Hormonal Control of Growth in the Infant Rat*

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Abstract
To obtain information on pituitary control of growth in infant rats, we employed a hypophysectomy technique which can be used on 4- to 6-day-old pups with an 80% success rate. A 30–50% reduction in body weight gain and a 30–40% decrease in tail growth were observed in the pups after hypophysectomy. The following hormone replacement injections were administered from days 7–15: ovine GH (oGH) or PRL (oPRL), at 1 or 4 μg/g BW, alone and in combination; T4 (40 ng/g for 4 days then 8 ng/g for 4 days) alone or in combination with the low dose of oPRL and/or oGH; and rat pituitary homogenate at 10 and 100 jug wet weight/g BW. Neither dose of oPRL increased weight gain or tail growth significantly. The high dose of oGH restored body weight gain and tail growth to 95% and 80%, respectively, of that in the sham controls. The low dose of GH increased both parameters to 75% of the sham values. Responses to injections of PRL and GH combined were intermediate between those of the hormones alone. T4 alone had no effect on body weight gain but restored skeletal growth (tail length) to 94% of that in the sham controls. T4 plus PRL were more effective than T4 alone, and T4 plus GH injections restored both parameters to normal. T4 plus oPRL plus oGH injection restored body weight gain to a level intermediate between that seen with injections of T4 plus PRL and T4 plus GH and increased tail growth to 100% of the sham value. The low dose of rat pituitary homogenate restored both parameters to 82% of the sham values, and the high dose stimulated weight gain and tail growth to 89% and 97%, respectively, of the sham control values. These data indicate that in the infant rat, PRL has weak growth-promoting activity, but it can antagonize the actions of somatotropin. GH is relatively more effective in promoting body weight gain than skeletal growth, and T4 has the opposite effect. The two hormones act synergistically to restore body growth to normal. The striking effect of T4 alone in restoring skeletal growth to normal in the hypophysectomized infant is of particular interest. This response contrasts with the limited effectiveness of T4 in promoting growth in older hypophysectomized rats. 176, 1981)