Modulation of Ca2+ channels by alpha 1A- and alpha 2A-adrenoceptors in vascular myocytes: involvement of different transduction pathways

Connective tissue mast cells store and release noradrenaline

Mast cells are present in mucosal and connective tissues throughout the body. They synthesize and release a wide variety of bioactive molecules, such as histamine, proteases, and cytokines. In this study, we found that a population of connective tissue mast cells (CTMCs) stores and releases noradrenaline, originating from sympathetic nerves. Noradrenaline-storing cells, not neuronal fibers, were predominantly identified in the connective tissues of the skin, mammary gland, gastrointestinal tract, bronchus, thymus, and pancreas in wild-type mice but were absent in mast cell-deficient W-sash c-kit mutant KitW-sh/W-sh mice. In vitro studies using bone marrow-derived mast cells revealed that extracellular noradrenaline was taken up but not synthesized. Upon ionomycin stimulation, noradrenaline was released. Electron microscopy analyses further suggested that noradrenaline is stored in and released from the secretory granules of mast cells. Finally, we found that noradrenaline-storing CTMCs express organic cation transporter 3 (Oct3), which is also known as an extraneuronal monoamine transporter, SLC22A3. Our findings indicate that mast cells may play a role in regulating noradrenaline concentration by storing and releasing it in somatic tissues.

Intravenous dexmedetomidine, morphine, or a combination can result in gallbladder wall thickening; with no significant association with plasma histamine concentrations

The gallbladder is routinely evaluated during ultrasonographic examinations in dogs. However, published studies describing the effects of sedative agents on gallbladder wall thickness are currently lacking. The aims of this prospective, blinded, randomized crossover pilot study were to test hypotheses that IV morphine would result in gallbladder wall thickening, that morphine administration would increase plasma histamine concentrations, and that combining IV morphine with dexmedetomidine would potentiate gallbladder wall thickening. Six healthy Beagle dogs were sedated with intravenous (IV) morphine 0.4 mg/kg (group M), dexmedetomidine 7 μg/kg (group D), or a combination of the two (group MD). Physiologic parameters were measured at baseline and at regular intervals until the last ultrasonographic scan. Ultrasonographic scans were performed at baseline, 90 s, and at 5, 15, 30, 45, 60, 90, and 120 min. Plasma histamine samples were taken at baseline, 90 s, and 5 and 60 min. Cochran's Q-test was used to compare gallbladder wall thickening between groups, while the association between histamine plasma concentration and gallbladder wall thickness was compared with a mixed-effects model. Baseline gallbladder wall thickness was not significantly different between groups. Six of 18 treatments/dogs (33%) developed gallbladder thickening, with no difference between groups. There was no significant difference in baseline plasma histamine concentrations between groups, and no association between plasma histamine concentration and gallbladder wall thickness. Gallbladder wall thickening was observed in at least one dog in each group, therefore caution is recommended for gallbladder wall thickness ultrasonographic interpretation in dogs when these drugs have been administered.

The Role of Two-Pore Channels in Norepinephrine-Induced [Ca2+]i Rise in Rat Aortic Smooth Muscle Cells and Aorta Contraction

Second messenger nicotinic acid adenine dinucleotide phosphate (NAADP) triggers Ca²⁺ release via two-pore channels (TPCs) localized in endolysosomal vesicles. The aim of the present work is to evaluate the role of TPCs in the action of norepinephrine (NE), angiotensin II (AngII), vasopressin (AVP), and 5-hydroxytriptamine (5-HT) on free cytoplasmic calcium concentration ([Ca²⁺]_i) in smooth muscle cells (SMCs) isolated from rat aorta and on aorta contraction. To address this issue, the NAADP structural analogue and inhibitor of TPCs, NED 19, was applied. We have demonstrated a high degree of colocalization of the fluorescent signals of cis-NED 19 and endolysosmal probe LysoTracker in SMCs. Both cis- or trans-NED 19 inhibited the rise of [Ca²⁺]_i in SMCs induced by 100 μM NE by 50–60%. IC₅₀ for cis- and trans-NED 19 were 2.7 and 8.9 μM, respectively. The inhibition by NED 19 stereoisomers of the effects of AngII, AVP, and 5-HT was much weaker. Both forms of NED 19 caused relaxation of aortic rings preconstricted by NE, with relative potency of cis-NED 19 several times higher than that of trans-NED 19. Inhibition by cis-NED 19 of NE-induced contraction was maintained after intensive washing and slowly reversed within an hour of incubation. Cis- and trans-NED 19 did not cause decrease in the force of aorta contraction in response to Ang II and AVP, and only slightly relaxed aorta preconstricted by 5-HT and by KCl. Suppression of TPC1 in SMCs with siRNA caused a 40% decrease in [Ca²⁺]_i in response to NE, whereas siRNA against TPC2 did not change NE calcium signaling. These data suggest that TPC1 is involved in the NE-stimulated [Ca²⁺]_i rise in SMCs. Inhibition of TPC1 activity by NED 19 could be the reason for partial inhibition of aortic rings contraction in response to NE.

Enhanced calcium entry via activation of NOX/PKC underlies increased vasoconstriction induced by methylglyoxal

Advanced glycation end-products (AGEs) play a pivotal role in macro- and micro-vascular diabetic complications. We investigated the mechanism by which methylglyoxal (an endogenous generator of AGEs) affects vascular contractility using the isolated artery technique. Contractile responses to vasoconstrictors phenylephrine (PE), angiotensin II (Ang II), vasopressin (VP) and KCl were measured in the isolated rat aorta following one-our exposure to methylglyoxal (50-200 μM). The perfused rat kidney was employed to confirm the effect of methylglyoxal on microvessels. Methylglyoxal-induced changes in cytosolic calcium were measured in the smooth muscle layer of the aorta with the calcium-sensing fluorophore Fluo-4 AM. Methylglyoxal significantly increased maximal contraction of the rat aorta to PE, Ang II and VP. Similar results were seen in response to the depolarizing vasoconstrictor KCl in macro and micro vessels. The methylglyoxal-induced increases in aortic contraction mediated by the agonist and KCl were endothelium independent. Methylglyoxal-induced increases in KCl-dependent aortic contraction were abolished after the removal of extracellular calcium or in the presence of the calcium channel blocker nifedipine. Incubation with the antioxidant N-acetyl-l-cysteine (NAC), apocynin (a nonselective NADPH oxidase (NOX) inhibitor) or chelerythrine (a protein kinase C (PKC) inhibitor) prior to methylglyoxal pre-treatment reversed the methylglyoxal-induced increases in the rat aortic contractility. In conclusion, the formation of AGEs increases vasoconstriction of both macro- and micro-vessels by increasing the voltage-activated calcium entry in vascular smooth muscles in a NOX and PKC dependent manner.

Physiological vs. pharmacological signalling to myosin phosphorylation in airway smooth muscle

KEY POINTS

Smooth muscle myosin regulatory light chain (RLC) is phosphorylated by Ca²⁺ /calmodulin-dependent myosin light chain kinase and dephosphorylated by myosin light chain phosphatase (MLCP). Tracheal smooth muscle contains significant amounts of myosin binding subunit 85 (MBS85), another myosin phosphatase targeting subunit (MYPT) family member, in addition to MLCP regulatory subunit MYPT1. Concentration/temporal responses to carbachol demonstrated similar sensitivities for bovine tracheal force development and phosphorylation of RLC, MYPT1, MBS85 and paxillin. Electrical field stimulation releases ACh from nerves to increase RLC phosphorylation but not MYPT1 or MBS85 phosphorylation. Thus, nerve-mediated muscarinic responses in signalling modules acting on RLC phosphorylation are different from pharmacological responses with bath added agonist. The conditional knockout of MYPT1 or the knock-in mutation T853A in mice had no effect on muscarinic force responses in isolated tracheal tissues. MLCP activity may arise from functionally shared roles between MYPT1 and MBS85, resulting in minimal effects of MYPT1 knockout on contraction.

ABSTRACT

Ca²⁺ /calmodulin activation of myosin light chain kinase (MLCK) initiates myosin regulatory light chain (RLC) phosphorylation for smooth muscle contraction with subsequent dephosphorylation for relaxation by myosin light chain phosphatase (MLCP) containing regulatory (MYPT1) and catalytic (PP1cδ) subunits. RLC phosphorylation-dependent force development is regulated by distinct signalling modules involving protein phosphorylations. We investigated responses to cholinergic agonist treatment vs. neurostimulation by electric field stimulation (EFS) in bovine tracheal smooth muscle. Concentration/temporal responses to carbachol demonstrated tight coupling between force development and RLC phosphorylation but sensitivity differences in MLCK, MYPT1 T853, MYPT1 T696, myosin binding subunit 85 (MBS85), paxillin and CPI-17 (PKC-potentiated protein phosphatase 1 inhibitor protein of 17 kDa) phosphorylations. EFS increased force and phosphorylation of RLC, CPI-17 and MLCK. In the presence of the cholinesterase inhibitor neostigmine, EFS led to an additional increase in phosphorylation of MYPT1 T853, MYPT1 T696, MBS85 and paxillin. Thus, there were distinct pharmacological vs. physiological responses in signalling modules acting on RLC phosphorylation and force responses, probably related to degenerate G protein signalling networks. Studies with genetically modified mice were performed. Expression of another MYPT1 family member, MBS85, was enriched in mouse, as well as bovine tracheal smooth muscle. Carbachol concentration/temporal-force responses were similar in trachea from MYPT1^SM+/+ , MYPT1^SM-/- and the knock-in mutant mice containing nonphosphorylatable MYPT1 T853A with no differences in RLC phosphorylation. Thus, MYPT1 T853 phosphorylation was not necessary for regulation of RLC phosphorylation in tonic airway smooth muscle. Furthermore, MLCP activity may arise from functionally shared roles between MYPT1 and MBS85, resulting in minimal effects of MYPT1 knockout on contraction.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

Mechanism and effect of shijueming (Concha Haliotidis) on serum calcium in spontaneously hypertensive rats

OBJECTIVE

To observe the impact of Shijueming (Concha Haliotidis) on spontaneously hypertensive rats via blood pressure, serum calcium, vascular smooth muscle membrane L-type calcium channel alpha1 C subunit (CaL-alpha1C), plasma membrane calcium-ATPase (PMCA) mRNA expression, and the L-type calcium channel in vascular smooth muscle cells.

METHODS

Twelve-week-old male rats with spontaneous hypertension were divided into three groups: a Shijueming (Concha Haliotidis) group (group 1), a nifedipine group (group 2), and a distilled water group (group 3). All were given a four-week treatment. Blood pressure and dissociative serum calcium were examined before treatment. Blood pressure was taken every week during treatment. Atomic absorption spectrometry was used to examine dissociative serum calcium. Reverse transcription-polymerase chain reaction was used to examine the expression of CaL-alpha1C and PMCA1 mRNA. The patch clamp technique was used to examine the electrophysiological characteristics of the vascular smooth muscle cell calcium channels.

RESULTS

After treatment, blood pressure of the Shijueming (Concha Haliotidis) group lowered but not significantly (P > 0.05). Blood pressure of the nifedipine group lowered significantly (P < 0.05). Blood pressure of the distilled water group remained high. The concentration of serum calcium in the Shijueming (Concha Haliotidis) and the distilled water groups lowered (P < 0.05). Expression of CaL-alpha1C mRNA in the nifedipine group decreased compared with the distilled water group (P < 0.01). There was the decreasing trend in the Shijueming (Concha Haliotidis) group, but it was not statistically significant. Shijueming (Concha Haliotidis) also had effects on the expression of PMCA1 mRNA but without statistical significance. However, there was a significant decreasing effect on vascular smooth muscle cell I(Ca)-L flow.

CONCLUSION

This study indicated that Shijueming (Concha Haliotidis) could increase serum calcium and decrease blood pressure. It may work by influencing calcium channels, expression of PMCA1 mRNA, and regulating ion calcium channels and calcium-ATPase.

Selective modulation of lymphoproliferation and cytokine production via intracellular signaling targets by α1- and α2-adrenoceptors and estrogen in splenocytes

The mechanistic implications of the presence of sympathetic noradrenergic innervation in lymphoid organs in synaptic association with lymphocytes open to the influence of hormonal fluctuations throughout reproductive age in females has not been investigated yet.

OBJECTIVES

The aim of the present study is to investigate the role of alpha-adrenoceptors (α-ARs) and estrogen in modulating immune responses in the spleen through intracellular signaling targets such as ERK 1/2, CREB, Akt, NF-κB.

METHODS

Splenocytes from young Sprague-Dawley rats were incubated with α1- and α2- AR specific agonists, phenylephrine and clonidine, without and with 17b-estradiol or specific antagonists prazosin and idazoxan to examine their effects on proliferation, cytokine production, nitric oxide production, and intracellular signaling molecules.

RESULTS

α1-AR stimulation inhibited lymphocyte proliferation and IFN-g production and enhanced IL-2, p-ERK and p-CREB expression. Co-stimulation using estrogen enhanced cytokine production and suppressed p-Akt expression. α1-AR blockade reversed agonist-induced IL-2 production alone. α2-AR stimulation inhibited lymphocyte proliferation, p-ERK and p-CREB expression, and increased p-NF-kB and p-Akt expression. Co-stimulation with estrogen increased IL-2 and suppressed p-CREB expression. α2-AR Idazoxan prevented IL-2 production in the absence and presence of estrogen, and reversed clonidine-induced increase in NO production and p-ERK and p-Akt expression in the presence of estrogen.

CONCLUSIONS

These results suggest that the cell-mediated immune responses are selectively modulated depending upon the subtypes of α-AR and further, these effects are differentially regulated in the presence of estrogen mediated through selective alteration in the intracellular signaling pathways involving ERK, CREB, Akt, and NF-κB.

Scientific rationale for the use of alpha-adrenergic agonists and glucocorticoids in the therapy of pediatric stridor

Purpose. The most common pharmacological therapies used in the treatment of stridor in children are glucocorticosteroids (GC) and alpha-adrenergic (αAR) agonists. Despite the long-standing reported efficacy of these medications, there is a paucity of data relating to their actual mechanisms of action in the upper airway. Summary. There is compelling scientific evidence supporting the use of αAR-agonists and GCs in pediatric stridor. αAR signaling and GCs regulate the vasomotor tone in the upper airway mucosa. The latter translates into better airflow dynamics, as delineated by human and nonhuman upper airway physiological models. In turn, clinical trials have demonstrated that GCs and the nonselective αAR agonist, epinephrine, improve respiratory distress scores and reduce the need for further medical care in children with stridor. Future research is needed to investigate the role of selective αAR agonists and the potential synergism of GCs and αAR-signaling in the treatment of upper airway obstruction and stridor.

Enhanced store-operated Ca²+ entry and TRPC channel expression in pulmonary arteries of monocrotaline-induced pulmonary hypertensive rats

Pulmonary hypertension (PH) is associated with profound vascular remodeling and alterations in Ca(2+) homeostasis in pulmonary arterial smooth muscle cells (PASMCs). Previous studies show that canonical transient receptor potential (TRPC) genes are upregulated and store-operated Ca(2+) entry (SOCE) is augmented in PASMCs of chronic hypoxic rats and patients of pulmonary arterial hypertension (PAH). Here we further examine the involvement of TRPC and SOCE in PH with a widely used rat model of monocrotaline (MCT)-induced PAH. Rats developed severe PAH, right ventricular hypertrophy, and significant increase in store-operated TRPC1 and TRPC4 mRNA and protein in endothelium-denuded pulmonary arteries (PAs) 3 wk after MCT injection. Contraction of PA and Ca(2+) influx in PASMC evoked by store depletion using cyclopiazonic acid (CPA) were enhanced dramatically, consistent with augmented SOCE in the MCT-treated group. The time course of increase in CPA-induced contraction corresponded to that of TRPC1 expression. Endothelin-1 (ET-1)-induced vasoconstriction was also potentiated in PAs of MCT-treated rats. The response was partially inhibited by SOCE blockers, including Gd(3+), La(3+), and SKF-96365, as well as the general TRPC inhibitor BTP-2, suggesting that TRPC-dependent SOCE was involved. Moreover, the ET-1-induced contraction and Ca(2+) response in the MCT group were more susceptible to the inhibition caused by the various SOCE blockers. Hence, our study shows that MCT-induced PAH is associated with increased TRPC expression and SOCE, which are involved in the enhanced vascular reactivity to ET-1, and support the hypothesis that TRPC-dependent SOCE is an important pathway for the development of PH.

Expression of alpha-AR subtypes in T lymphocytes and role of the alpha-ARs in mediating modulation of T cell function

OBJECTIVES

Previous work in our laboratory has shown that alpha-adrenoreceptors (alpha-ARs) and beta-ARs exist on lymphocytes from functional profile, and that the receptors mediate the regulation of lymphocyte function by catecholamines. In the present study, we directly examined the expression of alpha-AR subtypes, alpha(1)-AR and alpha(2)-AR mRNAs, in T lymphocytes and explored the roles of the alpha-AR subtypes and intracellular signal transduction mechanisms linked to the receptors in mediating the modulation of T lymphocyte function.

METHODS

T lymphocytes from mesenteric lymph nodes of rats were purified by using a nylon wool column. Reverse transcription polymerase chain reaction was used to detect the expression of alpha(1)-AR and alpha(2)-AR mRNAs in the freshly isolated T cells and the mitogen concanavalin A (Con A)-activated lymphocytes. Colorimetric methylthiazoletetrazolium assay was employed to measure lymphocyte proliferation induced by Con A. Interferon-gamma (IFN-gamma) and interleukin-4 (IL-4) levels in the Con A-stimulated lymphocyte culture supernatants were examined by enzyme-linked immunosorbent assay.

RESULTS

T cells expressed both alpha(1)-AR and alpha(2)-AR mRNAs. The expression of both alpha(1)-AR and alpha(2)-AR mRNAs was significantly higher in the Con A-activated lymphocytes than in the resting lymphocytes. Phenylephrine, a selective alpha(1)-AR agonist, had no evident effect on lymphocyte proliferation nor on IFN-gamma and IL-4 production induced by Con A. However, the selective alpha(2)-AR agonist clonidine attenuated Con A-induced lymphocyte proliferation as well as IFN-gamma and IL-4 production. The inhibited lymphocyte proliferation and IFN-gamma and IL-4 production by clonidine were blocked by yohimbine, an alpha(2)-AR antagonist. Either phospholipase C inhibitor U-73122 or protein kinase C inhibitor chelerythrine partially prevented the suppressive effect of clonidine on Con A-stimulated lymphocyte proliferation and IL-4 production.

CONCLUSIONS

T lymphocytes express both alpha(1)-ARs and alpha(2)-ARs, but only the alpha(2)-ARs participate in the suppressive modulation of lymphocyte proliferation and cytokine production in vitro. The inhibitory effect of alpha(2)-AR stimulation on lymphocyte function is partially mediated via the phospholipase C-protein kinase C pathway.

Adrenergic origin of very low-frequency blood pressure oscillations in the unanesthetized rat

Spectral analysis of cardiovascular signals has been extensively used to investigate circulatory homeostatic mechanisms. However, the nature of very low-frequency (VLF) fluctuations remains unclear. Because we previously observed enhanced VLF fluctuations in blood pressure (BP) in the sympathectomized rat (a model characterized by markedly increased plasma epinephrine levels), the aims of our study were to assess whether the genesis of VLF fluctuations in BP depends on circulating catecholamines and to determine which adrenergic receptor(s) and which membrane ion channel(s) are involved. We used continuous intra-arterial BP recordings from unanesthetized unrestrained rats to compute the power of VLF fluctuations in BP in the intact condition, during acute ganglionic blockade with hexamethonium, and after restoration of BP levels by infusion (in addition to hexamethonium) of adrenergic agonists (epinephrine, norepinephrine, and clonidine) or nonadrenergic vasoconstrictors (vasopressin). Effects of infusion of specific adrenergic receptor blockers (propranolol, prazosin, and yohimbine) with hexamethonium and catecholamines and infusion of various membrane ion channel blockers on VLF fluctuations in BP were also evaluated. Our results are as follows. 1) Ganglionic blockade drastically reduced BP levels and VLF fluctuations. 2) All vasoconstrictors restored BP levels, but only adrenergic vasoconstrictors generated striking VLF fluctuations in BP. 3) Catecholamine-induced fluctuations were abolished by alpha2-, but not alpha1- or beta-, adrenergic receptor blockade and by Ba2+-sensitive K+ channel or L-type Ca2+ channel, but not by other ion channel, blockers. We conclude that, in the conscious, unrestrained ganglion-blocked rat, catecholamine infusion generates VLF fluctuations in BP through stimulation of alpha2-receptors and activation of Ba2+-sensitive K+ channels. These fluctuations may have (patho)physiological relevance under conditions of disrupted circulatory homeostasis.

PI3K and PKC contribute to membrane depolarization mediated by alpha2-adrenoceptors in the canine isolated mesenteric vein

BACKGROUND

Norepinephrine (NE), a classic neurotransmitter in the sympathetic nervous system, induces vasoconstriction of canine isolated mesenteric vein that is accompanied by a sustained membrane depolarization. The mechanisms underlying the NE-elicited membrane depolarization remain undefined. In the present study we hypothesized that phosphatidylinositol 3-kinase (PI3K) and protein kinase C (PKC) are involved in the electrical field stimulation (EFS)-induced slow membrane depolarization (SMD) in canine isolated mesenteric vein. EFS (0.1-2 Hz, 0.1 ms, 15V, 10 s)-induced changes in the membrane potential were recorded with a conventional intracellular microelectrode technique and evaluated in the absence and presence of inhibitors of neuronal activity, alpha-adrenoceptors, membrane ion channels, PI3K, inositol 1,4,5-triphosphate (InsP3) receptors, and PKC. Activation of PI3Kgamma and PKCzeta in response to exogenous NE and clonidine in the absence and presence of receptor and kinase inhibitors were also determined.

RESULTS

Contractile responses to NE and clonidine (0.05 - 10 microM) were significantly diminished in the presence of yohimbine (0.1 microM). Exogenous NE (0.1 microM) and clonidine (1 microM) elicited SMD. The resting membrane potential of canine mesenteric vein smooth muscle cells was -68.8 +/- 0.8 mV. EFS elicited a biphasic depolarization comprised of excitatory junction potentials and SMD that are purinergic and adrenergic in nature, respectively. The magnitude of the SMD in response to EFS at 0.5 Hz was 9.4 +/- 0.7 mV. This response was reduced by 65-98% by the fast Na+ channel inhibitor tetrodotoxin (1 microM), by the inhibitor of N-type Ca2+ channels omega-conotoxin GVIA (5 nM), the non-selective alpha-adrenoceptor blocker phentolamine (1 microM), the selective alpha2-adrenoceptor blocker yohimbine (0.1 microM), the ion channel inhibitors niflumic acid (NFA, 100 microM), 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB, 30 microM), 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS, 200 microM), and Gd3+ (30 microM), and the PI3K inhibitors wortmannin (100 nM) and LY-294002 (10 microM). The SMD remained unchanged in the presence of the L-type Ca2+ channel blocker nicardipine (1 microM) and the InsP3 receptor blockers 2-aminoethoxydiphenylborate (2APB, 50 microM) and xestospongin C (3 microM). The inhibitor of PKC chelerythrine (1 microM), but not calphostin C (10 microM), diminished the SMD. Exogenous NE and clonidine (1 microM each) activated both PI3Kgamma and PKCzeta, and the activation of these kinases was abolished by preincubation of tissue with the alpha2-adrenoceptor blocker yohimbine.

CONCLUSION

Neuronally-released NE stimulates smooth muscle alpha2-adrenoceptors and activates PI3K and atypical PKC in the canine mesenteric vein. Events downstream of PKC lead to SMD and vasoconstriction. This represents a novel pathway for NE-induced membrane depolarization in a vascular smooth muscle preparation.

Neural control of muscle blood flow during exercise

Activation of skeletal muscle fibers by somatic nerves results in vasodilation and functional hyperemia. Sympathetic nerve activity is integral to vasoconstriction and the maintenance of arterial blood pressure. Thus the interaction between somatic and sympathetic neuroeffector pathways underlies blood flow control to skeletal muscle during exercise. Muscle blood flow increases in proportion to the intensity of activity despite concomitant increases in sympathetic neural discharge to the active muscles, indicating a reduced responsiveness to sympathetic activation. However, increased sympathetic nerve activity can restrict blood flow to active muscles to maintain arterial blood pressure. In this brief review, we highlight recent advances in our understanding of the neural control of the circulation in exercising muscle by focusing on two main topics: 1) the role of motor unit recruitment and muscle fiber activation in generating vasodilator signals and 2) the nature of interaction between sympathetic vasoconstriction and functional vasodilation that occurs throughout the resistance network. Understanding how these control systems interact to govern muscle blood flow during exercise leads to a clear set of specific aims for future research.

Involvement of signal transducing GTP-binding proteins in renal artery alpha 1-adrenoceptor mediated smooth muscle contraction

OBJECTIVE

To determine the participation of GTP-binding proteins (G-proteins) in the cellular mechanism of the phenylephrine-induced renal artery vasospasm by using swine renal artery smooth muscle rings in a standard organ baths, as increased noradrenaline release from perivasal and intramural sympathetic nerve endings during renal ischaemia results in increased vascular smooth muscle tone that is important in the loss of kidney function during renal transplantation and nephron-sparing surgery.

MATERIALS AND METHODS

Fresh swine kidneys were transported in cold calcium-free Tyrode solution to the laboratory. Adipose tissue around the arteries was removed, the organ de-capsulated and interlobar arteries dissected. The contractile properties of renal artery smooth muscle rings were assessed in a standard organ bath, the rings pre-tensioned at 2 g. Contractions were evoked by applying the alpha 1-adrenoceptor selective agonist phenylephrine (1 nmol/L to 0.3 mmol/L). Isometric contractions of the tissue were registered and stored digitally. Dose-response curves were obtained sequentially with a wash-out of 20 min between each concentration; the maximum contractility of an individual muscle ring was set at 100%. Dose-response curves of inhibitory agents (e.g. WB4101, cholera and pertussis toxins) were determined by comparing the remaining contractility after incubating with the respective drug with a control contraction that was evoked three times (10 mumol/L phenylephrine) and the mean set at 100%.

RESULTS

Phenylephrine induced dose-dependent and fully reversible isometric contractions with a threshold concentration of 100 nmol/L and an EC50 of 0.8 mumol/L. The receptor was identified as the alpha 1A-subtype by the selective antagonist WB4101. Pre-treatment of tissue rings with 5 micrograms/mL pertussis toxin (120 min, 37 degrees C) inhibited the control contraction by a mean (SEM) of 52.0 (4.6)%, whereas pre-treatment with 1 microgram/mL cholera toxin (60 min, 37 degrees C), leading to a permanent activation of the Gs-protein via blockade of the GTPase activity, decreased the response by 39.0 (8.2%).

CONCLUSION

These results suggest a coupling of alpha 1A-adrenoceptors in renal vascular tissue to the heterotrimeric Gs-protein and to heterotrimeric G-proteins of the G1- and/or G0-family in the phenylephrine-induced contraction.

Mechanism of acidic pH-induced contraction in spontaneously hypertensive rat aorta: role of Ca2+release from the sarcoplasmic reticulum

AIM

This study was conducted to investigate the mechanism of acidic pH-induced contraction (APIC) with regard to Ca2+ handling using isometric tension recording experiments.

RESULTS

Decreasing extracellular pH from 7.4 to 6.5 produced a marked and sustained contraction of spontaneously hypertensive rat (SHR) aorta, that was 128.7 +/- 2.0% of the 64.8 mm KCl-induced contraction. Verapamil, an inhibitor of voltage-dependent Ca2+ channels (VDCC) significantly inhibited the APIC. In Ca2+-deficient solution, sustained contraction induced by acidic pH was abolished completely, while a transient contraction was still observed suggesting the release of Ca2+ from intracellular site. Ryanodine (1 microm), a ryanodine receptor blocker, and 10 microm cyclopiazonic acid (CPA; a sarco/endoplasmic reticulum Ca2+ ATPase inhibitor) abolished the transient contraction induced by acidosis. In normal Ca2+-containing solution, ryanodine significantly decreased the rate of rise as well as maximum level of APIC. Interestingly, ryanodine and CPA showed an additive inhibitory effect with verapamil and the combined treatment of ryanodine or CPA with verapamil nearly abolished the APIC.

CONCLUSIONS

It is concluded that acidic pH induces Ca2+ release from ryanodine/CPA-sensitive store of sarcoplasmic reticulum in SHR aorta. This Ca2+ plays an important role in the facilitation of the rate of rise of APIC, as well as contributing to the sustained contraction via a mechanism which is independent of Ca2+ influx through VDCC.

Substance P modulates localized calcium transients and membrane current responses in murine colonic myocytes

1. Neurokinins contribute to the neural regulation of gastrointestinal (GI) smooth muscles. We studied responses of murine colonic smooth muscle cells to substance P (SP) and NK(1) and NK(2) agonists using confocal microscopy and the patch clamp technique. 2. Colonic myocytes generated localized Ca(2+) transients that were coupled to spontaneous transient outward currents (STOCs). SP (10(-10) M) increased Ca(2+) transients and STOCs. Higher concentrations of SP (10(-6) M) increased basal Ca(2+) and inhibited Ca(2+) transients and STOCs. 3. Effects of SP were due to increased Ca(2+) entry via L-type Ca(2+) channels, and were mediated by protein kinase C (PKC). Nifedipine (10(-6) M) and the PKC inhibitor, GF 109203X (10(-6) M) reduced L-type Ca(2+) current and blocked the effects of SP. 4. SP responses depended upon parallel stimulation of NK(1) and NK(2) receptors. NK(1) agonist ([Sar(9),Met(O(2))(11)]-substance P; SSP) and NK(2) agonists (neurokinin A (NKA) or GR-64349) did not mimic the effects of SP alone, but NK(1) and NK(2) agonists were effective when added in combination (10(-10)-10(-6) M). Consistent with this, either an NK(1)-specific antagonist (GR-82334; 10(-7) M) or an NK(2)-specific antagonist (MEN 10,627; 10(-7) M) blocked responses to SP (10(-6) M). 5. Ryanodine (10(-5) M) blocked the increase in Ca(2+) transients and STOCs in response to SP (10(-10) M). 6. Our findings show that low concentrations of SP, via PKC-dependent enhancement of L-type Ca(2+) current and recruitment of ryanodine receptors, stimulate Ca(2+) transients. At higher concentrations of SP (10(-6) M), basal Ca(2+) increases and spontaneous Ca(2+) transients and STOCs are inhibited.

Regulation of the L-type calcium channel by alpha 5beta 1 integrin requires signaling between focal adhesion proteins

The L-type calcium channel is the major calcium influx pathway in vascular smooth muscle and is regulated by integrin ligands, suggesting an important link between extracellular matrix and vascular tone regulation in tissue injury and remodeling. We examined the role of integrin-linked tyrosine kinases and focal adhesion proteins in regulation of L-type calcium current in single vascular myocytes. Soluble tyrosine kinase inhibitors blocked the increase in current produced by alpha(5) integrin antibody or fibronectin, whereas tyrosine phosphatase inhibition enhanced the effect. Cell dialysis with an antibody to focal adhesion kinase or with FRNK, the C-terminal noncatalytic domain of focal adhesion kinase, produced moderate (24 or 18%, respectively) inhibition of basal current but much greater inhibition (63 or 68%, respectively) of integrin-enhanced current. A c-Src antibody and peptide inhibitors of the Src homology-2 domain or a putative Src tyrosine phosphorylation site on the channel produced similar inhibition. Antibodies to the cytoskeletal proteins paxillin and vinculin, but not alpha-actinin, inhibited integrin-dependent current by 65-80%. Therefore, alpha(5)beta(1) integrin appears to regulate a tyrosine phosphorylation cascade involving Src and various focal adhesion proteins that control the function of the L-type calcium channel. This interaction may represent a novel mechanism for control of calcium influx in vascular smooth muscle and other cell types.

Metabolic modulation of sympathetic vasoconstriction in exercising skeletal muscle

The tight coupling of oxygen supply and utilization in exercising skeletal muscle is the result of complex interactions between local mechanisms that control muscle blood flow and substrate utilization and systemic mechanisms that control cardiac output and arterial pressure. The role of the sympathetic nervous system in the integration of these responses, specifically the interaction between sympathetic activation and local vasodilator mechanisms in exercising muscle, has been an active area of research for many years yet remains incompletely understood. The functional consequence of sympathetic activation in exercising skeletal muscle has been the subject of considerable debate. Previous studies in animals and humans have suggested that sympathetic vasoconstricton in active muscle is (a) well maintained and serves to limit active hyperaemia, thereby preventing muscle blood flow from outstripping cardiac output in order to preserve blood pressure and vital organ perfusion or (b) greatly attenuated in order to optimize muscle perfusion, a concept that has been termed 'functional sympatholysis'. Studies performed over the past 70 years have provided conflicting evidence regarding the relative importance of sympathetic vasoconstriction vs. functional sympatholysis in exercising skeletal muscle. The focus of this review is mainly on recent studies in anaesthetized animal preparations and in conscious humans that have provided evidence for the metabolic modulation of sympathetic vasoconstriction in contracting skeletal muscle and have identified a number of key underlying mechanisms that extend the initial concept of sympatholysis.

Farnesol inhibits L-type Ca2+ channels in vascular smooth muscle cells

Earlier experiments with animal and human arteries have shown that farnesol, a natural 15-carbon (C15) isoprenoid, is an inhibitor of vasoconstriction (Roullet, J.-B., Xue, H., Chapman, J., McDougal, P., Roullet, C. M., and McCarron, D. A. (1996) J. Clin. Invest. 97, 2384-2390). We report here that farnesol reduced KCl- and norepinephrine-dependent cytosolic Ca2+ transients in fura-2-loaded intact arteries. An effect on Ca2+ signaling was also observed in cultured aortic smooth muscle cells (A10 cells). In these cells, farnesol reduced KCl-induced [Ca2+]i transients and mimicked the inhibitory effect of Ca2+-free medium on the [Ca2+]i response to both 12,13-phorbol myristate acetate, a protein kinase C activator, and thapsigargin, a specific endoplasmic reticulum ATPase inhibitor. Perforated patch-clamp experiments further showed in two vascular smooth muscle cell lines (A10 and A7r5), a reversible, dose-dependent inhibitory effect of farnesol on L-type Ca2+ currents (IC50 = 2.2 microM). Shorter (C10, geraniol) and longer (C20, geranylgeraniol) isoprenols were inactive. L-type Ca2+ channel blockade also occurred under tight (gigaohm) seal configuration using cell-attached, single-channel analysis, thus suggesting a possible action of farnesol from within the intracellular space. We finally demonstrated that farnesol did not affect Ca2+-sensitive pathways implicated in smooth muscle contraction, as tested with alpha-toxin permeabilized arteries. Altogether, our results indicate that farnesol is an inhibitor of vascular smooth muscle Ca2+ signaling with plasma membrane Ca2+ channel blocker properties. The data have implications for the endogenous and pharmacological regulation of vascular tone by farnesol or farnesol analogues.

Comparison of the Ca2+ movement by activation of alpha1-adrenoceptor subtypes in HEK-293 cells

We studied the Ca2+ movement induced by activation of alpha1A-, alpha1B- and alpha1D-adrenoceptor subtypes in transfected HEK-293 cells with the fura-2 probe. All these alpha1-AR subtypes induced both Ca2+ release and Ca2+ entry. The effect on Ca2+ release in alpha1b transfected HEK-293 cells was bigger than that in alpha1a and alpha1d transfected HEK-293 cells, and the effects on Ca2+ entry were the same in alpha1a, alpha1b and alpha1d transfected HEK-293 cells. The Ca2+ entry was inhibited by 1 mM NiSO4, but not by nifedipine. Cyclopiazonic acid (CPA) produced a biphasic Ca2+ signal response in Ca2+ medium, and only induced a transient response in Ca2+-free medium. After depletion of CPA-sensitive Ca2+ pool by 10 microM CPA in Ca2+-free medium, 10 microM adrenaline (Adr) still transiently increased [Ca2+]i in three different alpha1-adrenoceptor subtype transfected HEK-293 cells. However, after depletion of adrenaline-sensitive Ca2+ pool by 10 microM Adr, CPA transiently elevated [Ca2+]i only in alpha1a and alpha1d transfected HEK-293 cells, not in alpha1b transfected HEK-293 cells. U73122, a phospholipase C (PLC) inhibitor, inhibited both Ca2+ release and Ca2+ entry induced by activation of alpha1A alpha1B and alpha1D subtypes in transfected HEK-293 cells. These results suggest that HEK-293 cell line contains two functionally separate intracellular Ca2+ pools, CPA-sensitive and Adr-sensitive pools. Activation of alpha1B-AR stimulates Ca2+ release from both CPA-sensitive and Adr-sensitive Ca2+ pools. Alpha1A and alpha1D subtypes induce Ca2+ release only from Adr-sensitive Ca2+ pool.

Increased vascular responsiveness to alpha 2-adrenergic stimulation during NOS inhibition-induced hypertension

Increased vascular resistance during systemic nitric oxide synthase (NOS) inhibition is dependent on adrenergic vasoconstriction. This study tested the hypothesis that increased vascular sensitivity to adrenergic agonists contributes to this vasoconstriction. Superior mesenteric arteries and thoracic aortae from male Sprague-Dawley rats drinking water containing N omega-nitro-L-arginine (L-NNA; 14 days, 60 mg.kg-1.day-1) and control rats were-cut into helical strips, and endothelium was removed for contractile experiments. L-NNA arteries were more sensitive to UK-14304 (alpha 2-adrenergic agonist) and norepinephrine (NE), whereas responses to phenylephrine (PE) were not different concentration causing 50% maximal response (EC50), L-NNA vs. control: UK-14304, 0.071 vs. 0.71 mumol/l; NE, 1.15 vs. 9.95 nmol/l]. Yohimbine, an alpha 2-selective antagonist, caused a concentration-dependent inhibition of contraction to NE only in L-NNA arteries (EC50 = 6.3 vs. 1.6 nmol/l at 1 nmol/l yohimbine), whereas prazosin shifted NE curves similarly in arteries from both groups. Yohimbine (10 nmol/l) inhibited contractions to UK-14304 (EC50 = 59 mumol/l vs. 17 mumol/l) but not contractions to PE, whereas prazosin inhibited both. These data indicate that L-NNA-induced hypertension leads to increased sensitivity of prazosin-sensitive alpha 2-adrenoceptors, an upregulation that could cause the increased vasoconstrictor response to NE in this model of hypertension.

Alpha-adrenoceptors and vascular regulation: molecular, pharmacologic and clinical correlates

This manuscript is intended to provide a comprehensive review of the alpha-adrenoceptors (ARs) and their role in vascular regulation. The historical development of the concept of receptors and the division of the alpha-ARs into alpha 1 and alpha 2 subtypes is traced. Emphasis will be placed on current understanding of the specific contribution of discrete alpha 1- and alpha 2-AR subtypes in the regulation of the vasculature, selective agonists and antagonists for these receptors, the second messengers utilized by these receptors, the myoplasmic calcium pathways activated to initiate smooth muscle contraction, as well as the clinical uses of agonists and antagonists that work at these receptors. New information is presented that deals with the molecular aspects of ligand interactions with specific subdomains of these receptors, as well as mRNA distribution and the regulation of alpha 1- and alpha 2-AR gene transcription and translation.

ATP-sensitive potassium channels mediate contraction-induced attenuation of sympathetic vasoconstriction in rat skeletal muscle

Sympathetic vasoconstriction is sensitive to inhibition by metabolic events in contracting rat and human skeletal muscle, but the underlying cellular mechanisms are unknown. In rats, this inhibition involves mainly alpha2-adrenergic vasoconstriction, which relies heavily on Ca2+ influx through voltage-dependent Ca2+ channels. We therefore hypothesized that contraction-induced inhibition of sympathetic vasoconstriction is mediated by ATP-sensitive potassium (KATP) channels, a hyperpolarizing vasodilator mechanism that could be activated by some metabolic product(s) of skeletal muscle contraction. We tested this hypothesis in anesthetized rats by measuring femoral artery blood flow responses to lumbar sympathetic nerve stimulation or intraarterial hindlimb infusion of the specific alpha2-adrenergic agonist UK 14,304 during KATP channel activation with diazoxide in resting hindlimb and during KATP channel block with glibenclamide in contracting hindlimb. The major new findings are twofold. First, like muscle contraction, pharmacologic activation of KATP channels with diazoxide in resting hindlimb dose dependently attenuated the vasoconstrictor responses to either sympathetic nerve stimulation or intraarterial UK 14,304. Second, the large contraction-induced attenuation in sympathetic vasoconstriction elicited by nerve stimulation or UK 14,304 was partially reversed when the physiologic activation of KATP channels produced by muscle contraction was prevented with glibenclamide. We conclude that contraction-induced activation of KATP channels is a major mechanism underlying metabolic inhibition of sympathetic vasoconstriction in exercising skeletal muscle.