β-adrenergic and muscarinic agonists modulate inactivation of l-type ca2+ Channel Currents in Guinea-Pig Ventricular Myocytes

The objective of this study was to examine the effects of isoproterenol (isoprenaline) and carbachol upon voltage-dependent inactivation of L-type Ca²⁺ current ( I _Ca,L). I _Ca,L was recorded in guinea-pig isolated ventricular myocytes in the presence and absence of extracellular Ca²⁺ to separate total inactivation and voltage-dependent inactivation. In the presence of Ca²⁺, isoproterenol and carbachol had 'competitive' effects upon the relationships between membrane voltage and I _Ca,L amplitude and inactivation. Neither agonist had a marked effect upon the decay of inward I _Ca,L carried by Ca²⁺. In the absence of Ca²⁺, isoproterenol severely reduced and slowed I _Ca,L inactivation; this effect was reversed by carbachol. Under control conditions decay was dominated by fast inactivation. Isoproterenol reduced fast-inactivating and increased time-independent currents in a dose-dependent manner. These effects were counteracted by carbachol. There was a reciprocal relationship between the amplitude of fast-inactivating and time-independent currents with agonist stimulation. It is concluded that agonist modulation of rapid voltage-dependent inactivation of L-type Ca²⁺ channels involves an 'on-off' switch.     

Tyrosine kinases enhance the function of glycine receptors in rat hippocampal neurons and human α1β glycine receptors

Glycine receptors (GlyRs) are transmitter-gated channels that mediate fast inhibitory neurotransmission in the spinal cord and brain. The GlyR β subunit contains a putative tyrosine phosphorylation site whose functional role has not been determined. To examine if protein tyrosine kinases (PTKs) regulate the function of GlyRs, we analysed whole-cell currents activated by applications of glycine to CA1 hippocampal neurons and spinal neurons. The role of a putative site for tyrosine phosphorylation at position 413 of the β subunit was examined using site-directed mutagenesis and expression of recombinant (α₁β^Y413F ) receptors in human embryonic kidney (HEK 293) cells. Lavendustin A, an inhibitor of PTKs, depressed glycine-evoked currents ( I _Gly) in CA1 neurons and spinal neurons by 31 % and 40 %, respectively. In contrast, the intracellular application of the exogenous tyrosine kinase, cSrc, enhanced I _Gly in CA1 neurons by 56 %. cSrc also accelerated GlyR desensitization and increased the potency of glycine 2-fold (control EC₅₀= 143 μ m ; cSrc EC₅₀= 74 μ m ). Exogenous cSrc, applied intracellularly, upregulated heteromeric α₁β receptors but not homomeric α₁ receptors. Substitution mutation of the tyrosine to phenylalanine at position β-413 prevented this enhancement. Furthermore, a selective inhibitor of the Src family kinases, PP2, down-regulated wild-type α₁β but not α₁β^Y413F receptors. Together, these findings indicate that GlyR function is upregulated by PTKs and this modulation is dependent on the tyrosine-413 residue of the β subunit.     

Subunit composition and role of Na+,K+-ATPases in adrenal chromaffin cells

Adrenal medullary (AM) cells are exposed to high concentrations of cortical hormones, one of which is a ouabain-like substance. Thus, the effects of ouabain on catecholamine secretion and distribution of Na⁺,K⁺-ATPase α and β subunits in rat and guinea-pig AM cells were examined using amperometry and immunological techniques. While exposure to 1 μ m ouabain did not have a marked effect on resting secretion, it induced an increase in secretion due to mobilization of Ca²⁺ ions that were stored during a 4 min interval between muscarine applications. Immunocytochemistry revealed that Na⁺,K⁺-ATPase α1 subunit-like and β3 subunit-like immunoreactive (IR) materials were distributed ubiquitously at the cell periphery, whereas α2- and β2-like IR materials were present in restricted parts of the cell periphery. The α1 and α2 subunits were mainly immunoprecipitated from AM preparations by anti-β3 and anti-β2 antisera, respectively. Peripheral BODIPY-FL-InsP₃ binding sites were localized below membrane domains with α2- and β2-like IR materials. The results indicate that in AM cells, α1β3 isozymes of Na⁺,K⁺-ATPase were present ubiquitously in the plasma membrane, while α2β2 isozymes were in the membrane domain closely associated with peripheral Ca²⁺ store sites. This close association of the α2β2 isozyme with peripheral Ca²⁺ store sites may account for the facilitation of mobilization-dependent secretion in the presence of 1 μ m ouabain.     

Regulation of cardiac excitation–contraction coupling by action potential repolarization: role of the transient outward potassium current (Ito)

The cardiac action potential (AP) is critical for initiating and coordinating myocyte contraction. In particular, the early repolarization period of the AP (phase 1) strongly influences the time course and magnitude of the whole-cell intracellular Ca²⁺ transient by modulating trans-sarcolemmal Ca²⁺ influx through L-type Ca²⁺ channels ( I _Ca,L) and Na-Ca exchangers ( I _Ca,NCX). The transient outward potassium current ( I _to) has kinetic properties that make it especially effective in modulating the trajectory of phase 1 repolarization and thereby cardiac excitation-contraction coupling (ECC). The magnitude of I _to varies greatly during cardiac development, between different regions of the heart, and is invariably reduced as a result of heart disease, leading to corresponding variations in ECC. In this article, we review evidence supporting a modulatory role of I _to in ECC through its influence on I _Ca,L, and possibly I _Ca,NCX. We also discuss differential effects of I _to on ECC between different species, between different regions of the heart and in heart disease.     

Expression of Integrin αvβ3 in Gliomas Correlates with Tumor Grade and Is not Restricted to Tumor Vasculature

In malignant gliomas, the integrin adhesion receptors seem to play a key role for invasive growth and angiogenesis. However, there is still a controversy about the expression and the distribution of α_vβ₃ integrin caused by malignancy. The aim of our study was to assess the extent and pattern of α_vβ₃ integrin expression within primary glioblastomas (GBMs) compared with low-grade gliomas (LGGs). Tumor samples were immunostained for the detection of α_vβ₃ integrin and quantified by an imaging software. The expression of α_vβ₃ was found to be significantly higher in GBMs than in LGGs, whereby focal strong reactivity was restricted to GBMs only. Subsequent analysis revealed that not only endothelial cells but also, to a large extent, glial tumor cells contribute to the overall amount of α_vβ₃ integrin in the tumors. To further analyze the integrin subunits, Western blots from histologic sections were performed, which demonstrated a significant difference in the expression of the β₃ integrin subunit between GBMs and LGGs. The presented data lead to new insights in the pattern of α_vβ₃ integrin in gliomas and are of relevance for the inhibition of α_vβ₃ integrin with specific RGD peptides and interfering drugs to reduce angiogenesis and tumor growth.     

Conformation of the T-Cell Antigen Receptor-β Chain C-Domain Contributes to Vβ3 Epitope Recognition by Monoclonal Antibody KJ25

The clonotypic T-cell antigen receptor (TCR)-β chain contains two extracellular intrachain disulfide bonds. It belongs to the immunoglobulin gene superfamily and is subdivided into variable (V), joining (J), diversity (D) and constant (C) region. Monoclonal antibody (MoAb) KJ25 is believed to recognize an epitope in the V-domain of TCR-β (Vβ₃) chain, but its epitope requirements are unknown. In this study of TCR-αβ chain interactions using chimeric recombinant TCR-β chains, the authors found that partial substitution of the Cβ-domain with that of interleukin-2 receptor α chain (Tac) sequences led to the loss of TCR-Vβ₃ epitope recognition by KJ25. These results suggest that epitope recognition of the TCR-Vβ₃ by KJ25 MoAb is dependent not only on the V-domain, but also on the close contact with the extracellular C-domain which influences the conformation and epitope recognition of the Vβ₃-region. This may not be unique to Vβ₃ and may be a general feature of TCR-β protein folding.     

cAMP microdomains and L-type Ca2+ channel regulation in guinea-pig ventricular myocytes

Many different receptors can stimulate cAMP synthesis in the heart, but not all elicit the same functional responses. For example, it has been recognized for some time that prostaglandins such as PGE1 increase cAMP production and activate PKA, but they do not elicit responses like those produced by β-adrenergic receptor (βAR) agonists such as isoproterenol (isoprenaline), even though both stimulate the same signalling pathway. In the present study, we confirm that isoproterenol, but not PGE1, is able to produce cAMP-dependent stimulation of the L-type Ca²⁺ current in guinea pig ventricular myocytes. This is despite finding evidence that these cells express EP₄ prostaglandin receptors, which are known to activate G_s-dependent signalling pathways. Using fluorescence resonance energy transfer-based biosensors that are either freely diffusible or bound to A kinase anchoring proteins, we demonstrate that the difference is due to the ability of isoproterenol to stimulate cAMP production in cytosolic and caveolar compartments of intact cardiac myocytes, while PGE1 only stimulates cAMP production in the cytosolic compartment. Unlike other receptor-mediated responses, compartmentation of PGE1 responses was not due to concurrent activation of a G_i-dependent signalling pathway or phosphodiesterase activity. Instead, compartmentation of the PGE1 response in cardiac myocytes appears to be due to transient stimulation of cAMP in a microdomain that can communicate directly with the bulk cytosolic compartment but not the caveolar compartment associated with βAR regulation of L-type Ca²⁺ channel function.     

Serial Measurements of Pregnancy-specific β1-glycoprotein (β1SP1) in Patients with Trophoblastic Disease

A radioimmunoassay for pregnancy-specific-β₁-glycoprotein (Bohn. 1971) has been established with a limit of detection of 2 μMg/litre in serum. The assay has been used to measure serial levels of pregnancy-specific-β₁-glycoprotein (β₁SP₁) in the serum of patients with choriocarcinoma and teratoma for comparison with measurements of the β-subunit of human chorionic gonadotrophin. The value of the assay for β₁SP₁, in the management of these patients is discussed.     

α1-Adrenoceptor-activated cation currents in neurones acutely isolated from rat cardiac parasympathetic ganglia

The noradrenaline (NA)-induced cation current was investigated in neurones freshly isolated from rat cardiac parasympathetic ganglia using the nystatin-perforated patch recording configuration. Under current-clamp conditions, NA depolarized the membrane, eliciting repetitive action potentials. NA evoked an inward cation current under voltage-clamp conditions at a holding potential of −60 mV. The NA-induced current was inhibited by extracellular Ca²⁺ or Mg²⁺, with a half-maximal concentration of 13 μ m for Ca²⁺ and 1.2 m m for Mg²⁺. Cirazoline mimicked the NA response, and prazosin and WB-4101 inhibited the NA-induced current, suggesting the contribution of an α₁-adrenoceptor. The NA-induced current was inhibited by U73122, a phospholipase C (PLC) inhibitor. The membrane-permeable IP₃ receptor blocker xestospongin-C also blocked the NA-induced current. Furthermore, pretreatment with thapsigargin and BAPTA-AM could inhibit the NA response while KN-62, phorbol 12-myristate 13-acetate (PMA) and staurosporine had no effect. These results suggest that NA activates the extracellular Ca²⁺- and Mg²⁺-sensitive cation channels via α₁-adrenoceptors in neurones freshly isolated from rat cardiac parasympathetic ganglia. This activation mechanism also involves phosphoinositide breakdown, release of Ca²⁺ from intracellular Ca²⁺ stores and calmodulin. The cation channels activated by NA may play an important role in neuronal membrane depolarization in rat cardiac ganglia.     

Gastric motility in soluble guanylate cyclase α1 knock-out mice

The principal target of the relaxant neurotransmitter nitric oxide (NO) is soluble guanylate cyclase (sGC). As the α₁β₁-isoform of sGC is the predominant one in the gastrointestinal tract, the aim of this study was to investigate the role of sGC in nitrergic regulation of gastric motility in male and female sGCα₁ knock-out (KO) mice. In circular gastric fundus muscle strips, functional responses and cGMP levels were determined in response to nitrergic and non-nitrergic stimuli. sGC subunit mRNA expression in fundus was measured by real-time RT-PCR; in vivo gastric emptying of a phenol red meal was determined. No changes were observed in sGC subunit mRNA levels between wild-type (WT) and KO tissues. Nitrergic relaxations induced by short trains of electrical field stimulation (EFS) were abolished, while those by long trains of EFS were reduced in KO strips; the latter responses were abolished by 1 H [1,2,4,]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). The relaxations evoked by exogenous NO and the NO-independent sGC activator BAY 41-2272 were reduced in KO strips but still sensitive to ODQ. Relaxations induced by vasoactive intestinal peptide (VIP) and 8-bromo-cGMP were not influenced. Basal cGMP levels were decreased in KO strips but NO, long train EFS and BAY 41-2272 still induced a moderate ODQ-sensitive increase in cGMP levels. Gastric emptying, measured at 15 and 60 min, was increased at 15 min in male KO mice. sGCα₁β₁ plays an important role in gastric nitrergic relaxation in vitro , but some degree of nitrergic relaxation can occur via sGCα₂β₁ activation in sGCα₁ KO mice, which contributes to the moderate in vivo consequence on gastric emptying.     

Cross-linking of Both Types of IgG Fc Receptors, FcγyRI and FcγyRII, Enhances Intracellular Free Ca2+ in the Monocytic Cell Line U937

We have studied the effects of Fc receptor triggering on the free cytosolic Ca²⁺ concentration, [Ca²⁺] _i , in U937. These cells express two types of IgG Fc receptors, FcγRI and FcγRII. Binding of several anti-FcγRI and anti-FcγRII mouse monoclonal antibodies (MoAb) to Quin2- or Indo-I-loaded U937 cells had no direct effect on [Ca²⁺] _i . After addition of a bridging anti-mouse Ig antibody however, transient increases in [Ca²⁺] _i were observed for both types of FcγR. One of the anti-FcγRII MoAb, CIKM5, was exceptional in that it could induce Ca²⁺ increases in U937 cells by itself. Studies with F(ab')₂ fragments of CIK M5 revealed that this MoAb simultaneously binds to FcγRII, via both its Fab and Fc fragments, which might induce cross-linking of two FcγRII molecules. One anti-FcγRI MoAb, 197, a mouse (m)IgG2a antibody directed to an epitope outside the IgG-binding region, remarkably also caused an immediate increase in [Ca²⁺] _i , but only when added to U937 precultured with gamma interferon (IFN-γ). FcγRI can bind monomeric human IgG as well as mIgG2a, and cross-linking of cytophilic Ig induced an increase in [Ca²⁺] _i . Our results show that [Ca²⁺] _i increases can be induced only after cross-linking of FcγR, either via anti-FcγR MoAb or via Fc-FcR interactions. Furthermore, we show that FcγR cross-linking results in activation of the Ca²⁺/phosphatidylinositol (PI) signal transduction pathway.     

Cerebral non-oxidative carbohydrate consumption in humans driven by adrenaline

During brain activation, the decrease in the ratio between cerebral oxygen and carbohydrate uptake (6 O₂/(glucose +  ¹/₂  lactate); the oxygen–carbohydrate index, OCI) is attenuated by the non-selective β-adrenergic receptor antagonist propranolol, whereas OCI remains unaffected by the β₁-adrenergic receptor antagonist metroprolol. These observations suggest involvement of a β₂-adrenergic mechanism in non-oxidative metabolism for the brain. Therefore, we evaluated the effect of adrenaline (0.08 μg kg⁻¹ min⁻¹ i.v. for 15 min) and noradrenaline (0.5, 0.1 and 0.15 μg kg⁻¹ min⁻¹ i.v. for 20 min) on the arterial to internal jugular venous concentration differences (a-v diff) of O₂, glucose and lactate in healthy humans. Adrenaline ( n = 10) increased the arterial concentrations of O₂, glucose and lactate ( P < 0.05) and also increased the a-v diff for glucose from 0.6 ± 0.1 to 0.8 ± 0.2 m m (mean ± s.d. ; P < 0.05). The a-v diff for lactate shifted from a net cerebral release to an uptake and OCI was lowered from 5.1 ± 1.5 to 3.6 ± 0.4 ( P < 0.05) indicating an 8-fold increase in the rate of non-oxidative carbohydrate uptake during adrenaline infusion ( P < 0.01). Conversely, noradrenaline ( n = 8) did not affect the OCI despite an increase in the a-v diff for glucose ( P < 0.05). These results support that non-oxidative carbohydrate consumption for the brain is driven by a β₂-adrenergic mechanism, giving neurons an abundant provision of energy when plasma adrenaline increases.     

Palmitate increases L-type Ca2+ currents and the size of the readily releasable granule pool in mouse pancreatic β-cells

We have investigated the in vitro effects of the saturated free fatty acid palmitate on mouse pancreatic β-cells by a combination of electrophysiological recordings, intracellular Ca²⁺ ([Ca²⁺]_i) microfluorimetry and insulin release measurements. Addition of palmitate (1 m m , bound to fatty acid-free albumin) to intact islets exposed to 15 m m glucose increased the [Ca²⁺]_i by ∼30% and insulin secretion 2-fold. Palmitate remained capable of increasing [Ca²⁺]_i and insulin release in the presence of tolbutamide and in islets depolarized by high K⁺ in combination with diazoxide, indicating that the stimulation occurs independently of closure of ATP-regulated K⁺ channels (K_ATP channels). Palmitate (0.5 m m ) augmented exocytosis (measured as an increase in cell capacitance) in single β-cells and increased the size of the readily releasable pool (RRP) of granules 2-fold. Whole-cell peak Ca²⁺ currents rose by ∼25% following addition of 0.5 m m palmitate, an effect that was abolished in the presence of 10 μ m isradipine indicating that the free fatty acid specifically acts on L-type Ca²⁺ channels. The actions of palmitate on exocytosis and Ca²⁺ currents were not mimicked by intracellular application of palmitoyl-CoA. We conclude that palmitate increases insulin secretion by a K_ATP channel-independent mechanism exerted at the level of exocytosis and that involves both augmentation of L-type Ca²⁺ currents and an increased size of the RRP.     

The Complete Amino Acid Sequence of Rat β2-Microglobulin

The primary structure of rat β₂-microglobulin (β₂m) was determined. It is a polypeptide of 99 amino acids with the following sequence: IQKTPQIQVY SRHPPENGKP NFLNCYVSQF HPPQIEIELL KNGKKIPNIE MSDLSFSKDW SFYILAHTEF TPTETDVYAC RVKHVTLKEP KTVTWDRDM. The primary structure was determined by NH₂-terminal sequence analysis together with sequence determination of one cyanogen bromide fragment and one tryptic peptide. Of other known β₂m sequences, rat β₂m is most homologous to mouse β₂m (83% identity). The rabbit, human, and guinea pig sequences are more distant, with 24,27, and 31% differences, respectively.     

Expression of α5 (CD49e) and α6 (CD49f) Integrin Subunits on T Cells in the Circulation and the Lamina Propria of Normal and Inflammatory Bowel Disease Colonic Mucosa

Intestinal lamina propria T cells are believed to be derived, via the systemic circulation, from gut-associated lymphoid tissue. After migration into the lamina propria, T cells are capable of luminally directed migration following the loss of surface epithelial cells. For adhesion and migration within the extracellular matrix, T cells are likely to utilize the integrin family of adhesion molecules. The aim of this study was to quantitatively and qualitatively investigate the expression of α₅ and α₆ integrin subunits on the surface of human T cells that: (a) migrated out of the lamina propria, (b) remained resident within the matrix and (c) were present in the circulation. In both subpopulations of CD4 and CD8-positive T cells, from both normal and inflamed (inflammatory bowel disease) colonic mucosa, there were significantly fewer α₅ and α₆-positive cells than in the peripheral blood. In addition, there were significantly fewer α₆ integrin molecules on the surface of CD4 and CD8-positive lamina propria T-cell subpopulations, compared with those in the circulation. Our studies suggest that, following migration into the lamina propria, there is down-regulation of α₅ and α₆ integrin-subunit expression on the surface of T cells. Molecules other than members of very late activation antigen-5 (VLA-5) (α₅β₁) and VLA-6 (α₆β₁) families of adhesion molecules are likely to be important in interactions with extracellular components in the lamina propria of normal and inflamed human colonic mucosa.     

Characterization of Monoclonal Antibodies to the αIIbβ3 Integrin on Bovine Platelets

A set of monoclonal antibodies (MoAbs) to leucocyte antigens is an essential tool to identify different cell types and functional membrane molecules involved in immune responses. Since no MoAbs existed to bovine integrins, except against the β₂ subfamily, we generated MoAbs to β₃ integrin after the immunization of mice with bovine platelets. Two MoAbs, IL-A164 (IgG_2a) and IL-A166 (IgG₁), were selected that reacted specifically with bovine platelets and detected the same membrane molecule. The antigen was a heterodimer of two polypeptide chains of 122 kDa and 95 kDa as resolved by SDS-PAGE under reducing conditions. Although the Mr of the smaller subunit is identical to that of β₂ integrin, pre-absorption with an antibody to β₂ (or CD18) did not remove the bovine antigen. Comparing the molecular masses of the two subunits in reduced and non-reduced forms showed a pattern that was similar to that of human GPIIb/IIIa (also called α_IIbβ₃ or CD41a). Reduction of the bovine molecule increased the apparent Mr of the light chain from 76 kDa to 95 kDa, while the heavy subunit changed from 136 kDa to 122 kDa. As with human GPIIb, the decrease in Mr of the α-subunit is probably a result of a small disulphide-linked polypeptide, although no additional evidence for this was detected for the bovine integrin. Sequencing of the N-terminal amino acids of both bovine polypeptides showed identity of the bovine integrin with human GPIIb/IIIa.     

Synaptotagmin-7 is a principal Ca2+ sensor for Ca2+-induced glucagon exocytosis in pancreas

Hormones such as glucagon are secreted by Ca²⁺-induced exocytosis of large dense-core vesicles, but the mechanisms involved have only been partially elucidated. Studies of pancreatic β-cells secreting insulin revealed that synaptotagmin-7 alone is not sufficient to mediate Ca²⁺-dependent insulin granule exocytosis, and studies of chromaffin cells secreting neuropeptides and catecholamines showed that synaptotagmin-1 and -7 collaborate as Ca²⁺ sensors for exocytosis, and that both are equally involved. As no other peptide secretion was analysed, it remains unclear whether synaptotagmins generally act as Ca²⁺ sensors in large dense-core vesicle exocytosis in endocrine cells, and if so, whether synaptotagmin-7 always functions with a partner in that role. In particular, far less is known about the mechanisms underlying Ca²⁺-triggered glucagon release from α-cells than insulin secretion from β-cells, even though insulin and glucagon together regulate blood glucose levels. To address these issues, we analysed the role of synaptotagmins in Ca²⁺-triggered glucagon exocytosis. Surprisingly, we find that deletion of a single synaptotagmin isoform, synaptotagmin-7, nearly abolished Ca²⁺-triggered glucagon secretion. Moreover, single-cell capacitance measurements confirmed that pancreatic α-cells lacking synaptotagmin-7 exhibited little Ca²⁺-induced exocytosis, whereas all other physiological and morphological parameters of the α-cells were normal. Our data thus identify synaptotagmin-7 as a principal Ca²⁺ sensor for glucagon secretion, and support the notion that synaptotagmins perform a universal but selective function as individually acting Ca²⁺ sensors in neurotransmitter, neuropeptide, and hormone secretion.     

Adenosine inhibition via A1 receptor of N-type Ca2+ current and peptide release from isolated neurohypophysial terminals of the rat

Effects of adenosine on voltage-gated Ca²⁺ channel currents and on arginine vasopressin (AVP) and oxytocin (OT) release from isolated neurohypophysial (NH) terminals of the rat were investigated using perforated-patch clamp recordings and hormone-specific radioimmunoassays. Adenosine, but not adenosine 5'-triphosphate (ATP), dose-dependently and reversibly inhibited the transient component of the whole-terminal Ba²⁺ currents, with an IC₅₀ of 0.875 μ m. Adenosine strongly inhibited, in a dose-dependent manner (IC₅₀= 2.67 μ m ), depolarization-triggered AVP and OT release from isolated NH terminals. Adenosine and the N-type Ca²⁺ channel blocker ω-conotoxin GVIA, but not other Ca²⁺ channel-type antagonists, inhibited the same transient component of the Ba²⁺ current. Other components such as the L-, Q- and R-type channels, however, were insensitive to adenosine. Similarly, only adenosine and ω-conotoxin GVIA were able to inhibit the same component of AVP release. A₁ receptor agonists, but not other purinoceptor-type agonists, inhibited the same transient component of the Ba²⁺ current as adenosine. Furthermore, the A₁ receptor antagonist 8-cyclopentyltheophylline (CPT), but not the A₂ receptor antagonist 3, 7-dimethyl-1-propargylxanthine (DMPGX), reversed inhibition of this current component by adenosine. The inhibition of AVP and OT release also appeared to be via the A₁ receptor, since it was reversed by CPT. We therefore conclude that adenosine, acting via A₁ receptors, specifically blocks the terminal N-type Ca²⁺ channel thus leading to inhibition of the release of both AVP and OT.     

Complex cardiovascular actions of α-adrenergic receptors expressed in the nucleus tractus solitarii of rats

Although both α₁- and α₂-adrenergic receptors (ARs) are known to be expressed in the nucleus of the solitary tract (NTS), the functional significance of these receptors is still not fully established. In this study, we microinjected α₁- and α₂-AR agonists into the NTS of urethane-anaesthetized Wister rats to study the cardiovascular effects in response to their activation. When the α₁-AR agonist phenylephrine was microinjected into the area where barosensitive neurons are dominantly located (baro-NTS), mean arterial pressure (MAP) and heart rate (HR) were significantly elevated. When tested in the area where chemosensitive neurons are dominantly located (chemo-NTS), however, MAP and HR were significantly decreased. Pretreatment with the non-specific α-AR antagonist phentolamine into the NTS inhibited the phenylephrine-induced cardiovascular responses. In contrast, microinjection of the α₂-AR agonist clonidine into either the baro-NTS or the chemo-NTS decreased MAP and HR; they were also inhibited by the α₂-adrenergic antagonist yohimbine. Moreover, we immunohistochemically identified that cardiovascular responses induced by α₁-ARs may be mediated by NTS neurons while those induced by α₂-ARs may be mediated by astrocytes located in the barosensitive and chemosensitive areas of the NTS. These results suggest that both types of α-AR expressed in the NTS may be involved in regulating cardiovascular homeostasis via modulation of input signals from baroreceptor and chemoreceptor afferents; however, cardiovascular responses produced by stimulation of α₁-ARs are strictly location specific within the NTS.