Download ADAPTATIONS OF RUMINANT GLUCOSE METABOLISM TO INCREASED GLUCOSE DEMAND

ADAPTATIONS OF RUMINANT GLUCOSE
METABOLISM TO INCREASED GLUCOSE DEMAND
IMPOSED EXPERIMENTALLY OR DURING THE
TRANSITION PERIOD OF DAIRY COWS
Thomas R. Overton, James K. Drackley, Cynthia J. Ottemann-Abbamonte,
A. Denise Beaulieu, Laurel S. Emmert, G. Neil Douglas,
and Jimmy H. Clark
TAKE HOME MESSAGES
♦ Demands for glucose increase tremendously during early lactation compared with the dry
period.
♦ Increased use of amino acids for synthesis of glucose by liver appears to be a major
adaptation of ruminants to suddenly increased glucose demand, which may impact the
protein needs of transition dairy cows.
♦ In addition to mobilizing body fat stores during early lactation, dairy cows also mobilize
protein through degradation of skeletal muscle.
INTRODUCTION
Glucose is required by the lactating mammary gland to synthesize lactose, which is present in
milk at essentially a constant percentage and is the primary controller of milk volume. As a
result, dairy cows are subjected almost overnight to a nearly overwhelming increase in demand
for glucose during early lactation compared with late gestation. Dry matter intake, and the
potential dietary contribution to glucose supply of the cow, increases more slowly than does the
demand for glucose after calving. Consequently, substantial coordination of metabolism must
occur to direct glucose to the mammary gland in support of milk synthesis. Some of this
coordination involves changes in nonmammary glucose utilization, but the liver, as the largest
producer of glucose through hepatic gluconeogenesis, also may adapt its metabolism in support
of this increased glucose demand. Propionate from ruminal fermentation is quantitatively the
major substrate used by liver to make glucose. Amino acids, lactate, and glycerol typically make
smaller contributions to glucose synthesis. Our hypothesis was that the contribution of amino
acids to hepatic gluconeogenesis would increase when glucose demand was increased in
ruminants. Our first approach was to establish a sheep model for increased glucose demand and
assess adaptations in liver glucose metabolism using isolated hepatocytes (liver cells). Finally,
we sought to determine whether similar adaptations occurred in liver of transition dairy cows.
EXPERIMENTS CONDUCTED
In the first experiment, we used four Dorset wethers in a Latin square design with 3-day periods.
Wether were injected at 8-hour intervals with 0 or .5, 1.0, or 2.0 grams per day of phlorizin, a
drug that causes animals to excrete glucose in their urine and so simulates increased glucose
demand. Wethers treated with phlorizin in this experiment excreted 50 percent or more of their
probable glucose production into the urine and response was maximal when 1.0 grams per day of
phlorizin was injected. Changes in concentrations of metabolites (increased urea nitrogen) and
hormones (increased glucagon; decreased insulin:glucagon ratio) in plasma samples collected
frequently during each 3-day period were consistent with increased gluconeogenesis from amino
acids to meet the increased glucose demand.
In our second experiment, we isolated hepatocytes from 10 Dorset wethers treated with 0 or 1.0
grams per day of phlorizin for 3 days. Hepatocytes were used to determine the effects of
increased glucose demand on conversion of [1- 14 C]propionate and [1- 14 C]alanine (two major
substrates for hepatic gluconeogenesis) to CO2 and glucose. Phlorizin tended to increase
conversion of substrates to CO2 . Conversion of [1- 14 C]alanine to glucose was increased more by
phlorizin treatment than was conversion of [1- 14 C]propionate (285 percent of controls vs. 166
percent of controls; Figure 1). These data suggested that, although utilization of propionate for
gluconeogenesis is extensive, the ability to use amino acids for glucose synthesis increases when
glucose demand is increased substantially.
In our third experiment, thirty-six Holstein cows entering their second or later lactation were fed
either a high grain diet or a high fat diet for ad libitum intake or at 80 percent of predicted
requirements for NEL from 60 days prepartum until calving, and then all were switched to a
lactation diet postpartum. Liver was biopsied at -21, +1, +21, and +65 days relative to calving
and liver slices were used to determine in vitro conversion of [1- 14 C]propionate and [114
C]alanine to CO2 and glucose.
Conversion of substrates to CO2 and glucose was increased at 1 and 21 days postpartum
compared with 21 days prepartum and 65 days postpartum. Rates of conversion of [114
C]propionate to CO2 and glucose always were greater than those of [1- 14 C]alanine. However,
rates of conversion of [1- 14 C]alanine to glucose at 1 and 21 days postpartum were 198 percent
and 150 percent, respectively, of the rate measured at 21 days prepartum whereas rates of
conversion of [1- 14 C]propionate to glucose at 1 and 21 days postpartum were 119 percent and
129 percent, respectively, of the rate measured at 21 days prepartum (Figure 2).
Complex interactions among diet, intake, substrate, and day for conversion to CO2 and glucose
indicated that metabolism of [1- 14 C]propionate could be modulated by source of energy and
intake of the prepartum diet, but that these factors had minimal effects on metabolism of [114
C]alanine. Overall, effects of diet and prepartum feed restriction were small compared with
changes occurring during the transition to lactation.
We used the ratio of 3-methyl histidine to creatinine in urine to provide an index of skeletal
muscle degradation. This ratio was markedly increased shortly after calving and then declined
until approximately 21 days postpartum (Figure 3), indicating a substantially increased
degradation rate of skeletal muscle during the first 3 weeks postcalving. In total, these results
suggest that amino acids are more important gluconeogenic substrates during the first 3 weeks of
lactation than at other times during the productive cycle. Furthermore, effects of source of
energy and moderate restriction of the prepartum diet on hepatic gluconeogenesis have more
influence on propionate metabolism than on alanine metabolism and appear to be minimal by 65
days postcalving.
IMPLICATIONS
This set of experiments is the first to establish that the liver increases its capacity to produce
glucose from amino acids when ruminants face a suddenly increased glucose demand such as
that occurring at the start of lactation. This response was quite similar both when increased
glucose demand was imposed experimentally in sheep and when it occurred at the beginning of
lactation in transition dairy cows. Furthermore, increased degradation of skeletal muscle protein
seems to accompany this increased capacity for liver conversion of amino acids to glucose,
which would supply amino acids to the liver for conversion into glucose. Finally, our results
point toward more careful consideration of diet formulation relative to protein and amino acid
nutrition of dairy cows during the transition period.
20.00
umol/h*g cell DM
Conversion to glucose,
25.00
15.00
Propionate
10.00
Alanine
5.00
0.00
Control
Phlorizin
Figure 1. Conversion of [1- 14 C]propionate and [1- 14 C]alanine to glucose by hepatocytes isolated
from control or phlorizin-treated wethers.
umol/h*g wet wt
Conversion to glucose,
4.00
3.50
3.00
2.50
2.00
1.50
1.00
0.50
0.00
Propionate
Alanine
-21
1
21
65
Day relative to calving
Ratio of 3-methyl histidine to
creatinine (nmol:mg)
Figure 2. Conversion of [1- 14 C]propionate and [1- 14 C]alanine to glucose in liver biopsied from
dairy cows during the transition period.
90
80
70
60
50
40
30
20
10
0
-35
-21
-14
-10
-7
-3
1
3
7
10
14
21
35
65
Day relative to calving
High grain, ad lib
High grain, restricted
High fat, ad lib
High fat, restricted
Figure 3. Ratio of 3-methyl histidine to creatinine in urine of cows fed high grain or high fat
diets for ad libitum or restricted (80 percent of NRC requirements for NEL) intakes during the
dry period.