Decrease in the Electrolytes Concentration and Change in Lactose and Solids Content of Milk from Cows Administered Thyroxine

Abstract

There are individual differences in the electrolytes concentration and the solids content in cow's milk and an inverse relationship exists between the two. In order to examine whether such inverse relationship exists also within milks from an individual cow when the composition of her milk changes, we injected thyroxine to cows in mid-lactation and changed milk composition of them experimentally. Six Holstein cows were injected 0.03mg thyroxine per kg body weight daily for 7 days intramuscularly and the milk samples from the healthy quarters were collected 3-4 times a week, during the pre-injection, injection and post-injection weeks. In all cows the lactose concentration began to increase with the start of thyroxine injection and reached a maximum at the end of the injection period and thereafter decreased gradually. Among the electrolytes, the concentration of Na in milk remained unchanged throughout the injection period and the post injection period. The concentration of Cl and K changed in an inverse manner to the change in the lactose concentration of milk (Fig. 1 and Table 2). The electrolytes, expressed as Na+K+Cl, decreased in an inverse manner to lactose, reached a minimum at the end of the injection period and increased thereafter. The correlation between the changes in lactose and electrolytes, which were expressed as the deviations from the pre-injection levels was -0.84 (P<0.001). While a gross agreements between the change in lactose mM/l and the change in the electrolytes in relation to the balance in the osmotic concentration of milk were observed in the milk samples, the latter was larger than the former (Fig. 2 and Fig. 3.). The decrease in electrolytes during the injection period was mainly due to the decrease of chloride; and this seemed to be attributable to the increased production of CO₂ in the mammary gland cells. Contrary, in the post injection period the decrease of K constituted a larger percentage of the total decrease in the electrolytes (Fig. 4.). The results were considered in relation to the theory on the scheme for ion and lactose secretion through mammary gland cells presented by Linzell and Peaker (1971), and Peaker (1978), and the cause of the inverse relation between the electrolytes and the main organic constituents, i. e., lactose, protein and fat was discussed.