1. Dietary magnesium deficiency is commonly associated with significant potassium depletion although the mechanisms responsible are unknown. Because the kidney has an important role in both magnesium and potassium homeostasis, clearance and micropuncture experiments were performed on thyroparathyroidectomized magnesium-deficient, normal and hypermagnesaemic rats to study the effect of body magnesium status on renal potassium handling.

2. Dietary magnesium restriction that reduced total-body magnesium by 30% did not alter renal potassium excretion despite a 10% reduction in total-body potassium. Graded magnesium infusions increased the fractional excretion of potassium in both magnesium-depleted and normal rats. However, the increase in the dietary depleted group was significantly less than in the control group (5-10 and then 13% compared with 7-19 and then 28% respectively). These changes in urine potassium excretion followed alterations in distal-tubule function. Parathyroid hormone did not alter potassium excretion in any of the experimental groups in contrast with its effect on magnesium excretion.

3. These data support the concept of distal tubular control of renal potassium homeostasis. The body magnesium status appears to exert some control over cellular potassium content and to alter indirectly distal-tubule potassium excretion.

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