Periodic growth in rats

Microknemometry, a novel non-invasive technique, allows the accurate measurements of the lower leg length in the conscious rat, not only daily but even in periods smaller than 24 hours. Its use revealed the presence of nonlinear growth increments (mini-growth spurts) with periods between 4 and 5 days, that presented a gradual decline in amplitude when the animals were getting older, and a maximal growth rate between 0600 h and 0900 h. A sexual dimorphic growth pattern could be established with females growing less and presenting spurts of lower amplitude and smaller duration than males. High doses of recombinant human Growth Hormone (rhGH) stimulated growth velocity in female rates, but did not show any effect on males. Neonatal Monosodium Glutamate (MSG) treatment reduced growth both in males and females. Growth hormone (GH) replacement therapy in MSG treated animals was capable of increasing growth velocity, from day 30 onwards. The recovery was partial in males and complete in females. In intact male rats growth blockade induced by fasting was not followed by a catch up effect after refeeding, although growth velocity tended to increase and a clear catch up effect on weight was detected. Male rats seemed to grow at a maximal speed over at least the first 60 days of life, that cannot be accelerated with GH treatment, whereas female rats did respond to exogenous GH.