Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

III. Starvation, Flight, and Postprandial Effects: Circadian and Circannual Rhythms
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parrots ( Baker, 1980 ; Murphey, 1992a ), raptors ( Cooper
and Forbes, 1983 ; Forbes and Cooper, 1993 ), and bustards
( Nichols et al. , 1997 ) and is known as fatty liver syndrome.
Although the exact mechanism has not been elucidated, it
seems that deficiencies of other nutrients, which are essential
in lipid metabolism, like the amino acids methionine
and cysteine and the vitamin biotin, may play a crucial role
in the pathophysiology of this syndrome ( Butler, 1976 ).
Because of the lack of these essential components for lipid
metabolism, a buildup of lipids occurs in the liver, which
eventually leads to liver failure. The need to conserve body
protein during starvation has been stressed in extremely
obese persons who were treated by starvation, because
slow loss of protein during complete starvation may lead
to sudden death because of a cumulative protein loss
( Le Maho et al ., 1988 ).
From a physiological point of view, birds thus seem to be
well equipped to deal with prolonged periods of starvation
through prolonged metabolism of fat as the major energy
source, provided they have sufficient fat stores and sufficient
essential amino acid and vitamin stores to facilitate
lipid catabolism. When clinically monitoring obese birds
during a forced starvation period, plasma concentrations of
corticosterone, β -hydroxybutyrate and uric acid can be used
to pinpoint the critical transition from phase 2 to phase 3 of
starvation.
When starving obese birds, which have a history of
malnutrition, to force them to change over to a balanced
diet, it seems prudent to give a multivitamin injection
and small amounts of a mixture of essential amino acids
to avoid a deficiency of lipotrophic factors and starvationrelated
hepatic lipidosis.
C . Biochemistry of Endurance Flight
After a 90- to 160-min flight of 48 km, homing pigeons
show marked changes in plasma chemistry, which include
increased glucagon like immunoreactivity (GLI), increased
concentrations of free fatty acids (FFA) and triglyceride
(TG), decreased thyroxine (T4), triiodothyronine (T3),
and T3/T4 ratio ( George et al. , 1989 ). Viswanathan et al.
(1987, 1988) observed significant increases in plasma glucose
and lactate, FFA, and growth hormone (GH), but not
corticosterone after a 80- to 90-min flight of 48 km. In contrast
to George et al. (1989) , they did not see changes in
T4 and T3. George et al. ( 1992) documented under similar
conditions a significant increase of plasma arginine vasotocine
(AVT) without change in plasma osmolality. However,
in free-flying tippler pigeons trained to fly continuously
for up to 5 h, Giladi et al. (1997) found three- to eightfold
increased plasma AVT (up to 100 pg/ml), increased plasma
osmolality and decreased hematocrit values.
Bordel and Haase (1993, 2000) studied the influence of
flight duration on blood parameters in homing pigeons that
returned after 2 to 22 h from release sites 113 to 620 km
away. Hematocrit values decreased from 54% in controls
to 51% in flown birds. Plasma FFA levels increased significantly
during flight, and TG concentrations gradually
decreased with progressive flight duration. Plasma concentrations
of glucose and lactate did not differ between
experimental and control birds. Immediately after takeoff
and up to 5 h of flight, plasma uric acid (UA) increased
in a linear manner and reached values of 1500 μ mol/after
flight duration 5 h to 22h (two to fourfold increase of
control values), whereas urea (UR) levels gradually rose
with flight duration to 400% of control values. Plasma protein
decreased in flown pigeons. The excretion of UR, uric
acid and N τ -methylhistidine was significantly higher in
flown birds compared to controls during 1 to 3 days immediately
following return, but immediately after flight N τ -
methylhistidine did not elevate.
These findings support the view that lipids are the main
energy source during flight. The increase in lactate during
short flights is compatible with the idea that carbohydrates
are utilized as fuel mainly in the initial phase of flight and
are used for the activity of the white glycolytic fibers in the
flight muscles. Furthermore, protein catabolism increases
during endurance flights. Because UR formation in pigeons
occurs mainly through arginolysis ( Bordel and Haase, 1998 )
and increased protein breakdown raises the availability of
arginine ( Robin et al. , 1987 ), the elevated plasma concentrations
of UR and UA can be attributed to an accelerated
protein breakdown during flight. The increased availability
of free amino acids and their conversion into metabolites of
the citric acid cycle could enhance the capacity of the tricarboxylic
acid cycle and thereupon the oxidation of acetyl-
CoA derived from lipolysis ( Dohm et al ., 1985 ). In addition,
protein degradation contributes to the prevention of dehydration
during flight because the catabolism of a mixture of
70% lipids and 30% protein yields 20% more water than
the catabolism of pure fat (Klaasen, 1996). Because the
methylated amino acid N τ -methylhistidine occurs almost
exclusive in actin and myosin filaments and is excreted after
myofilament breakdown, the findings suggest an increased
breakdown of myofibrillar proteins in the immediate period
after the flight, probably as a result of repair processes of
contractile elements in the muscles as a reaction to protein
breakdown during flight ( Bordel and Haase, 2000 ).
The AVT increase can be regarded as an overall homeostatic
mechanism during homing flights, whereby (1) lipid
is mobilized, (2) water is conserved, and (3) temperature
is regulated. There is a significant correlation between
postflight AVT and body mass loss (which in flying birds
represents mainly water loss). Water loss is related to the
duration of flight and the environmental temperature.
Despite substantial water loss, the hematocrit of flying
pigeons significantly decreases. This probably results
from expansion of plasma volume through a shift of water
from the interstitial fluid. The expanded plasma volume