Ability of corticotropin releasing hormone to stimulate cortisol secretion independent from pituitary adrenocorticotropin

Cortisol secretion by the adrenal cortex is thought to depend upon a preceding release of pituitary ACTH. This concept ignores a large number of observations suggesting important extrapituitary influences on adrenocortical function. The present study was designed to demonstrate the contribution of these extrapituitary mechanisms in the release of cortisol induced by human corticotropin releasing hormone (hCRH) in man. In patients with proven deficiency in pituitary ACTH the functional atrophy of the adrenals had been restored by pretreatment with long-acting ACTH. Fifty-eight hours after the second and last injection of ACTH a CRH test was performed (100 micrograms hCRH intravenously). Administration of hCRH induced a small but significant increase in plasma cortisol. Surprisingly, this rise was preceded by an increase in plasma ACTH similar to the ACTH response observed in the control group. It appeared that hCRH is able to stimulate cortisol release in the absence of pituitary ACTH, presumably by stimulating extrapituitary sources of ACTH.     

Linoleic acid-enriched phospholipids act through peroxisome proliferator-activated receptors alpha to stimulate hepatic apolipoprotein A-I secretion

A uniquely formulated soy phospholipid, phosphatidylinositol (PI), is under development as a therapeutic agent for increasing plasma high-density lipoprotein (HDL) levels. Soy PI has been shown to increase plasma HDL and apolipoprotein A-I (apoA-I) levels in phase I human trials. Low micromolar concentrations of PI increase the secretion of apoA-I in model human hepatoma cell lines, through activation of G-protein and mitogen-activated protein (MAP) kinase pathways. Experiments were undertaken to determine the importance of the PI head group and acyl chain composition on hepatic apoA-I secretion. Phospholipids with choline and inositol head groups and one or more linoleic acid (LA) acyl chains were shown to stimulate apoA-I secretion by HepG2 cells and primary human hepatocytes. Phospholipids containing two LA groups (dilinoleoylphosphatidylcholine, DLPC) were twice as active as those with only one LA group and promoted a 4-fold stimulation in apoA-I secretion. Inhibition of cytosolic phospholipase A2 with pyrrolidine 1 (10 microM) resulted in complete attenuation of PI- and DLPC-induced apoA-I secretion. Pretreatment with the peroxisome proliferator-activated receptor alpha (PPARalpha) inhibitor MK886 (10 microM) also completely blocked PI- and DLPC-induced apoA-I secretion. Hepatic PPARalpha expression was significantly increased by both PI and DLPC. However, in contrast to that seen with the fibrate drugs, PI caused minimal inhibition of catalytic activities of cytochrome P450 and UGT1A1 enzymes. These data suggest that LA-enriched phospholipids stimulate hepatic apoA-I secretion through a MAP kinase stimulation of PPARalpha. LA-enriched phospholipids have a greater apoA-I secretory activity than the fibrate drugs and a reduced likelihood to interfere with concomitant drug therapies.     

Metoclopramide fails to stimulate aldosterone secretion and inhibits serotonin-mediated aldosterone secretion in the rat

The acute and chronic effects of metoclopramide on aldosterone secretion in the rat model were examined. Metoclopramide 50 micrograms iv in dexamethasone-treated rats did not increase plasma aldosterone concentration. Chronic infusion of metoclopramide (72 micrograms/hr) over 5 days also did not show any increase in the plasma or urinary aldosterone concentration when compared with control rats. Metoclopramide in vitro showed no effect on aldosterone secretion from rat adrenal capsular cells but it inhibited serotonin-mediated aldosterone secretion from the same cells significantly.     

Cholinergic agonists transactivate EGFR and stimulate MAPK to induce goblet cell secretion

Conjunctival goblet cells are the primarysource of mucins in the mucous layer, the innermost layer of the tearfilm. Conjunctival goblet cell mucin secretion is under neural controlbecause exogenous addition of parasympathetic agonists stimulatesgoblet cell secretion. To elucidate the intracellular signal pathwaysused by cholinergic agonists to stimulate goblet cell mucin secretion,we determined whether p42/p44 mitogen-activated protein kinase (MAPK)is activated during cholinergic agonist-stimulated mucin secretion. Ratconjunctiva was removed, preincubated with or without antagonists, andstimulated with the cholinergic agonist carbachol (10⁴M). Carbachol statistically significantly stimulated thephosphorylation of MAPK in a time- and concentration-dependent manner.U-0126, an inhibitor of MAPK activation, completely inhibited both the activation of MAPK and goblet cell secretion stimulated by carbachol. The M₁ muscarinic antagonist pirenzepine, theM₂ muscarinic antagonist gallamine, and theM₁/M₃ muscarinic receptor antagonistN-(3-chloropropyl)-4-piperidinyl diphenylacetate (4-DAMP)also inhibited carbachol-stimulated MAPK activation. Increasing theintracellular Ca²⁺ concentration with a Ca²⁺ionophore increased MAPK activation, and chelation of extracellular Ca²⁺ inhibited carbachol-stimulated activation. Carbacholalso increased tyrosine phosphorylation of Pyk2, p60Src, and theepidermal growth factor receptor (EGFR). The Src inhibitor PP1 and theEGFR inhibitor AG-1478 completely inhibited carbachol-stimulated MAPKactivation. AG-1478 also inhibited goblet cell secretion. We concludethat carbachol transactivates the EGFR to activate MAPK, leading to conjunctival goblet cell secretion. In addition, carbachol also activates Pyk2 and p60Src that could play a role in the transactivation of the EGFR.

     

Ghrelin acts on the dorsal vagal complex to stimulate pancreatic protein secretion

Ghrelin receptors are present in the central nervous system. We hypothesized that ghrelin released from the stomach acts as an endocrine substance and stimulates brain stem vagovagal circuitry to evoke pancreatic secretion. In an in vivo anesthetized rat model, an intravenous infusion of ghrelin at doses of 5, 10, and 25 nmol increased pancreatic protein secretion from a basal level of 125 +/- 6 to 186 +/- 8, 295 +/- 12, and 356 +/- 11 mg/h, respectively. Pretreatment with atropine or hexamethonium or an acute vagotomy, but not a perivagal application of capsaicin, completely abolished pancreatic protein secretion responses to ghrelin. In conscious rats, an intravenous infusion of ghrelin at a dose of 10 nmol resulted in a 2.2-fold increase in pancreatic protein secretion over basal volume. Selective ablation of the area postrema abolished pancreatic protein secretion stimulated by intravenous infusion of ghrelin but did not alter the increase in pancreatic protein secretion evoked by diversion of bile-pancreatic juice. Immunohistochemical staining showed a marked increase in the number of c-Fos-expressing neurons in the area postrema, nucleus of the solitary tract, and dorsal motor nucleus of the vagus after an intravenous infusion of ghrelin in sham-lesioned rats; selective ablation of the area postrema eliminated this increase. In conclusion, ghrelin stimulates pancreatic secretion via a vagal cholinergic efferent pathway. Circulating ghrelin gains access to the brain stem vagovagal circuitry via the area postrema, which represents the primary target on which peripheral ghrelin may act as an endocrine substance to stimulate pancreatic secretion.     

Ghrelin acts in the central nervous system to stimulate gastric acid secretion

Ghrelin is a novel acylated peptide that functions in the regulation of growth hormone release and energy metabolism. It was isolated from rat stomach as an endogenous ligand for growth hormone secretagogue receptor. Ghrelin is also localized in the arcuate nucleus of rat hypothalamus. Intracerebroventricular (ICV) administration increases food intake and body weight. We examined the effect of ghrelin on gastric acid secretion in urethane-anesthetized rats and found that ICV administration of ghrelin increased gastric acid output in a dose-dependent manner. Vagotomy and administration of atropine abolished the gastric acid secretion induced by ghrelin. ICV administration of ghrelin also induced c-fos expression in the neurons of the nucleus of the solitary tract and the dorsomotor nucleus of the vagus, which are key sites in the central nervous system for regulation of gastric acid secretion. Our results suggest that ghrelin participates in the central regulation of gastric acid secretion by activating the vagus system.     

Stimulation of rat pancreatic exocrine secretion by urocortin and corticotropin releasing factor

Neural and hormonal mechanisms control pancreatic secretion. The effects of the corticotropin releasing factor (CRF) related neuropeptide urocortin (UCN) on pancreatic exocrine secretion were examined. In anesthetized male rats, pancreatic secretion volume and total protein were assayed. UCN increased pancreatic secretory volume and protein secretion and potentiated cholecytokinin-stimulated protein secretion. Astressin, a non-specific CRF receptor antagonist, inhibited UCN-stimulated protein output while CRF(2) receptor antagonist, antisauvagine-30, was without effect. Atropine, but not subdiaphragmatic vagotomy, inhibited UCN-mediated secretion. In acinar cells, UCN did not stimulate release of amylase nor intracellular cAMP. UCN is a pancreatic exocrine secreatagogue with effects mediated through cholinergic intrapancreatic neurons.     

G Protein beta gamma subunits act on the catalytic domain to stimulate Bruton's agammaglobulinemia tyrosine kinase.

G proteins are critical cellular signal transducers for a variety of cell surface receptors. Both alpha and betagamma subunits of G proteins are able to transduce receptor signals. Several direct effect molecules for Gbetagamma subunits have been reported; yet the biochemical mechanism by which Gbetagamma executes its modulatory role is not well understood. We have shown that Gbetagamma could directly increase the kinase activity of Bruton's tyrosine kinase (Btk) whose defects are responsible for X chromosome-linked agammaglobulinemia in patients. The well characterized interaction of Gbetagamma with the PH (pleckstrin homology)/TH (Tec-homology) module of Btk was proposed to be the underlying activation mechanism. Here we show that Gbetagamma also interacts with the catalytic domain of Btk leading to increased kinase activity. Furthermore, we showed that the PH/TH module is required for Gbetagamma-induced membrane translocation of Btk. The membrane anchorage is also dependent on the interaction of Btk with phosphatidylinositol 3,4,5-trisphosphate, the product of phosphoinositide 3-kinase. These data support a dual role for Gbetagamma in the activation of Btk signaling function, namely membrane translocation and direct regulation of Btk catalytic activity.     

Prostanoids stimulate K secretion and Cl secretion in guinea pig distal colon via distinct pathways

Short-circuit current (I(sc)) and transepithelial conductance (Gt) were measured in guinea pig distal colonic mucosa isolated from submucosa and underlying muscle layers. Indomethacin (2 microM) and NS-398 (2 microM) were added to suppress endogenous production of prostanoids. Serosal addition of PGE2 (10 nM) stimulated negative I(sc) consistent with K secretion, and concentrations >30 nM stimulated positive I(sc) consistent with Cl secretion. PGE2 also stimulated Gt at low and high concentrations. Dose responses to prostanoids specific for EP prostanoid receptors were consistent with stimulating K secretion through EP2 receptors, based on a rank order potency (from EC50 values) of PGE2 (1.9 nM) > 11-deoxy-PGE1 (8.3 nM) > 19(R)-hydroxy-PGE2 (13.9 nM) > butaprost (67 nM) > 17-phenyl-trinor-PGE2 (307 nM) > sulprostone (>10 microM). An isoprostane, 8-iso-PGE2, stimulated K secretion with an EC50 of 33 nM. Cl secretory response was stimulated by PGD2 and BW-245C, a DP prostanoid receptor-specific agonist: BW-245C (15 nM) > PGD2 (30 nM) > PGE2 (203 nM). Agonists specific for FP, IP, and TP prostanoid receptors were ineffective in stimulating I(sc) and Gt at concentrations <1 microM. These results indicate that PGE2 stimulated electrogenic K secretion through activation of EP2 receptors and electrogenic KCl secretion through activation of DP receptors. Thus stimulation of Cl secretion in vivo would occur either via physiological concentrations of PGD2 (<100 nM) or pathophysiological concentrations of PGE2 (>100 nM) that could occur during inflammatory conditions.     

Dissociation of oxytocin, vasopressin and corticotropin secretion during different types of stress

Oxytocin (OT), vasopressin (AVP), and corticotropin (ACTH) levels were measured in peripheral plasma of male rats subjected to one of three models of stress: restraint, cold, or ether. ACTH secretion was increased in all three groups compared to unstressed controls. OT secretion was increased in rats subjected to restraint or ether but not cold. AVP secretion was increased only by ether stress. The data suggest that the hypothalamic and neurohypophysial contribution to the control of ACTH secretion may vary in response to different types of stress.     

Beta-glucan and pectin derivatives stimulate prolactin secretion from hypophysis in vitro

Previous work has shown that various plant extracts administered to animals stimulate milk protein synthesis through the secretion of prolactin. It has also been shown that beta-glucan and pectin are the active molecules capable of stimulating prolactin release in vivo after intravenous injections. In this work, it is shown that beta-glucan and several pectin derivatives are able to stimulate prolactin secretion from hypophysis fragments incubated for 2 hr in a synthetic medium.     

Suppression of delayed-type hypersensitivity to syngeneic testicular cells in mice: involvement of suppressor T cells which act at the induction stage

Suppressor cells for delayed footpad reaction (DFR) against syngeneic testicular cells (TC) were detected in the spleen cells of donor mice immunized intravenously (iv) with viable syngeneic TC. Cyclophosphamide (CY)-pretreated recipients were given spleen cells from donors iv, immunized subcutaneously (sc) with syngeneic TC, and the footpad reaction at 24 hr was elicited with syngeneic TC 6 days after immunization. DFR in the recipients was suppressed by the transfer of spleen suppressor cells. The suppressor cells induced were Thy-1+, CY-sensitive, adult thymectomy (ATx)-resistant and act only at the induction stage. They directly suppress the generation of effector T cells for delayed-type hypersensitivity (DTH). When mice pretreated with CY were actively immunized with syngeneic TC, DFR could be provoked to a measurable level only when they were immunized sc. However, peritoneal exudate cells of those tolerant mice immunized sc without CY pretreatment or immunized iv with CY pretreatment also passively transferred DFR locally, suggesting the existence of effector T cells for DTH even in tolerant mice.