EFFECT OF THYROIDECTOMY ON THE SECRETORY PROFILES OF GROWTH HORMONE, THYROTROPIN AND CORTICOSTERONE IN THE RAT

Abstract

In view of the complicated manner of growth hormone secretion we tried to observe changes in the secretion profiles as well as GH, thyrotropin and corticosterone in the plasma from normal and thyroidectomized rats, which were obtained not in a single but sequential blood collections. These fluctuations date were analyzed by power spectral analysis.The most conspicuous change after thyroidectomy was a decrease in pituitary growth hormone contents. Pulsatile GH secretions were observed every 2.3 or 3.5 hr in normal rats, while thyroidectomy 3 weeks prior to experiments abolished the growth hormone burst. Corticosterone profiles from normal rats showed also pulsatile fashions which were more often observed in the night-time experiment. The episodic secretion of corticosterone seemed to inhibit the following growth hormone burst but not vice versa. Interrelation between integrated concentrations of growth hormone and corticosterone showed a negative correlation although the coefficient was not statistically significant. After thyroidectomy, the basal level of corticosterone decreased and the pulsatile secretion was blunched. There was no nyctohemeral variation in plasma thyrotropin level in normal rats except small and slow fluctuations. Thyroidectomy caused about ten times elevation in basal thyrotropin level.Power spectral analysis revealed that small components in growth hormone secretion in normal rats, which had period of 60, 40, and 36 min beside the hormone burst. The spectral analysis also made it clear that there were small and frequent components (40 or 36 min periods) in corticosterone secretion.By being expressed as the percentage of total power of each secretion profiles in the day experiments, the percentage of total variance accounted for by each period of interest can be directly estimated. With this management, it is suggested that these three hormones have the same spectral patterns on the episodic secretion profiles. Moreover, the effect of thyroidectomy on the spectral patterns was similar between these hormones.It is indicated that retarded growth in hypothyroid rats is due to decreased integration of the growth hormone secretion.