The effect of varying the sodium or potassium intake, or both, on magnesium status in the rat

Alterations in sodium or potassium intake, or both, affect the distribution and excretion of magnesium in man and animals. Experiments were performed on rats to investigate the effect of varying Na, K or Na + K intakes on Na, K and Mg excretion and plasma and tissue concentrations. In Expt 1, increasing the Na intake in a linear fashion produced a significant (P less than 0.05) quadratic response in urinary Mg excretion, with a decrease followed by an increase as Na intake rose. No effect was observed on faecal excretion of Mg. Plasma and tissues were sampled at the end of an 18 d collection period; concentrations of aldosterone in plasma and Mg, Na and K in plasma and tissue were determined. Increasing Na intake in a linear fashion produced a significant (P less than 0.05) quadratic effect on Mg concentration in heart and muscle, i.e. a decrease followed by an increase as Na intake rose; Na intake did not affect liver or bone Mg concentrations. There were no significant effects of Na intake on plasma Mg, Na or aldosterone but plasma K fell significantly (P less than 0.01) as Na intake increased. In Expt 2, constant amounts of four diets supplying adequate or high levels of Na and adequate or high levels of K but a constant intake of Mg were given to rats. The rats fed on the adequate-Na diet had a significantly (P less than 0.05) higher urinary Mg excretion than those fed on the high-Na diet; Na intake did not affect faecal Mg excretion.