The effect of implantation of osmotic pumps on rat thyroid hormone and testosterone levels in the plasma, an implication for tissue 'S' phase studies

In order to monitor the effect of the procedures required to s.c. implant osmotic pumps into rats on plasma thyroid and testosterone hormone levels, male Fischer 344 rats (8-10 weeks old) were divided into six groups of 10 rats and the groups treated in the following manner: (1) controls housed 5 per cage; (2) controls housed individually; (3) animals anaesthetised for surgery and individually housed; (4) anaesthetised, sham operated and individually housed; (5) anaesthetised, s.c. implanted with osmotic pumps containing saline and individually housed; (6) anaesthetised, s.c. implanted with osmotic pumps containing 5-bromo 2-deoxyuridine (BRDU) and individually housed. Four days after performing the surgery the study was terminated and the level of hormones in the plasma determined by radio immunoassay (RIA). Tri-iodothyronine (T3) and thyroxine (T4) plasma levels (free and total) were significantly decreased with each additional step in the procedure used for the s.c. implantation of an osmotic pump containing BRDU, when compared with the individually housed controls. Similarly, testosterone plasma levels were significantly decreased by the s.c. implantation of osmotic pumps, implying a 'stress' response might occur following implantation. These observations might need to be considered by investigators when performing toxicological research which, as part of the study, uses osmotic pumps for the delivery of the nucleotide precursor required for monitoring cells in 'S' phase.