Is calcitonin an active hormone in the onset and prevention of hypocalcemia in dairy cattle?

The objective of this study was to assess the potential importance of calcitonin (CALC) in the onset of subclinical hypocalcemia (experiment 1) and in the physiological mechanisms underlying the prevention of bovine hypocalcemia under metabolic acidosis (experiments 2 and 3). In experiment 1, 15 Holstein cows naturally incurring subclinical hypocalcemia during the first 5 d postpartum were classified as low subclinical hypocalcemia (LSH) when blood Ca concentrations were between 7.5 and 8.5 mg/dL, or as high subclinical hypocalcemia (HSH) when blood Ca concentrations were between 6.0 and 7.6 mg/dL. Blood samples were taken daily from d −5 to 5 relative to parturition to determine concentrations of parathyroid hormone (PTH), CALC, and 1,25(OH)2D3. In experiment 2, 24 Holstein bulls (497 ± 69 kg of body weight and 342 ± 10.5 d of age) were assigned to 2 treatments (metabolic acidosis or control). Metabolic acidosis was induced by an oral administration of ammonium chloride (2.5 mEq/d) during 10 d, and animals were slaughtered thereafter. Blood samples were collected before slaughter to determine CALC, PTH, 1,25(OH)2D3, and samples of urine, kidney, parathyroid, and thyroid glands were obtained immediately after slaughter to determine expression of several genes in these tissues. Last, in experiment 3, we tested the activity of CALC under metabolic acidosis in vitro using breast cancer cell (T47D) cultures. Although PTH tended to be greater in HSH than in LSH, the levels of 1,25(OH)2D3 were lower in HSH cows (experiment 1). Blood CALC concentration was not affected by the severity of subclinical hypocalcemia, but it was influenced by days from calving (experiment 1). The expression of PTH receptor (PTHR) in the kidney was increased under metabolic acidosis (experiment 2). Furthermore, the activity of CALC was impaired under acidic blood pH (experiment 3). In conclusion, the CALC rise in HSH cows after calving impaired the recovery of blood Ca concentrations because the PTHR response was not sufficient to activate 1,25(OH)2D3 and compensate for the CALC effect. Metabolic acidosis prevents hypocalcemia because the expression of PTHR is upregulated in the kidney, resulting in an increased PTH activity and a subsequent increase in 1,25(OH)2D3 serum concentrations. Moreover, an impairment of CALC activity at low pH enhances the hypercalcemic role of PTH.