Age-related changes in plasma immunoreactive growth hormone secretory patterns in broiler pullets

Apparent circulating plasma immunoreactive growth hormone (I-GH) concentrations were determined in Hubbard X Hubbard broiler pullets at weekly intervals from 1 through 12 weeks of age, in conjunction with measurement of relative weight gain (growth rate). Blood was obtained by cardiac puncture (wk 1) or venipuncture (weeks 2-12) from 12 birds (one sample per bird) at each age. Subsequently, in a second study, the growth hormone secretory pattern was quantitated for broiler pullets at 4, 8 and 12 weeks of age. At each of these ages, five birds were surgically prepared with indwelling venous catheters and blood samples withdrawn remotely at 15 or 30 min intervals for 6 h beginning at 0900 h. Plasma was assayed for I-GH and data subjected to a computer-based statistical algorithm for quantitation of secretory pattern characteristics. Plasma I-GH increased with age to peak at 4 weeks and declined thereafter, whereas relative weight gain declined overall. When adjusted for increasing body weight, however, plasma I-GH concentration per unit metabolic body size (an expression of relative tissue exposure to the hormone) was highly correlated with growth rate (P less than .007). At 4 weeks of age, chicks displayed relatively high amplitude peaks in plasma I-GH (35.0 +/- 3.20 ng/ml) (means +/- SEM) which occurred asynchronously between birds. These peaks were substantially reduced in amplitude (P less than .05) by 8 weeks (5.9 +/- 2.82 ng/ml) and remained low through 12 weeks of age (2.6 +/- 1.18 ng/ml). The baseline plasma concentration of I-GH also declined (P less than .05) from 4 to 8 weeks of age, and remained low thereafter. In contrast, the frequency and duration of peaks did not differ significantly with age, such that peaks occurred approximately every 1.75 h and were 1 h in duration, over all ages. These age-related changes in secretory pattern characteristics, and the strong association between plasma I-GH per unit of metabolic body size and growth rate, suggest that both pattern and level of tissue exposure may be important in the regulation of growth in these birds.