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Dietary Calcium, Calcium Kinetics and Plasma Parathyroid Hormone Concentration in Cows¹

Department of Clinical Studies, School of Veterinary Medicine, University of Pennsylvania, New Bolton Center, Kennett Square, Pennsylvania 19348, and Endocrine Unit, Massachusetts General Hospital, Boston, Massachusetts 02114

Kinetic analysis of radiocalcium data was combined with radioimmunoassays of plasma parathyroid hormone in nonpregnant nonlactating cows 4.5 to 7.8 years of age whose calcium intake was varied from 0.05% to 1.4% of the diet. Calcium intake had no significant effect on plasma concentration of calcium or parathyroid hormone. It had a slight but significant effect on the plasma phosphorus concentration. As calcium intake increased, calcium absorption increased but calcium removal from bone decreased so that total calcium transport into the exchangeable pool remained fairly constant. A decline in calcium clearance with age was accompanied by a fall in plasma parathyroid hormone concentration, and a decrease in clacium transport into the pool. These observations are consistent with feedback controls between plasma calcium, parathyroid hormone and calcium transport operating to maintain plasma calcium homeostasis as calcium clearance declined with advancing age.

KEY WORDS: • calcium kinetics • age • radioimmunoassay • bone • compartmental analysis

¹ This work was supported in part by USPHS Research Grants No. AM12112 and AM15695, AEC Contract No. AT-(11-1)-3400, USPHS Postdoctoral Fellowship No. AM31,380, and USPHS Research Career Development Award No. 1-K3-AM14,874.

² Present address: Department of Physiological Sciences. College of Veterinary Medicine, Oklahoma State University, Stillwater, Oklahoma 74074.

Manuscript received 18 December 1975.

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