The Effect of Route of Administration on the Metabolic Fate of Terbutaline in the Rat

1. The metabolic fate of [3H]terbutaline has been investigated in rats after oral, subcutaneous, intraperitoneal and intraportal administration (5 mg per kg).2. About half the administered radioactivity was excreted in the urine and the remainder in faeces regardless of route of administration. Urinary excretion was essentially complete in 24 h, but an additional 10% of the dose was excreted in the 24–48 h faeces.3. Only one metabolite, a glucuronide conjugate of terbutaline, was excreted in the urine along with unchanged drug. About 3% of the dose was excreted unchanged in urine following oral administration. Ratios of terbutaline glucuronide to free drug were 1 : 1, 2 : 1 and 13 : 1 after subcutaneous, intraperitoneal or intraportal, and oral administration respectively, suggesting that the orally administered drug is extensively conjugated in the intestinal mucosa.4. Measurement of the mobility-pH profile by high-voltage paper electrophoresis was utilized to characterize the conjugate.