COMPARISON OF DAILY VERSUS CONTINUOUS ADMINISTRATION OF SOMATOTROPIN ON GROWTH-RATE, FEED-INTAKE, AND BODY-COMPOSITION IN INTACT FEMALE RATS

The response to continuous delivery or daily bolus injection of porcin e somatotropin (pST) was compared in mature, pituitary-intact female r ats (225 g). Growth rate in control rats was approximately 1 g/d over the 14-d study. There was a dose-dependent (0, .4, 1.2, and 3.6 mg of pST/d; P < .001) increase in rate of gain with an interaction (P < .00 1) of dose and mode of delivery. The slope of the dose-response curve for growth rate was linear on a logarithmic scale for both modes of de livery but was greater for continuous delivery. At the low dose (.4 mg /d) pST stimulated gain (21.7 g/14 d above control, P < .05) when admi nistered by daily injection but failed to stimulate gain (6.0 g/14 d a bove control, NS) when delivered continuously. At the high dose (3.6 m g/d), gain (above that in control rats) was 49.1 and 79.7 g/14 d for d aily and continuous delivery; the two modes were different (P < .05) f rom each other. Feed intake and liver weights were also stimulated by pST in a dose-dependent manner. The increase in liver size was accompa nied by a dose-dependent increase in liver DNA, indicative of an incre ase in cell number. Increased carcass gain was largely accounted for b y increased carcass protein accretion. Rates of carcass lipid accretio n were lower than those for protein accretion and were further decreas ed by pST, particularly by the high dose administered by continuous de livery, where a negative lipid accretion value was observed. Circulati ng IGF-I was increased by pST (P < .001) but was not affected by the m ode of delivery. The results demonstrate that the increased gain obser ved in mature rats is largely due to lean tissue accretion and is acco mpanied by an increase in feed intake.