Calcium and strontium secretion from blood to milk

¹ Department of Physical Biology, New York State Veterinary College, Cornell University, Ithaca, New York

The mechanisms of Ca and Sr secretion from blood to milk in goats were studied using varying combinations of one radioisotope of Ca and two radioisotopes of Sr and continuous, long-term intravenous infusions as a means of maintaining desired blood levels of stable and radioisotopes of the two elements. Calcium and Sr secretion from blood to milk was the same whether radionuclides were given orally or intravenously; the same was true for radiostrontium excretion by the kidney. Stable Ca infusions gave plasma Ca levels as high as 36 mg %, causing marked decreases in blood and milk levels of ingested Ca⁴⁵ and Sr⁸⁵ and reduced renal Ca-Sr discrimination; these results are explainable in terms of a flooding of the renal tubule with stable Ca. Stable Sr infusion gave blood plasma and milk levels as high as 4.6 mg % and 39 mg %, respectively; it caused no change in blood and milk levels of ingested Ca⁴⁵ but increased levels of ingested Sr⁸⁹ by a factor of two. The relative movement of Ca and Sr from blood to milk was unaffected by infusions. After stable Ca infusions, plasma stable Ca consistently dropped below preinfusion levels; this may have been due to depression of the parathyroids by high blood Ca. After injection into the mammary gland via the teat canal, equal amounts of Ca⁴⁵ and Sr⁸⁵ moved into the blood stream, which indicated the existence of a unidirectional mechanism for preferential movement of calcium over strontium from blood to milk.