Comparative Metabolism of Calcium and Strontium in Lactation

Abstract

The comparative metabolism of Ca and Sr in the lactating and nonlactating goat was studied, using double tracer methods. At steady state, Ca was preferentially secreted into milk by a factor of about 11 over Sr in its passage from diet to milk. With corollary data from nonlactating animals, it was found that the largest amount of discrimination took place in the gastrointestinal absorption of these alkaline earths, Ca absorption being from 3 to 4 times that of Sr. The over-all discrimination of 0.09 between milk and diet resulted primarily from the following discrimination factors: absorption, 0. 24; urinary excretion, 0.60; transfer from plasma to milk, 0.61. Under steady state conditions, the relative Sr/Ca ratios were diet = 1, plasma = 0.23, skeleton = 0.23, milk = 0.09, and urine = 4.0. These values indicate that there is no differential movement of Ca and Sr between blood and bone, and that Sr is preferentially excreted in the urinary process.