Abstract
The hypocalcaemia caused by parturition and onset of lactation in high-production dairy cows was mimicked by subjecting cows to starvation periods before and after partus. The changes in plasma calcium, phosphate and magnesium were followed and compared with immunoreactive parathyroid hormone (iPTH) in 2 cows. During the starvation periods before partus, the cows developed hypocalcaemia with no or only small changes in the plasma concentration of magnesium. After the onset of hypocalcaemia, the concentration of iPTH increased on the average 3–4-fold and the raised hormone levels lasted about 24 h after start of refeeding. An increase in plasma phosphate occurred somewhat later than the rise in iPTH and lasted longer. After partus hypocalcaemia developed, together with smaller increases in iPTH concentration (about 2-fold). The post-partum starvation period again resulted in hypocalcaemia and raised iPTH concentrations.
In conclusion, starvation and parturition induced inverse changes in plasma calcium and iPTH in dairy cows. The increases in plasma iPTH were reversible and considered secondary to the hypocalcaemia. Through the effect of paratyroid hormone, plasma calcium was normalized and phosphate concentration increased. Therefore, fatal hypocalcaemia which may occur following the course of parturition and onset of lactation is not due to impaired PTH secretion.
Keywords: dairy cow, starvation, parturient hypocalcaemia, parathyroid hormone, calcium, magnesium, inorganic phosphate
Sammendrag
Konsentrasjonen av parathormon (PTH), kalsium, magnesium og uorganisk fosfat ble mlt i plasma hos 2 melkekyr i forbindelse med kalving og under innlagte fasteperioder.
I forbindelse med kalving økte konsentrasjonen av plasma PTH sekundært til en reduksjon i plasma kalsium konsentrasjonen. Parallelle, men mer uttalte forandringer i PTH og kalsium, ble mlt under fasteperioder.
Forandringer i konsentrasjonen av magnesium og uorganisk fosfat under kalving var varierende. Under fasteperioderne var det imidlertid et fall i plasma magnesium og en bifasisk økning i plasma fosfat.
Resultatene viser at melkefeber hos kyr ikke skyldes en primaer svikt i parathormon produksjon.
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Footnotes
This work was financially supported by a scholarship grant provided by the Norwegian Agricultural Research Council.
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