Feeding-fasting Cycles Research Articles

Blood glucose concentrations during breeding in Short-tailed Shearwaters

Blood glucose concentration was measured in breeding adult and non-breeding immature short-tailed shearwaters (Puffinus tenuirostris) at key stages during the breeding cycle on Phillip Island, Victoria. The species’ diet has a low carbohydrate content, but its mean whole blood glucose level of ∼14 mM L−1 was comparatively high; however, this may partly have resulted from the procedure of sampling birds relatively soon after they returned to the colony at night after daytime feeding. Mean whole blood glucose level was significantly, but not dramatically, higher (by about 3 mM) during the incubation stage than at other times during breeding. This pattern was not related to breeding energetics, because it was exhibited by both age-classes. It may have reflected systematic variation in the recent feeding history of the sampled birds over the breeding season. Coefficients of variation in whole blood glucose concentration within samples obtained across the breeding cycle ranged from 13.7 to 20.8%. This level of variation is a little higher than that observed during the fasting—feeding cycle of other bird species, but might be expected in a species that exploits patchily dispersed food resources.

Calcium as an Intracellular Regulator

The survival and reproduction of human life requires calcium, not only outside the cell for maintenance of skeletal and tissue architecture, and for key extracellular enzymes such as those involved in blood clotting or digestion, but also inside cells controlling their behaviour (Campbell 1983). The evolutionary success of eukaryotic cells has been critically dependent on the development of mechanisms which enable the whole organism to regulate its behaviour in response to both internal and external stimuli. Signals from the brain can provoke release of neurotransmitters from nerve terminals leading to muscle movement. Individual cell movement occurs in neutrophils, the first cells to appear at a site of infection. The daily fasting-feeding cycle requires regulation of intermediary metabolism by substrates and hormones, in order to channel nutrients through the pathways of lipid or carbohydrate oxidation necessary for supplying the energy needs of individual tissues and the body as a whole (Campbell and Hales 1976).

Determination of overt carnitine palmitoyltransferase by reversed-phase high-performance liquid chromatography

A method for the determination of carnitine palmitoyltransferase I (CPT I; EC 2.3.1.19) in isolated rat liver mitochondria by reversed-phase high-performance liquid chromatography is described. Enzyme activity is assayed by direct determination of coenzyme A (CoA) released from palmitoyl-CoA within 60 min by a linear gradient system. CPT I in rat liver mitochondria can be assayed from only 30 μg of mitochondrial protein per millilitre of assay mixture. The changes in the kinetic parameters of CPT I, including K i for malonyl-CoA, resulting from the fasting-feeding cycle are also discussed.

Differences in the sensitivity of carnitine palmitoyltransferase to inhibition by malonyl-CoA are due to differences in Ki values.

The hepatic carnitine palmitoyltransferase that is present on the outer surface of the mitochondrial inner membrane demonstrates hyperbolic substrate saturation curves with oleoyl-CoA in both fasted and fed rats. However, the addition of malonyl-CoA resulted in sigmoid substrate saturation curves, suggesting that malonyl-CoA induced the cooperative behavior. There was more of the outer carnitine palmitoyltransferase in liver mitochondria derived from fasted rats and that enzyme had a much greater Ki for malonyl-CoA than the enzyme from fed rats, but the Km values were apparently not different. The Dixon plot with mitochondria from fed rats, but not fasted rats, was curved upward, indicating cooperative inhibition by malonyl-CoA. Carnitine palmitoyltransferase of heart mitochondria had a Ki for malonyl-CoA that was much less than that of the liver enzyme and it did not change on fasting. Furthermore, no evidence for cooperative inhibition was found in the heart. The results of these studies indicate that carnitine palmitoyltransferase is not subject to substrate cooperativity and that malonyl-CoA is not a simple competitive inhibitor of this enzyme but inhibits by a mechanism involving cooperative inhibition. The fasting-feeding cycle induces changes in the liver enzyme that alter its affinity for malonyl-CoA without changing its affinity for its acyl-CoA substrate. Carnitine palmitoyltransferase from heart appears to be different from that of liver and is apparently not subject to the same control mechanisms.