Page 434 - Anatomy and Physiology of Farm Animals, 8th Edition

Nutrition and Metabolism / 419

liver gluconeogenesis and more lipolysis in gluconeogenesis. Rising levels of amino
acids and propionic acid (a VFA produced
adipose tissue. The decrease in insulin is
VetBooks.ir not detrimental to working skeletal mus- in the rumen) can stimulate glucagon
release, so presumably the continual absorp-
cle, because glucose uptake by working
muscle is less insulin dependent. tion of these from the ruminant gastrointes-
Anaerobic metabolism, by working tinal tract can maintain glucagon secretion.
skeletal muscles, raises the rate of lactic Propionic acid is one of the three major
acid production. The lactate ion can diffuse VFAs produced in the rumen that can be
into the blood from the skeletal muscle, used by the liver for gluconeogenesis.
and plasma levels of lactate increase during
strenuous and prolonged exercise. The Ketosis
liver can use blood lactate for gluconeo-
genesis, and glucose can then be returned Ketosis is a metabolic state characterized
to the blood to maintain blood glucose by an increase in blood ketones, a reduc-
levels.
tion in urine and blood pH, and the pres-
ence of ketones in the urine. The increase
Blood Glucose in Ruminants in the acidic ketones in the blood and urine
are responsible for the changes in pH.
Normal ranges for blood glucose levels in Ketosis may occur when fatty acid mobili-
mature ruminants are lower than in other zation from adipose tissue is elevated and
animals, even other herbivores (cattle, 45 glucose is deficient. The deficiency in glu-
to 80 mg/dL; dog, 70 to 110 mg/dL; and cose stimulates the release of glucagon and
horse, 60 to 110 mg/dL). The lower normal inhibits insulin release, and the increased
range for mature ruminants is associated ratio of glucagon to insulin promotes the
with the relatively small amount of glucose‐ formation of ketones by the liver from
yielding carbohydrate digestion in their readily available fatty acids.
small intestine. Most of the carbohydrates Ketosis may develop in dairy cattle at
that they consume undergo fermentative the peak of lactation, when the need for
digestion in the forestomach and result in glucose to synthesize lactose (milk sugar)
the production of short‐chain VFAs, is maximal. The rapid use of glucose by the
which are absorbed directly from the mammary glands reduces blood glucose
forestomach. and brings about these changes in gluca-
Without glucose readily available via gon and insulin. Ketosis may also develop
absorption from the gastrointestinal tract, as a result of type I diabetes mellitus, in
ruminants must have a continuous and a which the primary problem is a deficiency
relatively high rate of gluconeogenesis in the of insulin. In this case, the dominant effects
liver to maintain the blood glucose level. of glucagon on fatty acid mobilization and
Glucagon appears to be an important endo- ketone synthesis are primarily responsible
crine stimulant to maintain this rate of for the development of the ketosis.