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Effects of a Low Calcium Prepartal Diet on Calcium Homeostatic Mechanisms in the Cow: Morphologic and Biochemical Studies^1,2,3,

Department of Veterinary Pathobiology, The Ohio State University, Columbus, Ohio 43210, Miami Valley Laboratories, The Procter and Gamble Company, Cincinnati, Ohio 45247 and the Endocrine Unit, Department of Medicine, Massachusetts General Hospital, Boston, Massachusetts 02114

The effects of feeding a low calcium diet (9.5 g calcium, 25 g phosphorus/day) prepartum on calcium homeostatic mechanisms were investigated in pregnant cows, and compared to findings in cows fed a control diet supplying the required amounts of calcium (25 g) and phosphorus (25 g). Following a 4-hour EDTA challenge given 10 days prepartum, the rate of return of serum calcium was greater in cows fed the low calcium diet compared to control cows. At some intervals for 20 hours post-EDTA infusion values of plasma immunoreactive parathyroid hormone (iPTH), urinary cAMP and urinary hydroxyproline were significantly higher for cows fed the low calcium diet. The specific activity of intestinal and renal calcium-binding protein, percentage of bone surfaces undergoing resorption, and the number of cortical resorptive spaces in bone were significantly higher 1 day postpartum for cows fed the low calcium compared to the control diet. Urinary hydroxyproline excretion at 1 day postpartum was increased 553% above prepartal values for cows fed the low calcium diet compared to 345% for the control cows. Serum calcium and phosphorus remained within normal limits, while serum magnesium and urinary cAMP increased significantly from 30 days prepartum to 1 day postpartum in cows fed low calcium diet. Plasma iPTH levels increased with the approach of parturition in both groups of cows. The predominant chief cell in the parathyroid gland of cows receiving the low calcium diet was in the actively synthesizing phase of the secretory cycle and had large lamellar arrays of rough endoplasmic reticulum, a prominent Golgi apparatus, and large mitochondria. Secretory granules either were depleted from chief cells or were localized in small groups near the Golgi apparatus and plasma membrane. A more varied population of active and inactive chief cells with more numerous storage granules were present in the parathyroids of control parturient cows. Calcitonin activity in the thyroid gland was significantly greater in cows fed the low calcium diet and ultrastructurally, secretory granules were more numerous in thyroid C-cells. Calcium homeostasis in cows fed the low calcium diet prepartum appeared to be more directly under the fine control of PTH-mediated bone resorption with the approach of parturition. This greater reliance on skeletal calcium mobilization than on intestinal calcium absorption may be a significant factor in the preventative effects of low calcium diets on the development of parturient hypocalcemia.

KEY WORDS: • low calcium diet • dairy cow • calcium homeostasis • parturient hypocalcemia • parathyroid hormone

¹ Supported in part by Grants RR-05463 and GM-1052 from the National Institutes of Health, U.S. Public Health Service and by a grant from the Procter and Gamble Company, Cincinnati, Ohio.

² Senior author's present address is the Department of Veterinary Pathology and Hygiene, University of Illinois, Urbana, Illinois 61801.

³ Address reprint requests to Dr. Charles C. Capen, Department of Veterinary Pathobiology, 1925 Coffey Road, Goss Laboratory, The Ohio State University, Columbus, Ohio 43210.

Manuscript received 8 June 1977.