Labile Nitrogen Reserves and Plasma Nitrogen Fractions in Growing Cattle1'2
ABSTRACT The existence and magnitude of labile nitrogen (N) reserves were studied in growing cattle using a dietary N depletion-repletion technique. Blood parameters and urinary N excretion patterns were monitored. Blood hematocrit (Ht) did not respond to reduced ingested N until after week 3 of depletion at which time it began to fall. About 3 weeks of repletion regime was required before Ht values increased again. Plasma protein and albumin decreased from 6.05 and 2.70 g during standardization to 5.44 and 2.44 g/100 ml after 5 weeks of depletion and did not ap proach predepletion levels until week 6 of repletion. Plasma urea N decreased from 16 mg in standardization to 0.8 mg/100 ml in depletion and required 3 weeks of repletion treatment to attain a peak of 18 mg/100 ml. Labile N reserves were deter mined by integrating the areas from total urinary N excretion curves obtained during depletion and repletion periods. On a live body weight basis labile N represented 5.6% of total body N. When computed on an empty body weight basis (does not include weight of gastrointestinal tract), labile N totalled 6.0%. The magnitude of labile N stores as a percentage of total body N was 44% greater in steers with a mean body weight of 280 kg compared with animals weighing 144 kg. J. Nutr. 205: 1584-1591, 1975. INDEXING KEY WORDS nitrogen • depletion-repletion reserves •plasma nitrogen/protein •ruminants
• labile
nitrogen
According to Munro (1) the concept of labile stores of body protein originated in 1866, when Voit demonstrated that dogs voided considerably more urinary nitrogen (N) during the first few days of a fasting period than after 5 to 6 days. Based on this observation, Voit suggested that the greater loss of urinary N early in the fast represented a reserve store of labile N in the body, the magnitude of which was dependent on the level of protein found in the preceding diet. Since the classical work of Voit, many nonruminant studies have been conducted in which the existence of
cated that nonprotein N contributed significantly to the reserve component in adult cockerels. Based on a similar observation in male college students, Fisher et al. (8) suggested that the term, N stores, replace protein reserves. Regardless of the nature of the labile protein or N, Munro (1) proposed that the amount stored in the body would not exceed 5% of total body protein even under favorable nutritional conditions. Although the concept of labile protein or N stores has received a great deal of
aminprl aminea
»Paper of the Journal New University, Jersey Agrlcultural Experlment station, Series, Rutgers New
labile
protein
or protein
19_fi AB rlpfinprl (¿-O)."l AS aennea
protein 5" j that can
reserves i
pleted,
thereby
amino pleting
acid pool processes.
be
reserves
are -iiiiiij those tissue reversibly depleted contributing
was ex-
Viv Allicrm Dy Allison
17Ì (I ),
Received for publication May 19, 1975.
proteins
Brunswick, 08903.of 2 PreliminaryN.J. report
and
re-
at the Annual
to the
free
these results was presented of the American Society of
j-?1â„¢^.s(lebns^:)Blacksbur8>Va- 19T2-J" Anlm Scl'
of the body during deAlthough Such a défini-
tion implied that protein per se was the labile entity, Wessels and Fisher (5) indi-
Meeting
»Present address: Southeast Utilization Research r>r?ve.r'Unlversity of "Maryland eCampuse,rCo?Ã-egeepark!
JI
ABSTRACT The existence and magnitude of labile nitrogen (N) reserves were studied in growing cattle using a dietary N depletion-repletion technique. Blood parameters and urinary N excretion patterns were monitored. Blood hematocrit (Ht) did not respond to reduced ingested N until after week 3 of depletion at which time it began to fall. About 3 weeks of repletion regime was required before Ht values increased again. Plasma protein and albumin decreased from 6.05 and 2.70 g during standardization to 5.44 and 2.44 g/100 ml after 5 weeks of depletion and did not ap proach predepletion levels until week 6 of repletion. Plasma urea N decreased from 16 mg in standardization to 0.8 mg/100 ml in depletion and required 3 weeks of repletion treatment to attain a peak of 18 mg/100 ml. Labile N reserves were deter mined by integrating the areas from total urinary N excretion curves obtained during depletion and repletion periods. On a live body weight basis labile N represented 5.6% of total body N. When computed on an empty body weight basis (does not include weight of gastrointestinal tract), labile N totalled 6.0%. The magnitude of labile N stores as a percentage of total body N was 44% greater in steers with a mean body weight of 280 kg compared with animals weighing 144 kg. J. Nutr. 205: 1584-1591, 1975. INDEXING KEY WORDS nitrogen • depletion-repletion reserves •plasma nitrogen/protein •ruminants
• labile
nitrogen
According to Munro (1) the concept of labile stores of body protein originated in 1866, when Voit demonstrated that dogs voided considerably more urinary nitrogen (N) during the first few days of a fasting period than after 5 to 6 days. Based on this observation, Voit suggested that the greater loss of urinary N early in the fast represented a reserve store of labile N in the body, the magnitude of which was dependent on the level of protein found in the preceding diet. Since the classical work of Voit, many nonruminant studies have been conducted in which the existence of
cated that nonprotein N contributed significantly to the reserve component in adult cockerels. Based on a similar observation in male college students, Fisher et al. (8) suggested that the term, N stores, replace protein reserves. Regardless of the nature of the labile protein or N, Munro (1) proposed that the amount stored in the body would not exceed 5% of total body protein even under favorable nutritional conditions. Although the concept of labile protein or N stores has received a great deal of
aminprl aminea
»Paper of the Journal New University, Jersey Agrlcultural Experlment station, Series, Rutgers New
labile
protein
or protein
19_fi AB rlpfinprl (¿-O)."l AS aennea
protein 5" j that can
reserves i
pleted,
thereby
amino pleting
acid pool processes.
be
reserves
are -iiiiiij those tissue reversibly depleted contributing
was ex-
Viv Allicrm Dy Allison
17Ì (I ),
Received for publication May 19, 1975.
proteins
Brunswick, 08903.of 2 PreliminaryN.J. report
and
re-
at the Annual
to the
free
these results was presented of the American Society of
j-?1â„¢^.s(lebns^:)Blacksbur8>Va- 19T2-J" Anlm Scl'
of the body during deAlthough Such a défini-
tion implied that protein per se was the labile entity, Wessels and Fisher (5) indi-
Meeting
»Present address: Southeast Utilization Research r>r?ve.r'Unlversity of "Maryland eCampuse,rCo?Ã-egeepark!
JI
