CALCIUM HOMEOSTASIS IN THE FOETAL GUINEA PIG

¹ Department of Anatomy, University of Newcastle upon Tyne

Plasma total calcium levels have been studied in pregnant guinea pigs and their foetuses from 26 days of gestation to term. Between 28 days and 33-39 days the foetal plasma calcium falls from nearly the maternal level (9·6 mg/100 ml.) to 5·5 mg/ 100 ml. Thereafter the foetal plasma calcium rises, reaching the maternal level at term. No trend is found in the maternal levels. Foetal skeletal ossification was studied using an alizarin stain technique. Ossification centres appeared first in the clavicle, face and skull at 26 days of gestation. After 27 days gestation (foetal plasma protein = 1 g/100 ml.) there is a steady rise in foetal plasma protein level and at term foetal and maternal levels are approximately equal. Between 41 days gestation and term foetal blood is consistently more acid than maternal blood and has a higher pCO₂ and lower pO₂. There is no evidence of a progressive change during gestation in pH, pO₂ or pCO₂ in either foetal or maternal blood. It is concluded that the fall in foetal plasma calcium between 28 days and 33-39 days gestation may be due to calcification of the foetal skeleton before the full establishment of calcium homeostasis in the foetus. Increasing foetal plasma protein concentration or foetal parathyroid activity may account for the rise in foetal plasma calcium after 33-39 days.

Note:

Our thanks are due to Drs D. Reed and G. Dale for their invaluable assistance with blood gas analyses and pH determination, and estimation of plasma protein concentration respectively. Technical assistance from Messrs D. Auld, B. Layfield and D. McLean and his department is gratefully acknowledged. This investigation was supported by grants from the Medical Research Council and the British Empire Cancer Campaign.