Growth Hormone, Glucocorticoid and Thryoxine Response to Duration, Intensity and Wavelength of Light in Prepubertal Bulls2

Abstract

Effects of duration, intensity and wavelength of light on growth hormone (GH), glucocorticoids, thyrotropin (TSH) and thyroxine (T4) concentrations in serum were determined in prepubertal bulls maintained at 18 to 22 C and 60 tp 70% relative humidity. Mean GH was 8.3 +/- .8, 9.5 +/- 2.2 and 13.8 +/- 2.2 ng/ml (P greater than .10) during 6 weeks of exposure to 8 hr of light: 16 hr of dark (8L:16D), 16L:8D and 20L:4D, respectively. However, concentrations of GH were more variable (P less than .01) in bulle exposed to 16L:8D and 20L:4D than in bulls exposed to 8L:16D. Concentrations of GH averaged 10.6 +/- 1.3 and 13.6 +/- 2.6 ng/ml (P greater than .10) when intensities of light exposures were 22 and 540 lux. Variability of GH was greater (P less than .01) in bulls exposed to 540 lux. Increase in the duration of light by the addition of 8 hr of red (550 to 750 nm) or 8 of blue (300 to 427 nm) light to 8 hr of white (300 to 750 nm) light did not affect (P greater than .10) mean GH concentrations. Among experiments, concentrations of glucocorticoids decreased by 29 to 58% (P less than .05) when daily light exposures were increased from 8 to 15.7, 16 or 20 hours. Conversely, glucocorticoids increased by 118% (P less than .05) when light was decreased from 15.7 to 8 hr daily. Duration of light exposures did not effect concentrations of TSH or T4. Similarly, intensities of 22 and 540 lux and blue and red wavelengths of light did not affect average concentrations of glucocorticoids or T4. Injection of 33 microgram thyrotropin releasing hormone (TRH)/100 kg body weight increased (P less than .05) GH two- to five-fold above average basal values; however, duration, intensity and wavelength of light did not influence peak GH concentration or area of GH response curves after injection of TRH. We conclude that in comparison with 8 L, 16 L increased variance of GH without affecting average concentrations of GH, TSH or T4. In contrast, 16 L suppressed serum glococorticoids.