ENHANCEMENT OF HEPATIC RIBONUCLEOTIDE REDUCTASE AND OTHER COGNATE RESPONSES BY GROWTH HORMONE IN RATS

The conversion ofdeoxyribonucleotides to ribonucleotides, catalysed by ribonucleo-tide reductase (RR), may be a rate-limiting step in DNA synthesis and cell division(Elford, Freese, Passamani & Morris, 1970). As the administration of growth hor-mone (GH) to hypophysectomized rats stimulates growth and the synthesis of DNA, protein and RNA of many tissues, including liver (di Stephano & Diermeier, 1959; Korner, 1961; Munro & Korner, 1964), the response of RR activity to GH administra-tion was investigated. Several characteristic anabolic effects of GH were also determined. Male Sprague-Dawley rats were used 7-14 days after hypophysectomy. Experi¬ mental and control animals were pair fed and kept on a light cycle of 12 h darkness: 12 h light. Bovine GH (NIH-GH-B16) was dissolved in 0-9 % saline and the pH adjusted to 9. Rats were decapitated and cartilage was removed and incubated in duplicate with 4 /¿Ci Na235S04 for the assessment of sulphation activity (Liberti, 1973). One liver slice, prepared with a Stadie-Riggs apparatus, was obtained from each animal for the assessment of incorporation of [3H]uridine (2 /iCi/flask) into RNA and of [14C]leucine (0-5 /¿Ci/flask) into protein (Jezyk & Liberti, 1969; Liberti