Plasma testosterone levels in adult and neonatal female rats bearing testosterone propionate-filled silicone elastomer capsules for varying periods of time
Testosterone propionate (TP) was administered, by means of subcutaneous implanted silicone elastomer capsules, into adult and neonatal (aged 3 days) female rats. In the adult rats a dose-dependent increase in plasma testosterone was measured for capsules of three different sizes (5, 10 and 20 mm crystal length). Testosterone levels reached a peak 4–8 h after insertion (5 mm, 24·6±1·4 (s.e.m.) nmol/l; 10 mm, 34·0 ± 3±8; 20 mm, 44·4 ± 3·1) and returned to control levels within 4 h after removal: the calculated half-life of testosterone was 1 h for all sizes of capsule. In neonates, a capsule of 2·5 mm crystal length was removed after 4 h subcutaneous implantation (at day 3 of age) and produced peak testosterone levels of 126·2± 11·8 nmol/l: the calculated half-life was 8·6 h which compared with a half-life of 48 h after a subcutaneous injection of 312 μmol TP (in 0·05 ml arachis oil) which produced peak levels of testosterone in 4–8 h of 84·6 ±11·8 nmol/l. Chronic implants of TP-filled capsules (2·5 mm crystal length) at 3 days of age and left in situ for 15 weeks gave a half-life of 69 h. Removable silicone elastomer capsules were found to be a versatile vehicle for the administration of TP to rats of all ages where precise hormone treatment for a known period or prolonged administration is required. The duration and magnitude of plasma hormone levels should be established by assay in an in-vivo situation.