Effects of a Low Calcium Prepartal Diet on Calcium Homeostatic Mechanisms in the Cow: Morphologic and Biochemical Studies

Abstract

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.