Traditionally, endocrinology has focused on organs whose primary role is to secrete hormones. The pituitary gland, adrenal gland, thyroid gland, the gonads, the kidney and the pancreas all received much attention in the early years of endocrinology.
But in recent years there has been a paradigm shift to recognize that major metabolic tissues, including adipose tissue, liver, the gut, and even bone also serve as endocrine organs. In many cases, the hormones derived from these tissues are only now being characterized in transition dairy cattle and some have intriguing possible roles in regulating metabolism during this turbulent state of the production cycle, says Barry Bradford, animal science professor at Kansas State University.
For example bone cells produce a protein hormone called osteocalcin. Increased osteocalcin concentrations promote insulin release and also improve insulin sensitivity in adipose tissue. Combined, these responses promote increased adipose tissue fat deposition, notes Bradford. Osteocalcin also stimulates secretion of adiponectin, which is a key anti-inflammatory hormone produced in adipose tissue. The fact that osteocalcin is produced by osteoblasts suggests that its release would be greatest during skeletal deposition.
Data in growing and mature dairy cattle fits with this characterization, demonstrating that serum osteocalcin is highest early in life, declines with age, and declines further in the days immediately following calving. Interestingly, plasma osteocalcin is also decreased in heat-stressed lactating cows, consistent with both bone mobilization during heat-stress and the potential for increased inflammation.
"The developing story of osteocalcin in the transition cow will be interesting to watch," notes Bradford.
The dramatic remodeling of bone that occurs in the first week of lactation coincides with decreased insulin concentrations, the onset of insulin resistance and systemic inflammation (as indicated by acute phase proteins).
"We may find that strategies designed to promote calcium mobilization may have much broader effects on metabolic physiology through effects on osteocalcin," he notes.
Bradford provides a review of some of the most recent findings and discusses potential roles of newly-described hormones in the paper he presented at the California Animal Nutrition Conference.