PubMed Health⌕ Search

Biomedical subjects

G Dayanithi

Publications and source records attributed to G Dayanithi.

At least 55 records · Page 3Linked to original sources
PubMed1993

Glucocorticoids rapidly inhibit oxytocin-stimulated adrenocorticotropin release from rat anterior pituitary cells, without modifying intracellular calcium transients.

Glucocorticoid hormones suppress the secretion of ACTH evoked by secretagogues such as CRF and arginine vasopressin. In this study, we investigated the effects of glucocorticoids on ACTH release induced by oxytocin (OT) and on intracellular free calcium ion levels in corticotropes prepared from the adenohypophyses of female Wistar rats. Pulsatile additions of physiological concentration of OT (10 nM) to superfused anterior pituitary cells caused pulsatile ACTH release about 4-fold above basal secretion with similar peak amounts of ACTH during subsequent OT pulses. Exposure of the cells to corticosterone (100 nM) or to a selective glucocorticoid receptor agonist RU 28362 (100 nM) for 30 min suppressed OT-stimulated but not basal ACTH release by approximately 60%. Inhibition gradually disappeared during subsequent pulses of OT in the absence of corticosterone. Pretreatment with the selective antagonist RU 38486 (1 microM) completely blocked the inhibitory effect of corticosterone on OT-induced ACTH secretion. Changes in free cytosolic calcium levels in single cultured pituitary cells were measured using the calcium indicator Fura-2. OT caused calcium transients in corticotropes, which were identified by immunocytochemistry. They responded in a similar manner to a second OT stimulus when preincubated for 30 min with corticosterone (1 microM) or with RU 28362 (1 microM). Our data indicate that glucocorticoids, via glucocorticoid receptors, rapidly inhibit OT-stimulated ACTH secretion by corticotropes without affecting intracellular calcium transients due to OT. Therefore, we conclude that rapid inhibition of ACTH release by glucocorticoids interferes with cellular signal transduction beyond the step of calcium mobilization.

Adrenocorticotropic Hormone↗
PubMed1992

Endothelin regulation of neuropeptide release from nerve endings of the posterior pituitary.

We have investigated the role of endothelin (ET) in the stimulus-secretion coupling mechanism in the posterior pituitary. We report that isolated nerve endings contain immunoreactive endothelin, the level of which is regulated by homeostatic mechanisms involved in control of water balance. ET-1 and ET-3 potentiate vasopressin release induced by depolarization through interaction with specific receptors of the ETA subtype and this response is antagonized by sarafotoxin S6b. The second messenger for this effect, however, remains unknown since the potentiation of depolarization-induced vasopressin release occurs in the absence of an increase in cellular calcium.

Animals↗
PubMed1992

Intracellular calcium and hormone release from nerve endings of the neurohypophysis in the presence of opioid agonists and antagonists.

Rat neural lobes and isolated nerve terminals from the neurohypophysis were stimulated in the presence of different opioid agonists and antagonists. The secretion of arginine vasopressin and oxytocin and rise in cytoplasmic calcium induced by depolarization were analyzed by radioimmunoassay and the fluorescent probe fura-2, respectively. The kappa-agonists dynorphin A(1-13) and dynorphin A(1-8) did not affect electrically evoked release of vasopressin, although oxytocin release was slightly reduced. U-50 488, a relatively specific kappa-receptor agonist, had no effect on the amount of vasopressin or oxytocin secreted, although it significantly reduced K(+)-evoked changes in [Ca2+]i in isolated nerve endings. Two kappa-receptor antagonists, MR 2266 and diprenorphin, alone had no effect on vasopressin and oxytocin secretion from isolated nerve endings depolarized with potassium. Opioid agonists less selective for the kappa receptors, etorphin and ethylketocyclazocin, were found to inhibit the release of both vasopressin and oxytocin significantly. Naloxone, a nonselective opiate receptor antagonist, alone had no effect on vasopressin release but potentiated the electrically evoked release of oxytocin. Naloxone also could overcome the inhibitory effect of etorphin on oxytocin and vasopressin release observed after electrical stimulation of the neural lobe. A number of inconsistencies therefore exist between the effects of opioid agonists and antagonists on neuropeptide release and on the evoked changes in [Ca2+]i. In view of these inconsistencies and the high concentrations of opioid agonists and antagonists necessary to modify release, we conclude that it is doubtful that opioid molecules have a physiological role in controlling neurohypophysial secretion.

Animals↗
PubMed1992

G-proteins mediate inhibition and activation of Ca(2+)-induced exocytosis from SLO-permeabilized peptidergic nerve endings.

In SLO-permeabilized isolated nerve endings from the rat neurohypophysis, GTP, guanosine 5'[y-thio]triphosphate (GTPyS) and guanosine 5'(beta y-imido]triphosphate (GMPPNP) inhibit the Ca(2+)-evoked vasopressin release. Pretreatment with pertussis toxin enhances the inhibitory effects of both GTP-analogues. Omission of Mg2+ overcomes the effect of GMPPNP and reverses the inhibitory effect of GTP and GTPyS. In the absence of Mg2+, GTP and GTPyS now potentiate Ca(2+)-evoked secretion.

Animals↗
PubMed1992

The light chain of tetanus toxin inhibits calcium-dependent vasopressin release from permeabilized nerve endings.

The effects of tetanus toxin and its light and heavy chain subunits on vasopressin release were investigated in digitonin-permeabilized neurosecretory nerve terminals isolated from the neural lobe of the rat pituitary gland. Exocytosis was induced by challenging the permeabilized nerve endings with micromolar calcium concentrations. Tetanus toxin inhibited vasopressin release only in the presence of the reducing agent dithiothreitol. This effect was irreversible. The purified light chain of tetanus toxin strongly inhibited exocytosis in a dose-dependent manner with half-maximal effect at c. 10 nM. The action of the light chain was observed after only 2.5 min of preincubation. Separated heavy chain subunit had no effect on hormone secretion. Inhibition of vasopressin release could be prevented by preincubating the light chain of tetanus toxin with an immune serum against tetanus toxin. The data clearly demonstrate that in mammalian neurosecretory nerve endings tetanus toxin acts at a step downstream from the activation by Ca2+ of the exocytotic machinery and that the functional domain of this toxin is confined to its light chain.

Animals↗
PubMed1992

Possible role during exocytosis of a Ca(2+)-activated channel in neurohypophysial granules.

Ion channels from bovine neurohypophysial granules were incorporated into artificial lipid bilayers. The larger amplitude channel is permeable to cations and exhibits multiple conductances. The channel opens only in the presence of free Ca2+, but is inhibited by relatively high Ca2+ concentrations. Release of vasopressin from permeabilized neurohypophysial terminals also shows a similar biphasic dependence on Ca2+. Release is selectively inhibited by low concentrations of the long-chain alcohol octanol, but not by high concentrations of ethanol, as is the neurosecretory granule Ca(2+)-activated cation channel. Furthermore, Ca(2+)-evoked release and channel activity are both inhibited by the long-chain tetraethylammonium analogs decamethonium and decyl-triethyl ammonium bromide. The close correlation between channel and release properties lead us to conclude that the Ca(2+)-activated channel is involved in peptide secretion.

Alcohols↗
PubMed1992

Oxytocin at physiological concentrations evokes adrenocorticotropin (ACTH) release from corticotrophs by increasing intracellular free calcium mobilized mainly from intracellular stores. Oxytocin displays synergistic or additive effects on ACTH-releasing factor or arginine vasopressin-induced ACTH secretion, respectively.

The potency of oxytocin (OT) in evoking ACTH secretion by isolated, superfused rat adenohypophyseal corticotrophs and its enhancement by CRF and arginine vasopressin (AVP) were analyzed. Each secretagogue effectively released ACTH from adenohypophyseal cells when added separately in pulsatile fashion in physiological concentrations based on hypophyseal portal blood (OT, 10 nM; AVP, 0.5 nM; CRF, 0.1 nM). OT released ACTH at concentrations as low as 1 nM. Moreover, a dose-response relationship up to 10 microM was revealed. Combinations of a constant amount of CRF (0.1 nM) with increasing concentrations of OT exerted a synergistic effect on ACTH release. In contrast, OT given in various concentrations in combination with AVP (0.5 nM) produced an additive effect on ACTH release. To study the mechanism of action of OT on ACTH secretion, cytosolic free calcium levels in single pituitary cells exposed to OT or AVP were measured using the calcium-sensitive fluorescent indicator Fura-2. Corticotrophs among mixed adenohypophyseal cell types in the primary cultures were identified by immunocytochemistry. More than 500 cells were individually stimulated with OT or AVP. Basal cytosolic free calcium levels ranged between 80-130 nM free calcium. The addition of 100 nM OT or 1 microM AVP increased the cytosolic free calcium concentration within 3 sec to values ranging from 500-800 nM. An increase in intracellular calcium ranging from 200-500 nM due to OT could still be observed after extracellular calcium depletion. Taken together, our data demonstrate that physiological concentrations of OT stimulate ACTH secretion, independent of the other ACTH secretagogues, by mobilizing calcium mainly from intracellular stores.

Adrenocorticotropic Hormone↗
PubMed1991

Ethanol reduces vasopressin release by inhibiting calcium currents in nerve terminals.

Ingestion of ethanol (EtOH) is known to result in a reduction of plasma arginine-vasopressin (AVP) levels in mammals. We examined the basis for this effect using a combination of biochemical and electrophysiological techniques. Release of AVP from nerve terminals isolated from the rat neurohypophysis was very sensitive to EtOH, with significant reductions in AVP release evident in 10 mM EtOH. However, EtOH did not affect the release of AVP from terminals which had been permeabilized with digitonin, suggesting that voltage-gated calcium channels might be the target of EtOH's actions. Patch clamping of these terminals indicated that both inactivating and long-lasting calcium currents were reduced in EtOH, but the long-lasting currents were more sensitive (significant reductions in 10 mM EtOH). EtOH-induced decreases in plasma AVP levels can be explained by EtOH's inhibition of calcium currents in the nerve terminals.

Animals↗
PubMed1991

Calcium currents and peptide release from neurohypophysial terminals are inhibited by ethanol.

The effects of EtOH on peptide release and on high-threshold, voltage-activated calcium (Ca++) channels were examined in acutely dissociated rat neurohypophysial terminals. These terminals release the peptide hormones, arginine vasopressin (AVP) and oxytocin. Release of AVP from isolated intact neurohypophyses, induced by either electrical stimulation or elevated potassium, was inhibited by clinically relevant concentrations of EtOH. "Whole-cell" patch-clamp recording methods were used to study the effects of EtOH on voltage-activated Ca++ currents (ICa) in the peptidergic nerve terminals. Amplitudes of both fast-inactivating ICa and long-lasting ICa were reduced in EtOH, and the reduction in ICa did not result from a shift in its current-voltage or steady-state inactivation relationships. Only the fast-inactivating component recovered after removal of EtOH. The effects of EtOH on ICa could not be attributed to changes in osmolarity. In contrast to ICa, the fast, transient K+ current was insensitive to EtOH. These results suggest that EtOH-induced reduction of ICa in the peptidergic nerve terminals produces a decrease in AVP release, resulting in lowered plasma AVP levels.

Animals↗
PubMed1990

Release of vasopressin from isolated permeabilized neurosecretory nerve terminals is blocked by the light chain of botulinum A toxin.

The intracellular action on exocytosis of botulinum A toxin and constituent chains was studied using permeabilized isolated nerve endings from the rat neural lobe. The release of the neuropeptide vasopressin was measured by radioimmunoassay. In the presence of the reducing agent dithiothreitol, the two-chain form of botulinum A toxin inhibited vasopressin release induced by 10 microM free calcium. Half maximal inhibition was obtained with 15 nM botulinum A toxin. In the absence of the heavy chain the light chain of the toxin strongly inhibited exocytosis with a half maximal effect of 2.5 nM. The inhibitory effects on secretion could be prevented by incubating the light chain with an immune serum against botulinum A toxin. The heavy chain of botulinum A toxin did not affect vasopressin release. However, it prevented the inhibitory effects of the light chain on stimulated exocytosis. It is concluded that botulinum A toxin inhibits the calcium-dependent step leading to exocytosis by interfering with a target present in the isolated and permeabilized nerve terminals. The functional domain of this neurotoxin, which is responsible for the inhibition of vasopressin release, is present in its light chain.

Animals↗
PubMed1990

Evidence for distinct glucocorticoid and guanine 3',5'-monophosphate-effected inhibition of stimulated adrenocorticotropin release in vitro.

Previous work has shown that corticosterone, cell-membrane permeant analogs of cGMP, as well as activators of guanylyl cyclase inhibit secretagogue-stimulated ACTH release. In the present study we have examined whether cGMP mediates the inhibitory effect of corticosterone in perifused isolated rat anterior pituitary cells. A brief 22.5-min exposure to corticosterone strongly inhibited ACTH secretion evoked by arginine vasopressin (AVP), 48 mM KC1, and two types of combined stimuli, i.e. 41-residue CRF and AVP (0.05 and 0.5 nM, respectively; CRF/AVP), or ionomycin and phorbol-dibutyrate (200 and 10 nM, respectively; PdBu/IM). The time course of inhibition by corticosterone was similar in all cases; a rapid approximately 30% reduction in ACTH was evident within 25 min, which increased to 60% by 50-70 min and will be referred to as the delayed effect. The corticosteroid inhibition of PdBu/IM-induced ACTH release was fully antagonized by the glucocorticoid/progestin antagonist RU 38486, indicating that it is exerted through type II glucocorticoid receptors. In contrast to corticosterone, the cGMP derivative 8-bromo-cGMP failed to suppress ACTH release evoked by PdBu/IM, whereas it effectively inhibited the action of CRF/AVP. Furthermore, ionomycin reversed the reduction of CRF/AVP-stimulated ACTH release by 8-bromo-cGMP, but had no effect on the delayed inhibition caused by corticosterone. These data indicate that there are two distinct cellular pathways of inhibiting stimulus-evoked ACTH secretion in vitro. One of these is activated by corticosterone, whereas the other involves cGMP as a cellular messenger.

Adrenocorticotropic Hormone↗
PubMed1990

Atriopeptins are potent inhibitors of ACTH secretion by rat anterior pituitary cells in vitro: involvement of the atrial natriuretic factor receptor domain of membrane-bound guanylyl cyclase.

The current concept of the regulation of adenohypophysial corticotrophin secretion assigns a stimulatory role to neuropeptides of hypothalamic origin and an inhibitory function to adrenocortical glucocorticoid hormones. It has been shown that in isolated perifused rat anterior pituitary cells atriopeptins (1-28) and (5-28), but not (5-25), potently suppress corticotrophin secretion elicited by 41-residue corticotrophin-releasing factor and arginine vasopressin. The action of atriopeptins is associated with a marked increase in the cellular content of cyclic GMP. Moreover, 8-Br-cyclic GMP, a cell membrane permeant analogue of cyclic GMP, also inhibited secretagogue-stimulated corticotrophin release. It is suggested that atrio-peptins of hypothalamic and perhaps of cardiac origin may be functionally significant corticotrophin-release inhibiting factors.

Adrenocorticotropic Hormone↗
PubMed1990

Secretion of ACTH by perifused isolated rat anterior pituitary cells: pulses of secretagogue enhance the secretory response and modify the effect of atriopeptin.

The aim of the present study was to characterize the inhibitory action of atriopeptin on secretagogue-evoked ACTH release in vitro. Perifused isolated rat anterior pituitary cells were exposed to repeated pulses of 41-residue corticotrophin-releasing factor (CRF-41) or arginine vasopressin (AVP). The net ACTH secretory response to both neurohormones increased progressively with the number of pulses applied, until a maximum hormonal response was reached which was stable for the subsequent period of observation (2-3 h). The maximal secretagogue-evoked hormone release eventually achieved was 4 and 1.7 times greater than the initial response to AVP and CRF-41 respectively. The size of the ACTH response elicited by 50 pmol CRF-41/l and 500 pmol AVP/l (CRF/AVP) given together also underwent progressive enhancement. The number of secretagogue pulses required to reach the maximal response to a particular stimulus depended upon the concentration of the secretagogue peptides, higher concentrations favoured a more rapid development of the stable secretory response. The potency of 103-126 residue atriopeptin to inhibit CRF/AVP-induced ACTH release varied by about 1000-fold depending upon the prior treatment of the cells. In general, cells not previously exposed to secretagogues appeared largely resistant, those under a moderate secretagogue drive were strongly inhibited, and those under intense stimulation were again refractory to inhibition by atriopeptin. In contrast, cortico-sterone suppressed stimulated ACTH release regardless of the state of the cells. The data demonstrate that the conditions of cell maintenance are pivotal determinants of the inhibitory effect of atriopeptin on secretagogue-stimulated ACTH release in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenocorticotropic Hormone↗
PubMed1990

Blockage of K+ channels reverses the inhibitory action of atriopeptin on secretagogue-stimulated ACTH release by perifused isolated rat anterior pituitary cells.

The aim of the present study was to investigate how atriopeptin inhibits secretagogue-stimulated ACTH secretion in vitro. Perifused isolated rat anterior pituitary cells were used throughout; the ACTH content of the perifusate was measured by radioimmunoassay. In the presence of a constant (0.05 nmol/l) concentration of 41-residue corticotrophin-releasing factor (CRF), arginine vasopressin (AVP; 0.05-50 nmol/l) stimulated ACTH secretion in a concentration-dependent manner, the combination of 0.05 nmol CRF/l and 0.5 nmol AVP/l (CRF/AVP) stimulated ACTH release to six- to eightfold above baseline. The effect of CRF/AVP was not modified by tetrodotoxin, but was abolished by CoCl2 and reduced to about 70% of the control stimulus by nifedipine. Application of 103-126 residue atriopeptin for 10 min before and 2.5 min during the CRF/AVP stimulus strongly suppressed the evoked release of ACTH, the maximal inhibition was 75-90% at 10 nmol atriopeptin/l. The calcium ionophore ionomycin (200 nmol/l) reversed the effect of atriopeptin while it had no secretagogue activity of its own, and did not enhance the response to CRF/AVP. A variety of blockers of K+ channels, 4-amino pyridine, tetraethylammonium, apamine, quinine, but not tolbutamide, effectively antagonized the inhibitory action of atriopeptin (10 nmol/l). None of these drugs altered ACTH release evoked by CRF/AVP. In concentration-response experiments, the half effective concentration of 4-aminopyridine and tetraethylammonium were around 1 mmol/l and 10 nmol/l for apamine. Finally, tetraethylammonium and apamine also antagonized the inhibition of CRF/AVP-evoked ACTH release by 8-Br-cGMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenocorticotropic Hormone↗
PubMed1989

Chloride and magnesium dependence of vasopressin release from rat permeabilized neurohypophysial nerve endings.

The role of Cl- and Mg+ ions has been studied on the secretory mechanism leading to the release of vasopressin from digitonin permeabilized nerve endings isolated from the rat neurohypophysis. Secretion was triggered by challenging the permeabilized nerve endings with 1.1 microM free Ca2+. Magnesium enhances secretion and its maximal effect occurred at a concentration of about 2 mM. Further increase of this divalent cation concentration however led to an inhibition of secretion. Chloride ions are necessary for the final steps in exocytosis and this effect of Cl- was inhibited by the chloride channel antagonist N144. It is concluded that in neurosecretory nerve endings magnesium and chloride ions are crucial components for exocytosis to occur.

Animals↗
PubMed1989

Guanosine 3':5'cyclic monophosphate and activators of guanylate cyclase inhibit secretagogue-induced corticotropin release by rat anterior pituitary cells.

The secretion of corticotropin by perfused rat anterior pituitary cell columns was studied. Forty-one residue corticotropin releasing factor, vasopressin and high extracellular KC1 all stimulated the secretion of corticotropin. The hormonal response to corticotropin-releasing factor (10(-10) mol/l), vasopressin (10(-9) mol/l) as well as KC1 (48 mmol/l) was reduced by membrane permeant analogs of cGMP, such as 8-BrcGMP and dibutyryl-cGMP. The 8-BrcGMP analog (10(-5) mol/l) inhibited corticotropin release in response to corticotropin-releasing factor by 30%, that to vasopressin by 70%, and that to KCl by 50%. Atriopeptin1-28 (10(-8) and 10(-7) mol/l), a peptide known to activate membrane-bound guanylate cyclase in the anterior pituitary gland, decreased the release of corticotropin induced by vasopressin to about 30% of control. Similarly, activators of soluble guanylate cyclase, such as glyceryltrinitrate and sodium nitroprusside (10(-5) mol/l) inhibited vasopressin-stimulated corticotropin release by 60%. In conclusion, the data show that purported activators of particulate and soluble guanylate cyclase, as well as derivatives of cGMP itself are strong inhibitors of secretagogue-induced corticotropin release by corticotroph cells of the anterior pituitary gland.

Adrenocorticotropic Hormone↗