Alteration of the sodium to potassium ratio in milk and the effect on milk secretion in goats

Saanen goats were used to determine the effect of the alteration of the intramammary Na to K ratio on milk secretion. Udders were infused via the teat with an isosmotic solution that was high in Na or K to increase or decrease, respectively, the intramammary Na to K ratio. Control glands received an isosmotic sucrose solution. To ensure that the results were not confounded by a decrease in milk secretion as a result of enhanced permeability of mammary tight junctions, the latter was monitored throughout the experiments. An increase in the Na to K ratio caused a significant transient reduction in milk secretion. Therefore, an increase in Na and a decrease in K in milk, commonly observed as a result of the leakiness of tight junctions, may at least partially explain the reduction in milk secretion when the permeability of tight junctions was increased. These experiments further showed that the adverse effects on secretion were not due to a high intracellular concentration per se but were related to a change in the Na to K ratio because a reduction in the ratio also lowered milk secretion. These data support the evidence for activity of Na+-K+-ATPase in the basolateral secretory cell membranes and passive movement of these ions across the apical cell membranes.