Ultrastructure of parathyroid glands and plasma immunoreactive parathyroid hormone in pregnant cows fed normal and high calcium diets
Black, H.E.; Capen, C.C.; Arnaud, C.D.
Laboratory Investigation; A Journal of Technical Methods and Pathology 29(2): 173-185
1973
ISSN/ISBN: 0023-6837 PMID: 4353416 Document Number: 57527
Parturient paresis in dairy cows, with the known metabolic disturbances, low serum Ca and P, abnormal levels of parathyroid hormone and calcitonin in blood and low rate of bone turnover, provided the background to this study of the effect of Ca content in diet on the regulation of Ca metabolism at parturition.Experimental animals, 5 Jersey cows 5 to 8 years old, from 60 days before expected parturition, were given a diet with 150 g Ca and 25 g P daily; 2 controls a diet with the NRC recommended allowance of 25 g of each daily. The diet was of 64.6% ground maize and 16.1% maize cob with soya bean meal 1.7%, cottonseed hulls 16.5% and minerals and vitamins; it provided 8.15% protein and Ca and P were brought to the desired levels with calcium carbonate and monosodium phosphate. Of that diet the cows, in metabolism stalls, had 12 lb daily with 4 lb oat straw and 10 g chromic oxide as marker in the metabolism study. Urine was collected by indwelling catheter in 2 portions, one under toluene for estimation of hydroxyproline, the other in glacial acetic acid for cyclic adenosine monophosphate (cAMP) estimation; or with both for estimation of available Ca reserves. Also estimated in urine was creatinine, to which hydroxyproline was related. In serum Ca and Mg were measured by atomic spectrophotometry or, during and immediately after infusion of EDTA, by citric acid titration. Parathyroid hormone (iPTH) in plasma was estimated by radioimmunoassay. Cows were electrocuted 48 h after parturition for a detailed examination. The parathyroid glands were prepared for electron microscopy and sections are shown. Quantitative results are graphed.Compared with the normally fed cows those on high-Ca diet showed the following abnormalities: consistently low value of parathyroid hormone in plasma up to parturition and a further decline to 48 h post partum; no rise of hydroxyproline or cAMP in urine near parturition to show mobilisation of bone salt; low serum Ca; less reserve Ca and less reponse to low blood Ca after infusion of EDTA and iPTH; chief cells in the parathyroid inactive or even atrophic. From these differences it was deduced that the cows on high-Ca diet were dependent on absorption from the gut to adjust blood Ca at parturition, while controls derived the necessary Ca from bone, at once, as required.