Involvement of a Na-Ca exchange mechanism in contraction induced by low-Na solution in isolated guinea-pig aorta

Ca(2+) regulation in guinea-pig colonic smooth muscle: the role of the Na(+)-Ca(2+) exchanger and the sarcoplasmic reticulum

To study the contribution of the Na(+)-Ca(2+) exchanger to Ca(2+) regulation and its interaction with the sarcoplasmic reticulum (SR), changes in cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) were measured in single, voltage clamped, smooth muscle cells. Increases in [Ca(2+)](c) were evoked by either depolarisation (-70 mV to 0 mV) or by release from the SR by caffeine (10 mM) or flash photolysis of caged InsP(3) (InsP(3)). Depletion of the SR of Ca(2+) (verified by the absence of a response to caffeine and InsP(3)) by either ryanodine (50 microM), to open the ryanodine receptors (RyRs), or thapsigargin (500 nM) or cyclopiazonic acid (CPA, 10 microM), to inhibit the SR Ca(2+) pumps, reduced neither the magnitude of the Ca(2+) transient nor the relationship between the influx of and the rise in [Ca(2+)](c) evoked by depolarisation. This suggested that Ca(2+)-induced Ca(2+) release (CICR) from the SR did not contribute significantly to the depolarisation-evoked rise in [Ca(2+)](c). However, although Ca(2+) was not released from it, the SR accumulated the ion following depolarisation since ryanodine and thapsigargin each slowed the rate of decline of the depolarisation-evoked Ca(2+) transient. Indeed, the SR Ca(2+) content increased following depolarisation as assessed by the increased magnitude of the [Ca(2+)](c) levels evoked each by InsP(3) and caffeine, relative to controls. The increased SR Ca(2+) content following depolarisation returned to control values in approximately 12 min via Na(+)-Ca(2+) exchanger activity. Thus inhibition of the Na(+)-Ca(2+) exchanger by removal of external Na(+) (by either lithium or choline substitution) prevented the increased SR Ca(2+) content from returning to control levels. On the other hand, the Na(+)-Ca(2+) exchanger did not appear to regulate bulk average Ca(2+) directly since the rates of decline in [Ca(2+)](c), following either depolarisation or the release of Ca(2+) from the SR (by either InsP(3) or caffeine), were neither voltage nor Na(+) dependent. Thus, no evidence for short term (seconds) control of [Ca(2+)](c) by the Na(+)-Ca(2+) exchanger was found. Together, the results suggest that despite the lack of CICR, the SR removes Ca(2+) from the cytosol after its elevation by depolarisation. This Ca(2+) is then removed from the SR to outside the cell by the Na(+)-Ca(2+) exchanger. However, the exchanger does not contribute significantly to the decline in bulk average [Ca(2+)](c) following transient elevations in the ion produced either by depolarisation or by release from the store.

Characteristics and possible mechanisms of low-Na+ induced contractions in rat aorta

The influence of reducing external Na+ concentration ([Na+]ex) upon vascular smooth muscle contractility was investigated using the rat isolated aorta. NaCl from the physiological saline solution (PSS) was replaced with either choline-Cl, sucrose, or LiCl to give the following [Na+]ex (mM): 115, 85, 55, and 25 (115NaPSS to 25NaPSS). Small reductions in [Na+]ex (115NaPSS) induced a biphasic contraction, comparable in amplitude with the control one induced by phenylephrine 10(-6) M. Elimination of the endogenous catecholamine participation using either phentolamine 10(-5) M or guanethidine 3.10(-6) M similarly reduces these contractions to 25% (sucrose replacement). A similar relaxing effect was obtained with D600 10(-5) M, an antagonist of the voltage operated Ca2+ channels (25-30% residual tension for all the substitutes). Large reductions in [Na+]ex (25NaPSS) induced contractions comparable in amplitude and shape, but less sensitive to phentolamine and guanethidine (residual tension 65-75%, sucrose replacement) and insensitive to D600 (all the substitutes). The Na+/K+ ATPase inhibitor ouabain (10(-4) M) elicited slowly developing contractions, the amplitude being 115% of the phenylephrine 10(-6) M control. Phenylephrine further contracted the 115NaPSS precontracted preparations, but was significantly less effective in 25NaPSS, although the precontraction levels were similar for the same substitute used. The amplitude of the superimposed phenylephrine contractions exhibited [Na+]ex dependence. Phenylephrine 10(-6) M failed to further contract the ouabain 10(-4) M precontracted rings. We conclude that relatively small reductions in [Na+]ex are able to induce contractions of rat aorta primarily through release of endogenous catecholamines, probably through neural Na+/Ca2+ exchange. Larger reductions in [Na+]ex appear to cause contraction through muscular Na+/Ca2+ exchange.

Regulation of vascular smooth muscle contraction by extracellular Na+

1. The effects of extracellular Na+ removal on agonist-induced contraction of vascular smooth muscle in vitro are reviewed. 2. The effects of extracellular Na+ removal on contraction vary depending upon the agonist and vessel. 3. Factors that may influence the effects of extracellular Na+ removal on agonist-induced contraction include the compound substituted for Na+, time of tissue exposure to lowered extracellular Na+, concentration of extracellular Na+, agonist concentration, presence of the vascular endothelium and presence of basal tone. 4. The potential influence of these factors needs to be determined and minimized, in studies that investigate the role of extracellular Na+ in agonist-induced contraction.

Vasoconstriction by angiographic contrast media in isolated canine arteries

To study the mechanism of the pain produced by contrast media (CM) in peripheral arteriography, we examined the direct effects of low concentrations (1.85-100 mg I/ml) of CM (diatrizoate, iopamidol, ioxaglate, and iotrolan) on helically cut strips of canine blood vessels taken from six different regions. We found that: (a) low concentrations of CM induced vasoconstriction. (b) This occurred immediately after the application of CM and produced sustained constriction. (c) The constriction produced in arteries was dose-dependent. (d) The production and intensity of constriction in the arterial strips differed as follows: cranial mesenteric artery > renal artery > femoral artery > common carotid artery > thoracic aorta > coronary artery. The effects of the CM were, in order of magnitude: diatrizoate > iopamidol > ioxaglate > iotrolan. Differences between CM corresponded with the differences in osmolality of the CM solutions. (e) Low concentrations of meglumine and mannitol also produced vasoconstriction. (f) Constriction caused by all drug samples used was reversible, but the process of relaxation to the original tension was much slower in CM-treated arterial strips than in the other strips. From these results, we confirmed that the incidence and degree of vasoconstriction produced by all drug samples used in this experiment depended on solution osmolality, rather than on chemotoxicity or ionicity. We discuss the physiological mechanism of these results and stress the importance of CM hyperosmolality in vasoconstriction and vascular pain production.

Na+/Ca2+ exchange mediation in the ouabain-induced contraction in human placental vessels

1. Ouabain induced concentration-dependent contractions in segments of human placental arteries and veins, which were practically abolished in a Ca(2+)-free medium and not modified by the calcium antagonist nifedipine or the calcium agonist Bay K 8644. 2. Ouabain (10(-4) M) elicited a time-dependent enhancement of the 45Ca2+ uptake, which remained equal in presence of nifedipine or Bay K 8644. 3. The Na+/Ca2+ exchange blocker amiloride reduced both the contractions and the 45Ca2+ uptake induced by ouabain, whereas the Na ionophore monensin produced a parallel shift to the left of the concentration-response curve to ouabain. 4. These results suggest that ouabain-induced contractions in these vessels are dependent on the extracellular Ca2+, which mainly enters into the cell through the Na+/Ca2+ exchange system.

Cellular Mg++ accumulation is altered by extracellular Na+ and directly affects agonist-induced mobilization of Ca++ in vascular smooth muscle

Effects of altered Na+, Ca++ and Mg++ concentrations on 45Ca and 28Mg distribution and binding as well as of changes in cellular Mg++ on mobilization of Ca++ by added norepinephrine (NE) were examined in the rabbit aortic media-intimal layer. Uptake of 45Ca at cellular high affinity sites was decreased by Mg++ much more than 28Mg uptake was altered by Ca++. Substitution of Na+ affects 45Ca uptake primarily at extracellular (La( )-accessible) binding sites. Muscles were pre-loaded with Mg++ by incubation in a low-Na+ solution (75% Na+ replaced isosmotically with sucrose) for 30 min followed by a 90 min exposure to a similar solution also containing 15 mM MgCl2. These tissues, upon examination in normal (154 mM) Na(+)-containing solution, indicated decreased retention of that cellular, high-affinity Ca++ fraction important for NE-induced contractile response. Accordingly, release of 45Ca from this site and associated tension responses to added NE were attenuated in these muscles. These results suggest that variations in extracellular Na+ concentration modulate binding and subsequent mobilization of activator Ca++ by agonists through alterations in cellular Mg++ content in vascular smooth muscle.

Pharmacological properties of contraction caused by sodium removal in muscle strips isolated from canine coronary artery

Contractions produced by Na+ removal were studied in muscle strips isolated from canine coronary artery. In the presence of 20 mM K+ and 0.5 mM Ca2+, rapid contractions were observed repeatedly on complete replacement of NaCl with sucrose. This contraction in the absence of Na+ (0-Na) was not affected by phentolamine but was strongly inhibited by verapamil. Ouabain slowly potentiated the O-Na contraction and markedly reduced the inhibition due to verapamil. The O-Na contraction was dependent on external Ca2+ both with and without ouabain. Bepridil had effects very similar to those of verapamil. Amiloride and excess Mg2+ reduced the O-Na contraction and the degree of their inhibition was similar after ouabain treatment. The decrease in verapamil susceptibility could suggest that the O-Na contraction has verapamil-sensitive and -insensitive components. The former is probably due to Ca2+ influx through voltage-dependent channels and the latter to Ca2+ influx through an Na(+)-Ca2+ exchange process. Ouabain is considered to increase the contribution of Na(+)-Ca2+ exchange to the O-Na contraction. Mg2+ may inhibit both verapamil-sensitive and -insensitive pathways. Amiloride probably exerts its inhibitory effect on the contractile machinery.

Calcium-stimulated sodium efflux from rabbit vascular smooth muscle

1. The effects of the addition of Ca2+ on ouabain-resistant 22Na+ efflux from Na+-loaded strips of rabbit portal anterior mesenteric vein in Ca2+-free media have been studied. 2. Na+ efflux into Li+ media containing 5 mM-KCl is rapidly and transiently stimulated some 4- to 5-fold on the addition of Ca2+ (1.2 mM). No stimulation is observed if the Li+ medium is K+ free or if Na+ replaces Li+ ions. This Ca2+-activated Na+ efflux is not obligatorily coupled to Na+ influx. 3. The stimulation of Na+ efflux could also be triggered by the addition of 5 mM-K+ to a Ca2+-containing K+-free medium. The Ca2+-activated increase in Na+ efflux also occurred when K+ was the sole monovalent extracellular cation. Rb+ could substitute for the K+ requirement. Thus the Na+ efflux is not mediated by a system which has a specific requirement for counter-transport of Li+ or one in which Li+ but not K+ are counter-transported such as the familiar Na+-H+ exchange system. Acidification of the external medium reduced the Ca2+-stimulated Na+ efflux, in keeping with the conclusion that this efflux was not due to Na+-H+ exchange. 4. Progressive reduction of external [Ca2+] increased the time-lag to peak activation of Na+ efflux, suggesting that the effects of added Ca2+ were mediated by a rise in intracellular Ca2+. Under experimental conditions which did not result in activation of the Na+ efflux by the addition of extracellular Ca2+ alone (e.g. in Na+ media), addition of Ca2+ plus the Ca2+ ionophore, ionomycin, stimulated Na+ efflux. This further confirms that intracellular sites for Ca2+ are critical for the activation of Na+ efflux. In the absence of ionophore, in Na+ media, intracellular Ca2+ is not sufficiently increased when extracellular Ca2+ is added. A partial (40%) block of Ca2+-activated Na+ efflux by amiloride (2 X 10(-3) M) could also be overcome by the addition of ionomycin. 5. The lack of effect of a variety of inhibitors suggests that the Ca2+-stimulated Na+ efflux mechanism is not mediated via a Na+-K+-Cl- co-transport system or a Na+-H+ counter-transport system, or Na+-Ca2+ exchange. 6. The activation of Na+ efflux in smooth muscle by Ca2+ ions seems to involve Ca2+ entry partially via an extracellular Ca2+-intracellular Na+ exchange and also through other parallel pathway(s), followed by a rise in intracellular Ca2+ that activates Na+ efflux through a Ca2+-sensitive Na+ channel or other transport pathway.

Noradrenaline release induced by ouabain and vanadate in cat cerebral and peripheral arteries

The effects of 10(-4) M ouabain and 10(-3) M vanadate (Na3VO4) on [3H]noradrenaline release from cat cerebral and femoral arteries was studied. Ouabain induced tritium secretion in cerebral arteries, but not in femoral ones, which was reduced by Ca suppression and potentiated by extracellular Na reduction to 11.9 mM. However, vanadate evoked tritium release from both kinds of vessels was unaffected under these experimental conditions. These data suggest: ouabain elicited secretion from adrenergic nerve endings is likely due to inhibition of the Na, K-ATPase and subsequent Ca influx through Na-Ca exchange, and vanadate action is mediated by another mechanism different to the Na pump blockade.

Intracellular sodium, membrane potential, and contractility of rat mesenteric small arteries

We have investigated effects of altered extracellular sodium, intracellular sodium concentration, and membrane potential on the contractile responses of rat isolated mesenteric small arteries (internal diameter ca. 200 microns), when mounted as ring preparations on an isometric myograph. To avoid possible neural effects, all vessels were denervated in vitro using 6-hydroxydopamine. In unstimulated vessels, exposure to low-Na+ solutions (25 mM sodium, sucrose, or choline-substituted) did not cause any response nor did exposure to ouabain (1 mM) for 1 hour [when intracellular sodium concentration increased to 64 mmol/(liter-cell)]. However, a response was obtained if ouabain-exposed vessels were subjected to low-sodium solutions (ca. 15% of maximal response). The magnitude of the response was dependent on the ratio of intracellular to extracellular sodium and was not inhibitable by the calcium blockers, felodipine (1 nM) or D600 (10 microM). This response could therefore be explained in terms of Na-Ca exchange mechanism. The responses of activated vessels to ouabain and to low-sodium solutions were also investigated. The responses of vessels to submaximal doses of noradrenaline or potassium were potentiated acutely by ouabain (by 10-30% of the maximal response), even if the extracellular sodium was reduced to 25 mM. In all cases, the potentiation by ouabain was accompanied by a depolarization (3-12 mV). However, only in the case of noradrenaline-activated vessels with normal extracellular sodium was the potentiation accompanied by an increase in intracellular sodium [by ca. 7 mmol/(liter-cell)]. Moreover, the latter response was inhibited by felodipine and D600. The results suggest that Na-Ca exchange mechanisms may be present in these vessels but that they only play a role under extreme conditions; under normal conditions the effect of ouabain on activated vessels seems to be primarily due to its depolarizing effect, and not to its effect on intracellular sodium.

Action of the Na+ ionophore monensin on vascular smooth muscle of guinea-pig aorta

The effects of the Na+ ionophore monensin on contractile responses were investigated in guinea-pig aorta in normal and high K+ solutions. In normal K+ (5.4 mM) solution, monensin (2 X 10(-5) M) produced a rapid increase in tension followed by slow relaxation. This contraction was markedly inhibited by phentolamine (10(-5) M) or prazosin (10(-6) M) and was accompanied by an increase in tritium efflux from tissue preloaded with [3H]norepinephrine. In the presence of phentolamine, monensin (1-2 X 10(-5) M) or ouabain (1-2 X 10(-5) M) caused only a small and slowly developing contraction. Simultaneous application of these agents caused a more rapid and greater contraction. Either monensin or ouabain gradually increased cellular Na+ and decreased cellular K+ content. When monensin was applied simultaneously with ouabain, there was a rapid increase in cellular Na+ and loss of cellular K+. In high K+ (65.4 mM) solution, monensin (10(-6) M) slightly reduced the increased tension level but when external glucose was omitted monensin markedly inhibited the contraction. A significant decrease in tissue ATP content was observed only when monensin was applied in glucose-free solution. Similarly, hypoxia (N2 bubbling) markedly inhibited the high K+ contraction and decreased the tissue ATP content only in the absence of glucose. These results suggest that monensin produces a neurogenic contraction due to the release of endogenous catecholamines and also produces a myogenic contraction by a decrease in transmembrane Na+ and K+ gradients when the Na+-K+ pump is inhibited by ouabain, and that monensin inhibits aerobic energy metabolism of vascular smooth muscle.

Sodium/calcium exchange in smooth-muscle microsomal fractions

The existence of Na+ -dependent Ca2+ transport was investigated in microsomal fractions from the longitudinal smooth muscle of the guinea-pig ileum and from the rat aorta, and its activity was compared with that of the plasmalemmal ATP-dependent Ca2+ pump previously identified in these preparations. The rate of Ca2+ release from plasmalemmal vesicles previously loaded with Ca2+ through the ATP-dependent Ca2+ pump was transiently faster in the presence of 150 mM-NaCl in the medium than in the presence of 150 mM-KCl or -LiCl or 300 mM-sucrose. Na+-loaded vesicles took up Ca2+ when an outwardly directed Na+ gradient was formed across the membrane. The Ca ionophore A23187 induced a rapid release of 85% of the sequestered Ca2+, whereas only 15% was displaced by La3+. Ca2+ accumulated by the Na+-induced Ca2+ transport was released by the addition of NaCl, but not KCl, to the medium. Ca2+ uptake in Na+-loaded vesicles was inhibited in the presence of increasing NaCl concentration in the medium. Half-maximum inhibition was observed with 28 mM-NaCl. Data fitted the Hill equation, with a Hill coefficient (h) of 1.9. Na+-induced Ca2+ uptake was a saturable function of Ca2+ concentration in the medium. Half-maximum activity was obtained with 18 microM-Ca2+ in intestinal-smooth-muscle microsomal fraction and with 50 microM-Ca2+ in aortic microsomal fraction. The results suggest that in these membrane preparations a transmembrane movement of Ca2+ can be driven by a Na+ gradient. However, the Na+-induced Ca2+ transport had a lower capacity, a lower affinity and a slower rate than the ATP-dependent Ca2+ pump.

Effects of the Na ionophore monensin on the contractile response and the movements of monovalent cations in the vascular smooth muscle of rabbit aorta

The effects of monensin, a Na ionophore, on the muscle contraction and the movements of monovalent cations were investigated in rabbit aorta. Experiments were conducted in the presence of phentolamine (10(-6) M) to avoid the vasoactive effect of monensin due to the release of endogenous catecholamine. Both monensin (2 X 10(-5) M) and ouabain (2 X 10(-5) M), added separately, produced a small and slowly developing contraction, whereas simultaneous application of these agents produced more rapid and greater contraction. Verapamil (10(-6) M) decreased the contraction by 75%. Ouabain gradually increased cellular Na content. Monensin augmented the ouabain-induced Na increase. Further, the loss of cellular Na into Na deficient solution was enhanced by monensin. Low temperature (0.5 degrees C) inhibited the monensin-induced increase in Na permeability. The relaxation of noradrenaline-contraction induced by a K-readmission was inhibited by ouabain but not by monensin. These results suggest that monensin increases Na movement down its electrochemical gradient, augments cellular Na accumulation when the Na pump is inhibited by ouabain, and induces muscle contraction, and that the contraction induced by monensin and ouabain is mainly due to an increased Ca influx through voltage sensitive Ca channels.