Changes in norphenylephrine concentration in the serum of the rabbit, rat, and guinea pig were measured and the half-life of norphenylephrine was determined. Orally administered norphenylephrine was rapidly absorbed in the serum with concentration reaching the maximum level with in 1 hour (Fig. 1, 4, and 5), and no detectable amount was found 24 hours. The half-life of norphenylephrine was found to be 3.5, 4.6 and 4.2 hr in the rabbit, rat, and guinea pig, respectively. Urinary excretion rate of norphenylephrine was studied using
3H-labeled norphenylephrine in the rat, and it was found that 12.3% of
3H-norphenylephrine was excreted within 24 hours with approximately 85% of this amount appearing within 4 hours after intraperitoneal administration. Vasopressive reaction was observed after oral and enteric administration of norphenylephrine, but this was particularly marked in the case of enteric administration (Fig. 8). These results indicated that norphenylephrine administered orally was mainly absorbed through the intestine, and that the norphenylephrine absorbed exerted a vasopressive action. Determination of urinary epinephrine showed that the administration of norphenylephrine had no effect on epinephrine excretion.
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