Sequential blood samples were obtained from undisturbed freely-behaving male rats bearing chronic intracardiac venous cannulae. Blood was withdrawn every 15 min for periods of 4-24 h; plasma was separated, and saline-resuspended red cells were reinjected. Plasma GH was determined by radioimmunoassay. Pulsatile GH secretion was evident in each animal with most peak values greater than 200 ng/ml and most trough values less than ng/ml. The GH secretory episodes occurred at approximately 3 h intervals, and this rhythmic pattern of GH secretion persisted unchanged across all phases of a 12-h light-dark (L-D) cycle. Seven major episodes of GH secretion were observed during a single 24-h period. The mean period, or time interval between episodes, in 24 animals was 3.32 +/- 0.07 (SEM)h. The timing of the pulses with respect to the L-D cycle was similar in most animals, indicating that the rhythm may be entrained to the L-D cycle. The role of environmental lighting was further assessed in 14 animals exposed to constant light for 7 weeks. The results show that the basic rhythm was unchanged (mean period 3.18 +/- 0.06 h, peaks greater than 200 ng/ml, troughs less than 1 ng/ml), although entrainment to time of day was not evident. Subsequent exposure to the 12-h L-D cycle resulted in reversion to an entrained rhythm. These results suggest 1) that GH secretion in the rat is governed by an endogenous ultradian rhythm, with a periodicity of approximately 3.3 h, and 2) that the alternation of light and darkness probably serves as a Zeitgeber which sets the biological "clock" for GH secretion, but is not necessary for maintenance of the basic rhythm.

To further delineate the relationship between GH-releasing factor (GRF) and somatostatin (SRIF) in generation of the ultradian rhythm of GH secretion, we used two GRF peptides, human pancreas (hp) GRF-44 and rat hypothalamic (rh) GRF, and studied their interaction with SRIF by passive immunization with a specific antiserum (AS) to SRIF. Freely moving, chronically cannulated male rats were given 10 micrograms of either hpGRF-44 or rhGRF, iv, during peak (1100 h) and trough (1300 h) periods of the GH rhythm. Six-hour plasma GH profiles were obtained after pretreatment with either SRIF AS or normal sheep serum (NSS) as a control. In NSS-treated rats, the plasma GH responses to both hpGRF-44 and rhGRF were significantly greater when the peptides were administered during peak than during trough periods. Immunoneutralization with SRIF AS eliminated these differences and permitted marked GH release in response to both peptides at 1300 h. In addition, SRIF AS augmented the GRF-induced GH response at 1100 h compared with that in NSS controls. The rhGRF peptide caused significantly more GH release than hpGRF under both conditions. These results demonstrate that 1) the GH-releasing abilities of the GRF peptides vary markedly according to the time of injection; 2) the weak GRF-induced GH response observed during trough periods of the GH rhythm is due to antagonization by endogenous circulating SRIF; and 3) the rat-derived GRF may be a more potent GH secretagogue than the human-derived peptide in the rat. The findings reported here together with the available evidence provide support for the hypothesis that GRF and SRIF are secreted tonically from the hypothalamus into the hypophyseal portal blood, and that superimposed upon this steady state release is an additional 3- to 4-h rhythmic surge of each peptide, providing for integration of the ultradian rhythm of GH secretion, as observed in peripheral blood.

High-fat feeding induces insulin resistance and increases the risk for the development of diabetes and coronary artery disease. Glucocorticoids exacerbate this hyperinsulinemic state, rendering an individual at further risk for chronic disease. The present studies were undertaken to determine whether dietary fat-induced increases in corticosterone (B) reflect alterations in the regulatory components of the hypothalamic-pituitary-adrenal (HPA) axis. Adult male rats were maintained on a high-fat (20%) or control (4%) diet for varying periods of time. Marked elevations in light-phase spontaneous basal B levels were evident as early as 7 days after fat diet onset, and B concentrations remained significantly elevated up to 21 days after fat diet onset compared with controls. In contrast, there were no significant effects on any parameters of spontaneous growth hormone secretory profiles, thus providing support for the specificity of the effects on the HPA axis. In a second study, all groups of rats fed the high-fat diet for 1, 9, or 12 wk exhibited significantly elevated levels of plasma adrenocorticotropic hormone, B, fatty acid, and glucose before, during, and/or at 20, 60, and/or 120 min after the termination of a restraint stress. Furthermore, 12-wk fat-fed animals showed a significant resistance to insulin compared with normally fed controls. There were no differences in negative feedback efficacy in high-fat-fed rats vs. controls. Taken together, these results suggest that dietary fat intake acts as a background form of chronic stress, elevating basal B levels and enhancing HPA responses to stress.