No abstract

High doses of lipopolysaccharide (LPS) induce transient hyperglycemia, then chronic hypoglycemia and increased insulin resistance. In addition, appetite is reduced, while body temperature and concentrations of cortisol and tumor necrosis factor alpha (TNF ) are elevated. Furthermore, concentrations of GH and IGF-I are reduced in cattle. The objectives of this study were to determine whether a gonadal steroid implant (20 mg estrogen and 200 mg progesterone) given to endotoxemic steers would: (1) reduce hyperglycemia, reduce hypoglycemia, reduce insulin resistance, (2) reduce changes in concentrations of GH and IGF-I, (3) reduce inappetence and reduce concentrations of blood urea nitrogen (BUN) and nonesterified fatty acids (NEFA), and (4) reduce fever and concentrations of TNF and cortisol. Holstein steers were assigned within a 2 2 factorial arrangement of treatments as follows (n=5 per group): C/C, no steroid and vehicle; S/C, steroid and vehicle; C/E, no steroid and LPS (1 µg/kg body weight (BW), i.v.); S/E, steroid and endotoxin. Steroid implants were given at 20 weeks of age (day 0) and serial blood samples (15 min) were collected on day 14 for 8 h, with vehicle or LPS injected after 2 h. Intravenous glucose tolerance tests (100 mg/kg BW) were carried out at 6 h and 24 h. Hyperglycemia was 67% lower (P<0·05) in S/E-compared with C/E-treated steers between 30 and 150 min after i.v. injection of LPS.Hypoglycemia developed after 4 h and insulin resistance was greater in S/E-compared with C/E-treated steers (P<0·05) at 6 and 24 h. Concentrations of IGF-I were restored earlier in steroid-treated steers than in controls. Concentrations of GH were not affected by steroids, but increased 1 h after injection of LPS, then were reduced for 2 h. Appetite was greater (P<0·05) in S/E-(2·1% BW) compared with C/E-treated steers (1·1% BW) (pooled ...=0·3). Concentrations of NEFA increased after injecting LPS, but concentrations were lower (P<0·05) in S/E-compared with C/E-treated steers. LPS did not affect concentrations of BUN, but concentrations were lower in steroid-treated steers. Steroids did not affect body temperature or concentrations of TNF and cortisol. In summary, gonadal steroids reduce hyperglycemia, reduce inappetence and tissue wasting, but increase insulin resistance. Furthermore, concentrations of IGF-I are restored earlier in steroid-treated than in non-steroid-treated steers injected with LPS. It is concluded that gonadal steroids reduce severity of some endocrine and metabolic parameters associated with endotoxemia. However, it is unlikely that gonadal steroids acted via anti-inflammatory and immunosuppressive actions of glucocorticoids or through reducing concentrations of cytokines.

No abstract

Secretion of growth hormone (GH) is synchronized among castrate male cattle (steers) around feeding when access to feed is restricted to a 2-hr period each day. Typically, concentrations of GH increase before and decrease after feeding. Our objectives were to determine whether i) concentrations of GH decrease in blood after start of feeding; ii) activity of immunoreactive growth hormone-releasing hormone (GHRH-ir) neurons decreases in the arcuate nucleus (ARC) after feeding; iii) activity of immunoreactive somatostatin (SS-ir) neurons in the periventricular nucleus (PeVN) and ARC increase after feeding; and iv) GHRH stimulates release of GH to a similar magnitude at 0900 and at 1300 hr, in steers fed between 1000 and 1200 hr. Blood samples were collected at 20-min intervals from 0700 to 1300 hr. Groups of steers were euthanized at 0700, 0900, 1100, and 1300 hr (n = 5 per group). Dual-label immunohistochemistry was performed on free-floating sections of hypothalami using antibodies directed against Fos and Fos-related antigens (Fos/FRA) as a marker of neuronal activity in immunoreactive GHRH and SS neurons. Concentrations of GH were high before and decreased after feeding. The percentage of SS-ir neurons containing Fos/FRA-ir in the PeVN was 50% lower (P<0.01) at 1100 hr and 36% lower (P<0.05) at 1300 hr than at 0900 hr. There was no change in percentage of SS-ir neurons containing Fos/FRA-ir in the ARC. The percentage of GHRH-ir neurons containing Fos/FRA-ir in the ARC was 66% lower (P<0.05) at 1100 hr and 65% lower (P<0.05) at 1300 hr than at 0700 hr. In contrast, the number of GHRH-ir neurons increased from 0700 to 1300 hr. GHRH-induced release of GH was suppressed at 1300 hr compared with 0900 hr. In conclusion, reduced basal and GHRH-induced secretion of GH after feeding was associated with decreased activity of GHRH neurons in the ARC and decreased activity of SS neurons in the PeVN.

The purpose of this experiment was to determine the role of growth hormone-releasing hormone (GHRH) and somatostatin (SRIH) neurons in mediating α₂-adrenergic receptor-induced stimulation of growth hormone (GH) secretion in cattle. Our first objective was to determine if stimulation of α₂-adrenergic receptors increases activity of GHRH neurons in the arcuate nucleus (ARC) and/or decreases activity of SRIH neurons in periventricular (PeVN) and ARC nuclei. Clonidine (an α₂-adrenergic agonist) or vehicle (saline) were injected i.v. into steers and dual-label immunohistochemistry was performed to quantify the number of GHRH and SRIH neurons expressing Fos and Fos-related antigens (Fos/FRA) as markers of neuronal activity. Clonidine increased concentrations of GH in serum and decreased activity of SRIH neurons in the PeVN, but not in the ARC. Clonidine did not alter activity of GHRH neurons in the ARC. Our second objective was to determine if clonidine decreases secretion of SRIH from perifused slices of hypothalami, which contain perikarya and terminals of GHRH and SRIH neurons, and from explants of hypophysial stalk alone, which contain only terminals of GHRH and SRIH neurons. Clonidine failed to alter release of GHRH or SRIH from hypothalamic slices, but stimulated release of GHRH from explants of hypophysial stalk. Blockade of SRIH receptors enabled clonidine to stimulate release of GHRH from slices of hypothalami, but also stimulated release of SRIH. These results suggest that α₂-adrenergic-induced secretion of GH occurs via a dual mechanism involving inhibition of SRIH neurons in the PeVN and direct stimulation of GHRH release from axon terminals in the median eminence.