Adenosine A1 receptor-mediated inhibition of in vitro prolactin secretion from the rat anterior pituitary

In previous studies, we demonstrated biphasic purinergic effects on prolactin (PRL) secretion stimulated by an adenosine A2 agonist. In the present study, we investigated the role of the activation of adenosine A1 receptors by (R)-N6-(2-phenylisopropyl)adenosine (R-PIA) at the pituitary level in in vitro PRL secretion. Hemipituitaries (one per cuvette in five replicates) from adult male rats were incubated. Administration of R-PIA (0.001, 0.01, 0.1, 1, and 10 æM) induced a reduction of PRL secretion into the medium in a U-shaped dose-response curve. The maximal reduction was obtained with 0.1 æM R-PIA (mean ± SEM, 36.01 ± 5.53 ng/mg tissue weight (t.w.)) treatment compared to control (264.56 ± 15.46 ng/mg t.w.). R-PIA inhibition (0.01 æM = 141.97 ± 15.79 vs control = 244.77 ± 13.79 ng/mg t.w.) of PRL release was blocked by 1 æM cyclopentyltheophylline, a specific A1 receptor antagonist (1 æM = 212.360 ± 26.560 ng/mg t.w.), whereas cyclopentyltheophylline alone (0.01, 0.1, 1 æM) had no effect. R-PIA (0.001, 0.01, 0.1, 1 æM) produced inhibition of PRL secretion stimulated by both phospholipase C (0.5 IU/mL; 977.44 ± 76.17 ng/mg t.w.) and dibutyryl cAMP (1 mM; 415.93 ± 37.66 ng/mg t.w.) with nadir established at the dose of 0.1 æM (225.55 ± 71.42 and 201.9 ± 19.08 ng/mg t.w., respectively). Similarly, R-PIA (0.01 æM) decreased (242.00 ± 24.00 ng/mg t.w.) the PRL secretion stimulated by cholera toxin (0.5 mg/mL; 1050.00 ± 70.00 ng/mg t.w.). In contrast, R-PIA had no effect (468.00 ± 34.00 ng/mg t.w.) on PRL secretion stimulation by pertussis toxin (0.5 mg/mL; 430.00 ± 26.00 ng/mg t.w.). These results suggest that inhibition of PRL secretion after A1 receptor activation by R-PIA is mediated by a Gi protein-dependent mechanism.

Evaluation of vasopressin mediated effects on hemostatic mechanisms: relationship with aquaporins and caveolin proteins.

With a view to evaluate the role of AQP-1 and caveolin proteins in the hemostatic actions of vasopressin, hemostasis was evaluated by bleeding and clotting time respectively.Groups of mice and guinea pigs were treated with arginine vasopressin (AVP) and 1-deamino-8D-AVP (DDAVP) to evaluate their effects on the hemostasis. DDAVP and AVP were able to appreciably reduce the bleeding and clotting time after sodium thiopentone, but not effectively after TEA treatment. Animal groups were pretreated with aquaporin-1 (AQP-1) blockers or water deprived to enhance the expression of AQP-1 water channels. Another group of animals were treated with caveolin protein modulators, cholera toxin (CTX) and the effect of vasopressin analogues evaluated. The results suggest that AQP-1 water channels and caveolin proteins contribute to modulate the hemostatic mechanisms of vasopressin.

IP3 production in the hypersensitive response of lemon seedlings against Alternaria alternata involves active protein tyrosine kinases but not a G-protein

IP3 increase and de novo synthesis of scoparone are produced in the hypersensitive response (HR) of lemon seedlings against the fungus Alternaria alternata. To elucidate whether a G-protein and/or a protein tyrosine kinase (PTK) are involved in signal transduction leading to the production of such a defensive response, we studied the HR in this plant system after treatment with G-protein activators alone and PTK inhibitors in the presence of fungal conidia. No changes in the level of IP3 were detected in response to the treatment with the G-protein activators cholera toxin or mastoparan, although the HR was observed in response to these compounds as determined by the scoparone synthesis. On the contrary, the PTK inhibitors lavendustin A and 2,5-dihidroxy methyl cinnamate (DHMC) not only prevented the IP3 changes observed in response to the fungal inoculation of lemon seedlings but also blocked the development of the HR. These results suggest that the IP3 changes observed in response to A. alternata require a PTK activity and are the result of a G-protein independent Phospholipase C activity, even though the activation of a G-protein can also lead to the development of a HR. Therefore, it appears that more than one signaling pathway may be activated for the development of HR in lemon seedlings: one involving a G-protein and the other involving a PTK-dependent PLC.

Cholera toxin mediated regulation of the expression of Gq alpha and G11 alpha GTP binding proteins

Previously it has been shown that persistent activation of the stimulatory adenylyl cyclase pathway with cholera toxin (CT) downregulates the Gs alpha polypeptide (80%) in a cAMP-independent manner in C6 glioma cells (Shah, 1997). This study was conducted to examine the short and long term effects of CT on the regulation of pertussis toxin-sensitive and -insensitive G proteins and their transcripts in C6 glioma cells. Treatment of C6 cells with CT (100 ng/ml) up to 16 h had no effect on either Gi or Gq/11 alpha proteins. However, prolonged exposure (24-48 h) caused increased expression of Gi (20-30%) and Gq/11 alpha proteins (40%). Urea gradient gels, which can separate Gq alpha and G11 alpha proteins, revealed that prolonged CT treatment increased the expression of both of these G proteins. The CT-mediated enhanced expression of Gq alpha and G11 alpha proteins was accompanied by increased mRNA levels of these proteins as determined by RT/PCR. Cyclic-AMP elevating agents like forskolin (10 microM) and db-cAMP (1 mM) mimicked the effect of CT on Gi but not Gq/11 alpha proteins. These studies show long term cAMP-dependent regulation of Gi and cAMP-independent expression of Gq/11 alpha proteins in C6 glioma cells.

Nutrition-physiology link: need to increase basic research

Comentam-se alguns mecanismos complexos da absorçäo intestinal em relaçäo ao seu elo com interaçöes entre nutrientes, ou ao sistema alimentar, em si. Considera-se também a biodisponibilidade de nutrientes em relaçäo ao estado nutricional dos indivíduos, afim de enfatizar a necessidade de maior conhecimento da fisiologia para a melhor compreensäo da nutriçäo, e vice-versa

Is asparagine-linked protein glycosylation an obligatory requirement for angiogenesis?

Dependence of protein N-glycosylation on capillary endothelial cell proliferation has been studied. Amphomycin, a potent N-glycosylation inhibitor, inhibited capillary endothelial cell proliferation in a dose-dependent manner. beta-Agonist isoproterenol as well as other intracellular cAMP enhancing agents, viz. cholera toxin, prostaglandin E1 and 8Br-cAMP, also enhanced capillary endothelial cell proliferation. In addition to cell proliferation, isoproterenol also enhanced protein glycosylation in these cells. Isoproterenol effect was mediated by beta-adrenoreceptors, as it got reduced on pre-treatment of cells with either atenolol or ICI 118, 551 or propranolol. Furthermore, isoproterenol stimulation of protein glycosylation by exogenous dolichyl monophosphate and its inhibition by tunicamycin (GlcNAc-1P transferase inhibitor) supported the concept that isoproterenol specifically stimulated protein N-glycosylation event(s) in the cell.

Secretory effect of serotonin on jejunal response to cholera toxin, in dog

A substância mediadora liberada pela toxina da cólera e que estimula a secreçäo intestinal é ainda desconhecida. Sabe-se que a serotonina está envolvida no estímulo secretor intestinal de água e eletrólitos. Tendo em vista a avaliaçäo de um provável papel da serotonina na induçäo secretora jejunal pela toxina da cólera, calcularam-se os volumes de água, sódio, potássio e cloreto, bem como os níveis imunorreativos de serotonina, em alça de Thiry-Vella canina. A administraçäo de toxina provocou um aumento na secreçäo de todos os eletrólitos e do fluxo de serotonina. Esses resultados sugerem que a toxina da cólera induz à liberaçäo de serotonina na luz intestinal, talvez como um mediador da secreçäo hidroeletrolítica entérica