Effects of phototherapy on the activity of microsomal mixed-function monooxygenases and the parmacokinetics of [¹⁴C] hexobarbital in jaundiced and heterozygous Gunn rats¹

We assessed the effects of in vivo phototherapy of Gunn rats on the activity of hepatic microsomal mixed-function monoxygenases and on the in vivo pharmacokinetics of [¹⁴C]HB. In experiment 1 no serial changes were seen in activities of hexobarbital hydroxylase or benzo(a)pyrene hydroxylase in hepatic microsomes isolated after 2, 4 or 7 days from homozygous jaundiced female Gunn rats exposed to continuous phototherapy or in matched Gunn rats maintained under dim light. In experiment 2 homozygous jaundiced (jj) and heterozygous nonjaundiced (Jj) Gunn rats of both sexes each received i.v. [¹⁴C]HB on 2 successive days. In random order, each was exposed on the first or second day to phototherapy for 5.5 hr, beginning 0.5 hr before the administration of HB; otherwise, each was kept under dim light. Plasma [¹⁴C]HB in arterial blood samples was separated chromatographically from its labeled metabolites, and biexponential plasma disappearance curves for [¹⁴C]HB were analyzed by a SAAM-23 computer program. Clearances in female rats were much slower. In both sexes, the total body clearance and volume of distribution of HB were decreased by 20% during phototherapy of the jj but not the Jj rats; terminal plasma half-life was unchanged. In experiment 3 direct in vitro illumination of [¹⁴C]HB did not cause photodegradation of this compound, despite the presence of albumin with or without bilirubin.