Hypomagnesaemia following theophylline or furosemide injection in ewes: renal versus extrarenal effect.

1. Intravenous theophylline infusion (30 mg/kg body weight during 90 min) induces in normal ewes a magnesuria associated with hypomagnesaemia. Intravenous furosemide infusion (1 mg/kg body weight during 90 min) induces in the same animals a similar magnesuria with only a slight hypomagnesaemia.2. In magnesium deficient ewes, theophylline induces a hypomagnesaemia, without a significant rise in magnesuria.3. Theophylline infusion, with simultaneous reinjection of the excess magnesium excreted in urine, induces a significant hypomagnesaemia.4. Thyroidectomy does not inhibit the effect of theophylline on magnesaemia.5. It is concluded that the effect of theophylline on magnesaemia is mainly of extrarenal origin, while the effect of furosemide is only renal. The effect of theophylline is not mediated through calcitonin secretion.