Hypo-osmolarity stimulates and high sodium concentration inhibits thyrotropin-releasing hormone secretion from rat hypothalamus

The Effect of Swelling on TRH and Oxytocin Secretion From Hypothalamic Structures

1. Cell swelling induces exocytosis of material stored in secretory vesicles resulting in a secretory burst of peptidic hormones or enzymes from various types of cells including endocrine cells and neurons. We have previously shown that swelling-induced exocytosis possesses limited selectivity; hypotonic medium evokes TRH but not oxytocin release from hypothalamic paraventricular nucleus (PVN) and neurohypophysis (NH). 2. It is the aim of this study to ascertain whether the swelling-induced oxytocin secretion could be unmasked by the inhibition of specific osmotic response using Ca(2+)-free medium and GdCl(3), an inhibitor of stretch activated channels. 3. Oxytocin release from the PVN was stimulated by the hypotonic medium only in the presence of 50 or 100 microM GdCl(3.) Oxytocin release from supraoptic nucleus (SON) was also stimulated by the Ca(2+)-free hypotonic medium in the presence of GdCl(3). Oxytocin secretion from the NH was not stimulated even in the presence of GdCl(3), both in Ca(2+) containing and Ca(2+)-free medium. TRH response to swelling-inducing stimulus was not affected by the presence of GdCl(3). 4. An intranuclear oxytocin secretion to hyposmotic stimulation within the PVN and the SON could be unmasked by the inhibiting specific response by GdCl(3). At these conditions general secretory response to swelling-inducing stimuli emerged. Secretion of oxytocin from the NH was not affected by any of these treatments. 5. Peptides and proteins released after cell swelling can play an important role in the pathophysiology of ischemia and could be mediators of local or remote preconditioning. Disruption of mechanosensitive gating in magnocellular neurosecretory cells could result in an inadequate secretory response (e.g. stimulation instead of inhibition and vice versa) of hormones engaged in water and salt metabolism regulation.

Different Signaling Pathways Involved in Glucose- and Cell Swelling-Induced Insulin Secretion by Rat Pancreatic Islets in Vitro

BACKGROUND

The objective was to compare signal transduction pathways exploited by glucose and cell swelling in stimulating insulin secretion.

METHODS

Isolated rat (Wistar) pancreatic islets were stimulated in vitro by 20 mmol/l glucose or 30% hypotonic medium (202 mOsm/kg) in various experimental conditions.

RESULTS

Glucose did not stimulate insulin release in calcium free medium. Cell swelling-induced insulin release in calcium free medium, even in the presence of the membrane permeable calcium chelator BAPTA/AM (10 micromol/l). Protein kinase C (PKC) inhibitor bisindolylmaleimide VIII (1 micromol/l) abolished the stimulation of insulin secretion by glucose but did not affect the swelling-induced insulin release. PKC activator phorbol 12-13-dibutyrate (1 micromol/l) stimulated insulin secretion in medium containing Ca2+ and did not potentiate insulin secretion stimulated by hypotonic extracellular fluid. Dilution of the medium (10-30%) had an additive effect on the glucose-induced insulin secretion. Noradrenaline (1 micromol/l) abolished glucose-induced insulin secretion but did not inhibit hypotonic stimulation either in presence or absence of Ca2+.

CONCLUSION

Glucose- and swelling-induce insulin secretion through separate signal transduction pathways. Hyposmotic stimulation is independent from both the extracellular and intracellular Ca2+, does not involve PKC activation, and could not be inhibited by noradrenaline. These data indicate a novel signaling pathway for stimulation of insulin secretion exploited by cell swelling.

Cell swelling-induced peptide hormone secretion

Cell volume changes induced in various ways (anisosmotic environment, hormones, oxidative stress, substrate uptake) are an integral part of a signal transduction network regulating cell function. Cell swelling has received increasing attention as a stimulus for a variety of intracellular phenomena. One of the most remarkable effects of cell swelling is its powerful effect in inducing exocytosis of material in intracellular secretory vesicles. Secretion of essentially all so-packaged hormones including those from hypothalamus (thyrotropin-releasing hormone, TRH; gonadotropin-releasing hormone, GnRH), pituitary (LH, FSH, ACTH, MSH, TSH, prolactin, beta endorphin), pancreas (insulin, somatostatin, glucagon), heart (atrial natriuretic hormone) and kidney (renin) are stimulated in a concentration-related manner by medium hyposmolarity or isosmolar medium containing permeant molecules such as ethanol or urea (reviewed in Ref. 21). Cell swelling-induced exocytosis is not restricted to endocrine cells and hormones; medium hyposmolarity also induces secretion of exocrine pancreatic enzymes and myeloperoxidase from human polymorphonuclear leukocytes.

Effect of the disruption of body fluid balance on pyroglutamyl aminopeptidase (Type-1) in rat brain structures

The activity of soluble and membrane-bound pyroglutamyl aminopeptidase Type-1 (PAP I) was evaluated in the hypothalamus, hippocampus, thalamus, brain cortex, and pituitary gland of rats after applying certain hydromineral challenges. Compared to euhydrated rats, decreased enzyme activity was found in the hypophysis of rats deprived of water for 48 h, or rats drinking ad libitum hypertonic sodium chloride solution (2%) for 6 days or distilled water for 6 days and then submitted to acute water overload. PAP I cleaves the pGlu-amino acid bond of neuropeptides such as thyroliberin, luliberin, neurotensin, and bombesin. The decay of particulate PAP I activity may cause an increase of these pyroglutamate peptides in the whole pituitary. Although the deleterious or pro-homeostatic influence of this decay remains to be elucidated, the present data provide evidence for the involvement of this enzyme activity at this anatomical location in the water-electrolyte imbalance.

Hyposmotic shock stimulates insulin secretion by two distinct mechanisms. Studies with the betaHC9 cell

Exposure of betaHC9 cells to a Krebs-Ringer bicarbonate-HEPES buffer (KRBH) made hypotonic by a reduction of 25 mM NaCl resulted in a prompt stimulation of insulin release. The stimulation was transient, and release rates returned to basal levels after 10 min. The response resembles that of the first phase of glucose-stimulated insulin release. The response did not occur if the reduction in NaCl was compensated for by the addition of an equivalent osmolar amount of sorbitol, so the stimulation of release was due to the osmolarity change and not the reduction in NaCl. The hyposmotic shock released insulin in KRBH with or without Ca(2+). The L-type Ca(2+) channel blocker nitrendipine inhibited the response in normal KRBH but had no effect in KRBH without Ca(2+) despite the latter response being larger than in the presence of extracellular Ca(2+). Similar data were obtained with calciseptine, which also blocks L-type channels. The T-type Ca(2+) channel blocker flunarizine was without effect, as was the chloride channel blocker DIDS. In parallel studies, the readily releasable pool of insulin-containing granules was monitored. Immunoprecipitation of the target-SNARE protein syntaxin and co-immunoprecipitation of the vesicle-SNARE VAMP-2 was used as an indicator of the readily releasable granule pool. After hypotonic shock in the presence of extracellular Ca(2+), the amount of VAMP-2 coimmunoprecipitated by antibodies against syntaxin was much reduced compared with controls. Therefore, under these conditions, hypotonic shock stimulates exocytosis of the readily releasable pool of insulin-containing granules. No such reduction was seen in the absence of extracellular Ca(2+). In conclusion, after reexamination of the effect of hyposmotic shock on insulin secretion in the presence and absence of Ca(2+) (with EGTA in the medium), it is clear that two different mechanisms are operative under these conditions. Moreover, these two mechanisms may be associated with the release of two distinct pools of insulin-containing granules.

Cell swelling induced secretion of TRH by posterior pituitary, hypothalamic paraventricular nucleus and pancreatic islets: effect of L-canavanine

The aims of this study were to test if ethanol induces thyrotropin-releasing hormone (TRH) secretion in vitro from the posterior pituitary and hypothalamic explants by a mechanism involving cell swelling, and to characterize the pathway of stimulated secretion. Ethanol, at a concentration of 80 mM, stimulated the release of TRH from the posterior pituitary, the hypothalamic paraventricular nucleus, the median eminence, and the brain septum, when administered only in isosmolar but not in hyperosmolar medium. This indicates the involvement of a cell swelling-inducing mechanism. L-canavanine in a concentration of 3 mM, increased the basal and hyposmosis-induced TRH secretion from the posterior pituitary and the paraventricular nucleus, and both basal and ethanol-induced TRH secretion from isolated pancreatic islets. This indicates the presence of both constitutive and regulatory secretory pathways. Our results suggest that cell swelling induces exocytosis from clathrin coated granules.