COMPARISON OF THE GROWTH-PROMOTING PROPERTIES OF DAILY VERSUS CONTINUOUS ADMINISTRATION OF SOMATOTROPIN IN FEMALE RATS WITH INTACT PITUITARIES

The effect of dose and mode of delivery of recombinant bovine (rbST) or porcine somatotropin (rpST) on the growth rate of female rats with intact pituitaries was investigated in a series of experiments. The overall objective of these studies was to compare the relationship of dose and mode of delivery on the growth response to somatotropin. The equivalence of rbST and rpST in stimulation of growth was demonstrated. In an initial study, treatment of rats with 0, .67 (3 mg/kg), or 2.0 mg/d (9 mg/kg) of rbST for 28 d resulted in ADG of .8, 2.6, and 3.8 g/d (P <.0001, SEM.1 g). A log-linear dose response to rbST in the range of .07 to 32 mg/d (.3 to 145 mg/kg) for the stimulation of gain was observed. Continuous delivery of rpST in bicarbonate buffer was achieved using osmotic pumps. Sustained, log-linear growth for doses of 0, .5, 1, 2, and 4 mg of rpST/d for 28 d was demonstrated. In a direct comparison of bolus injection and continuous delivery at doses of 0, .22, .67, 2.0, and 6.0 mg of rpST/d, a significant difference m slope (P < .0001) of the modes of delivery was observed; continuous delivery had a greater slope. Depending on the dose administered, bolus injection was either more (< .67 mg/d) or less efficacious (> .67 mg/d) than continuous delivery in its ability to stimulate growth in female rats with intact pituitaries. It is hypothesized that the difference in the slope of the dose-response curves can be accounted for by the animal's ability to utilize the hormone effectively.