The fate of [7-3H]isoproterenol in cats after intravenous administration.

The fate of intravenously administered [7-3H]isoproterenol was investigated in cats. Ten minutes after injection the concentration of 3H was highest in the heart, lungs, adrenals, and kidneys, but after 5 hr most of the radioactivity was found in the liver. The concentration of the unchanged drug in the serum declined in a biphasic manner with half-lives of 2.1--2.5 min for the first phase, and 58--77 min for the second phase. The drug was rapidly metabolized to 3-O-methylisoproterenol (MISP) and then conjugated. In 5 hr 44--55% of the administered 3H was excreted in the urine, 2--2.5% as unchanged drug, 21--41% as MISP, and 12--22% as conjugated MISP. Conjugated MISP was also found in the bile. The results indicate that the rate of formation of MISP in cats is much faster than its rate of conjugation and excretion.