In vivo and in vitro effects of X-irradiation and histamine-PO4 on rat and bovine pineal HIOMT acitivty and melatonin synthesis

Localized irradiation to the heads of adult male rats with 450 R increased pineal gland HIOMT activity while the same amount of irradiation restricted to the body diminished the activity of the enzyme. Anesthesia had no effect on this enzyme. Extremely high doses of irradiation were required (3,870 R) to inhibit HIOMT activity of bovine enzyme preparations in vitro. Localized irradaition of the head of rats with 450 R increased melatonin biosynthesis from tryptophan-3-¹⁴C, but irradiation of the body only had no such an effect. Injections of histamine-PO₄ into rats or the addition of it to the incubation media of bovine enzyme preparations inhibited melatonin synthesis and bovine HIOMT activity, respectively.