Arachidonic acid metabolism and thyrotropin secretion in vitro

Relationship between fatty acids and the endocrine and neuroendocrine system

Significant interactions exist between fatty acids and the endocrine system. Dietary fatty acids alter both hormone and neuropeptide concentrations and also their receptors. In addition, hormones affect the metabolism of fatty acids and the fatty acid composition of tissue lipids. The principal hormones involved in lipid metabolism are insulin, glucagon, catecholamines, cortisol and growth hormone. The concentrations of these hormones are altered in chronic degenerative conditions such as diabetes and cardiovascular disease, which in turn leads to alterations in tissue lipids. Lipogenesis and lipolysis, which modulate fatty acid concentrations in plasma and tissues, are under hormonal control. Neuropeptides are also involved in lipid metabolism in brain and other tissues. Polyunsaturated fatty acids are also precursors for eicosanoids including prostaglandins, leucotrienes, and thromboxanes, which have hormone-like activities. Fatty acids in turn affect the endocrine system. Saturated and trans fatty acids decrease insulin concentration leading to insulin resistance. In contrast, polyunsaturated fatty acids increase plasma insulin concentration and decrease insulin resistance. In humans, omega3 polyunsaturated fatty acids alter the levels of opioid peptides in plasma. Free fatty acids have been reported to inhibit glucagon release. Fatty acids also affect receptors for hormones and neuropeptides.

Role of nitric oxide in the metabolism of arachidonic acid in the rat anterior pituitary gland

Nitric oxide (NO) affects cyclooxygenase (COX) and lipooxygenase (LOX) activities in several tissues. The aim of this study was to investigate the effect of NO on the AA metabolism in the anterior pituitary. LOX and COX products from anterior pituitaries of Wistar male rats were determined by [14C]-AA radioconversion method. Sodium nitroprusside (NP, 0.5 mM) and DETA NONOate (1 mM), NO donors, decreased 5-hydroxy-5,8,11,14-eicosatetraenoic acid (5-HETE) synthesis (P<0.05), effects that were reversed by hemoglobin. L-arginine also inhibited LOX activity. To the contrary, the inhibition of NO synthase by L-NAME (0.5 mM) or aminoguanidine (0.5 mM) increased 5-HETE production (P<0.05). COX activity was slightly stimulated by NP and L-arginine. However, DETA NONOate induced a stimulation of the synthesis of all prostanoids (P<0.05), this effect being reversed by hemoglobin. Neither NOS inhibitors nor hemoglobin modified basal prostanoids synthesis. These results indicate that NO inhibits LOX activity and stimulates COX activity in the anterior pituitary gland. The inhibition of LOX by NO may be another mechanism involved in the effects of NO on hormone release in the anterior pituitary.

A phospholipase A2-related snake venom (from Crotalus durissus terrificus) stimulates neuroendocrine and immune functions: determination of different sites of action

Immune neuroendocrine interactions are vital for the individual's survival in certain physiopathological conditions, such as sepsis and tissular injury. It is known that several animal venoms, such as those from different snakes, are potent neurotoxic compounds and that their main component is a specific phospholipase A type 2 (PLA2). It has been described recently that the venom from Crotalus durissus terrificus [snake venom (SV), in the present study] possesses some cytotoxic effect in different in vitro and in vivo animal models. In the present study, we investigated whether SV and its main component, PLA2 (obtained from the same source), are able to stimulate both immune and neuroendocrine functions in mice, thus characterizing this type of neurotoxic shock. For this purpose, several in vivo and in vitro designs were used to further determine the sites of action of SV-PLA2 on the hypothalamo-pituitary-adrenal (HPA) axis function and on the release of the pathognomonic cytokine, tumor necrosis factor alpha (TNF alpha), of different types of inflammatory stress. Our results indicate that SV (25 microg/animal) and PLA2 (5 microg/animal), from the same origin, stimulate the HPA and immune axes when administered (i.p.) to adult mice; both preparations were able to enhance plasma glucose, ACTH, corticosterone (B), and TNF alpha plasma levels in a time-related fashion. SV was found to activate CRH- and arginine vasopressin-ergic functions in vivo and, in vitro, SV and PLA2 induced a concentration-related (0.05-10 microg/ml) effect on the release of both neuropeptides. SV also was effective in changing anterior pituitary ACTH and adrenal B contents, also in a time-dependent fashion. Direct effects of SV and PLA2 on anterior pituitary ACTH secretion also were found to function in a concentration-related fashion (0.001-1 microg/ml), and the direct corticotropin-releasing activity of PLA2 was additive to those of CRH and arginine vasopressin; the corticotropin-releasing activity of both SV and PLA2 were partially reversed by the specific PLA2 inhibitor, manoalide. On the other hand, neither preparation was able to directly modify spontaneous and ACTH-stimulated adrenal B output. The stimulatory effect of SV and PLA2 on in vivo TNF alpha release was confirmed by in vitro experiments on peripheral mononuclear cells; in fact, both PLA2 (0.001-1 microg/ml) and SV (0.1-10 microg/ml), as well as concavalin A (1-100 microg/ml), were able to stimulate TNF alpha output in the incubation medium. Our results clearly indicate that PLA2-dependent mechanisms are responsible for several symptoms of inflammatory stress induced during neurotoxemia. In fact, we found that this particular PLA2-related SV is able to stimulate both HPA axis and immune functions during the acute phase response of the inflammatory processes.

Inhibitors of the lipoxygenase arachidonic acid pathway impair glycocholate efflux in isolated rat hepatocytes

Lipoxygenase arachidonic acid metabolites mediate secretory processes in several tissues, but their possible involvement in liver transport functions is still unknown. This study evaluated the influence of the lipoxygenase inhibitor nordihydroguayaretic acid (NDGA), the cyclooxygenase inhibitor indomethacin (INDO), and the dual cyclo and lipoxygenase inhibitors 3-amino-1-[m-(trifluoromethyl)-phenyl]-2-pyrazoline (BW 755c) and eicosatetraynoic acid (ETYA) on the handling of glycocholic acid (GC) by isolated rat hepatocytes. No drug modified cell viability or oxygen consumption in hepatocytes. In 30-min incubations with 50 microM GC the initial rate of GC uptake (V0) in control hepatocytes was 1.15 +/- 0.09 nmol.mg protein-1.min-1. The cellular GC content remained constant from 10 to 30 min (steady-state phase), the 30-min value being 6.63 +/- 0.35 nmol.mg protein-1. NDGA (10-50 microM), BW 755c (25-200 microM) and ETYA (5-100 microM), prevented the steady-state phase occurring, thus determining a progressive accumulation of GC in cells with time. As compared to controls, 50 microM NDGA (+37%, p less than 0.01), 200 microM BW 755c (+39%, p less than 0.01) and 5 microM ETYA (+19%, p less than 0.05) induced the highest increases in the amount of GC in cells at 30 min, in all cases V0 being unchanged. Concentrations of BW 755c and ETYA above those indicated also decreased V0. Both V0 and the amount of cellular GC in the steady-state phase were proportionally decreased by high INDO concentrations (25-100 microM) which did not modify the morphology of the uptake curve. Since experiments with dual and lipoxygenase inhibitors suggested an impairment of GC efflux, the initial rate of GC efflux (V0ef) was measured in hepatocytes preloaded with 50 microM GC and transferred to a GC-free medium. In controls, V0ef was 1.12 +/- 0.12 nmol.mg protein-1.min-1. BW 755c (200 microM) and NDGA (50 microM) reduced V0ef by 45 and 38%, respectively. The kinetic analysis of the effect of 200 microM BW 755c on the efflux process using hepatocytes preloaded with GC from 5 to 200 microM disclosed a non-competitive inhibition. Vmax was reduced from 1.37 +/- 0.15 to 0.89 +/- 0.10 (p less than 0.01), whereas Km was unchanged (3.79 +/- 0.33 vs. 4.25 +/- 0.54, N.S.). In summary, inhibitors of the lipoxygenase arachidonic acid pathway impaired the efflux of GC from isolated rat hepatocytes. The hypothesis is raised that oxidized metabolites of arachidonic acid may participate on the secretion of bile salts in these cells.

Possible involvement of lipoxygenase pathway of arachidonic acid in rat pituitary hormone release in vitro

The roles of arachidonic acid (AA) and its lipoxygenase products in control of secretion of anterior pituitary hormones were studied in vitro using cultured cells. AA (10(-4)M) and 5-hydroxy-eicosatetraenoic acid (5HETE) (5 x 10(-6)M) significantly (p less than 0.05) stimulated the releases of LH, TSH, GH, PRL, ACTH and beta-endorphin (beta-E). Added leukotriene B4 (LTB4) (5 x 10(-6)M) also caused significant increases in the secretions of LH, GH, ACTH and beta-E. The other lipoxygenase metabolites tested, 12HETE, 15HETE, LTA4, LTC4 and LTD4, had no effect on the releases of anterior pituitary hormones. These results suggest that AA and 5-lipoxygenase metabolites may be involved in the control of the releases of anterior pituitary hormones.

Effect of prostaglandin E2 on ACTH and beta-endorphin release from rat adenohypophysis in vitro after secretagogues which can mimic various first or second messengers

The purpose of the present study was to further characterize the inhibition by prostaglandin E2 (PGE2) of adrenocorticotropin (ACTH) and beta-endorphin release from rat anterior pituitary fragments in vitro. Peptide hormone release was induced by vasopressin, which initiates secretion via cell surface receptors, or by secretagogues which can mimic various post-receptor mechanisms and the effect of PGE2 was examined. Concentration-response curves of the effect of vasopressin on the release of beta-endorphin-like (beta-End-IR) and ACTH-like immunoreactivity (ACTH-IR) were constructed in the absence or presence of a fixed concentration of PGE2. The concentration-response curve of vasopressin was shifted to the right about 8-fold by PGE2 (1 mumol/l) without altering the maximum effect. PGE2 (60 nmol/l-1 mumol/l) markedly reduced beta-End-IR release induced by 8-bromoadenosine-3',5'-cyclic-monophosphate (8Br-cAMP) (1 mmol/l). Omission of Ca2+ from the incubation medium did not prevent PGE2-induced inhibition of 8Br-cAMP-evoked secretion. 4 beta-Phorbol, 12 beta-myristate, 13 alpha-acetate (PMA) stimulated beta-End-IR and ACTH-IR release in a concentration-dependent manner. This effect was not blocked by indometacin or eicosatetraynoic acid. PG E2 (greater than 100 nmol/l) reduced PMA (100 nmol/l)-elicited secretion by about 50%. PG E2 (1 mumol/l) almost halved beta-End-IR release caused by K+ (30 mmol/l). After pre-incubation in Ca2+-free medium, re-introduction of Ca2+ (1.3 mmol/l) elicited beta-End-IR release. This response was abolished by PG E2 (1 mumol/l). The addition of Ba2+ (10 mmol/l) to a Ca2+-free medium markedly enhanced beta-End-IR release.(ABSTRACT TRUNCATED AT 250 WORDS)

A possible role of arachidonate metabolism in the mechanism of prolactin release

The cleavage of arachidonate from pituitary phospholipids may contribute to the process that regulates the release of prolactin. To test this hypothesis, primary cultures of anterior pituitary cells from female rats were preincubated with [3H]arachidonate to label their phospholipid-containing components. The cells were then washed and incubated with vehicle or test agents and the release into the medium of prolactin and [3H]arachidonate cleaved from the phospholipids was measured. Thyrotropin-releasing hormone (TRH) and neurotensin significantly increased the release of both [3H]arachidonate and prolactin. Although basal [3H]arachidonate release was not affected by dopamine or somatostatin, both of these agents reduced [3H]arachidonate release induced by TRH. The relationship between calcium mobilization and arachidonate release was investigated by exposing the cells to agents that modify calcium balance. Maitotoxin, a calcium channel activator, stimulated prolactin and arachidonate release. In contrast cobalt, a calcium channel blocker, penfluridol, a calcium-binding protein inhibitor, and low-calcium medium decreased basal and TRH-induced prolactin release and diminished the TRH-induced release of arachidonate. RHC 80267, an inhibitor of diacylglycerol lipase, decreased TRH-induced prolactin and arachidonate release. BW755c, an inhibitor of the conversion of arachidonate to its metabolites, decreased TRH-induced prolactin release but predictably increased arachidonate release. These findings support the hypothesis that arachidonate metabolites may be involved in the process regulating prolactin release.