATP-sensitive K(+) (K(ATP)) channels in ischemia-induced electrophysiological alterations has not been thoroughly established. Using mice with homozygous knockout (KO) of Kir6.2 (a pore-forming subunit of cardiac K(ATP) channel) gene, we investigated the potential contribution of K(ATP) channels to electrophysiological alterations and extracellular K(+) accumulation during myocardial ischemia. Coronary-perfused mouse left ventricular muscles were stimulated at 5 Hz and subjected to no-flow ischemia. Transmembrane potential and extracellular K(+) concentration ([K(+)](o)) were measured by using conventional and K(+)-selective microelectrodes, respectively. In wild-type (WT) hearts, action potential duration (APD) at 90% repolarization (APD(90)) was significantly decreased by 70.1 +/- 5.2% after 10 min of ischemia (n = 6, P < 0.05). Such ischemia-induced shortening of APD(90) did not occur in Kir6.2-deficient (Kir6.2 KO) hearts. Resting membrane potential in WT and Kir6.2 KO hearts similarly decreased by 16.8 +/- 5.6 (n = 7, P < 0.05) and 15.0 +/- 1.7 (n = 6, P < 0.05) mV, respectively. The [K(+)](o) in WT hearts increased within the first 5 min of ischemia by 6.9 +/- 2.5 mM (n = 6, P < 0.05) and then reached a plateau. However, the extracellular K(+) accumulation similarly occurred in Kir6.2 KO hearts and the degree of [K(+)](o) increase was comparable to that in WT hearts (by 7.0 +/- 1.7 mM, n = 6, P < 0.05). In Kir6.2 KO hearts, time-dependent slowing of conduction was more pronounced compared with WT hearts. In conclusion, the present study using Kir6.2 KO hearts provides evidence that the activation of K(ATP) channels contributes to the shortening of APD, whereas it is not the primary cause of extracellular K(+) accumulation during early myocardial ischemia.     

A possible role of 20-hydroxyecdysone in embryonic development of the silkworm Bombyx mori

It has been well established that eggs of insects, including those of the silkworm Bombyx mori, contain various ecdysteroids and the amounts of these ecdysteroids fluctuate during embryonic development. In order to know the function of egg ecdysteroids in embryonic development of B. mori, we examined the biological activities of various egg ecdysteroids by in vitro ligand-binding assay and bioassay using B. mori eggs. First, using the ecdysteroid receptor of B. mori (BmEcR-B1/BmUSP heterodimer) prepared by yeast and Escherichia coli expression systems, the interaction between the ecdysteroid receptor and various egg ecdysteroids of B. mori was analyzed. The relative binding affinities of egg ecdysteroids to the BmEcR-B1/BmUSP heterodimer decreased in the order of 20-hydroxyecdysone > 2-deoxy-20-hydroxyecdysone > 22-deoxy-20-hydroxyecdysone > ecdysone > 2-deoxyecdysone > ecdysone 22-phosphate. Next, several egg ecdysteroids of B. mori were injected into the prospective diapause eggs, which show a very low level of free ecdysteroids at the onset of embryonic diapause (gastrula stage). Approximately 7% of them (P < 0.002, chi(2)-test) developed beyond the gastrula stage without entering diapause by the injection of 20-hydroxyecdysone (25 ng/egg). In contrast, the injection of other ecdysteroids was not effective in inducing embryonic development. These results suggest that 20-hydroxyecdysone, via the ecdysteroid receptor, is responsible for the developmental difference between diapause and non-diapause in B. mori embryos. Furthermore, it was suggested that continuous supply of 20-hydroxyecdysone may be required to induce embryonic development.     

Sequences of primate insulin genes support the hypothesis of a slower rate of molecular evolution in humans and apes than in monkeys.

The chimpanzee and African green monkey insulin genes have been cloned and sequenced. These two sequences together with the previously reported sequences for the human and owl monkey insulin genes provide additional support for the hominoid-rate-slowdown hypothesis, i.e., a slower rate of nucleotide substitution in humans and apes than in monkeys. When these sequences and other primate sequences available for the relative-rate test were considered together, the substitution rate in the Old World monkey lineage was shown to be significantly higher than the rates in the human and chimpanzee lineages. This was true regardless of whether the eta-globin pseudogene was included in the analysis. Therefore, in contrast to the claim by Easteal, the hominoid-rate-slowdown is not unique to the eta-globin pseudogene but appears to be a rather general phenomenon. On average, the substitution rate at silent sites is about 1.5 times higher in the Old World monkey lineage than in the human and chimpanzee lineages.