RENAL RESPONSE TO AMINOPHYLLINE IN RELATION TO HYDRATION AND EXOGENOUS VASOPRESSIN

¹ Department of Pharmacology and Therapeutics, University of Florida College of Medicine, Gainesville, Florida

A constant dose of aminophylline (27 mg/kg i.v.) was tested in dogs with different states of hydration or with simultaneous administration of different doses of vasopressin when the water load was constant. Standard clearance methods were used in conscious female beagles. Both dehydration and exogenous vasopressin at doses which reduce glomerular filtration rate (GFR, creatinine clearance), predispose the dog to an increase in GFR by aminophylline. The effect of aminophylline on electrolyte excretion and Cl^- reabsorption is augmented by the two procedures. The ionuretic effect of aminophylline is greatly reduced by heavy hydration without much change in GFR. Aminophylline with large doses of exogenous vasopressin, but not during dehydration, alkalinizes the urine and causes a small increase in HCO₃^- excretion. Moderate hydration or doses of vasopressin, which reduce urine flow but do not reduce GFR, have little or no modifying effect on the response to aminophylline. The reduction of urine flow due to exogenous vasopressin is completely antagonized by aminophylline. Indirect evidence sugests that aminophylline, similar to vasopressin, reduces free water clearance. The effect of hydrochlorothiazide is largely independent of exogenous vasopressin. The data may help to explain the variable effects of methyl xanthines reported from different laboratories.