Calcium homeostasis represents a series of mechanisms that maintainextracellular calcium concentrations within narrow limits. This includes intestinal
calcium absorption, renal reabsorption, and turnover of bone in response to calcium
demand. Numerous hormones (vitamin D, parathyroid hormone, and calcitonin) and
calcium transport genes (TRPV5, TRPV6, CB9, CB28, NCX, PMCA, and VDR) are
known to play major rolls in this process. Although calcium physiology is known to vary
significantly among species, the genetic basis for these differences is poorly understood.
We are interested in horses as a species because of their unique in calcium regulation, and
because disorders of calcium regulation are common in horses with various pathological
conditions. In order to improve our knowledge of equine calcium metabolism, we
performed two studies to address specific questions on equine calcium regulation.
In part 1, we quantified the expression of genes involved in epithelial calcium
transport in various equine tissues. Messenger RNA was cloned for transcellular calcium
transport genes, including transient receptor potential vanilloid member 5 (TRPV5),
transient potential receptor vanilloid member 6 (TRPV6), calbindin D9k (CB9), calbindin
D28k (CB28), sodium calcium exchanger (NCX), plasma membrane calcium ATPase
(PMCA), and vitamin D receptor (VDR). Comparative gene sequence analysis was
performed to reveal high homology with archived sequences for mammals, marsupials,
birds, and fish. Real-time polymerase chain reaction (PCR) assays were designed to
quantify the expression of these genes in equine kidney, and gastrointestinal tract
(stomach, duodenum, jejunum, ileum, cecum, right ventral colon, left ventral colon,
pelvic flexure, left dorsal colon, right dorsal colon, transverse colon and rectum). TRPV5,
TRPV6, and CB9 were expressed at higher levels in the duodenum and proximal
jejunum, compared to the distal jejunum, ileum and large colon. PMCA, NCX and CB28
were expressed at similar levels throughout the intestine of the horse. Finally, VDR was
expressed at higher levels in the colon compared to the small intestine.
In part 2, serum calcitonin, (a hypocalcemia hormone) concentrations were
measured in healthy horses in response to experimentally-induced hypercalcemia. This
study revealed that horses respond to increased serum calcium with rapid release of
calcitonin. It also provided validation of a human calcitonin assay for use in horses in the
research setting.