Effects of pHi on isometric force and [Ca2+]i in porcine coronary artery smooth muscle

Contribution of elevated intracellular calcium to pulmonary arterial myocyte alkalinization during chronic hypoxia

In the lung, exposure to chronic hypoxia (CH) causes pulmonary hypertension, a debilitating disease. Development of this condition arises from increased muscularity and contraction of pulmonary vessels, associated with increases in pulmonary arterial smooth muscle cell (PASMC) intracellular pH (pHi) and Ca(2+) concentration ([Ca(2+)]i). In this study, we explored the interaction between pHi and [Ca(2+)]i in PASMCs from rats exposed to normoxia or CH (3 weeks, 10% O2). PASMC pHi and [Ca(2+)]i were measured with fluorescent microscopy and the dyes BCECF and Fura-2. Both pHi and [Ca(2+)]i levels were elevated in PASMCs from hypoxic rats. Exposure to KCl increased [Ca(2+)]i and pHi to a similar extent in normoxic and hypoxic PASMCs. Conversely, removal of extracellular Ca(2+) or blockade of Ca(2+) entry with NiCl2 or SKF 96365 decreased [Ca(2+)]i and pHi only in hypoxic cells. Neither increasing pHi with NH4Cl nor decreasing pHi by removal of bicarbonate impacted PASMC [Ca(2+)]i. We also examined the roles of Na(+)/Ca(2+) exchange (NCX) and Na(+)/H(+) exchange (NHE) in mediating the elevated basal [Ca(2+)]i and Ca(2+)-dependent changes in PASMC pHi. Bepridil, dichlorobenzamil, and KB-R7943, which are NCX inhibitors, decreased resting [Ca(2+)]i and pHi only in hypoxic PASMCs and blocked the changes in pHi induced by altering [Ca(2+)]i. Exposure to ethyl isopropyl amiloride, an NHE inhibitor, decreased resting pHi and prevented changes in pHi due to changing [Ca(2+)]i. Our findings indicate that, during CH, the elevation in basal [Ca(2+)]i may contribute to the alkaline shift in pHi in PASMCs, likely via mechanisms involving reverse-mode NCX and NHE.

Comparative effectiveness of intravenous immunoglobulin for children with Kawasaki disease: a nationwide cohort study

Introduction

Different immunoglobulin manufacturing processes may influence its effectiveness for Kawasaki disease. However, nationwide studies with longitudinal follow-up are still lacking. The aim of this study was to evaluate the comparative effectiveness of immunoglobulin preparations from a nationwide perspective.

Materials and Methods

This is a nationwide retrospective cohort study with a new user design. Data came from the National Health Insurance Research Database of Taiwan. From 1997 to 2008, children under 2 years old who received immunoglobulin therapy for the first time under the main diagnosis of Kawasaki disease were enrolled. The manufacturing processes were divided into β-propiolactonation, acidification and those containing IgA. The endpoints were immunoglobulin non-responsiveness, acute aneurysm, prolonged use of anti-platelets or anti-coagulants, and recurrence.

Results

In total, 3830 children were enrolled. β-propiolactonation had a relative risk of 1.45 (95% CI 1.08∼1.94) of immunoglobulin non-responsiveness, however, the relative risks for acidification and containing IgA were non-significant. For acute aneurysms, acidification had a relative risk of 1.49 (95% CI 1.17∼1.90), however the relative risks for β-propiolactonation and containing IgA were non-significant. For prolonged use of anti-platelets or anti-coagulants, β-propiolactonation had a relative risk of 1.44 (95% CI 1.18∼1.76), and acidification protected against them both with a relative risk of 0.82 (95% CI 0.69∼0.97), whereas the relative risk for containing IgA was non-significant. For recurrence, all three factors were non-significant.

Conclusions

The effectiveness of immunoglobulin may differ among different manufacturing processes. β-propiolactonation had a higher risk of treatment failure and prolonged use of anti-platelets or anti-coagulants. Acidification may increase the risk of acute coronary aneurysms.

Protective role of acidic pH-activated chloride channel in severe acidosis-induced contraction from the aorta of spontaneously hypertensive rats

Severe acidic pH-activated chloride channel (I_Cl,acid) has been found in various mammalian cells. In the present study, we investigate whether this channel participates in reactions of the thoracic aorta to severe acidosis and whether it plays a role in hypertension. We measured isometric contraction in thoracic aorta rings from spontaneously hypertensive rats (SHRs) and normotensive Wistar rats. Severe acidosis induced contractions of both endothelium-intact and -denuded thoracic aorta rings. In Wistar rats, contractions did not differ at pH 6.4, 5.4 and 4.4. However, in SHRs, contractions were higher at pH 5.4 or 4.4 than pH 6.4, with no difference between contractions at pH 5.4 and 4.4. Nifedipine, I_Cl,acid blockers 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) and 4,4′-diisothiocyanatostilbene-2, 2′-disulfonic acid (DIDS) inhibited severe acidosis-induced contraction of aortas at different pH levels. When blocking I_Cl,acid, the remnant contraction was greater at pH 4.4 than pH 5.4 and 6.4 for both SHRs and Wistar rats. With nifedipine, the remnant contraction was greatly reduced at pH 4.4 as compared with at pH 6.4 and 5.4. With NPPB or DIDS, the ratio of remnant contractions at pH 4.4 and 5.4 (R_4.4/5.4) was lower for SHRs than Wistar rats (all <1). However, with nifedipine, the R_4.4/5.4 was higher for SHRs than Wistar rats (both >1). Furthermore, patch clamp recordings of I_Cl,acid and intracellular Ca²⁺ measurements in smooth muscle cells confirmed these findings. I_Cl,acid may protect arteries against excess vasoconstriction under extremely acidic extracellular conditions. This protective effect may be decreased in hypertension.

Intravenous immunoglobulin preparation type: association with outcomes for patients with acute Kawasaki disease

To determine whether two different intravenous immunoglobulin (IVIG) preparations were equally efficacious in the treatment of Kawasaki disease (KD). Single centre retrospective review of all patients treated with IVIG for KD between January 1990 and April 2007. Comparison of IVIG (dose 2 g/kg) from two commercial preparations; Iveegam stabilized with sugar (lyophilized, 5 g/ml glucose, pH 6.4-7.2, IgA 10 microg/ml, 5% IgG/ml) and Gamimune stabilized through acidification (no sugar, pH 4.0-4.5, IgA 270 microg/ml, 5% 1990-1999, 10% 1999-2007 IgG/ml). Propensity-adjusted differences in duration of fever after treatment initiation, frequency of retreatment with IVIG, hospital stay and maximum coronary artery z-score. A total of 954 patients were included, 862 (90%) were treated with Iveegam and 92 (10%) were treated with Gamimune. Patients' demographic, clinical and laboratory characteristics were similar between the two groups. In propensity-adjusted models, Iveegam was found to be associated with higher probability of non-response to IVIG (12% vs. 5%, p = 0.05) and longer median duration of fever after IVIG [1 (1-27) vs. 1 (1-8) days, p = 0.02] than Gamimune. Nevertheless, Gamimune was found to be associated with longer median duration of hospital stay [5 (2-49) vs. 4 (2-76) days, p < 0.0001] and higher median maximum coronary artery z-score both at the end of the acute phase (+1.4 vs. +0.8, p < 0.0001) and 6-8 weeks after the acute phase (+0.7 vs. +0.4, p < 0.0001). IVIG preparations with lower IgA content and stabilized with glucose appear to be associated with improved coronary artery outcomes for patients with KD.

Hydrogen sulfide and oxygen sensing: implications in cardiorespiratory control

Although all cells are variously affected by oxygen, a few have the responsibility of monitoring oxygen tensions and initiating key homeostatic responses when P(O2) falls to critical levels. These ;oxygen-sensing' cells include the chemoreceptors in the gills (neuroepithelial cells), airways (neuroepithelial bodies) and vasculature (carotid bodies) that initiate cardiorespiratory reflexes, oxygen sensitive chromaffin cells associated with systemic veins or adrenal glands that regulate the rate of catecholamine secretion, and vascular smooth muscle cells capable of increasing blood flow to systemic tissues, or decreasing it through the lungs. In spite of intense research, and enormous clinical applicability, there is little, if any, consensus regarding the mechanism of how these cells sense oxygen and transduce this into the appropriate physiological response. We have recently proposed that the metabolism of hydrogen sulfide (H2S) may serve as an 'oxygen sensor' in vertebrate vascular smooth muscle and preliminary evidence suggests it has similar activity in gill chemoreceptors. In this proposed mechanism, the cellular concentration of H2S is determined by the simple balance between constitutive H2S production in the cytoplasm and H2S oxidation in the mitochondria; when tissue oxygen levels fall the rate of H2S oxidation decreases and the concentration of biologically active H2S in the tissue increases. This commentary briefly describes the oxygen-sensitive tissues in fish and mammals, delineates the current hypotheses of oxygen sensing by these tissues, and then critically evaluates the evidence for H2S metabolism in oxygen sensing.

Sivelestat relaxes porcine coronary artery via inhibition of Ca2+ sensitization induced by a receptor agonist

Sivelestat sodium hydrate (sivelestat) is a novel synthetic drug and specific inhibitor of neutrophil elastase that has been approved in Japan as a treatment for acute lung injury associated with systemic inflammatory response syndrome. There are no reports on the effects of sivelestat on the contractile regulation of vascular smooth muscle. The purpose of the present study was to assess the effects of sivelestat on porcine coronary artery. Sivelestat induced concentration-dependent (3 x 10 to 3 x 10 M) vasorelaxation in U46619 (100 nM)-precontracted porcine coronary artery with or without endothelium. Simultaneous measurements of tension and the cytosolic Ca concentration ([Ca]i) revealed that sivelestat shifted the [Ca]i-tension curve to the right and downward during stimulation with 118 mM K and 100 nM U46619. In beta-escin-permeabilized arterial strips, sivelestat abolished GTP plus U46619-induced contractions at constant [Ca]i, whereas it had no effect on Ca-induced contractions. Thus, sivelestat relaxes porcine coronary artery smooth muscle via the selective inhibition of Ca sensitization induced by a receptor agonist, without affecting Ca-induced contraction.

Hydrogen sulfide as an oxygen sensor in trout gill chemoreceptors

O2 chemoreceptors elicit cardiorespiratory reflexes in all vertebrates, but consensus on O2-sensing signal transduction mechanism(s) is lacking. We recently proposed that hydrogen sulfide (H2S) metabolism is involved in O2 sensing in vascular smooth muscle. Here, we examined the possibility that H2S is an O2 sensor in trout chemoreceptors where the first pair of gills is a primary site of aquatic O2 sensing and the homolog of the mammalian carotid body. Intrabuccal injection of H2S in unanesthetized trout produced a dose-dependent bradycardia and increased ventilatory frequency and amplitude similar to the hypoxic response. Removal of the first, but not second, pair of gills significantly inhibited H2S-mediated bradycardia, consistent with the loss of aquatic chemoreceptors. mRNA for H2S-synthesizing enzymes, cystathionine beta-synthase and cystathionine gamma-lyase, was present in branchial tissue. Homogenized gills produced H2S enzymatically, and H2S production was inhibited by O2, whereas mitochondrial H2S consumption was O2 dependent. Ambient hypoxia did not affect plasma H2S in unanesthetized trout, but produced a PO2-dependent increase in a sulfide moiety suggestive of increased H2S production. In isolated zebrafish neuroepithelial cells, the putative chemoreceptive cells of fish, both hypoxia and H2S, produced a similar approximately 10-mV depolarization. These studies are consistent with H2S involvement in O2 sensing/signal transduction pathway(s) in chemoreceptive cells, as previously demonstrated in vascular smooth muscle. This novel mechanism, whereby H2S concentration ([H2S]) is governed by the balance between constitutive production and oxidation, tightly couples tissue [H2S] to PO2 and may provide an exquisitely sensitive, yet simple, O2 sensor in a variety of tissues.

Transgenic mice expressing Na+-K+-ATPase in smooth muscle decreases blood pressure

The Na(+)-K(+)-ATPase (NKA) is a transmembrane protein that sets and maintains the electrochemical gradient by extruding three Na(+) in exchange for two K(+). An important physiological role proposed for vascular smooth muscle NKA is the regulation of blood pressure via modulation of vascular smooth muscle contractility (5). To investigate the relations between the level of NKA in smooth muscle and blood pressure, we developed mice carrying a transgene for either the NKA alpha(1)- or alpha(2)-isoform (alpha(1 sm+) or alpha(2 sm+) mice) driven by the smooth muscle-specific alpha-actin promoter SMP8. Interestingly, both alpha-isoforms, the one contained in the transgene and the one not contained, were increased to a similar degree at both protein and mRNA levels. The total alpha-isoform protein was increased from 1.5-fold (alpha(1 sm+) mice) to 7-fold (alpha(2 sm+) mice). The increase in total NKA alpha-isoform protein was accompanied by a 2.5-fold increase in NKA activity in alpha(2 sm+) gastric antrum. Immunocytochemistry of the alpha(1)- and alpha(2)-isoforms in alpha(2 sm+) aortic smooth muscle cells indicated that alpha-isoform distributions were similar to those shown in wild-type cells. alpha(2 sm+) Mice (high expression) were hypotensive (109.9 +/- 1.6 vs. 121.3 +/- 1.4 mmHg; n = 13 and 11, respectively), whereas alpha(1 sm+) mice (low expression) were normotensive (122.7 +/- 2.5 vs. 117.4 +/- 2.3; n = 11 or 12). alpha(2 sm+) Aorta, but not alpha(1 sm+) aorta, relaxed faster from a KCl-induced contraction than wild-type aorta. Our results show that smooth muscle displays unique coordinate expression of the alpha-isoforms. Increasing smooth muscle NKA decreases blood pressure and is dependent on the degree of increased alpha-isoform expression.

The effects of endothelial factor inhibition on the time course of responses of isolated rat coronary arteries to intraluminal flow

The aims of this study were to investigate, for the first time, the effects of endothelial factor inhibition on both the magnitude and dynamics of the response of isolated small coronary arteries to intraluminal flow. Isolated rat coronary arteries were mounted on a pressure myograph and left to develop myogenic tone. Flow was introduced and maintained until stable diameters were attained. Dilatory responses were observed which were maximal at low flow rates (5-10 microl/min) and thus shear stresses (1-2 dyn/cm(2)). These responses were transient in nature. Transient dilations were also observed upon cessation of flow. All responses (to 5 microl/min) were endothelium dependent and were completely abolished by addition of charybdotoxin (100 nM) and apamin (100-500 nM) suggesting an important role for a hyperpolarizing mechanism most likely involving an endothelium-derived hyperpolarizing factor. However, inhibitors of nitric oxide synthase (L-NNA; 100 microM) or cyclo-oxygenase (indomethacin; 10 microM) also modulated the response causing an increase and decrease in maximum vasodilation, respectively. By examining the time course we showed that both agents also made the response significantly more transient in nature. These results show that inhibition of endothelial factor pathways can influence both the magnitude and dynamics of the response of isolated rat coronary arteries to flow.

Regulation of membrane excitability by intracellular pH (pHi) changers through Ca2+-activated K+ current (BK channel) in single smooth muscle cells from rabbit basilar artery

Employing microfluorometric system and patch clamp technique in rabbit basilar arterial myocytes, regulation mechanisms of vascular excitability were investigated by applying intracellular pH (pH(i)) changers such as sodium acetate (SA) and NH(4)Cl. Applications of caffeine produced transient phasic contractions in a reversible manner. These caffeine-induced contractions were significantly enhanced by SA and suppressed by NH(4)Cl. Intracellular Ca(2+) concentration ([Ca(2+)](i)) was monitored in a single isolated myocyte and based the ratio of fluorescence using Fura-2 AM (R (340/380)). SA (20 mM) increased and NH(4)Cl (20 mM) decreased R (340/380) by 0.2 +/- 0.03 and 0.1 +/- 0.02, respectively, in a reversible manner. Caffeine (10 mM) transiently increased R (340/380) by 0.9 +/- 0.07, and the ratio increment was significantly enhanced by SA and suppressed by NH(4)Cl, implying that SA and NH(4)Cl may affect [Ca(2+)](i) (p < 0.05). Accordingly, we studied the effects of SA and NH(4)Cl on Ca(2+)-activated K(+) current (IK(Ca)) under patch clamp technique. Caffeine produced transient outward current at holding potential (V (h)) of 0 mV, caffeine induced transient outward K(+) current, and the spontaneous transient outward currents were significantly enhanced by SA and suppressed by NH(4)Cl. In addition, IK(Ca) was significantly increased by acidotic condition when pH(i) was lowered by altering the NH(4)Cl gradient across the cell membrane. Finally, the effects of SA and NH(4)Cl on the membrane excitability and basal tension were studied: Under current clamp mode, resting membrane potential (RMP) was -28 +/- 2.3 mV in a single cell level and was depolarized by 13 +/- 2.4 mV with 2 mM tetraethylammonium (TEA). SA hyperpolarized and NH(4)Cl depolarized RMP by 10 +/- 1.9 and 16 +/- 4.7 mV, respectively. SA-induced hyperpolarization and relaxation of basal tension was significantly inhibited by TEA. These results suggest that SA and NH(4)Cl might regulate vascular tone by altering membrane excitability through modulation of [Ca(2+)](i) and Ca(2+)-activated K channels in rabbit basilar artery.

Acidosis Augments Myogenic Constriction in Rat Coronary Arteries

The myogenic response is a process by which blood vessels autoregulate vascular smooth muscle tone in response to changes in transmural pressure. It is characterized by vessel contraction or dilation with increased or decreased pressure, respectively. We sought to identify whether acidosis impacts the myogenic response in rat coronary resistance arteries. Ventricular septal arteries were isolated from male Sprague-Dawley rats and mounted on a pressure myograph. The myogenic response was assessed by measuring the arterial diameter at pressures of 10-120 mm Hg. The fluorescence indicators 2',7'-bis-(carboxyethyl)-5(and-6)-carboxyfluorescein and Fura-2 were utilized to measure intracellular pH (pH(i)) and intracellular free calcium concentration ([Ca(2+)](i)), respectively. A decrease in the extracellular pH (pH(o)) from 7.4 to 6.9 produced a fall in pH(i) and an increase in the myogenic response. Under nominally HCO (3) (-) /CO(2)-free conditions at a constant pH(o), blockade of the sodium-hydrogen exchanger with HOE694 also resulted in a fall in pH(i) and a similar enhancement of myogenic activity. Concentration response curves were constructed to measure the potencies of the HOE694 effects: the EC(50) was 34 microM for the pH(i) change and 19 microM for vessel constriction. Apparent [Ca(2+)](i) remained unchanged during HOE694-induced intracellular acidification. Furthermore, in the presence of HCO (3) (-) , HOE694 did not markedly affect pH(i) and vascular tone remained unaltered. Our data demonstrate that acidosis augments myogenic constriction of rat coronary arteries. These effects are due to a fall in pH(i) consequent upon the reduction in pH(o) and may reflect an increased myofilament [Ca(2+)](i) sensitivity within vascular smooth muscle cells.

Low Extracellular Cl– Environment Attenuates Changes in Intracellular pH and Contraction following Extracellular Acidosis in Wistar Kyoto Rat Aorta

This study was conducted to investigate the influence of extracellular Cl- ([Cl-]o) on the intracellular pH (pHi) regulation and the contractile state of the isolated aorta from Wistar Kyoto (WKY) rats. Isometric tension recording and fluorometry techniques were utilized to measure contractile response and pHi in isolated aortic strips. Decreasing extracellular pH (pHo) from 7.4 to 6.5 produced a marked contraction, which was 75.8 +/- 5.6% of the 64.8 mmol/l KCl-induced contraction. The acidosis-induced contraction was significantly attenuated in low [Cl-]o solution, the magnitude of which was 56.0 +/- 3.0% of the 64.8 mmol/l KCl-induced contraction. Decreasing pHo of the normal solution to 6.5 rapidly decreased pHi in aortic smooth muscle cells and produced a corresponding contraction. When the pHo was decreased in low [Cl-]o solution, a rapid fall in pHi followed by reversal of pHi changes, in a time-dependent manner was observed, despite low pHo. Omission of HCO3- from the low [Cl-]o solution restored the contractile response to acidosis, which was comparable to that in normal solution. Similarly, following decrease in pHo to 6.5, no recovery of intracellular acidosis was observed. We conclude that low [Cl-]o environment causes activation of extracellular HCO3- -dependent pHi-regulating mechanism, that results in the rapid recovery of pHi following acidosis, and the attenuation of acidosis-induced contraction of WKY aorta.

Chronic hypoxia elevates intracellular pH and activates Na+/H+ exchange in pulmonary arterial smooth muscle cells

Chronic hypoxia (CH), caused by many lung diseases, results in pulmonary hypertension due, in part, to increased muscularity of small pulmonary vessels. Pulmonary arterial smooth muscle cell (PASMC) proliferation in response to growth factors requires increased intracellular pH (pHi) mediated by activation of Na+/H+ exchange (NHE); however, the effect of CH on PASMC pHi homeostasis is unknown. Thus we measured basal pHi and NHE activity and expression in PASMCs isolated from mice exposed to normoxia or CH (3 wk/10% O2). pHi was measured using the pH-sensitive fluorescent dye BCECF-AM. NHE activity was determined from Na+-dependent recovery from NH4-induced acidosis, and NHE expression was determined by RT-PCR and immunoblot. PASMCs from chronically hypoxic mice exhibited elevated basal pHi and increased NHE activity. NHE1 was the predominate isoform present in mouse PASMCs, and both gene and protein expression of NHE1 was increased following exposure to CH. Our findings indicate that exposure to CH caused increased pHi, NHE activity, and NHE1 expression, changes that may contribute to the development of pulmonary hypertension, in part, via pH-dependent induction of PASMC proliferation.

Hypoxic vasorelaxation: Ca2+-dependent and Ca2+-independent mechanisms

The mechanisms of oxygen sensing in vascular smooth muscle have been studied extensively in a variety of tissue types and the results of these studies indicate that the mechanism of hypoxia-induced vasodilation probably involves several mechanisms that combined to assure the appropriate response. After a short discussion of the regulatory mechanisms for smooth muscle contractility, we present the evidence indicating that hypoxic vasorelaxation involves both Ca2+-dependent and Ca2+-independent mechanisms. More recent experiments using proteomic approaches in organ cultures of porcine coronary artery reveal important changes evoked by hypoxia in both Ca2+-dependent and Ca2+-independent pathways.

Ca2+-independent hypoxic vasorelaxation in porcine coronary artery

To demonstrate a Ca(2+)-independent component of hypoxic vasorelaxation and to investigate its mechanism, we utilized permeabilized porcine coronary arteries, in which [Ca(2+)] could be clamped. Arteries permeabilized with beta-escin developed maximum force in response to free Ca(2+) (6.6 microm), concomitant with a parallel increase in myosin regulatory light chain phosphorylation (MRLC-P(i)), from 0.183 +/- 0.023 to 0.353 +/- 0.019 MRLC-P(i) (total light chain)(-1). Hypoxia resulted in a significant decrease in both force (-31.9 +/- 4.1% prior developed force) and MRLC-P(i) (from 0.353 to 0.280 +/- 0.023), despite constant [Ca(2+)] buffered by EGTA (4 mm). Forces developed in response to Ca(2+) (6.6 microm), Ca(2+) (0.2 microm) + GTPgammaS (1 mM), or in the absence of Ca(2+) after treatment with ATPgammaS (1 mM), were of similar magnitude. Hypoxia also relaxed GTPgammaS contractures but importantly, arteries could not be relaxed after treatment with ATPgammaS. Permeabilization with Triton X-100 for 60 min also abolished hypoxic relaxation. The blocking of hypoxic relaxation after ATPgammaS suggests that this Ca(2+)-independent mechanism(s) may operate through alteration of MRLC-P(i) or of phosphorylation of the myosin binding subunit of myosin light chain phosphatase. Treatment with the Rho kinase inhibitor Y27632 (1 microm) relaxed GTPgammaS and Ca(2+) contractures; but the latter required a higher concentration (10 microm) for consistent relaxation. Relaxations to N(2) and/or Y27632 averaged 35% and were not additive or dependent on order. Our data suggest that the GTP-mediated, Rho kinase-coupled pathway merits further investigation as a potential site of this novel, Ca(2+)-independent O(2)-sensing mechanism. Importantly, these results unambiguously show that hypoxia-induced vasorelaxation can occur in permeabilized arteries where the Ca(2+) is clamped at a constant value.

Contribution of store-operated Ca2+ entry to pHo-dependent changes in vascular tone of porcine coronary smooth muscle

Vascular smooth muscle contracts on increases of extracellular pH (pH(o)) and relaxes on pH(o) decreases possibly resulting from changes in transsarcolemmal Ca(2+) influx. Therefore, we studied store-operated Ca(2+) entry (SOCE; i.e. capacitative Ca(2+) entry (CCE)) during acidification (pH(o)=6.5) and alkalinization (pH(o)=8.0) in isolated porcine coronary smooth muscle cells (SMCs) by monitoring cytoplasmic Ca(2+) ([Ca(2+)](i)) and divalent cation entry (Mn(2+) quench) with fura-2/AM-fluorometry. Additionally, we evaluated the contribution of SOCE to pH(o)-dependent changes in isometric tension of porcine coronary smooth muscle strips. SOCE elicited in SMCs by the SERCA inhibitor BHQ was strongly modulated by pH(o) showing a decrease upon acidification and vice versa an increase upon alkalinization. BHQ-mediated tension of smooth muscle strips also revealed strong pH(o) dependence. In contrast, L-VOC-dependent tension ([K(+)](o)=20 and 40 mmol l(-1)) was remarkably less affected by pH(o) changes. Moreover, refilling of depleted Ca(2+) stores after repeated M(3)-cholinergic receptor stimulation could be almost completely inhibited by SKF 96365 and was markedly reduced by acidification and considerably enhanced by alkalinization pointing to a major role of SOCE in refilling. We conclude that vascular tone particularly responds to alterations in pH(o) whenever SOCE substantially contributes to the amount of activator Ca(2+) for contraction.

Intracellular alkalinization induces Ca2+ influx via non-voltage-operated Ca2+ channels in rat aortic smooth muscle cells

In smooth muscle, the cytosolic Ca2+ concentration ([Ca2+](i)) is the primary determinant of contraction, and the intracellular pH (pH(i)) modulates contractility. Using fura-2 and 2',7'-biscarboxyethyl-5(6) carboxyfluorescein (BCECF) fluorometry and rat aortic smooth muscle cells in primary culture, we investigated the effect of the increase in pH(i) on [Ca2+](i). The application of the NH(4)Cl induced concentration-dependent increases in both pH(i) and [Ca2+](i). The extent of [Ca2+](i) elevation induced by 20mM NH(4)Cl was approximately 50% of that obtained with 100mM K(+)-depolarization. The NH(4)Cl-induced elevation of [Ca2+](i) was completely abolished by the removal of extracellular Ca2+ or the addition of extracellular Ni2+. The 100mM K(+)-induced [Ca2+](i) elevation was markedly inhibited by a voltage-operated Ca2+ channel blocker, diltiazem, and partly inhibited by a non-voltage-operated Ca2+ channel blocker, SKF96365. On the other hand, the NH(4)Cl-induced [Ca2+](i) elevation was resistant to diltiazem, but was markedly inhibited by SKF96365. It is thus concluded that intracellular alkalinization activates the Ca2+ influx via non-voltage-operated Ca2+ channels and thereby increases [Ca2+](i) in the vascular smooth muscle cells. The alkalinization-induced Ca2+ influx may therefore contribute to the enhancement of 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.

Cardiac neural crest ablation alters aortic smooth muscle force and voltage-sensitive Ca2+ responses

Ablation of the premigratory cardiac neural crest (CNC) from the chick embryo results in a malformed outflow tract vasculature termed persistent truncus arteriosus (PTA). In addition, loss of the CNC disrupts myocardial excitation-contraction (EC) coupling, decreases intracellular Ca2+ transients, and depresses force generation. We examined if similar defects occurred in the neural crest-derived smooth muscle of the aortic arch in a test of the hypothesis that loss of elements from the CNC disrupts EC coupling and force production in the smooth muscle of the tunica media of the aortic arch. Aortic arch segments from chicks (embryonic day 15) displaying PTA generated approximately 43% of stress generated by the aortic arch from sham-operated control embryos during potassium depolarization. The depressed force response was associated with a twofold lower Fura-2 transient. In contrast, force and steady-state Fura-2 signals during endothelin-1 stimulation were unchanged. The differences seen in stress generation with potassium depolarization between sham and PTA displaying embryos were not seen in the descending aorta, a tissue not derived from the neural crest. Protein content and immunostaining revealed no differences in the content of actin, myosin, or dihydropyridine receptor from sham or PTA aortic arch. Our results suggest that the CNC is required for normal aortic arch smooth muscle function and support the hypothesis that the loss of CNC impacts the force generating ability, in part by disruption of the EC-coupling processes and altering Ca(2+)-handling.

The effects of pH change on Ca(++) signaling and force in pregnant human myometrium

OBJECTIVE

This study was designed to determine the effects of both intracellular and extracellular pH change on contractile activity and intracellular Ca(++) during spontaneous contractions, oxytocin, and depolarization-induced stimulation of human myometrium.

STUDY DESIGN

Human myometrial tissue was obtained at elective caesarean delivery at term (37-41 completed weeks of gestation). Longitudinal strips were dissected and loaded with the calcium sensitive indicator Indo-1. Statistical significance was tested with the Student t test.

RESULTS

Both intracellular and extracellular acidification significantly reduces or even abolishes phasic activity, whether it arises spontaneously or in the presence of oxytocin. These contractile changes can be accounted for by the changes in intracellular Ca(++). Alkalinization produced the opposite effects. However, baseline or maintained tension changes could not be accounted for by changes in intracellular Ca(++).

CONCLUSION

We suggest that the effects on phasic activity are due to the inhibition of L-type calcium entry and that, during maintained or baseline activity, pH-sensitive Ca(++) release, possibly from the sarcoplasmic reticulum occurs; but it is insufficient to overcome the inhibitory effects at the myofilaments. We conclude that alterations of pH significantly affect calcium signaling and force production in the human myometrium and may contribute to dysfunction in labor.

Extracellular acidosis results in higher intracellular acidosis and greater contraction in spontaneously hypertensive rat aorta

Acidic pH induces a contraction in aorta from spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats. The contractile response to acidic pH in SHR aorta is greater than that in WKY aorta. The purpose of this study was to investigate the correlation among extracellular pH (pH(o)), intracellular pH (pH(i)) and contraction in order to understand the exaggerated contractile response to acidic pH in SHR aorta. pH(i) measurement showed that at pH(o) 6.5, intracellular acidification was greater in SHR aorta than in WKY aorta. Decreasing pH(o) further to 6.2 in WKY aorta produced intracellular acidification close to that achieved at pH(o) 6.5 in SHR aorta, and at this level, the difference in contractile response between the two strains was also abolished. These results suggest that acidic pH(i), but not pH(o), is closely correlated with the contractile response and that the exaggerated contractile response in SHR aorta is due to a greater fall in pH(i).

Stimulated tyrosine phosphorylation of phosphatidylinositol 3-kinase causes acidic pH-induced contraction in spontaneously hypertensive rat aorta

Acidic pH induced a contraction (APIC) in isolated aortas from spontaneously hypertensive (SHR) and Wistar Kyoto rats, but failed to produce any response in age-matched Wistar rat aorta. This study was conducted to test the hypothesis that tyrosine phosphorylation of proteins is a molecular mechanism underlying the APIC. Tyrosine kinase inhibitors, genistein and tyrphostin 23 inhibited the APIC in a concentration-dependent manner. APIC was inhibited by phosphatidylinositol 3-kinase (PI3-kinase) inhibitors, LY-294002 [2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one hydrochloride] and wortmannin. Consistent with the results from tension measurement experiments, Western blot analysis showed that acidic pH induced an appreciable increment of tyrosine phosphorylation of 85-kDa protein (p85) in SHR aorta, which was completely inhibited by tyrphostin 23, whereas in Wistar rat aorta, the protein tyrosine phosphorylation was not observed. Further investigations using immunoprecipitation followed by Western blotting confirmed an increase in the tyrosine phosphorylation of p85. Analysis by SDS-polyacrylamide gel electrophoresis followed by silver staining of the gel revealed that amounts of multiple proteins with molecular sizes of 120, 130, 210, and 225 kDa were increased at acidic pH, which were immunoprecipitated with anti-phosphotyrosine antibody. Western blotting using a specific anti-PI3-kinase antibody identified the p85 as the regulatory subunit of PI3-kinase, whereas 120-, 130-, and 225-kDa proteins were identified by mass spectrometry as pro-alpha2 (I) collagen, collagen alpha1 (I) chain, and fibernectin I, respectively. As assayed by Western blotting using anti-myosin light chain (MLC) antibody, acidic pH induced a stimulation of MLC phosphorylation, and the stimulated MLC phosphorylation was abolished by tyrphostin 23 and LY-294002. These results suggest that acidic pH induces an increase in tyrosine phosphorylation of PI3-kinase, resulting in the MLC phosphorylation-dependent contraction of SHR aorta.

Functional role of Cl- channels in acidic pH-induced contraction of the aorta of spontaneously hypertensive and Wistar Kyoto rats

pH regulates various cellular functions. Previously, we have described that acidic pH produces depolarization and contraction in isolated aorta from spontaneously hypertensive (SHR) and Wistar Kyoto (WKY) rats [Br. J. Pharmacol. 118 (1996) 485]. The aim of the present study was to investigate the involvement of Cl- channels in acidic pH-induced contraction. Changing the pH of the bathing solution from 7.4 to 6.5 induced a contraction in both SHR and WKY aorta, which was 127.50+/-13.32% and 79.27+/-0.94% of the 64.8 mM KCl-induced contraction, respectively. The acidic pH-induced contraction was partially inhibited by the voltage-dependent Ca2+ channel (VDCC) blockers, verapamil (1 microM) and nifedipine (0.1 microM). The Cl- channel inhibitors, diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) (0.5 mM), 9-anthracene chloride (0.5 mM), indanyloxyacetic acid (30 microM) and niflumic acid (3 microM) also inhibited the acidic pH-induced contraction and the degree of attenuation was comparable to that of VDCC blockers. DIDS, 9-anthracene chloride and niflumic acid at concentrations used to inhibit the acidic pH-induced contraction also inhibited the 10 microM phenylephrine-induced contraction partially, without affecting the 64.8 mM KCl-induced contraction, whereas both the contractions were inhibited by indanyloxyacetic acid with equal efficacy. Indanyloxyacetic acid but not DIDS, 9-anthracene chloride or niflumic acid inhibited the 24.8 mM KCl-induced contraction. Simultaneous measurement of cytosolic Ca2+ and tension showed that niflumic acid reversed the increase in intracellular Ca2+ level and inhibited the contraction caused by acidic pH. Similarly, acidic pH depolarized the cultured vascular smooth muscle cells from SHR and the depolarization was completely reversible after the administration of niflumic acid. All these results suggest that the activation of Cl- channels is an important mechanism underlying the depolarization and contraction induced by acidic pH in SHR and WKY aortas.

Measurement of interstitial lactate during hypoxia-induced dilatation in isolated pressurised porcine coronary arteries

Lactate is formed in the coronary arterial wall and in the myocardium as a consequence of ischaemia and infarction. We combined direct measurement of coronary artery diameter and interstitial arterial wall lactate concentration ex vivo in order to ascertain the possible role of lactate in hypoxia-induced vasodilatation. The wall of porcine coronary arteries, precontracted during an intraluminal pressure of 40 mmHg by addition of prostaglandin F2alpha, was cannulated using a microdialysis catheter, and exposed to hypoxia for 60 min, followed by 45 min of reoxygenation. The exchange fraction of [14C]lactate over the microdialysis membrane increased from 0.38 +/- 0.04 to 0.52 +/- 0.05 (P < 0.001) during the study period. Coronary artery diameter increased by 15.5 +/- 2.0 % (n = 20) during hypoxia (P < 0.001, compared to normoxic controls) and interstitial lactate concentration rose from 1.07 +/- 0.21 to 2.50 +/- 0.40 mmol x l(-1) during hypoxia (P < 0.01) and was unchanged in controls. The increase in coronary artery diameter correlated with the increase in interstitial lactate concentration in the period between 30 and 60 min of hypoxia (r = 0.62; P = 0.02). Dichloroacetate (10(-5) M), an agent that reduces lactate generation by activating pyruvate dehydrogenase, abolished hypoxia-induced lactate production, but caused a further increase in coronary arterial diameter (30.2 +/- 4.4 %, n = 9; P < 0.001 vs. hypoxia and no dichloroacetate). Under control conditions, the addition of L-lactate (10(-3)-10(-2) M) increased dose-dependently coronary arterial diameter by 22.0 +/- 4.2 % (n = 5) and interstitial lactate concentration from 0.52 +/- 0.04 to 5.70 +/- 0.66 mmol x l(-1) (P < 0.001). There was a correlation between the increase in coronary artery diameter and interstitial lactate concentration (r = 0.60; P = 0.02). The present observations represent the first direct measurements of metabolites by microdialysis in a blood vessel wall. The lactate concentration may affect, but is not essential for, hypoxia-induced vasodilatation in porcine coronary arteries.

Hypoxic vasodilation in porcine coronary artery is preferentially inhibited by organ culture

Hypoxia (95% N2-5% CO2) elicits an endothelium-independent relaxation (45-80%) in freshly dissected porcine coronary arteries. Paired artery rings cultured at 37 degrees C in sterile DMEM (pH approximately 7.4) for 24 h contracted normally to KCl or 1 microM U-46619. However, relaxation in response to hypoxia was sharply attenuated compared with control (fresh arteries or those stored at 4 degrees C for 24 h). Hypoxic vasorelaxation in organ cultured vessels was reduced at both high and low stimulation, indicating that both Ca2+-independent and Ca2+-dependent components are altered. In contrast, relaxation to G-kinase (sodium nitroprusside) or A-kinase (forskolin and isoproterenol) activation was not significantly affected by organ culture. Additionally, there was no difference in relaxation after washout of the stimulus, indicating that the inhibition is specific to acute hypoxia-induced relaxation. Simultaneous force and intracellular calcium concentration ([Ca2+]i) measurements indicate the reduction in [Ca2+]i concomitant with hypoxia at low stimulus levels in these tissue is abolished by culture. Our results indicate that organ culture at 37 degrees C specifically attenuates hypoxic relaxation in vascular smooth muscle by altering dynamics of [Ca2+]i handling and decreasing a Ca2+-independent component of relaxation. Thus organ culture can be a novel tool for investigating the mechanisms of hypoxia-induced vasodilation.

Effects of microtubule disruption on force, velocity, stiffness and [Ca(2+)](i) in porcine coronary arteries

Force generated by smooth muscle cells is believed to result from the interaction of actin and myosin filaments and is regulated through phosphorylation of the myosin regulatory light chain (LC(20)). The role of other cytoskeleton filaments, such as microtubules and intermediate filaments, in determining the mechanical output of smooth muscle is unclear. In cultured fibroblasts, microtubule disruption results in large increases in force similar to contractions associated with LC(20) phosphorylation (15). One hypothesis, the "tensegrity" or "push-pull" model, attributes this increase in force to the disruption of microtubules functioning as rigid struts to resist force generated by actin-myosin interaction (9). In porcine coronary arteries, the disruption of microtubules by nocodazole (11 microM) also elicited moderate but significant increases in isometric force (10-40% of a KCl contracture), which could be blocked or reversed by taxol (a microtubule stabilizer). We tested whether this nocodazole-induced force was accompanied by changes in coronary artery stiffness or unloaded shortening velocity, parameters likely to be highly sensitive to microtubule resistance elements. Few changes were seen, ruling out push-pull mechanisms for the increase in force by nocodazole. In contrast, the intracellular calcium concentration, measured by fura 2 in the intact artery, was increased by nocodazole in parallel with force, and this was inhibited and/or reversed by taxol. Our results indicate that microtubules do not significantly contribute to vascular smooth muscle mechanical characteristics but, importantly, may play a role in modulation of Ca(2+) signal transduction.

Energy state, pH, and vasomotor tone during hypoxia in precontracted pulmonary and femoral arteries

To assess effects of smooth muscle energy state and intracellular pH (pH(i)) on pulmonary arterial tone during hypoxia, we measured ATP, phosphocreatine, P(i), and pH(i) by (31)P-NMR spectroscopy and isometric tension in phenylephrine-contracted rings of porcine proximal intrapulmonary arteries. Hypoxia caused early transient contraction followed by relaxation and late sustained contraction. Energy state and pH(i) decreased during relaxation and recovered toward control values during late contraction. Femoral arterial rings had higher energy state and lower pH(i) under baseline conditions and did not exhibit late contraction or recovery of energy state and pH(i) during hypoxia. In pulmonary arteries, glucose-free conditions abolished late hypoxic contraction and recovery of energy state and pH(i), but endothelial denudation abolished only late hypoxic contraction. NaCN had little effect at 0. 1 and 1.0 mM but caused marked vasorelaxation and decreases in energy state and pH(i) at 10 mM. These results suggest that 1) regulation of tone, energy state, and pH(i) differed markedly in pulmonary and femoral arterial smooth muscle, 2) hypoxic relaxation was mediated by decreased energy state or pH(i) due to hypoxic inhibition of oxidative phosphorylation, 3) recovery of energy state and pH(i) in hypoxic pulmonary arteries was due to accelerated glycolysis mediated by mechanisms intrinsic to smooth muscle, and 4) late hypoxic contraction in pulmonary arteries was mediated by endothelial factors that required hypoxic recovery of energy state and pH(i) for transduction in smooth muscle or extracellular glucose for production and release by endothelium.

Acidosis-induced coronary constriction in the rat heart: evidence for the activation of L-type calcium channels

Perfused rat hearts were used to study the effects of acidosis on coronary tone. When pH was decreased, over the range pH 7.4 to pH 6.2, by reducing perfusate bicarbonate levels, under constant flow conditions, there was a transient decrease in coronary perfusion pressure (CPP), followed by a sustained acidosis-dependent increase in CPP, which reversed when pH was returned to pH 7.4. This increase in CPP was seen at perfusion rates of 5, 10, and 20 ml/min(-1). When using constant pressure perfusion acidosis reduced coronary flow. In a HEPES-buffered bicarbonate-free solution, acidosis did not cause a transient fall in CPP but it did produce a sustained increase in CPP. Addition of ammonium chloride (10 mM) reduced CPP, while washout of ammonium chloride increased CPP. The acidosis-induced increase in CPP was not affected by indomethacin, nitro-L-arginine, the nonselective adenosine receptor antagonist, 8-phenyl theophylline, or the thromboxane receptor antagonist, ZD 1542. The acidosis-induced increase in CPP was independent of the myocardial depressant effects of acidosis, but was attenuated by three different L-type calcium channel blockers. These results demonstrate that the coronary circulation of the rat constricts in response to acidosis. Experiments performed with L-type calcium channel blockers, and the calcium channel activator BAY K8644, suggest that constriction occurs via activation of L-type calcium channels. This would not be expected on the basis of electrophysiological studies, which have shown an inhibition of L-type calcium channels by acidosis.

Carbonic anhydrase I inhibition by nitric oxide: implications for mediation of the hypercapnia-induced vasodilator response

1. At present, CO2 is considered to be the most important factor in regulating cerebral blood flow by modification of the interstitial fluid and extracellular pH, but the mechanism by which hypercapnia produces vasodilation is still controversial. In the present paper we investigated the effect of hypercapnia on carbonic anhydrase (CA) activity. We also studied the combined effects of CO2 with either indomethacin or an L-arginine analogue on CA activity. 2. Nine groups of 12 rabbits each were established. Groups 1-4 were ventilated with a mixture of 10% CO2, 21% O2 and 69% N2 for 20, 60, 120 and 180 min. Group 5 rabbits received 15 mg/kg bodyweight, i.v., indomethacin and, after 1 h, were ventilated with a mixture of 10% CO2, 21% O2 and 69% N2 for 2 h. Group 6 animals were ventilated with a mixture of 10% CO2, 21% O2 and 69% N2 for 2 h and then received indomethacin. Group 7 rabbits received 100 mg/kg bodyweight, i.v., NG-monomethyl-L-arginine (L-NMMA) and, after 1 h, were ventilated with a mixture of 10% CO2, 21% O2 and 69% N2 for 2 h. Group 8 rabbits were ventilated for 2 h with a mixture of 10% CO2, 21% O2 and 69% N2 and were then administered L-NMMA. Group 9 rabbits received L-NMMA treatment concomitant with ventilation for 2 h with a mixture of 10% CO2, 21% O2 and 69% N2. In all groups, the erythrocyte CA activity was measured, as well as PaCO2 before and after ventilation or treatment. 3. The present study shows that CO2 reduces CA I activity down to complete inhibition and antagonizes the activating effects of indomethacin and L-NMMA on this isozyme. Our data prove that nitric oxide- and prostaglandin-induced CA I inhibition is involved in the vasodilation produced by hypercapnia. These results suggest that, due to subsequent pH changes, CA I is directly implicated in the modulation of vascular processes in the organism.

Effects of insulin and metformin on glucose metabolism in rat vascular smooth muscle

Glucose metabolism in vascular smooth muscle cells (VSMCs) is characterized by substantial lactate production even in fully oxygenated conditions. Insulin and metformin, an insulin-sensitizing agent, have direct effects on the vascular tissue metabolism. We investigated whether insulin or metformin can induce a switch in VSMC glucose metabolism from lactate production to pyruvate oxidation, by measuring lactate oxidation as determined by the conversion of [1-14C]-D,L-lactate to [1-14C]-pyruvate and subsequent oxidation to acetyl coenzyme A and 14CO2 by pyruvate dehydrogenase (PDH). Lactate oxidation was measured in control rat aortic cultured VSMCs incubated for 30 minutes in media with and without additional glucose compared with VSMCs cultured in the presence of insulin or metformin. The addition of glucose to VSMCs decreased lactate oxidation (4.6+/-1.7 v 9.6+/-2.4 pmol/cell/min, P < .001). In the absence of additional glucose, metformin decreased lactate oxidation in VSMCs compared with controls (4.9+/-1.4 v 9.6+/-2.4 pmol/cell/min, P < .01). Metformin in the presence of glucose caused the greatest decline in lactate oxidation (2.5+/-0.4 pmol/cell/min, P < .001). In contrast to the effects of metformin, insulin increased lactate oxidation both with (12.9+/-1.5 pmol/cell/min, P < .001) and without (17.9+/-4.4, P < .01) additional glucose. This suggests that insulin facilitates VSMC utilization of lactate as a source of pyruvate and energy production even during noncontractile periods.

[Ca2+]i homeostasis and cyclic nucleotide relaxation in aorta of phospholamban-deficient mice

Phospholamban (PLB), a protein localized in the sarcoplasmic reticulum (SR), inhibits the SR Ca2+-ATPase; phosphorylation of PLB relieves this inhibition. We previously reported significant differences in contractility in aorta from mice in which the gene for PLB was ablated (PLB-). In this study, we measured intracellular Ca2+ concentration ([Ca2+]i) with fura 2 in the intact mouse aorta to more directly test the hypothesis that these changes are ascribable to altered SR function in vivo. Ten micromoles per liter of the alpha-agonist phenylephrine (PE) increased [Ca2+]i monotonically to a steady state in the wild-type aorta. In contrast, in PLB- aorta there was an initial rapid increase to a peak [Ca2+]i, which then decreased to a steady state that was lower than that in the wild type. Upon removal of the stimulus (either PE or KCl), the decrease in [Ca2+]i was two times as fast in the PLB- as in the wild-type aorta. There were no significant differences between PLB- and wild-type aortas in the concentration vs. force relations or the time courses of relaxation in response to forskolin or sodium nitroprusside. Interestingly, stimulation of the cAMP pathway before cGMP pathway activation resulted in a significant increase in sensitivity and a difference in relaxation parameters between PLB- and wild-type aortas. Western blot analysis indicated that the PLB-to-sarcoendoplasmic reticulum Ca2+ATPase ratio in the mouse aorta was similar to that in the heart; 20-fold more aortic than heart homogenate was required to achieve a similar level of immunoreactivity. Our data indicate that PLB can play a major role in modulating smooth muscle [Ca(2+)](i) but only a minor role, if any, in cyclic nucleotide-mediated relaxation.

The role of phospholamban and SERCA3 in regulation of smooth muscle-endothelial cell signalling mechanisms: evidence from gene-ablated mice

It is generally agreed that intracellular Ca2+ stores, the sarco(endo)plasmic-reticulum (SER), affect Ca2+ homeostasis and thus contractility of vascular smooth muscle. There is, however, no general consensus as to the magnitude of the SER contribution to Ca2+ handling, the basis for isoforms of the SER Ca(2+)-ATPases (SERCAs) or the role of an SER-associated regulatory protein, phospholamban (PLB). Although the biochemical and cell biological roles of the SER have been intensely studied in vitro, the development of gene-targeted and transgenic mouse models enables one to extend our information to the in vivo levels. A brief review of the role of PLB and SERCA function in vascular and endothelial cell function is presented. Studies on the PLB gene-ablated mouse indicate that vascular contractility is considerably altered. This is mirrored by changes in intracellular Ca2+. Moreover, differences in contractility of the gene-ablated tissues are eliminated by treatment with cyclopiazonic acid, which pharmacologically abolishes SER function by inhibiting the Ca(2+)-ATPase. Thus PLB modulation of sarcoplasmic reticulum (SR) Ca2+ uptake plays a major role in modulating vascular contractility. It is interesting that endothelium-dependent relaxation was decreased in the PLB-deficient aorta. This is surprising in light of the PLB distribution, thought to be limited to cardiac, slow skeletal and smooth muscle. Our data indicate the presence of PLB in endothelial cells and point to an unrecognized pathway for modulation of endothelial cell [Ca2+]i and vascular contractility. Data from smooth muscle tissues of the SERCA3 gene-ablated mouse demonstrate that this isoform affects endothelium-dependent function, but not that of smooth muscle, consistent with its known distribution. This isoform appears to perform a modulatory function, rather than the more essential role of SERCA2. Gene-targeted and transgenic models provide an important avenue for understanding the role of SER in vascular signalling.

Effects of hypoxia on energy state and pH in resting pulmonary and femoral arterial smooth muscles

To determine the effects of hypoxia on energy state and intracellular pH (pHi) in resting pulmonary and systemic arterial smooth muscles, we used 31P nuclear magnetic resonance spectroscopy and colorimetric and enzymatic assays to measure pHi; intracellular concentrations of ATP, phosphocreatine, creatine, and Pi; and phosphorylation potential in superfused tissue segments from porcine proximal intrapulmonary and superficial femoral arteries. Under baseline conditions (PO2 467 +/- 12.1 mmHg), energy state and total creatine (phosphocreatine + creatine) concentration were lower and pHi was higher in pulmonary arteries. During hypoxia (PO2 23 +/- 2.4 mmHg), energy state deteriorated more in femoral arteries than in pulmonary arteries. pHi fell in both tissues but was always more alkaline in pulmonary arteries. Reoxygenation reversed the changes induced by hypoxia. These results suggest that production and/or elimination of ATP and H+ was different in resting pulmonary and systemic arterial smooth muscles under baseline and hypoxic conditions. Because energy state and pHi affect a wide variety of cellular processes, including signal transduction, contractile protein interaction, and activities of ion pumps and channels, further investigation is indicated to determine whether these differences have functional significance.

Synergistic actions of insulin and troglitazone on contractility in endothelium-denuded rat aortic rings

Insulin attenuates vascular contraction via inhibition of voltage-operated Ca2+ channels and by enhancement of endothelium-dependent vasodilation. Thus it has been suggested that hypertension-associated insulin resistance results from an insensitivity to the hormone's effects on vascular reactivity. This hypothesis has been strengthened by reports that thiazolidinediones, a class of insulin-sensitizing agents, lower blood pressure and improve insulin responsiveness in hypertensive, insulin-resistant animal models. We tested the hypothesis that troglitazone enhances the vasodilating effect of insulin via inhibition of voltage-operated Ca2+ channels in vascular smooth muscle cells. Rat thoracic aortic rings (no endothelium) were suspended in tissue baths for isometric force measurement. Rings were incubated with 0.1 DMSO vehicle (control), troglitazone (10(-5) M), insulin (10(-7) U/l), or both troglitazone and insulin (1 h) and then contracted with phenylephrine (PE), KCl, or BAY K 8644. Troglitazone increased the EC50 values for PE and KCl. Contractions to BAY K 8644 in troglitazone-treated rings were virtually abolished. Insulin alone had no effect on contraction. However, when insulin was combined with troglitazone, the EC50 values for PE and KCl were further increased. Additionally, the maximum contractions to both PE (14 +/- 4% of control) and KCl (12 +/- 2% of control) were reduced. Measurement of Ca2+ concentration ([Ca2+]) with fura 2-AM in dispersed vascular smooth muscle cells indicated that neither insulin nor troglitazone alone altered PE-induced increases in intracellular [Ca2+]. However, troglitazone and insulin together caused a significant reduction in PE-induced increases in intracellular [Ca2+] (expressed as percentage of preincubation stimulation to PE: 47 +/- 10%, treated; 102 +/- 13%, vehicle). These results demonstrate that troglitazone inhibits Ca2+ influx and that it acts synergistically with insulin to attenuate further vascular contraction via inhibition of voltage-operated Ca2+ channels.

Developmental changes in the effect of acidosis on contraction, intracellular pH, and calcium in the rabbit mesenteric small artery

The purpose of the present study was to determine developmental changes in the effect of respiratory acidosis on vascular smooth muscle contraction. Vessel diameter, intracellular pH (pHi), and calcium concentration ([Ca]i) were measured in a cannulated preparation of the small mesenteric artery of newborn and adult rabbits. In the artery precontracted by high KCl, acidosis caused a vasorelaxation both in the newborn and the adult; the vasorelaxation was greater in the newborn than in the adult. The fura-2 fluorescence ratio, an indicator of [Ca]i, decreased transiently during acidosis and the decrease was similar in the two age groups. In the artery precontracted by norepinephrine, acidosis caused a transient vasoconstriction in the adult and a vasorelaxation in the newborn. In these vessels, the fura-2 fluorescence ratio increased transiently during acidosis; the increase was similar in the two groups. Upon induction of acidosis, pHi fell rapidly in the artery precontracted by norepinephrine or high KCl, and the depression of pHi was similar in the two groups. In the skinned smooth muscle preparation, a tension-[Ca] relationship curve at pH 7.1 was not significantly different from that at pH 6.8 in the adult. In the newborn, the tension-[Ca] curve at pH 6.8 was shifted to the right, compared with that at pH 7.1. These data suggest that the vasorelaxant effect of respiratory acidosis in the premature vessel is greater than in the adult. The greater vasorelaxation in the newborn cannot be explained by the age-related difference in pHi or [Ca]i during acidosis. The greater sensitivity of myofibrils to low pHi in the newborn may, at least in part, be responsible for the greater vasorelaxation in this age group.

Effect of acidosis on contraction, intracellular pH, and calcium in the newborn and adult rabbit aorta

This study investigated the effect of acidosis on intracellular pH (pHi), intracellular calcium concentration ([Ca]i), and vascular contraction in the aorta of the newborn and adult rabbit. Isometric tension, pHi, and [Ca]i were measured in an isolated ring preparation. After the vascular contraction was induced with 50mM KC1, the effect of respiratory acidosis produced by elevation of PCO2 was studied. Respiratory acidosis caused a transient depression followed by a recovery of contractile tension. The decrease in developed tension was greater in the newborn than in the adult. The decrease in pHi during acidosis was similar in the two age groups. [Ca]i increased during acidosis and the increase was greater in the newborn than in the adult. These data show that the vasorelaxant effect of acidosis in the newborn aorta is greater than that in the adult aorta. The greater vasodilation in the newborn cannot be explained by the difference in pHi or [Ca]i.

Linkage of aerobic glycolysis to sodium-potassium transport in rat skeletal muscle. Implications for increased muscle lactate production in sepsis

Although a linkage between aerobic glycolysis and sodium-potassium transport has been demonstrated in diaphragm, vascular smooth muscle, and other cells, it is not known whether this linkage occurs in skeletal muscle generally. Metabolism of intact hind-leg muscles from young rats was studied in vitro under aerobic incubation conditions. When sodium influx into rat extensor digitorum longus (EDL) and soleus muscles was facilitated by the sodium ionophore monensin, muscle weight gain and production of lactate and alanine were markedly stimulated in a dose-dependent manner. Although lactate production rose in both muscles, it was more pronounced in EDL than in soleus. Monensin-induced lactate production was inhibited by ouabain or by incubation in sodium-free medium. Preincubation in potassium-free medium followed by potassium re-addition also stimulated ouabain-inhibitable lactate release. Replacement of glucose in the incubation medium with pyruvate abolished monensin-induced lactate production but exacerbated monensin-induced weight gain. Muscles from septic or endotoxin-treated rats exhibited an increased rate of lactate production in vitro that was partially inhibited by ouabain. Increases muscle lactate production in sepsis may reflect linked increases in activity of the Na+, K+-ATPase, consumption of ATP and stimulation of aerobic glycolysis.

The mechanism of acidic pH-induced contraction in aortae from SHR and WKY rats enhanced by increasing blood pressure

1. Effect of pH on vascular smooth muscle contraction was analyzed by use of biochemical and pharmacological techniques. 2. In the aorta isolated from spontaneously hypertensive rats (SHR) decreasing extracellular pH (pH0) caused a rapid acidification of intracellular pH accompanied by a pH0-dependent increase in tension. The contraction of the SHR aorta was remarkable compared with that of the Wistar Kyoto rat (WKY) aorta. 3. Removal of NH4Cl caused a transient decrease in intracellular pH followed by a marked increase in tension. 4. Both contraction and intracellular Ca2+ mobilization induced by acidic pH0 were markedly inhibited by removal of extracellular Ca2+, verapamil and adenosine, whereas these were not affected by tetrodotoxin or Gd3+, a stretch-activated cation channel blocker. Furthermore, cromakalim (a K+ channel opener) inhibited acidic pH0-induced contraction (APIC). 5. Acidic pH0 induced a depolarization of cultured smooth muscle cells from SHR aorta. 6. Blood pressure elevated with increasing age of WKY and SHR accompanied by an increase in APIC. Feeding WKY with NG-nitro-L-arginine, an inhibitor of nitric oxide synthases caused a marked elevation of their blood pressure followed by an increase in APIC. 7. These results suggest that APIC is caused by Ca2+ influx mediated through the activation of voltage-sensitive Ca2+ channels mainly due to acidic pH0-induced depolarization of the plasma membrane of smooth muscle cells. It is also suggested that APIC is strengthened by the elevation of blood pressure.

NH4Cl-induced contraction of porcine coronary artery involves activation of dihydropyridine-sensitive Ca2+ entry

The role of voltage-dependent, dihydropyridine-sensitive Ca2+ channels in NH4Cl-induced vasoconstriction was investigated in isolated porcine coronary arteries by measuring in parallel isometric tone and 45Ca2+ uptake. NH4Cl (10-80 mM) concentration dependently induced tonic contractions which were preceded by a time lag of several minutes. Contractile responses to high (60 mM) as well as low (25 mM) concentrations of NH4Cl were markedly inhibited by 1 microM nifedipine or removal of extracellular Ca2+. The contractile effect of 25 mM NH4Cl was substantially enhanced by increasing extracellular K+ to 14.7 mM or by pretreatment of coronary arteries with either 5 mM tetraethylammonium chloride or 0.1 microM 1,4-dihydro- 2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)-phenyl]-3-pyridine carboxylic acid methyl ester (BAY K8644). NH4Cl (60 mM) significantly increased 45Ca2+ uptake with a lag time of more than 5 min. The increase in 45Ca2+ uptake induced by 60 mM NH4Cl was abolished in the presence of 1 microM nifedipine. Although NH4Cl (25 mM) did not detectably stimulate 45Ca2+ uptake in normal K+ solution, it significantly augmented 45Ca2+ uptake when extracellular K+ was increased to 14.7 mM. Furthermore, NH4Cl (20 mM) potentiated histamine-induced contraction of coronary arteries. This potentiating effect of NH4Cl was completely antagonized by nifedipine. Our results suggest an involvement of nifedipine-sensitive Ca2+ channels in NH4Cl-induced vasoconstriction of porcine coronary artery.

The effects of extracellular pH and calcium change on force and intracellular calcium in rat vascular smooth muscle

1. In order to investigate the mechanism whereby changes in external pH (pHo) alter tone in rat mesenteric resistance vessels, we have made simultaneous measurements of tension and intracellular Ca2+ [Ca2+]i. Strips of mesenteric artery were loaded with the Ca(2+)-sensitive indicator indo-1 and superfused with physiological salt solution at pH 7.4 and 37 degrees C. 2. An increase of pHo from 7.4 to 7.9 produced an increase in tension. This was accompanied by an increase in [Ca2+]i in resting and high-K(+)-depolarized vessels. Acidification to 6.9 reduced tension and was associated with a fall in [Ca2+]i. Over the pHi range examined, 6.6-7.9, parallel changes in [Ca2+]i and tension were found in K(+)-activated vessels. 3. In contrast to the relatively slow change in [Ca2+]i, pHi and tension with change of pHo, depolarization produced rapid changes in [Ca2+]i and tension, consistent with a more direct action on Ca2+ mobilization. 4. Reducing the external [Ca2+] below 1 mM produced a pronounced fall in [Ca2+]i and force. Changes in [Ca2+]i, produced by alteration of external [Ca2+] (Cao2+) were used to examine the relation between [Ca2+]i and tension. A linear relation was found. Alteration of pHo to 6.9 or 7.9 did not significantly change this relation. When the tension data were normalized to their own maxima, no shift in the tension-Ca2+ relation occurred, suggesting little or no effect of pH on the Ca2+ sensitivity of force production by the contractile proteins. 5. To determine further whether the changes in [Ca2+]i produced by alteration of pHo could account for all the changes observed in tension, [Ca2+]i was restored to control levels while maintaining an altered pHo. When this was done, restoration of [Ca2+]i led to restoration of force. Thus, in this preparation, the changes in [Ca2+]i produced by altering pHo in depolarized vessels can account for the changes in vascular tone.