Starved-fed cycle

Under normal feeding patterns the rate of tissue protein catabolism is more or less constant throughout the day it is only in cachexia that there is an increased rate of protein catabolism. There is net protein catabolism in the postabsorptive phase of the feeding cycle and net protein synthesis in the absorptive phase, when the rate of synthesis increases by about 20-25%. The increased rate of protein synthesis is, again, a response to insulin action. Protein synthesis is an energy-expensive process, accounting for up to almost 20% of energy expenditure in the fed state, when there is an ample supply of amino acids from the diet, but under 9% in the starved state. 

Our studies of the ERT cell cycles show that they are regulated by nutrition (Britton Edgar 1998). If the newly hatched larva is starved for dietary amino acids, DNA replication in most ERTs is not initiated. Under starvation conditions these tissues express low levels of cyclin E and E2F, the transcription factor which is probably responsible for cyclin E expression. If either E2F or cyclin E is induced in starved larvae, DNA replication in the ERTs is activated, and thus expression of these genes appears to limit the ERT cell cycle. When nutrient-deprived larvae are fed, expression of E2F and cyclin E mRNAs increases approximately sixfold, and DNA replication is initiated in most ERT cells. If the animal is first fed and then starved, the ERT cell cycle is activated and then inactivated quite rapidly. These experiments all indicate that the ERT cell cycle is nutrition-responsive, rather than controlled by a rigid developmental program. 

These changes in demand for urea cycle activity are met over the long term by regulation of the rates of synthesis of the four urea cycle enzymes and carbamoyl phosphate synthetase I in the liver. All five enzymes are synthesized at higher rates in starving animals and in animals on veiy-high-protein diets than in well-fed animals eating primarily carbohydrates and fats. Animals on protein-free diets produce lower levels of urea cycle enzymes. 

More recently, Drynan et al. have used MCA to investigate the role of CPT I in controlling fluxes (from palmitate) through P-oxidation, ketogenesis and the Krebs cycle (Jfiox, Jkb, Jk bs) in hepatocytes from adult rats in different metabolic states (fed, starved, starved/refed, starved/insulin treated). This enabled them to derive flux control coefficients which describe quantitatively the control exerted by CPT I... 

Under ideal conditions the transfer of the cold slurry would occur the moment after the chiller target temperature has been reached. In practice two conditions may exist. If the chiller final chill temperature has been reached and the filters are not ready to accept the new batch, then the slurry is held in die chiller and the slurry waits for the filters. This is referred to as wait time . If the filters are ready to accept the next batch of feed but the chiller has not reached it s final chill temperature so that it is ready for transfer the filter levels will drop and the filters will be starved for feed. This is referred to as starve time . Chilled slurry is transferred to the filters. After transfer has been completed the chiller is warmed by pressuring with propane, which condenses on the walls of the chiller. The wall temperature is warmed-up to a high enough temperature so that when the prechilled slurry from the warm solution drum is fed to the chiller shock chilling at the wall will not occur. Warm-up takes time which limits production. Hie steps in the chiller cycle are ... 

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