Na+-Ca2+ exchange in cultured vascular smooth muscle cells

Importance of Altered Levels of SERCA, IP3R, and RyR in Vascular Smooth Muscle Cell

Calcium cycling between the sarcoplasmic reticulum (SR) and the cytosol via the sarco-/endoplasmic reticulum Ca-ATPase (SERCA) pump, inositol-1,4,5-triphosphate receptor (IP₃R), and Ryanodine receptor (RyR), plays a major role in agonist-induced intracellular calcium ([Ca²⁺]_cyt) dynamics in vascular smooth muscle cells (VSMC). Levels of these calcium handling proteins in SR get altered under disease conditions. We have developed a mathematical model to understand the significance of altered levels of SERCA, IP₃R, and RyR on the intracellular calcium dynamics of VSMC and to understand how variation in protein levels that arise due to diabetes contribute to different VSMC behavior and thus vascular disease. SR is modeled as a single continuous entity with homogeneous intra-SR calcium. Model results show that agonist-induced intracellular calcium dynamics can be modified by changing the levels of SERCA, IP₃R, and/or RyR. Lowering SERCA level will enable intracellular calcium oscillations at low agonist concentrations whereas lowered levels of IP₃R and RyR need higher agonist concentration for intracellular calcium oscillations. This research suggests that reduced SERCA level is the main factor responsible for the reduced intracellular calcium transients and contractility in VSMCs.

Mechanism of alkalosis-induced constriction of rat cerebral penetrating arterioles

Cerebral arterioles are in close contact with the supplied tissue and are strong regulators of cerebrovascular tone. Transient ischemia can cause brain intracellular alkalosis producing vasoconstriction. However, the mechanisms of alkalosis-induced cerebral arteriolar constriction are poorly understood. Here, we determined the vascular responses to alkalosis under different conditions by monitoring the internal diameter of pressurized penetrating arterioles isolated from the rat cerebrum with an operating microscope. The roles of Na+/H+ exchanger (NHE), Na+/Ca²+ exchanger (NCX), Na+/K+-adenosine triphosphatase (NKA), and potassium (K+) channels during alkalosis were examined using specific inhibitors. Our results indicated that the extent of constriction of the penetrating arterioles was dependent on alkaline pH. Moreover, the alkalosis-induced vasoconstriction was significantly attenuated by inhibitors of NHE, NCX, and NKA, but not K+ channel inhibitors. Therefore, we concluded that NHE, NKA, and NCX are important regulators involved in alkalosis-induced vasoconstriction of rat cerebral penetrating arterioles.

Attenuation of maitotoxin-induced cytotoxicity in rat aortic smooth muscle cells by inhibitors of Na+/Ca2+ exchange, and calpain activation

The highly potent marine toxin maitotoxin (MTX) evoked an increase in cytosolic Ca(2+) levels in fura-2 loaded rat aortic smooth muscle cells, which was dependent on extracellular Ca(2+). This increase was almost fully inhibited by KB-R7943, a potent selective inhibitor of the reverse mode of the Na(+)/Ca(2+) exchanger (NCX). Cell viability was assessed using ethidium bromide uptake and the alamarBlue cytotoxicity assay. In both assays MTX-induced toxicity was attenuated by KB-R7943, as well as by MDL 28170, a membrane permeable calpain inhibitor. Maitotoxin-evoked contractions of rat aortic strip preparations in vitro, which persist following washout of the toxin, were relaxed by subsequent addition of KB-R7943 or MDL 28170, either in the presence of, or following washout of MTX. These results suggest that MTX targets the Na(+)/Ca(2+) exchanger and causes it to operate in reverse mode (Na(+) efflux/Ca(2+) influx), thus leading to calpain activation, NCX cleavage, secondary Ca(2+) overload and cell death.

Functional role of the Na+/H+ exchanger in the regulation of cerebral arteriolar tone in rats

OBJECT

In vascular smooth-muscle cells, the Na+/H+ exchanger (NHE) is involved in the regulation of [Na+]i, pHi through [H+], and cell volume. Recently, investigations have determined that this exchanger contributes to ischemia and reperfusion injury in coronary circulation. Nonetheless, there is limited information on this glycoprotein in cerebral circulation, especially microcirculation. Thus, the authors in the present study examined the role of NHE in the regulation of cerebral arteriolar tone and its related mechanisms in vitro.

METHODS

The internal diameter of isolated pressurized intracerebral arterioles in rats was monitored with the aid of a microscope. To examine the basal activity of NHE two kinds of Na+/H+ exchange inhibitors (FR183998 and 5-[N,N-hexamethylene]amiloride) were administered in the arterioles. Furthermore the authors studied the effects of nitric oxide (NO) synthase inhibitor (NG methyl-L-arginine), Na+/K+ -adenosine triphosphatase (NKA) inhibitor (ouabain), and the Na+/Ca++ exchange inhibitor (SEA0400) on the vascular response induced by either of the Na+/H+ exchange inhibitors. Both of the Na+/H+ exchange inhibitors constricted the arteriole. Subsequent application of NO synthase inhibitor further decreased the diameter of the arterioles. The Na+/H+ exchange inhibitor-induced constriction was completely abolished in the presence of ouabain and SEA0400.

CONCLUSIONS

The NHE is active in the basal condition and regulates cerebral arteriolar tone through NKA and the Na+/Ca++ exchanger. Endogenous NO is not related to the activity of NHE in basal conditions.

[Ca2+]i changes in guinea pig tracheal smooth muscle cells in culture: effects of Na+ and ouabain

The objective of this work was to confirm that the contractile effects of ouabain and Na(+)-free solutions in guinea pig tracheal rings are associated with increments in the cytosolic free Ca2+ concentration ([Ca2+]i) in cultured tracheal smooth muscle (TSM) cells. Cultured cells were alpha-actin positive. Histamine (50 microM) and Na(+)-free solution elicited a transient increase in [Ca2+]i, while the responses to thapsigargin (1 microM) and ouabain (1 mM) were long lasting. However, carbachol (10, 200, and 500 mM) and high K(+)-solution produced no effect on [Ca2+]i, suggesting that cultured guinea pig TSM cells display a phenotype change but maintain some of the tracheal rings physiological properties. The transient rise in [Ca2+]i in response to the absence of extracellular Na+ and the effect of ouabain may indicate the participation of the Na(+)/Ca2+ exchanger (NCX) in the regulation of [Ca2+]i.

Transforming growth factor-beta1 modulates angiotensin II-induced calcium release in vascular smooth muscle cells from spontaneously hypertensive rats

OBJECTIVES

To investigate the role of transforming growth factor-beta1 (TGF-beta1) on Ca2+-dependent mechanisms elicited by angiotensin II in aortic vascular smooth muscle cells (VSMC) of Wistar- Kyoto (WKY) rats and spontaneously hypertensive rats (SHR).

METHODS

Cai2+ release induced by angiotensin II (1 micromol/ l) was studied in cultured VSMC isolated from the aortas of 6-week-old WKY rats and SHR. Intracellular Ca2+ (Cai2+) was assessed in Fura-2 loaded cells using fluorescent imaging microscopy. Angiotensin II receptors were analysed by binding studies.

RESULTS

Pretreatment of VSMC for 24 h with TGF-beta1 significantly increased angiotensin II-induced Cai2+ mobilization from internal stores in SHR, while Ca2+ influx was not altered. This effect involves tyrosine kinase and is not due to an increase in angiotensin II binding sites, or a change in the affinity of the receptors. By contrast, TGF-beta1 did not modify the response of VSMC from WKY rats to angiotensin II.

CONCLUSIONS

These results help our understanding of the interactions between the pathways activated by TGF-beta1 and the G protein-coupled receptor signalling pathway, and their role in genetic hypertension.

ANG II-induced Ca(2+) increase in smooth muscle cells from SHR is regulated by actin and microtubule networks

We hypothesized that the cytoskeletal network in vascular smooth muscle cells (VSMC) is critical to the signaling pathways from angiotensin (ANG) II-receptor subtype 1 (AT(1)) activation to intracellular Ca(2+) (Ca(2+)(i)) release from internal stores and Ca(2+) influx. This was tested in spontaneously hypertensive rats (SHR), in which differences were reported in cultured aortic VSMC Ca(2+)(i) regulation and G protein function compared with those in normotensive Wistar-Kyoto (WKY) rats. In cultured aortic VSMC, disorganization of actin filaments with cytochalasin D (2 micromol/l) decreased the ANG II-induced Ca(2+)(i) release from internal stores and the ANG II-induced Ca(2+) influx in SHR in a reversible fashion, whereas it was without effect in WKY rats. On the other hand, blocking the dynamic state of the microtubule network significantly reduced ANG II-induced Ca(2+)(i) release from internal stores but was without effect on Ca(2+) influx in either SHR or WKY rats. This study demonstrates for the first time that, in the SHR, actin filaments play a major role in linking AT(1)-receptor activation to both Ca(2+)(i) release mechanisms and capacitative Ca(2+) influx. Furthermore, a functionally intact microtubule system is a necessary prerequisite for ANG II-induced Ca(2+)(i) release in both strains.

Altered protein kinase C activation of Na+/Ca2+ exchange in mesangial cells from salt-sensitive rats

The purpose of these studies was to determine whether there is a defect in protein kinase C (PKC) regulation of the Na+/Ca2+ exchanger in cultured mesangial cells (MC) from Dahl/Rapp salt-sensitive (S) and salt-resistant (R) rats. R and S MCs were cultured, grown on coverslips, and loaded with fura 2 for measurement of single cell cytosolic calcium concentration ([Ca2+]i) in a microscope-based photometry system. Studies were performed in cells that were exposed to serum (serum fed) and in cells that were serum deprived for 24 h. Baseline [Ca2+]i values measured in a Ringer solution containing 150 mM NaCl were similar between R and S MCs in both serum-fed and serum-deprived groups, although baseline [Ca2+]i values were uniformly higher in the serum-deprived groups. Exchanger activity was assessed by reducing extracellular Na (Nae) from 150 to 2 mM, which resulted in movement of Na+ out of and Ca2+ into these cells (reverse-mode Na+/Ca2+ exchange). PKC was activated in these cells with 15-min exposure to 100 nM phorbol 12-myristate 13-acetate (PMA). In the absence of PMA, the change in [Ca2+]i (Delta[Ca2+]i) with reduction in Nae was similar between R and S MCs in both serum-fed and serum-deprived groups, although the magnitude of Delta[Ca2+]i was enhanced by serum deprivation. In both serum-fed and serum-deprived groups, PMA significantly increased Delta[Ca2+]i in R but not S MCs. Upregulation of exchanger activity in R MCs could be abolished by prior 24-h exposure to PMA, a maneuver that downregulates PKC activity. Other studies were performed to evaluate exchanger protein expression using monoclonal and polyclonal antibodies. Immunoblots of PMA-treated cells revealed an increase in the levels of 70- and 120-kDa proteins in the crude membrane fraction of R but not S MCs, an increase which was abrogated by prior 24-h PMA pretreatment and corresponded to reduction in the 70-kDa protein in the crude cytosolic fraction. These data demonstrate that PKC enhances Na+/Ca2+ exchange activity in MCs from R but not from S rats, suggesting that there may be a defect in the PKC-Na+/Ca2+ exchange regulation pathway in MCs of S rats.

Renal arteriolar Na+/Ca2+ exchange in salt-sensitive hypertension

The present studies were performed to assess Na+/Ca2+ exchange activity in afferent and efferent arterioles from Dahl/Rapp salt-resistant (R) and salt-sensitive (S) rats. Renal arterioles were obtained by microdissection from S and R rats on either a low-salt (0.3% NaCl) or high-salt (8.0% NaCl) diet. On the high-salt diet, S rats become markedly hypertensive. Cytosolic calcium concentration ([Ca2+]i) was measured in fura 2-loaded arterioles bathed in a Ringer solution in which extracellular Na (Nae) was varied from 150 to 2 mM (Na was replaced with N-methyl-D-glucamine). Baseline [Ca2+]i was similar in afferent arterioles of R and S rats fed low- and high-salt diet. The change in [Ca2+]i (Delta[Ca2+]i) during reduction in Nae from 150 to 2 mM was 80 +/- 10 and 61 +/- 3 nM (not significant) in afferent arterioles from R rats fed the low- and high-salt diet, respectively. In afferent arterioles from S rats on a high-salt diet, Delta[Ca2+]i during reductions in Nae from 150 to 2 mM was attenuated (39 +/- 4 nM) relative to the Delta[Ca2+]i of 79 +/- 13 nM (P < 0.05) obtained in afferent arterioles from S rats on a low-salt diet. In efferent arterioles, baseline [Ca2+]i was similar in R and S rats fed low- and high-salt diets, and Delta[Ca2+]i in response to reduction in Nae was also not different in efferent arterioles from R and S rats fed low- or high-salt diets. Differences in regulation of the exchanger in afferent arterioles of S and R rats were assessed by determining the effects of protein kinase C (PKC) activation by phorbol 12-myristate 13-acetate (PMA, 100 nM) on Delta[Ca2+]i in response to reductions in Nae from 150 to 2 mM. PMA increased Delta[Ca2+]i in afferent arterioles from R rats but not from S rats. These results suggest that Na+/Ca2+ exchange activity is suppressed in afferent arterioles of S rats that are on a high-salt diet. In addition, there appears to be a defect in the PKC-Na+/Ca2+ exchange pathway that might contribute to altered [Ca2+]i regulation in this important renal vascular segment in salt-sensitive hypertension.

The effect of sodium based hypo-osmolality on arterial smooth muscle reactivity in vitro

The study tested the hypothesis that the reduced [Na+]e and hypo-osmolality of normal pregnancy are causally linked to the attenuation of vascular smooth muscle reactivity in vitro. Aortic rings from nonpregnant female rats were incubated in physiological medium containing 114 mM NaCl/l and the contractile responses to phenylephrine, KCl and CaCl2 as well as the relaxations to acetylcholine and KCl were compared with those of rings incubated in normal medium containing 119 mM NaCl/l. There was no solute substituted for the lowered [Na+]. Experiments with phenylephrine were repeated using de-endothelialized rings and intact rings pretreated with indomethacin. Contractile responses of intact rings (n = 11) in hypo-osmolar solution to phenylephrine were significantly (P < 0.001) lower than of those in normal medium (n = 11). Responses were partially restored by endothelial denudation but not in the presence of indomethacin. Relaxations to acetylcholine (n = 7 for hypo-osmolar; n = 6 for normal solution) and KCl (n = 7 for each of hypo- and normal osmolar) were significantly enhanced (P < 0.05) in rings incubated in hypo-osmolar solution. There was no significant difference between the responses of the rings to KCl, and CaCl2 in either solution. These effects are similar to some of those previously described for vascular smooth muscle in normal pregnancy suggesting that the reduced [Na+]e and hypo-osmolarity of normal pregnancy may be contributing to the diminished vascular reactivity.

Small changes in extracellular sodium influence myogenic tone in rabbit facial vein by changing its sensitivity to calcium

Extracellular Na+ concentration ([Na+]e) significantly effects the regulation of myogenic tone in isolated blood vessels. We examined the effect of small changes in [Na+]e on simultaneous changes in stretch-activated myogenic tone in rabbit facial vein and 45Ca2+ unidirectional influx and net uptake. Decreasing [Na+]e from 150 to 120 mmol/l augmented myogenic tone (control: 3.15 +/- 0.27 mN, n = 22) by 89 +/- 29%, while raising [Na+]e to 165 mmol/l attenuated myogenic tone to 80 +/- 2% of control. Changes in myogenic tone induced by alterations in [Na+]e were not accompanied by proportional changes in 45Ca2+ net uptake. 45Ca2+ unidirectional influx per unit of wall force (10.2 +/- 1.0 pmol/mg per mN force, n = 22, control) was decreased to 6.1 +/- 0.6 pmol/mg per mN (n = 20, P < 0.05) and increased to 21.0 +/- 2.5 pmol/mg per mN (n = 14, P < 0.05) when [Na+]e was 120 or 165 mmol/l, respectively, suggesting that decreasing [Na+]e is related to an increased sensitivity to calcium. We conclude that, in the rabbit facial vein, the sensitivity of myogenic tone to changes in [Na+]e may reflect changes in the sensitivity of smooth muscle to Ca2+ through a change in mechanoreceptor sensitivity.

Characterization of sodium-calcium exchange in rabbit renal arterioles

Experiments were performed to test the hypothesis that renal arterioles exhibit Na-Ca exchange capability and that this process is regulated by protein kinase C (PKC). Glomeruli with attached arterioles were dissected from rabbit kidney and loaded with fura-2 for measurement of intracellular calcium concentration ([Ca2+]i) using microscope-based photometry. In tissue bathed in Ringer's solution containing 150 mM Na+ and 1.5 mM Ca2+, afferent and efferent arteriolar [Ca2+]i averaged 136 +/- 6 and 154 +/- 7 nM, respectively. Removal of extracellular Na+ increased afferent arteriolar [Ca2+]i by 70 +/- 7 mM, while efferent arteriolar [Ca2+]i only increased by 39 +/- 5 nM (P < 0.01 vs. afferent arteriole). These responses were inhibited by 6 nM Ni2+ and required extracellular Ca2+, but were unaffected by 10 microM diltiazem. After incubation in 500 microM ouabain, 5 microM monensin, and 5 microM nigericin, [Ca2+]i responses to removal of extracellular Na+ were exaggerated significantly, averaging 174 +/- 50 nM in afferent arterioles and 222 +/- 82 nM in efferent arterioles (NS vs. afferent arterioles). Moreover, responses to removal of extracellular Na+ were enhanced by 100 nM phorbol 12-myristate 13-acetate, an affect which was blocked by PKC inhibition (25 nM K252b). These data indicate that both afferent and efferent arterioles express the Na-Ca exchanger, and that PKC activity impacts on exchange capacity in these vessels.

Altered sodium-calcium exchange in afferent arterioles of the spontaneously hypertensive rat

Studies were performed to determine if there is a derangement in Na-Ca exchange activity in afferent (AA) and efferent (EA) arterioles from 3- and 9-week-old spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats. Cytosolic calcium concentration ([Ca2+]i) was assessed using microscope-based photometry in fura-2 loaded arterioles bathed in a Ringer's solution. Baseline [Ca2+]i was similar in the AA of 3- and 9-week-old WKY and SHR. In AA from 3-week-old rats, [Ca2+]i increased by 89 +/- 15 nM in WKY and by 73 +/- 13 nM in SHR during decreases in bath sodium concentration ([Na+]e) from 150 to 2 mM (Na+ replaced with n-methyl-D-glucamine). In 9-week-old hypertensive SHR (SBP = 150 mm Hg), increases in [Ca2+]i were attenuated (24 +/- 3 nM) relative to 3-week-old WKY and SHR, and 9-week-old WKY (90 +/- 9 nM; P < 0.05). Likewise, the rate of removal of Ca2+ in the continued presence of 2 mM Nae (Ca2+ sequestration and/or extrusion) was markedly reduced in AA of 9-week-old SHR (-0.15 +/- 0.03 nM/second) versus 3-week-old SHR (-0.72 +/- 0.12 nM/second) and 3- and 9-week-old WKY (-0.49 +/- 0.10 and -0.67 +/- 0.14 nM/second). In other experiments, AAs were preincubated in 1 mM ouabain to increase intracellular [Na+]. This maneuver augmented the increase in [Ca2+]i obtained with removal of Na+e; however, the responses obtained in 9-week-old SHR arterioles were still attenuated compared to those obtained in arterioles for 3- and 9-week-old WKY and 3-week-old SHR. These results suggest that exchanger number and/or sensitivity to the transmembrane Na gradient was reduced in the SHR AA. In EA, baseline [Ca2+]i was similar in 3- and 9-week-old WKY and SHR. In contrast to AA, the magnitude of Na-dependent and Na-independent changes in [Ca2+]i was not different in the EA of 3- and 9-week-old WKY and SHR. These results indicate that regulation of Na-Ca exchange activity may differ between AA and EA segments. Furthermore, diminished Na-Ca exchange and Na-independent Ca2+ sequestering/extrusion mechanisms could contribute to altered AA [Ca2+]i in the SHR.

Effects of dietary salt and angiotensin-converting enzyme inhibitor on ATP-driven Ca pump and Na/Ca exchange in aortic rings of Dahl rats

We examined effects of dietary salt and cilazapril on ATP-driven Ca pump and Na/Ca exchange system in arterial smooth muscle of Dahl salt-sensitive (DS) rats. Twenty-four DS rats were assigned to 4 groups. H- and H+ rats were fed a high-salt diet. L- and L+ rats were fed a low-salt diet. H+ and L+ were administered cilazapril. Aortic rings were superfused with physiological saline, and isometric tension was measured. Relaxation rates of low-Na(+)-induced contractions were promoted by the removal of external Ca. Cilazapril significantly decreased blood pressure in both the high- and low-salt diet groups. Salt loading reduced relaxation rates in 1.2 mmol/l Na+, and enhanced the increase in relaxation rates from 1.2 mmol/l to normal Na+. A negative correlation was observed between Ca extrusion by the Ca pump and blood pressure, and a positive correlation was observed between Ca extrusion by Na/Ca exchange and blood pressure. The results suggest that the decrease of Ca2+ extrusion by the ATP-driven Ca pump resulting from a high-salt diet might lead to an elevation in the concentration of cellular Ca2+.

Vasorelaxant properties of norbormide, a selective vasoconstrictor agent for the rat microvasculature

1. The effects of norbormide on the contractility of endothelium-deprived rat, guinea-pig, mouse, and human artery rings, and of freshly isolated smooth muscle cells of rat caudal artery were investigated. In addition, the effect of norbormide on intracellular calcium levels of A7r5 cells was evaluated. 2. In resting rat mesenteric, renal, and caudal arteries, norbormide (0.5-50 microM) induced a concentration-dependent contractile effect. In rat caudal artery, the contraction was very slowly reversible on washing, completely abolished in the absence of extracellular calcium, and antagonized by high concentrations (10-800 microM) of verapamil. The norbormide effect persisted upon removal of either extracellular Na+ or K+. The contractile effect of norbormide was observed also in single, freshly isolated smooth muscle cells from rat caudal artery. 3. In resting rat and guinea-pig aortae, guinea-pig mesenteric artery, mouse caudal artery, and human subcutaneous resistance arteries, norbormide did not induce contraction. When these vessels were contracted by 80 mM KCl, norbormide (10-100 microM) caused relaxation. Norbormide inhibited the response to Ca2+ of rat aorta incubated in 80 mM KCl/Ca2(+)-free medium. Norbormide (up to 100 microM) was ineffective in phenylephrine-contracted guinea-pig and rat aorta. 4. In A7r5 cells, a cell line from rat aorta, norbormide prevented high K(+)- but not 5-hydroxytryptamine-induced intracellular calcium transients. 5. These findings indicate that in vitro, norbormide induces a myogenic contraction, selective for the rat small vessels, by promoting calcium entry in smooth muscle cells, presumably through calcium channels. In rat aorta and arteries from other mammals, norbormide behaves like a calcium channel entry blocker.

Alterations in sodium metabolism as an etiological model for hypertension

An adequate matching for race, sex, stage of the menstrual cycle, family history of hypertension, and the amount of sodium and other electrolytes in the diet should be a prerequisite for valid conclusions when interpreting the erythrocyte concentration and fluxes of sodium in essential hypertensive patients in comparison with normal subjects. Alterations in intracellular sodium concentration and transmembrane sodium transport systems as causes of essential hypertension are postulated. This review article describes how this abnormal sodium and calcium metabolism translates into increased systemic vascular resistance through altered vasoactive responses and/or vasculature structural changes.

Effect of subthreshold ouabain on the tone of guinea-pig aortic strips following repeated noradrenaline stimulation

1. The effect of ouabain at a concentration (0.8 microM) that does not induce contractile response in guinea-pig aortic strips has been studied on endothelium-denuded strips repeatedly stimulated with 1 microM noradrenaline or 60 mM K+ applied for 5 min every 30 min. 2. The resting tone (i.e. the tone between one noradrenaline stimulation and the following) of the aortic strips exposed to ouabain increased progressively, whereas the control strips (no ouabain) completely relaxed on washout of the agonist. In the aortic strips stimulated by 60 mM K+, the resting tone did not increase. 3. The calcium antagonist, verapamil, did not affect the increase in tone, that was nevertheless strictly dependent on external calcium, since the contracted strips completely relaxed on calcium removal and promptly contracted again on calcium readdition. This finding indicates a mechanism independent of voltage-gated calcium channels. 4. Caffeine-induced contractions, taken as a measure of sarcoplasmic reticulum calcium content, were amplified by the presence of ouabain in aortic strips either stimulated by noradrenaline or unstimulated, with a larger increase in the former. 5. These results suggest that the repeated stimulation of guinea-pig aortic strips by noradrenaline in the presence of ouabain, by raising both intracellular Na+ and Ca2+, decreases the ouabain threshold concentration required for contraction, thus increasing the responsiveness of vascular smooth muscle to the glycoside.

Role of Na(+)-Ca2+ exchange in agonist-induced changes in cytosolic Ca2+ in vascular smooth muscle cells

Changes in cytosolic free calcium concentration ([Ca2+]i) induced by angiotensin II (ANG II), arginine vasopressin (AVP), angiotensin III (ANG III), norepinephrine (NE), or thapsigargin were investigated after inhibition of the Na(+)-Ca2+ exchange in vascular smooth muscle cells (VSMC) from Wistar-Kyoto rats by use of the fluorescent dye technique. The ANG II-induced peak [Ca2+]i increase was significantly enhanced after inhibition of Na(+)-Ca2+ exchange by NiCl2 or 1,3-dimethyl-2-thiourea (DMTU): control, 99 +/- 9 (SE) nM (n = 64); NiCl2, 181 +/- 23 nM (n = 23; P < 0.01); DMTU, 182 +/- 35 nM (n = 10; P < 0.05). In the absence of external calcium, the inhibition of the Na(+)-Ca2+ exchange by NiCl2 also enhanced the ANG II-induced [Ca2+]i increase. Inhibition of Na(+)-Ca2+ exchange by removal of external sodium, which was replaced by choline, augmented the ANG II-induced [Ca2+]i increase to 174 +/- 26 nM (n = 11; P < 0.05 compared with control). The inhibition of the protein kinase C activity by isoquinoline-sulfonyl-O-2-methylpiperazine blocked the enhancing effect of NiCl2 on ANG II-induced [Ca2+]i increase. The inhibition of the Na(+)-Ca2+ exchange did not enhance the increase in [Ca2+]i induced by ANG III, NE, or thapsigargin. The AVP-induced changes in [Ca2+]i were not significantly different in the presence or absence of NiCl2. It is concluded that the recovery of resting [Ca2+]i after stimulation by ANG II is mediated by calcium efflux via the Na(+)-Ca2+ exchange.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial natriuretic peptide and cGMP inhibit Na+/H+ antiporter in vascular smooth muscle cells in culture

The aim of the present paper was to study the mechanisms of the inhibitory effect of atrial natriuretic peptide (ANP) on the sustained contraction phase of vascular smooth muscle cells (VSMC). Specifically, the potential role of ANP on the Na+/H+ antiporter and Na+ transport systems was investigated. Both ANP and 8-bromo cGMP inhibited 22Na+ uptake and decreased intracellular Na ([Na+]i) in VSMC, an effect that was mimicked by the specific Na+/H+ antiporter inhibitor, hexamethylen amiloride (HMA). The effect of ANP was not additive with HMA, therefore suggesting that both inhibit the same 22Na+ transport pathway. On the other hand, the inhibition of 22Na+ accumulation by ANP was additive with the inhibition by furosemide or bumetanide, thus suggesting that both drugs act on different Na+ exchange systems. In HEPES-buffered medium, ANP, cGMP, and HMA significantly inhibited the AVP-induced intracellular alkalinization, an effect which was associated with significant inhibition of the AVP-induced shape change. In bicarbonate buffered medium, ANP and cGMP decreased the pH level below the baseline after application of AVP, and an inhibition by ANP and cGMP of AVP-induced VSMC shape change was also observed. The recovery of cellular pH after three different types of acid load, namely, ammonium chloride pulse, nigericin clamp and lowering of extracellular pH, was significantly decreased by ANP and cGMP. Taken together, these results indicate that ANP/cGMP inhibit the activity of the Na+/H+ antiporter in VSMC, either in hormone- or pH-stimulated conditions.

Enhancement of vascular action of arginine vasopressin by diminished extracellular sodium concentration

The present study was undertaken to examine the effects of diminished extracellular sodium concentration on the vascular action of arginine vasopressin (AVP) in cultured rat vascular smooth muscle cells (VSMC). The preincubation of cells with the 110 mM extracellular Na+ ([Na+]e) solution supplemented with 30 mM choline chloride for 60 minutes enhanced the effect of AVP- (1 x 10(-8) M) induced VSMC contraction. The treatment of 110 mM [Na+]e solution also enhanced the cellular contractile response to the protein kinase C (PKC) activators, phorbol 12-myristate 13-acetate and 1-oleoyl-2-acetyl-glycerol. Furthermore, preincubation with the 110 mM [Na+]e solution also potentiated the effect of 1 x 10(-8) M AVP, but not 1 x 10(-6) M, to increase the cytosolic-free Ca2+ ([Ca2+]i) concentration. The 110 mM [Na+]e media decreased the basal intracellular Na+ concentration and increased intracellular 45Ca2+ accumulation, basal [Ca2+]i and AVP-produced 45Ca2+ efflux. These effects of 110 mM [Na+]e solution to enhance the vascular action of AVP were abolished by using Ca(2+)-free 110 mM [Na+]e solution during the preincubation period. The preincubation with the 110 mM [Na+]e solution did not change either the Kd and Bmax of AVP V1 receptor of VSMC or the AVP-induced production of inositol 1,4,5-trisphosphate. The present in vitro results therefore indicate that the diminished extracellular fluid sodium concentration within a range observed in clinical hyponatremic states enhances the vascular action of AVP.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular cations modulate noradrenaline-stimulated calcium entry into smooth muscle cells of rat portal vein

1. The action of noradrenaline (NA, 10 microM) was studied in single patch-clamped smooth muscle cells of rat portal vein where free internal Ca2+ concentration in the cell (Ca2+i) was estimated using the emission from the dye Indo-1. 2. In the presence of 50 microM D600, a blocker of voltage-dependent Ca2+ channels, NA applied to the cell evoked an initial peak in Ca2+i followed by a smaller sustained rise. The initial rise in Ca2+i was associated with the activation of Ca(2+)-dependent Cl- channels. 3. The maintained rise in Ca2+i induced by NA was enhanced by increasing the external Ca2+ concentration and was abolished in Ca(2+)-free solution. The transient rise was resistant to the absence of external Ca2+. 4. Following the transient rise in Ca2+i induced by NA, the mechanisms extruding and/or sequestering cytoplasmic Ca2+ were stimulated. This stimulation was measured during the sustained rise in Cai and was maintained for a few seconds after NA was removed. 5. Unlike the transient rise in Ca2+i, the sustained rise in Ca2+i produced by NA was affected by changing the cell membrane potential. 6. Changing the Na+ gradient showed that the Na(+)-Ca2+ exchange was not involved in the sustained rise in Ca2+i. 7. The sustained increase in Ca2+i produced by NA was modulated by intracellular cations. This phase could be observed with 130 mM Na+ or 130 mM K+ in the pipette solution, but was severely reduced when the only cation in the intracellular solution was Cs+ and abolished with NMDG (N-methyl-D-glucamine) or TEA. However, inclusion of only 10 mM Na+ or 50 mM K+ in the pipette solution was sufficient to obtain a sustained rise in Ca2+i of maximal amplitude, similar to that obtained with 130 mM Na+i or 130 mM K+i during NA application. 8. In portal vein smooth muscle cells, NA induced a two-phase increase in Ca2+i similar to the two phases which have been previously observed upon muscarinic receptor activation by carbachol in intestinal smooth muscle. The transient rise was due to Ca2+ store release whereas the sustained rise was due to an increased Ca2+ entry into the cell down its electrochemical gradient, but not through voltage-dependent Ca2+ channels. The Ca2+ permeability pathway involved in the sustained rise in Ca2+i induced by NA was modulated by the intracellular cations.

Veratridine activates a silent sodium channel in rat isolated aorta

To investigate the existence of silent Na+ channels, isolated rat aorta was treated with veratridine (0.1 mM) and the resulting Ca2+ uptake was determined. After 30-min incubation the total tissue uptake of Ca2+ and Ca2+ uptake increased from 2.325 +/- 0.017 to 2.614 +/- 0.080 nmol/mg wet weight (ww) and from 162.6 +/- 9.7 to 218.1 +/- 13.0 pmol/mg ww, respectively. The veratridine-induced Ca2+ uptake was blocked by tetrodotoxin (1 microM; to 17 +/- 5%) but not altered by amiloride (10 microM-1 mM). Activation of Na+/Ca2+ exchange by Na+ removal increased Ca2+ uptake from 74.2 +/- 4.5 to 97.3 +/- 5.3 pmol/mg ww, which was suppressed by amiloride (10 microM-1 mM). Nifedipine (10 nM) and verapamil (0.1 microM) at concentrations at which depolarization-induced Ca2+ uptake was diminished did not attenuate veratridine-induced Ca2+ uptake. Phenytoin at 0.1 mM reduced the Ca2+ uptake induced by veratridine or by depolarization. R 56865 (0.1 microM) and R 59494 (1 microM), novel anti-ischemic compounds inhibiting slowly inactivating Na+ channels, suppressed the veratridine-induced but not the depolarization-induced Ca2+ uptake. Guanidinium uptake was increased by veratridine (0.1 mM) from 371.2 +/- 7.2 to 574.8 +/- 45.9 pmol/mg ww. These results suggest that the rat aorta possesses a Na+ channel which is electrically silent under normal conditions but could be activated by veratridine.

Magnesium regulates intracellular free ionized calcium concentration and cell geometry in vascular smooth muscle cells

Regulatory effects of extracellular magnesium ions ([Mg2+]o) on intracellular free ionized calcium ([Ca2+]i) were studied in cultured vascular smooth muscle cells (VSMCs) from rat aorta by use of the fluorescent indicator fura-2 and digital imaging microscopy. With normal Mg2+ (1.2 mM)-containing incubation media, [Ca2+]i in VSMCs was 93.6 +/- 7.93 nM with a heterogeneous cellular distribution. Lowering [Mg2+]o to 0 mM or 0.3 mM (the lowest physiological range) resulted in 5.8-fold (579.5 +/- 39.99 nM) and 3.5-fold (348.0 +/- 31.52 nM) increments of [Ca2+]i, respectively, without influencing the cellular distribution of [Ca2+]i. Surprisingly, [Mg2+]o withdrawal induced changes of cell geometry in many VSMCs, i.e., the cells rounded up. However, elevation of [Mg2+]o up to 4.8 mM only induced slight decrements of [Ca2+]i (mean = 72.0 +/- 4.55 nM). The large increment of [Ca2+]i induced by [Mg2+]o withdrawal was totally inhibited when [Ca2+]o was removed. The data suggest that: (1) [Mg2+]o regulates the level of [Ca2+]i in rat aortic smooth muscle cells, and (2) [Mg2+] acts as an important regulatory ion by modulating cell shapes in cultured VSMc and their metabolism to control vascular contractile activities.

Role of sarcolemmal membrane sodium-calcium exchange in vascular smooth muscle tension

A body of information obtained by experiments with intact tissues, isolated cells, and sarcolemmal vesicles indicates, beyond a reasonable doubt, that a specific Na(+)-Ca2+ exchange system exists in vascular smooth muscle. However, its role in the regulation of cytosolic free-Ca2+ concentration and cell tension under physiological conditions remains unclear. Under pharmacological conditions in which the Na(+)-K+ pump is inhibited either by digitalis glycosides or K(+)-free medium, Na(+)-Ca2+ exchange may be modulated by increases in cytosolic free Na+ to increase the cytosolic free-Ca2+ concentration and cell tension. Under pathological conditions in which the cytosolic Na+ concentration is increased as a result of inhibition of the Na(+)-K+ pump by endogenous ouabain or a digitalis-like factor, or activation of the Na(+)-H+ exchange or passive permeability of Na+, the Na(+)-Ca2+ exchange activity of vascular smooth muscle and the nerve terminal may play an important role in the development and/or maintenance of hypertension. These and other premises remain to be confirmed or discounted.

Effect of inhibition of Na+/K(+)-ATPase on the vasopressin-induced increase in intracellular Na+ concentration in vascular smooth muscle cells

The effect of inhibition of Na+/K(+)-ATPase by ouabain on the arginine vasopressin (AVP)-induced increase in intracellular Na+ concentration [( Na+]i) was examined in cultured rat vascular smooth muscle cells (VSMC) by the direct measurement of [Na+]i using a fluorescent indicator dye. AVP at a concentration of 1 x 10(-9) M or higher increased [Na+]i in a dose-dependent manner in cultured rat VSMC. The preincubation of cells with 1 x 10(-4) M ouabain for 1 hr at 37 degrees C did not affect the basal [Na+]i but enhanced the 1 x 10(-6) M AVP-induced increase in [Na+]i. The preincubation was not necessary because similar results were obtained after the simultaneous administration of AVP and ouabain. The treatment with ouabain did not affect the intracellular pH changes induced by AVP. These results therefore indicate that the inhibition of Na+/K(+)-ATPase enhances the AVP-induced increase in [Na+]i by decreasing cellular Na+ efflux in cultured rat VSMC.

Identification and characteristics of a Na+/Ca2+ exchanger in cultured human mesangial cells

Excitable cells express Na+/Ca2+ exchange activity among other mechanisms modulating rapid fluctuations of cytosolic free Ca2+ ([Ca2+]i). We studied functions and regulation of a Na+/Ca2+ exchanger in cultured human glomerular mesangial cells. Fura-2-loaded confluent monolayers reacted to removal of ambient Na+ with an immediate, transient elevation of [Ca2+]i, assessed by single/dual wavelength fluorometry. Peak [Ca2+]i was inversely correlated with the extracellular Na+ concentration. Ca2+ influx was the sole mechanism implicated, as the [Ca2+]i rise was prevented by EGTA. The process was inhibited by 1 mM amiloride, but not by blockers of voltage-operated Ca2+ channels. Re-addition of Na+ resulted in a rapid decrease of [Ca2+]i, indicating bimodal operation of the exchanger. Na(+)-loading the cells with monensin and ouabain enhanced Ca2+ uptake. Prior stimulation of [Ca2+]i with the thromboxane A2 mimetic, U-46619, or angiotensin II also increased Ca2+ uptake upon subsequent Na+ removal, suggesting induction of the exchanger by vasoconstrictors. Moreover, the magnitude of agonist-induced [Ca2+]i transients was amplified by Na+ removal, indicating that the exchanger modulates the effects of vasoconstrictors. These results demonstrate that an inducible Na+/Ca2+ antiporter is operative in resting and stimulated human mesangial cells, further confirming their smooth muscle origin and potential regulatory role on glomerular hemodynamics.

Effect of vasopressin on Na+ kinetics in cultured rat vascular smooth muscle cells

The effect of arginine vasopressin (AVP) on Na+ kinetics was examined in cultured rat vascular smooth muscle cells (VSMC) and rat renal papillary collecting tubule cells (RPCT) by the direct measurement of intracellular sodium concentration [(Na+]i) using fluorescence dye; SBFI. AVP increased [Na+]i in a dose-dependent manner at a concentration of 10(-9) M or higher in rat VSMC but did not affect [Na+]i in rat RPCT. The calcium (Ca2+)-free solution completely blocked the increasing effect of AVP on [Na+]i in rat VSMC. A Ca2+ ionophore, ionomycin (1-2 x 10(-6) M) increased [Na+]i both in rat VSMC and RPCT. The Ca2(+)-free solution abolished the ionomycin-increased [Na+]i both in rat VSMC and RPCT. These results therefore indicate that after binding the V1 receptor AVP increases [Na+]i mediated through an increase in cellular Ca2+ uptake in VSMC.

Nifedipine antagonizes ouabain-induced ST-segment changes and derangement of epicardial activation pattern in isolated rabbit hearts

Isolated rabbit hearts, perfused according to the Langendorff technique were treated with 0.2 mumol/l ouabain or with a combination of 0.2 mumol/l ouabain and 3 nmol/l nifedipine. In a second series of experiments, a combination of 0.2 mumol/l ouabain with 0.2 mumol/l nitroglycerin was examined. Under this treatment, coronary flow and left ventricular pressure were continuously measured and, furthermore, the epicardial potential distribution was mapped with high temporal (4 kHz/channel) and spatial resolution (256 electrodes, arranged in 4 plates with 64 electrodes each, 8 x 8 matrix with 1 mm mesh size). Ouabain led to the expected positive inotropy of 10%, but also to a decrease in coronary flow of 15%. Besides these changes a nearly generalized ST-segment elevation could be observed. Moreover, the epicardial activation pattern was disturbed by changes in the location of epicardial breakthrough-points. Concomitantly, the epicardial activation vector field was deranged. Under the additional influence of nifedipine, coronary flow was reduced by only 5%, whereas the positive inotropic effect remained unchanged. The epicardial ST-elevation was diminished significantly and there was no derangement in the process of epicardial activation. Nitroglycerin led to an increase in relative coronary flow comparable to that observed under nifedipine but did not antagonize the disturbances of the process of activation by ouabain and only partially inhibited ST-elevation. Hence, it is concluded that ouabain already provokes coronary vasoconstriction in therapeutic concentrations leading to ST-elevation and prearrhythmic changes in the process of excitation. These changes could be diminished by additional treatment with nifedipine, but not with nitroglycerin.

Effect of inhibition of Na+/K(+)-adenosine triphosphatase on vascular action of vasopressin

The present study was undertaken to examine the cellular interaction between a Na+/K(+)-ATPase inhibitor, ouabain, and arginine vasopressin (AVP) in rat vascular smooth muscle cells (VSMC) in culture. Preincubation with 10(-5) M ouabain for 60 min increased basal cytosolic free Ca2+ [( Ca2+]i) concentration and intracellular 45Ca2+ uptake. Ouabain, however, did not affect basal 45Ca2+ efflux or AVP-stimulated 45Ca2+ efflux. As assessed by cell shape change, preincubation with 10(-5) M ouabain for 60 min also enhanced the sustained cellular contractile effect of a submaximal (10(-8) M AVP, 21.5% vs. 30.5%, P less than 0.01) but not maximal dose of 10(-6) M AVP. Preincubation with 10(-5) M ouabain for 60 min did not change AVP-induced V1-specific surface receptor binding or AVP-induced inositol phosphate production but did however potentiate the mobilization of [Ca2+]i induced by a submaximal (10(-8) M AVP, 301 vs. 385 nM, P less than 0.01) but not a maximal dose of AVP. These effects of ouabain on the mobilization of [Ca2+]i were abolished by incubation in Ca2(+)-free buffer or 5 X 10(-5) M verapamil. Ouabain (10(-5) M) also enhanced the sustained cellular contractile effect of a direct protein kinase C activator, phorbol 12-myristate 13-acetate. The present results therefore indicate that the inhibition of Na+/K(+)-ATPase may enhance the vascular action of AVP, and perhaps other vasoconstrictors, by increasing the AVP-induced mobilization of [Ca2+]i and by potentiating the activity of protein kinase C stimulated by AVP through enhancing basal and AVP-stimulated cellular Ca2+ uptake.

Leucocyte Na+/H+ antiport activity in type 1 (insulin-dependent) diabetic patients with nephropathy

The development of proteinuria in Type 1 (insulin-dependent) diabetic patients may depend on predisposition to essential hypertension in addition to poor glycaemic control. Previous work has shown increased leucocyte Na+/H+ antiport activity in essential hypertension and increased erythrocyte Li+/Na+ exchange in Type 1 diabetic patients with proteinuria. To test whether susceptibility to nephropathy in Type 1 diabetes was linked to abnormalities of leucocyte Na+/H+ antiport activity, we measured the intracellular pH and kinetics of the Na+/H+ antiport in 19 Type 1 diabetic subjects with, and 15 diabetic subjects without albuminuria and compared them to 25 matched normal control subjects. Intracellular pH (mean +/- SD 7.59 +/- 0.14) and maximal transport capacity of the antiport (Vmax 87.7 +/- 24.9 mmol.1-1.min-1) were higher in diabetic subjects with albuminuria compared to normotensive control subjects (pH 7.44 +/- 0.09; Vmax 55.6 +/- 10.3 mmol.l-1.min-1; p less than 0.001 for both), similar to the defect described in essential hypertension. These differences were not seen in diabetic subjects with normal urinary albumin/creatinine ratios (pH 7.46 +/- 0.09; Vmax 61.0 +/- 13.6 mmol.l-1.min-1). Buffering characteristics of the leucocytes at different pH in the Type 1 diabetic subjects with albuminuria differed from normal control subjects and diabetic subjects with normal urinary albumin/creatinine ratios. We conclude that increased leucocyte Na+/H+ antiport activity, a known marker of essential hypertension, is usually associated with nephropathy in Type 1 diabetes.

Alterations in [Ca2+]i mediated by sodium-calcium exchange in smooth muscle cells isolated from the guinea-pig ureter

1. Sodium-calcium exchange was studied in single enzymatically isolated cells of the guinea-pig ureter using the Ca2(+)-sensitive fluorescent dye Indo-1 to monitor the intracellular Ca2+ concentration ([Ca2+]i). Patch pipettes containing Indo-1 were used to introduce the dye into cells, to set the intracellular Na+ concentration ([Na+]i) and control the membrane potential during experiments. 2. With [Na+]i set at 11-12 mM and a membrane potential of -60 or -70 mV, brief depolarization of ureter cells elicited typical voltage-gated inward currents associated with rapid increases in [Ca2+]i which showed a bell-shaped potential dependence. If Ca2+ currents were blocked with nifedipine, depolarization led to slower rises in [Ca2+]i. The rates and amplitudes of these increased monotonically with progressively larger depolarizations up to +120 mV. 3. The nifedipine-resistant rises in [Ca2+]i elicited by depolarization were potentiated when the extracellular sodium concentration ([Na+]o) was reduced. Basal levels of [Ca2+]i also increased as [Na+]o was reduced, although the dependence of this effect on [Na+]o was smaller than would be predicted if [Ca2+]i was set only by a Na(+)-Ca2+ exchange process. 4. The nifedipine-insensitive rises in [Ca2+]i elicited by depolarization were potentiated at higher basal levels of [Ca2+]i. 5. The ability of cells to reduce [Ca2+]i rapidly following Ca2+ loading during voltage-gated transients was markedly inhibited if the Na+ concentration gradient was reversed, but was little affected if the Na+ gradient was decreased by 25 or 50%. Recovery from a Ca2+ load caused by reversal of the Na+ gradient could be induced by removal of Cao2+ in the continuing absence of Nao+, indicating the importance of a Na(+)-independent [Ca2+]i-lowering system. 6. The results demonstrate that Na(+)-Ca2+ exchange can modulate [Ca2+]i when [Na+]i and the membrane potential are set at or near their physiological levels in these smooth muscle cells. [Ca2+]i does not, however, appear to be markedly sensitive to the Na+ concentration gradient under the conditions employed for these experiments, suggesting that a Na(+)-independent Ca2+ extrusion system is mainly responsible for regulating [Ca2+]i under normal conditions.

Spontaneously hypertensive rat vascular smooth muscle cells in culture exhibit increased growth and Na+/H+ exchange

The cellular mechanisms responsible for abnormalities in spontaneously hypertensive rat (SHR) vascular smooth muscle cell (VSMC) growth and vasoreactivity are not defined. Because Na+/H+ exchange, which we have previously demonstrated in cultured VSMC, plays an essential role in mediating growth factor responses, we hypothesized that abnormalities in SHR growth regulation might be reflected in the activity of this transporter. To test this hypothesis, we studied DNA synthesis and Na+/H+ exchange (measured as the rate of amiloride-sensitive intracellular alkalinization or Na+ influx) in early subcultures (less than 6) of aortic VSMC from 12-wk-old SHR and Wistar Kyoto (WKY) animals. Serum-deprived SHR VSMC grew more rapidly in response to 10% serum with an increase in [3H]thymidine incorporation of 439% compared with 191% in WKY controls. Basal intracellular pH (pHi) values determined by fluorescent pH measurements were 7.37 +/- 0.04 and 7.27 +/- 0.03 (P less than 0.05) in early passage SHR and WKY, respectively. Acid recovery (initial pHi = 6.8) by SHR VSMC was faster than by WKY VSMC as measured by alkalinization (1.8 +/- 0.6 vs. 0.8 +/- 0.2 mmol H+/liter.min, P less than 0.05) or by amiloride-sensitive 22Na+ influx (14.5 +/- 1.2 vs. 4.0 +/- 0.5 nmol Na+/mg protein.min, P less than 0.05). In comparison to WKY cells early passage SHR VSMC exhibited 2.5-fold greater alkalinization and amiloride-sensitive 22Na+ influx in response to 100 nM angiotensin II. During serial passage, WKY cells acquired enhanced Na+/H+ exchange and growth rates so that by passage 6, these differences were no longer present. These findings in early cultures of SHR VSMC, removed from the in vivo neurohumoral milieu, suggest that increased Na+/H+ exchange in SHR may reflect alterations in Na+ homeostasis that might contribute to altered SHR VSMC function such as enhanced growth and vasoreactivity.