Feasting, fasting and fermenting. Glucose sensing in yeast and other cells

Glucose is the primary fuel for most cells. Because the amount of available glucose can fluctuate wildly, organisms must sense the amount available to them and respond appropriately. Altering gene expression is one of the major effects glucose has on cells. Two different glucose sensing and signal transduction pathways in the yeast S. cerevisiae--one for repression, and one for induction of gene expression--have recently come into focus. What we have learned about these glucose sensing and signaling mechanisms might shed light on how other cells sense and respond to glucose.     

Ontogeny of enzymatic activities in fed and fasting turbot, Scophthalmus maximus L.

The presence and localization of acid and alkaline phosphatase, non-specific proteases, aminopeptidase, amylase, non-specific esterase and lipase was investigated by histoenzymologic methods in fed and fasting turbot from day 1 to day 40 post-hatching and compared with published data. Alkaline phosphatase and aminopeptidase activities were delected at day 1 in the distal region of the developing digestive tube. At day 3 (opening of the mouth) aminopeptidase and alkaline phosphatase activities were found all along the intestine. Sites of non-specific esterase and protease activities became apparent in the digestive tract at days 2 and 3 respectively. Amylase was present in the exocrine pancreas at day 3 and in the lumen of the intestine at day 4. Acid phosphatase was active in the cellular structure surrounding the yolk stores and in the lipid droplets at day 1 and in the intestinal epithelium at day 3. Lipase was found at day 15 when the larvae metamorphose into juveniles.
All the investigated enzymes were detected in fasting animals, except for lipase. However, the intensities of the enzymatic activities were weaker in the fasting specimens relative to the fed specimens between days 7 and 10.     

Short-term fasting and lipolytic activity in rat adipocytes.

The aim of this experiment was to study the influence of 18-hour food deprivation on basal and stimulated lipolysis in adipocytes obtained from young male Wistar rats. Fat cells from fed and fasted rats were isolated from the epididymal adipose tissue by collagenase digestion. Adipocytes were incubated in Krebs-Ringer buffer (pH 7.4, 37 degrees C) without agents affecting lipolysis and with different lipolytic stimulators (epinephrine, forskolin, dibutyryl-cAMP, theophylline, DPCPX, amrinone) or inhibitors (PIA, H-89, insulin). After 60 min of incubation, glycerol and, in some cases, also fatty acids released from adipocytes to the incubation medium were determined. Basal lipolysis was substantially potentiated in cells of fasted rats in comparison to adipocytes isolated from fed animals. The inhibition of protein kinase A activity by H-89 partially suppressed lipolysis in both groups of adipocytes, but did not eliminate this difference. The agonist of adenosine A (1) receptor also did not suppress fasting-enhanced basal lipolysis. The epinephrine-induced triglyceride breakdown was also enhanced by fasting. Similarly, the direct activation of adenylyl cyclase by forskolin or protein kinase A by dibutyryl-cAMP resulted in a higher lipolytic response in cells derived from fasted animals. These results indicate that the fasting-induced rise in lipolysis results predominantly from changes in the lipolytic cascade downstream from protein kinase A. The antagonism of the adenosine A (1) receptor and the inhibition of cAMP phosphodiesterase also induced lipolysis, which was potentiated by food deprivation. Moreover, the rise in basal and epinephrine-stimulated lipolysis in adipocytes of fasted rats was shown to be associated with a diminished non-esterified fatty acids/glycerol molar ratio. This effect was presumably due to increased re-esterification of triglyceride-derived fatty acids in cells of fasted rats. Comparing fed and fasted rats for the antilipolytic effect of insulin in adipocytes revealed that short-term food deprivation resulted in a substantial deterioration of the ability of insulin to suppress epinephrine-induced lipolysis.     

Lipoprotein lipase in rat lung. The effect of fasting.

We measured lipoprotein lipase activity in dried defatted preparations of rat lung using doubly labeled chylomicron triglyceride as substrate. The enzyme activity was linear for the first hour of incubation at 37 degrees C, had a pH optimum of 8.1 and was completely inhibited by 0.5 M NaC1. Lungs from fed rats hydrolyzed chylomicron triglyceride at a rate of 13.00 mumoles/g per h; the activity rate was unchanged by fasting 8-72 h. Heparin infusion into isolated lungs caused immediate release of lipoprotein lipase to the venous effluent. The activity released was equivalent to about 10% of total lung lipoprotein lipase activity in both fed and fasted rats. Since the ability to remove blood triglyceride is directly related to the level of lipoprotein lipase activity, these findings indicate that the lung is one of the few tissues able to remove efficiently blood triglyceride during fasting.     

Satiety, fasting, and refeeding study of absorption in rats

In the rat, small intestine preparation was studied with the aid of our modification of Na(+)-dependent nutrient absorption short-circuit current method. In experiments on rats, it was shown that reaction of the gut to animal state changes (fasting, satiety and refeeding) depended on its medial or distal localization. Active Na+ absorption in medial part of small intestine after refeeding rose 3-6-fold depending on period of previous fasting (2 or 5 days). Two states of satiety were elucidated: when the rats were in cage with meal and after refeeding following a 5-day fasting; at least in distal small intestine, absorption of nutrients in the latter state was much higher. Fast nutrient adaptation (approximately 30 min) of absorption was revealed, second responses of short-circuit current to glyala were 3.4-fold higher than the first one: 33.4 +/- 9.7 (n = 6) and 9.9 +/- 2.9 microA/cm2 (n = 6) (P < 0.05). It is possible that increased nutrients (glucose and aminoacids) entering in mucose after the 5th day refeeding play role as a primary signal for change of animal behavior.     

The effects of fasting, diabetes, and hypophysectomy on adipose lactic dehydrogenase isozymes.

Lactic dehydrogenase isozyme distributions have been examined in epididymal adipose tissue of fasting, diabetic, and hypophysectomized rats. In each of these conditions the electrophoresis patterns show an enhancement of H subunits relative to M subunits suggestive of an induction of H subunits. Careful assessment of the total lactic dehydrogenase activity of adipose tissue from fasted, diabetic, and hypophysectomized rats reveals, however, that these conditions decrease total lactic dehydrogenase activity to approximately one-half those of normal animals. When this factor and the relative activity of the M and H subunits are taken into consideration it is apparent that the pattern shift from normal in these three states is due to a disproportionate loss of M subunits. An explanation for these shifts is that some degradative process, possibly activation of a protease, is induced by each of these stress conditions which selectively degrades M subunits. An interesting reversal by phosphate of the electrophoresis patterns of fasting, hypophysectomized, and diabetic rats is demonstrated.     

Sodium sparing effect of alanine in fasting obese females.

In obese females, 50 g of alanine given orally on the fifth day of fasting, produced a decrease in urinary sodium that persisted the following day. Plasma renin did not increase the day following alanine intake.     

Hyperlipoproteinemia in fasting ponies

Ponies fasted for up to 8 days showed, both by agarose electrophoresis and preparative ultracentrifugation, the appearance of a pre-beta-migrating, very low density lipoprotein fraction in plasma. This lipoprotein differs from the very low density lipoprotein found in humans and rats in that it contains a relatively smaller amount of total cholesterol, 85% of which is present in the unesterified form. By the 8th day of fasting, plasma triglyceride concentrations had increased from a prefasting level of 20 mg/dl to as high as 1000 mg/dl. The increase in plasma lipid concentrations as a result of fasting was highly variable. Accumulation of plasma cholesterol and triglyceride after injection of Triton WR 1339 was not related to the degree of fasting hyperlipidemia. This suggests that the hyperlipoproteinemia of fasting may result from an impaired utilization of very low density lipoproteins.     

Size at birth, fasting glucose and insulin levels and insulin resistance in adult twins.

Studies in singletons have found an association between birthweight and Type 2 diabetes in adult life. The aim of this study was to investigate whether this association could also be seen in twins. 59 monozygotic (MZ) and 69 dizygotic (DZ) same-sex twin pairs aged 19-50 years and 89 singleton controls matched for age, gestational age, gender, maternal age and parity were recruited from a local obstetric database. Associations between adult glucose, HbA(1)C and insulin levels and insulin resistance and birthweight were assessed by linear regression with adjustment for confounding variables. Twins were significantly lighter at birth than singleton controls, but there were no significant differences in adult weight, glucose, HbA(1)C and insulin levels or insulin resistance between twins and controls. The relationship between birthweight and fasting glucose and insulin levels, and insulin resistance was not significantly different from zero in either twins or controls, but birthweight was significantly negatively associated with HbA(1)C only in controls. There was no evidence of a difference between MZ and DZ twins in unpaired or within-pair analysis. These results provide little evidence that low birthweight in twins increases the risk of impaired glucose-insulin metabolism in young adults or that genetic factors can account for the association observed in singletons.