Biomedical subjects

G Dayanithi

Publications and source records attributed to G Dayanithi.

At least 19 recordsLinked to original sources

PubMed2006

Rapid inhibition of Ca2+ influx by neurosteroids in murine embryonic sensory neurones.

The non-genomic role of neuroactive steroids on [Ca2+]i transients induced by GABA receptor activation was investigated in cultured dorsal root ganglia (DRG) neurones at embryonic stage E13. [Ca2+]i measurements were performed with Fura-2 fast fluorescence microfluorimetry. Application of the GABAA receptor agonist muscimol (Musci) evoked an increase in [Ca2+]i, confirming the excitatory effect of GABA at this embryonic stage. The muscimol-induced [Ca2+]i response was inhibited by progesterone (Proges) and its primary metabolite allopregnanolone (Allo) in a rapid, reversible and dose-dependent manner. These calcium transients were suppressed in the absence of external Ca2+ or in the presence of Ni2+ + Cd2+ suggesting an involvement of voltage-activated Ca2+ channels. In contrast, none of these steroids affected the resting [Ca2+]i nor exhibited any inhibitory effect on 50 mM KCl-induced [Ca2+]i increases. In view of the well-established potentiation of GABAA receptor by direct binding of neurosteroids, the inhibitory effects described in this study seem to involve distinct mechanisms. This new inhibitory effect of progesterone is observed at low and physiological concentrations, is rapid and independent of RU38486, an antagonist of the classic progesterone receptor, probably involving a membrane receptor. Using RT-PCR, we demonstrated the expression of progesterone receptor membrane component 1 (Pgrmc1), encoding 25-Dx, a membrane-associated progesterone binding protein in DRG neurones at different stages of development. In conclusion, we describe for the first time a rapid effect of progestins on embryonic DRG neurones involving an antagonistic effect of progesterone and allopregnanolone on GABAA receptors.

Animals↗

PubMed2006

Exaggerated response of arginine vasopressin-enhanced green fluorescent protein fusion gene to salt loading without disturbance of body fluid homeostasis in rats.

We examined the effects of chronic salt loading on the hypothalamic expressions of the enhanced green fluorescent protein (eGFP), arginine vasopressin (AVP) and oxytocin (OXT) genes in AVP-eGFP transgenic rats that expressed eGFP in the hypothalamic AVP-containing neurones. In these rats, salt loading for 5 days caused a marked increase of the eGFP fluorescence in the magnocellular divisions of the paraventricular nucleus (PVN), the supraoptic nucleus (SON) and the internal layer of the median eminence. Expression of the eGFP gene was increased seven- to eight-fold in the PVN and SON of salt-loaded rats in comparison with euhydrated rats. By contrast, none of these changes were observed in the suprachiasmatic nucleus. The expression of the AVP and OXT genes was increased 1.5- to two-fold in the PVN and SON of salt-loaded nontransgenic (control) and transgenic rats. There were no differences in the expression levels of the AVP and OXT genes in the PVN and SON between nontransgenic (control) and transgenic animals under normal conditions and after salt loading. In the posterior pituitary gland, the intensity of the eGFP fluorescence did not change after salt loading for 5 days, but increased after 10 days of salt loading. Upon salt loading, significant increases in the plasma AVP concentrations, plasma osmolality and plasma Na+ were observed. Furthermore, there were no significant differences in changes of water intake, food intake, urine volume, urine osmolality, urine Na+ concentrations, and the body weights in both models under normal or salt-loaded conditions. Our results show that the response of the AVP-eGFP fusion gene to chronic salt loading is exaggerated, and humoral responses such as AVP and OXT and the body fluid homeostasis are maintained in AVP-eGFP transgenic rats. The AVP-eGFP transgenic rat gives us a new opportunity to study the dynamics of the AVP system in vivo.

Animals↗

PubMed2005

Structural difference between heteromeric somatic and homomeric axonal glycine receptors in the hypothalamo-neurohypophysial system.

Glycine receptors are ionotropic receptors formed by either the homomeric assembly of ligand-binding alpha subunits or the heteromeric combination of an alpha subunit and the auxiliary beta subunit. Glycine receptors in the brain are found at either pre- or post-synaptic sites. Rat supraoptic nucleus neurons express glycine receptors on the membrane of both their soma and dendrites within the supraoptic nucleus, and their axon terminals in the neurohypophysis. Taking advantage of the well-separated cellular compartments of this system, we correlated the structural properties of the receptors to their subcellular localization. Immunohistochemical study using the generic mAb4a antibody revealed that somatodendritic receptors were clustered, whereas axonal glycine receptors showed a more diffuse distribution. This was paralleled by the presence of clusters of the glycine receptor aggregating protein gephyrin in the supraoptic nucleus and its complete absence in the neurohypophysis. Moreover, another antibody recognizing the alpha1/alpha2 subunits similarly labeled the axonal glycine receptors, but did not recognize the somatodendritic receptor clusters of supraoptic nucleus neurons, indicative of structural differences between somatic and axonal glycine receptors. Furthermore, the subunits composing the somatic and axonal receptors have different molecular weight. Functional study further differentiated the two types of glycine receptors on the basis of their sensitivity to picrotoxin, identifying somatic receptors as alpha/beta heteromers, and axonal receptors as alpha homomers. These results indicate that targeting of glycine receptors to axonal or somatodendritic compartment is directly related to their subunit composition, and set the hypothalamo-neurohypophysial system as an excellent model to study the mechanisms of targeting of proteins to various neuronal cellular compartments.

Animals↗

PubMed2005

Micro-opioid receptor preferentially inhibits oxytocin release from neurohypophysial terminals by blocking R-type Ca2+ channels.

Oxytocin release from neurophypophysial terminals is particularly sensitive to inhibition by the micro-opioid receptor agonist, DAMGO. Because the R-type component of the neurophypophysial terminal Ca2+ current (ICa) mediates exclusively oxytocin release, we hypothesised that micro-opioids could preferentially inhibit oxytocin release by blocking this channel subtype. Whole-terminal recordings showed that DAMGO and the R-type selective blocker SNX-482 inhibit a similar ICa component. Measurements of [Ca2+]i levels and oxytocin release confirmed that the effects of DAMGO and SNX-482 are not additive. Finally, isolation of the R-type component and its associated rise in [Ca2+]i and oxytocin release allowed us to demonstrate the selective inhibition by DAMGO of this channel subtype. Thus, micro-opioid agonists modulate specifically oxytocin release in neurophypophysial terminals by selectively targeting R-type Ca2+ channels. Modulation of Ca2+ channel subtypes could be a general mechanism for drugs of abuse to regulate the release of specific neurotransmitters at central nervous system synapses.

Analgesics, Opioid↗

PubMed2004

Intracellular calcium increase and somatodendritic vasopressin release by vasopressin receptor agonists in the rat supraoptic nucleus: involvement of multiple intracellular transduction signals.

Vasopressin neurones of the supraoptic nucleus are autoregulated by vasopressin released from their soma and dendrites. Vasopressin binds to specific autoreceptors to trigger an influx of Ca(2+), and this response involves both phospholipase C (PLC) and adenylate cyclase (AC) pathways that, in the periphery, are activated by V(1) (V(1a) and V(1b))- and V(2)-type receptors. To investigate the pathways involved in the [Ca(2+)](i) response, [Ca(2+)](i) measurements were made on freshly dissociated neurones using Fura-2 microspectrofluorimetry, and vasopressin release was measured from isolated supraoptic nuclei. The [Ca(2+)](i) increase and vasopressin release induced by the V(1a) agonist were strongly inhibited by a PLC blocker, an IP(3) receptor antagonist, and a PKC blocker. An AC inhibitor did not affect the V(1a) response, while PKA inhibitors significantly reduced the V(1a)-induced [Ca(2+)](i) and release responses. The [Ca(2+)](i) increase and vasopressin release elicited by the V(2) agonist were attenuated not only by AC pathway blockers, but also by PLC inhibitors. Surprisingly, the V(1b) agonist showed no [Ca(2+)](i) or vasopressin release response. In conclusion, the V(1a) agonist activates both PLC and AC pathway, confirming the functional expression of a V(1a) vasopressin receptor on vasopressin neurones. The V(2) agonist activation of both PLC and AC pathways could result from an action on the PLC-linked unknown receptor, and/or the AC-linked dual angiotensin II-vasopressin receptor.

Adenylyl Cyclases↗

PubMed2003

Impaired somatodendritic responses to pituitary adenylate cyclase-activating polypeptide (PACAP) of supraoptic neurones in PACAP type I -receptor deficient mice.

The role of pituitary adenylate cyclase-activating polypeptide (PACAP) type I receptor (PAC1 receptor) in regulating hypothalamic supraoptic neurones was investigated using PAC1 receptor-deficient male mice (PAC1-/-). The effects of PACAP on [Ca2+]i were investigated in freshly dissociated supraoptic neurones and on the somatodendritic release of vasopressin and oxytocin, examined on intact supraoptic nuclei. In supraoptic neurones from wild-type mice (PAC1+/+), 100 nm PACAP induced an increase in [Ca2+]i and release of vasopressin and oxytocin, whereas in heterozygous (PAC1+/-) and null-mutant mice (PAC1-/-), PACAP was much less effective. PACAP had no effect on these two parameters when applied to isolated neurohypophysial nerve terminals of PAC1+/+ and PAC1-/- mice, and rats. In conclusion, the PAC1 receptor is solely responsible for the PACAP-induced [Ca2+]i signalling and secretion of vasopressin and oxytocin in the somatodendritic region of supraoptic neurones.

Animals↗

PubMed2003

mu-Opioid receptor modulates peptide release from rat neurohypophysial terminals by inhibiting Ca(2+) influx.

The activation of opioid receptors in neurones of the central nervous system leads to a variety of effects including the modulation of diuresis and parturition, processes that are directly controlled by the hypothalamic-neurohypophysial system (HNS). The effects of mu-opioid receptor activation on peptide release, voltage-gated Ca2+ currents and intracellular calcium levels ([Ca2+]i) were studied in isolated nerve terminals of the HNS. The mu-receptor agonist, DAMGO ([d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin) inhibited high K+-induced peptide release in a dose-dependent manner, with oxytocin release being more sensitive to block than vasopressin release at all concentrations tested. The addition of the mu-receptor antagonist CTOP (d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr amide) was able to overcome the inhibitory effects of DAMGO. By contrast to previous results, voltage-gated Ca2+ currents were sensitive to blockage by DAMGO and this inhibition was also prevented by CTOP. Furthermore, [Ca2+]i measurements with Fura-2 corroborated the inhibition by DAMGO of calcium entry and its reversal by the micro -receptor antagonist in these nerve terminals. Thus, the decrease in neuropeptide release, particularly for oxytocin, induced by the activation of mu-opioid receptors in neurohypophysial terminals is mediated, at least in part, by a corresponding decrease in Ca2+ entry due to the inhibition of voltage-gated Ca2+ channels.

Animals↗

PubMed2001

Osmoregulation of vasopressin secretion via activation of neurohypophysial nerve terminals glycine receptors by glial taurine.

Osmotic regulation of supraoptic nucleus (SON) neuron activity depends in part on activation of neuronal glycine receptors (GlyRs), most probably by taurine released from adjacent astrocytes. In the neurohypophysis in which the axons of SON neurons terminate, taurine is also concentrated in and osmo-dependently released by pituicytes, the specialized glial cells ensheathing nerve terminals. We now show that taurine release from isolated neurohypophyses is enhanced by hypo-osmotic and decreased by hyper-osmotic stimulation. The high osmosensitivity is shown by the significant increase on only 3.3% reduction in osmolarity. Inhibition of taurine release by 5-nitro-2-(3-phenylpropylamino)benzoic acid, niflumic acid, and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid suggests the involvement of volume-sensitive anion channels. On purified neurohypophysial nerve endings, activation of strychnine-sensitive GlyRs by taurine or glycine primarily inhibits the high K(+)-induced rise in [Ca(2+)](i) and subsequent release of vasopressin. Expression of GlyRs in vasopressin and oxytocin terminals is confirmed by immunohistochemistry. Their implication in the osmoregulation of neurohormone secretion was assessed on isolated whole neurohypophyses. A 6.6% hypo-osmotic stimulus reduces by half the depolarization-evoked vasopressin secretion, an inhibition totally prevented by strychnine. Most importantly, depletion of taurine by a taurine transport inhibitor also abolishes the osmo-dependent inhibition of vasopressin release. Therefore, in the neurohypophysis, an osmoregulatory system involving pituicytes, taurine, and GlyRs is operating to control Ca(2+) influx in and neurohormone release from nerve terminals. This elucidates the functional role of glial taurine in the neurohypophysis, reveals the expression of GlyRs on axon terminals, and further defines the role of glial cells in the regulation of neuroendocrine function.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗

PubMed2001

Interaction of SNX482 with domains III and IV inhibits activation gating of alpha(1E) (Ca(V)2.3) calcium channels.

We have investigated the action of SNX482, a toxin isolated from the venom of the tarantula Hysterocrates gigas, on voltage-dependent calcium channels expressed in tsa-201 cells. Upon application of 200 nM SNX482, R-type alpha(1E) calcium channels underwent rapid and complete inhibition, which was only poorly reversible upon washout. However, upon application of strong membrane depolarizations, rapid and complete recovery from inhibition was obtained. Tail current analysis revealed that SNX482 mediated an approximately 70 mV depolarizing shift in half-activation potential, suggesting that the toxin inhibits alpha(1E) calcium channels by preventing their activation. Experiments involving chimeric channels combining structural features of alpha(1E) and alpha(1C) subunits indicated that the presence of the domain III and IV of alpha(1E) is a prerequisite for a strong gating inhibition. In contrast, L-type alpha(1C) channels underwent incomplete inhibition at saturating concentrations of SNX482 that was paralleled by a small shift in half-activation potential and which could be rapidly reversed, suggesting a less pronounced effect of the toxin on L-type calcium channel gating. We conclude that SNX482 does not exhibit unequivocal specificity for R-type channels, but highly effectively antagonizes their activation.

Animals↗

PubMed2001

ATP induces intracellular calcium increases and actin cytoskeleton disaggregation via P2x receptors.

The consequences of purinoceptor activation on calcium signalling, inositol phosphate metabolism, protein secretion and the actin cytoskeleton were demonstrated in the WRK-1 cell line. Extracellular ATP was used as a secretagogue to induce a rise in intracellular Ca(2+) concentration ([Ca(2+)](i)), acting via P2x purinergic receptors, which causes actin skeleton disaggregation and protein secretion. ATP bound specifically to purinergic receptors, with Ki of 0.8 microM. The magnitude order for binding of different nucleotides was alpha beta-Met-ATP >or= dATPalphaS > ATP >or= ADP > UTP > AMP > suramin. No increase in inositol phosphates (IPs) was observed after ATP application suggesting that the purinergic sites in WRK-1 cells are not of a P2y type. ATP (1-100 microM) caused a concentration-dependent increase in [Ca(2+)](i)(EC(50)= 30 microM). The responses were reproducible without any desensitization over several applications. The response to ATP was abolished when extracellular calcium ([Ca(2+)](e)) was reduced to 100 nM. A non-specific purinergic antagonist, suramin, reversibly inhibited the ATP-response suggesting that ATP is able to bind to P2x purinergic sites to trigger Ca(2+) entry and increase of [Ca(2+)](i). ATP induced a concentration-dependent disaggregation of actin and exocytotic release of proteins both, which were dependent upon [Ca(2+)](e). Similarly, alpha,beta-Met-ATP, a potent P2x agonist also stimulated Ca(2+) mobilization, actin network destructuration, and protein release. In the isolated rat neurohypophysial nerve terminals, ATP was shown to act as a physiological stimulus for vasopressin release via Ca(2+) entry through a P2x receptor [6]. Here, we show that in these nerve terminals, ATP is also able to induce actin disaggregation by a Ca(2+) dependent mechanism. Thus, actin cytoskeleton alterations induced by ATP through activation of P2x receptors could be a prelude to exocytosis.

Actins↗

PubMed2000

Developmental regulation of a local positive autocontrol of supraoptic neurons.

Mature oxytocin (OT) and vasopressin (AVP) magnocellular neurons of the hypothalamic supraoptic nuclei (SON) autocontrol their electrical activity via somatodendritic release of their respective peptides. Because OT and AVP are synthesized early in development and could play an important role in the maturation of these neurons, we checked whether the peptides are released within the SON and act on their secreting neurons during 3 weeks of postnatal development. We used patch-clamp recordings from SON neurons in rat hypothalamic horizontal slices to show that the spontaneous electrical activity of immature SON neurons is blocked by OT or AVP receptor antagonists, demonstrating a basal somatodendritic release of the peptides. Application of OT or AVP depolarizes SON neurons and stimulates activity typical of the corresponding mature neurons. This effect is directly on SON neurons because it is recorded in dissociated neurons. Radioimmunoassays from isolated SON were used to show that each peptide facilitates its own release at a somatodendritic level, exhibiting a self-sustaining positive feedback loop. This autocontrol is not uniform during development because the proportion of neurons depolarized by the peptides, the amplitude of the depolarization, and the propensity of the peptides to facilitate their own release are maximal during the second postnatal week and decrease thereafter. These data are consistent with a role of autocontrol in the maturation of SON neurons because it is maximal during the delimited period of postnatal development that corresponds to the period of major synapse formation.

Action Potentials↗

PubMed2000

Intracellular calcium signalling in magnocellular neurones of the rat supraoptic nucleus: understanding the autoregulatory mechanisms.

Oxytocin and vasopressin, released at the soma and dendrites of neurones, bind to specific autoreceptors and induce an increase in [Ca2+]i. In oxytocin cells, the increase results from a mobilisation of Ca2+ from intracellular stores, whereas in vasopressin cells, it results mainly from an influx of Ca2+ through voltage-dependent channels. The response to vasopressin is coupled to phospholipase C and adenylyl-cyclase pathways which are activated by V1 (V1a and V1b)- and V2-type receptors respectively. Measurements of [Ca2+]i in response to V1a and V2 agonists and antagonists suggest the functional expression of these two types of receptors in vasopressin neurones. The intracellular mechanisms involved are similar to those observed for the action of the pituitary adenylyl-cyclase-activating peptide (PACAP). Isolated vasopressin neurones exhibit spontaneous [Ca2+]i oscillations and these are synchronised with phasic bursts of electrical activity. Vasopressin modulates these spontaneous [Ca2+]i oscillations in a manner that depends on the initial state of the neurone, and such varied effects of vasopressin may be related to those observed on the electrical activity of vasopressin neurones in vivo.

Animals↗

PubMed2000

T-type calcium currents in rat cardiomyocytes during postnatal development: contribution to hormone secretion.

T-type Ca(2+) channels have been suggested to play a role in cardiac automaticity, cell growth, and cardiovascular remodeling. Although three genes encoding for a T-type Ca(2+) channel have been identified, the nature of the isoform(s) supporting the cardiac T-type Ca(2+) current (I(Ca,T)) has not yet been determined. We describe the postnatal evolution of I(Ca,T) density in freshly dissociated rat atrial and ventricular myocytes and its functional properties at peak current density in young atrial myocytes. I(Ca,T) displays a classical low activation threshold, rapid inactivation kinetics, negative steady-state inactivation, slow deactivation, and the presence of a window current. Interestingly, I(Ca,T) is poorly sensitive to Ni(2+) and insensitive to R-type current toxin SNX-482. RT-PCR experiments and comparison of functional properties with recombinant Ca(2+) channel subtypes suggest that neonatal I(Ca,T) is related to the alpha(1G)-subunit. Atrial natriuretic factor (ANF) secretion was measured using peptide radioimmunoassays in atrial tissue. Pharmacological dissection of ANF secretion indicates an important contribution of I(Ca,T) to Ca(2+) signaling during the neonatal period.

Aging↗

PubMed2000

Tolerance to acute ethanol inhibition of peptide hormone release in the isolated neurohypophysis.

BACKGROUND: Acute ethanol (EtOH) exposure reduces the evoked release of vasopressin (AVP) and oxytocin (OT) from excised neurohypophyses and from dissociated neurohypophysial terminals of the rat. METHODS AND RESULTS: Rats placed on a diet that maintained blood levels of 30 mM EtOH for 20 to 40 days developed tolerance to acute EtOH inhibition of release. In the presence of 10 mM EtOH, high (50 mM) K+-induced release of AVP from isolated neurohypophysial terminals of EtOH-naive rats was reduced by 77.7+/-1.4%, whereas in the chronic EtOH group, release was reduced by only 9.4+/-8.7%. Similar tolerance was evident during acute challenge with 75 mM EtOH, as well as for release of OT from isolated terminals. Animals treated with an intraperitoneal injection of EtOH and sacrificed 90 min postinjection did not exhibit the reduced EtOH inhibition of release from dissociated terminals during a 75 mM EtOH acute challenge. CONCLUSIONS: The altered component responsible for the tolerance to inhibition of release resides in the isolated terminal, because tolerance measured in vitro from intact neurohypophyses was similar to that seen in isolated terminals. The failure of EtOH-injected animals to exhibit reduced inhibition of release in response to an acute EtOH challenge indicates that short-term elevated blood alcohol level does not induce this tolerance. The finding of tolerance to EtOH-induced inhibition of release from the intact neurohypophysis and isolated terminals provides a physiological preparation in which to examine the molecular targets of acute drug action modified after chronic exposure to the drug.

Animals↗

PubMed1999

An R-type Ca(2+) current in neurohypophysial terminals preferentially regulates oxytocin secretion.

Multiple types of voltage-dependent Ca(2+) channels are involved in the regulation of neurotransmitter release (Tsien et al., 1991; Dunlap et al., 1995). In the nerve terminals of the neurohypophysis, the roles of L-, N-, and P/Q-type Ca(2+) channels in neuropeptide release have been identified previously (Wang et al., 1997a). Although the L- and N-type Ca(2+) currents play equivalent roles in both vasopressin and oxytocin release, the P/Q-type Ca(2+) current only regulates vasopressin release. An oxytocin-release and Ca(2+) current component is resistant to the L-, N-, and P/Q-type Ca(2+) channel blockers but is inhibited by Ni(2+). A new polypeptide toxin, SNX-482, which is a specific alpha(1E)-type Ca(2+) channel blocker (Newcomb et al., 1998), was used to characterize the biophysical properties of this resistant Ca(2+) current component and its role in neuropeptide release. This resistant component was dose dependently inhibited by SNX-482, with an IC(50) of 4.1 nM. Furthermore, SNX-482 did not affect the other Ca(2+) current types in these CNS terminals. Like the N- and P/Q-type Ca(2+) currents, this SNX-482-sensitive transient Ca(2+) current is high-threshold activated and shows moderate steady-state inactivation. At the same concentrations, SNX-482 blocked the component of oxytocin, but not of vasopressin, release that was resistant to the other channel blockers, indicating a preferential role for this type of Ca(2+) current in oxytocin release from neurohypophysial terminals. Our results suggest that an alpha(1E) or "R"-type Ca(2+) channel exists in oxytocinergic nerve terminals and, thus, functions in controlling only oxytocin release from the rat neurohypophysis.

Animals↗

PubMed1999

Regulation of Ca2+ homeostasis by atypical Na+ currents in cultured human coronary myocytes.

Primary cultured human coronary myocytes (HCMs) derived from ischemic human hearts express an atypical voltage-gated tetrodotoxin (TTX)-sensitive sodium current (I(Na)). The whole-cell patch-clamp technique was used to study the properties of I(Na) in HCMs. The variations of intracellular calcium ([Ca2+]i) and sodium ([Na+]i) were monitored in non-voltage-clamped cells loaded with Fura-2 or benzofuran isophthalate, respectively, using microspectrofluorimetry. The activation and steady-state inactivation properties of I(Na) determined a "window" current between -50 and -10 mV suggestive of a steady-state Na+ influx at the cell resting membrane potential. Consistent with this hypothesis, the resting [Na+]i was decreased by TTX (1 micromol/L). In contrast, it was increased by Na+ channel agonists that also promoted a large rise in [Ca2+]i. Veratridine (10 micromol/L), toxin V from Anemonia sulcata (0.1 micromol/L), and N-bromoacetamide (300 micromol/L) increased [Ca2+]i by 7- to 15-fold. This increase was prevented by prior application of TTX or lidocaine (10 micromol/L) and by the use of Na(+)-free or Ca(2+)-free external solutions. The Ca(2+)-channel antagonist nicardipine (5 micromol/L) blocked the effect of veratridine on [Ca2+]i only partially. The residual component disappeared when external Na+ was replaced by Li+ known to block the Na+/Ca2+ exchanger. The resting [Ca2+]i was decreased by TTX in some cells. In conclusion, I(Na) regulates [Ca2+]i in primary cultured HCMs. This regulation, effective at baseline, involves a tonic control of Ca2+ influx via depolarization-gated Ca2+ channels and, to a lesser extent, via a Na+/Ca2+ exchanger working in the reverse mode.

Adult↗

PubMed1999

V1a- and V2-type vasopressin receptors mediate vasopressin-induced Ca2+ responses in isolated rat supraoptic neurones.

1. The pharmacological profile of receptors activated by vasopressin (AVP) in freshly dissociated supraoptic magnocellular neurones was investigated using specific V1a- and V2-type AVP receptor agonists and antagonists. 2. In 97 % of AVP-responding neurones (1-3000 nM) V1a or V2 receptor type agonists (F-180 and dDAVP, respectively) elicited dose-dependent [Ca2+]i transients that were suppressed by removal of external Ca2+. 3. The [Ca2+]i response induced by 1 microM F-180 or dDAVP was selectively blocked by 10 nM of V1a and V2 antagonists (SR 49059 and SR 121463A, respectively). The response to V1a agonist was maintained in the presence of the V2 antagonist, and the V2 agonist-induced response persisted in the presence of the V1a antagonist. 4. The [Ca2+]i response induced by 1 microM AVP was partially (61 %) blocked by 10 nM SR 121463A. This blockade was increased by a further 31 % with the addition of 10 nM SR 49059. Similarly, the AVP-induced response was partially (47 %) decreased by SR 49059, and a further inhibition of 33 % was achieved in the presence of SR 121463A. 5. We demonstrate that AVP acts on the magnocellular neurones via two distinct types of AVP receptors that exhibit the pharmacological profiles of V1a and V2 types. However, since V2 receptor mRNA is not expressed in the supraoptic nucleus (SON), and since V1b receptor transcripts are observed in the SON, we propose that the V2 receptor agonist and antagonist act on a 'V2-like' receptor or a new type of AVP receptor that remains to be elucidated. The possibility that V2 ligands act on the V1b receptor cannot be excluded.

Animals↗

PubMed1999

Vasopressin(4-9) fragment activates V1a-type vasopressin receptor in rat supraoptic neurones.

The effect of vasopressin fragment 4-9 (AVP(4-9)) was investigated on freshly dissociated rat supraoptic neurones by measuring changes in intracellular calcium concentration ([Ca2+]i) using fura-2 microspectrofluorimetry. In 60% of neurones responding to vasopressin, AVP(4-9) induced a transient rise in [Ca2+]i that was dose-dependent in the concentration range 10 nM to 1 microM AVP(4-9) and strongly decreased in Ca2+-free buffer (84% inhibition). This [Ca2+]i response was completely and reversibly abolished by SR 49059 (1O nM), a specific V1a receptor antagonist, but not by SR 121463A, a specific V2 receptor antagonist. Our results demonstrate the presence of functional receptors activated by AVP(4-9) on vasopressin-sensitive neurones that possess the apparent pharmacological profile of the V1a-type vasopressin receptor.

Animals↗