Effects of calcium and sodium on contracture tension in the smooth muscle of the rat portal vein

[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.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Calcium dependency of the AT1-receptor mediated effects in the rat portal vein: influence of calcium antagonists

The calcium dependency of AT1-receptor mediated contractions was studied in isolated rat portal vein preparations. The spontaneous phasic contractile force of the rat portal vein was increased (ED50 = 1.76 mmol/l) and the frequency of contractions decreased by raising the extracellular calcium concentration. The Ang II-induced rise in phasic contractile force (mediated by AT1-receptors, Zhang et al. 1993) proved most pronounced at 0.9 mmol/l of calcium chloride, but it was weaker at either lower or higher calcium concentrations. The maximal increases in the phasic contractile force induced by Ang II were 2.4 +/- 0.4, 14.8 +/- 0.9 and 5 +/- 0.5 mN at calcium concentrations of 0.5, 0.9 and 2.5 mmol/l, respectively. Calcium antagonists reduced at the lower and abolished at the higher concentrations (nifedipine 2 x 10(-8) or 10(-7) mol/l; verapamil 10(-7) or 5 x 10(-7) mol/l; diltiazem 3 x 10(-7) or 10(-6) mol/l) the spontaneous contractile force. All of these calcium antagonists caused a strong inhibition or suppression of the phasic contractions induced by Ang II. The rank order of potency was nifedipine > verapamil > diltiazem. Ang II (10(-6) mol/l) elicited a tonic contraction which was abolished by the AT1-receptor antagonist losartan 10(-6) mol/l but not by the AT2-receptor antagonist PD 123177 (10(-5) mol/l). Very high concentrations of nifedipine (10(-6) mol/l), verapamil (5 x 10(-6) mol/l) and diltiazem (5 x 10(-6) mol/l) almost suppressed the tonic effect evoked by the activation of AT1-receptors. In a nominally Ca2+ "free", EGTA-containing solution, a single supra-maximal concentration of Ang II (10(-6) mol/l) caused a transient contraction, also mediated by AT1-receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro characterization of a novel Ca2+ entry blocker: SR 33805

In this study, SR 33805 was shown to inhibit competitively [3H]fantofarone binding to cardiac sarcolemmal membranes. In contrast, SR 33805 was shown to inhibit allosterically [3H](+)-PN200-110, [3H](-)-D888 and cis-(+)-[3H]diltiazem binding. In isolated rabbit atrial preparations, SR 33805 was shown to be the least potent of fantofarone, nifedipine, verapamil and diltiazem in terms of both negative chronotropic and inotropic responses (IC50's 6 and 12 microM, respectively). In superfused rat aortic strips, SR 33805 like other Ca2+ channel antagonists, caused a significant inhibition of both K(+)-induced 45Ca2+ influx and contractile responses. In addition this agent was shown to antagonize Ca(2+)-induced contractions in K(+)-depolarized aorta with a pA2, value of 8.39 +/- 0.02. In femoral, renal and basilar arteries, SR 33805 was equiactive to the other Ca2+ channel antagonists studied in antagonizing K(+)-induced contractions (IC50 approximately 40 nM), but unlike the reference Ca2+ channel antagonists, was equiactive in antagonizing serotonin-induced contractions (IC50 approximately 250 nM). This suggests that the effects of SR 33805 depend mainly on membrane potential. In conclusion, SR 33805 is a potent Ca2+ channel antagonist which, unlike fantofarone, verapamil and diltiazem, is highly selective for vascular smooth muscle and devoid of any potent negative inotropic actions.

Role of sodium in antigen-induced contraction of tracheal smooth muscle in dogs

We examined the role of Na+ influx in the airway response to antigen (ragweed pollen extract) in sensitized dogs, using amiloride analogs to block Na(+)-dependent processes. In in vivo studies, respiratory resistance was measured in amiloride treated and untreated groups. The resistance increased by 9.3 cmH2O.L-1.sec in response to ragweed aerosol in the untreated group, but increased only by 5.2 cmH2O.L-1.sec in the treated group. In in vitro studies, isometric tension was measured in ragweed pollen sensitized tracheal strips. Tissues were treated with amiloride or its derivatives (50 microM) for specifically blocking Na+ channels (phenamil), Na(+)-H+ exchanger [5-(N-methyl-N-guanidinocarbonyl methyl)-amiloride] or Na(+)-Ca2+ exchanger [5-(4-chlorobenzyl)-2',4'-dimethylbenzamil]. In untreated strips, tension increased in response to ragweed by 1.9 +/- 0.5 mN/mg. The increase was reduced by phenamil (95.2 +/- 2.5%; P < 0.01) and amiloride (41.7 +/- 13.1%; P < 0.01), but not by the other two agents. Furthermore, phenamil also inhibited histamine-induced tension response and histamine-induced 22Na+ uptake of the muscle. We conclude that antigen-induced airway response is attenuated by blocking Na+ influx in smooth muscle.

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.

Electrical and mechanical properties of spontaneous contraction in hypertensive rat portal vein

Contractile and electrical activities of longitudinal smooth muscle of portal vein from normotensive Wistar Kyoto rats (WKY) and stroke-prone spontaneously hypertensive rats (SHRSP) were compared. Amplitude and duration of spontaneous contraction of SHRSP portal vein were greater than those of WKY portal vein. No significant differences were observed in the resting membrane potentials between these preparations. Spontaneous spike activity appeared as a form of bursts. Duration of the burst and the number of spikes in each burst was greater in the portal vein of SHRSP than that of WKY. The amplitude of phasic and tonic components of K-contracture was also greater in SHRSP portal vein. Adrenergic and cholinergic nerves were not involved in the differences in contractions of the portal vein of these animal strains. Cross-sectional area of the longitudinal muscle layer was greater in SHRSP portal vein. These results suggest that the differences in spontaneous electrical activity are the cause of the differences in force and duration of the spontaneous contraction of portal vein from WKY and SHRSP, although the difference in excitation-contraction coupling of smooth muscle may be involved in much less extent.

Regulation of cell calcium and contractility in mammalian arterial smooth muscle: the role of sodium-calcium exchange

1. The contraction and relaxation of rings of rat thoracic aorta and bovine tail artery were examined as a function of changes in the Na+ electrochemical gradient in order to determine the role of Na-Ca exchange in the control of contractility. 2. Inhibition of the Na+ pump in rat aorta by K+-free media or a low concentration (5 x 10(-5) M) of strophanthidin reversibly increased the contractile responses to caffeine and noradrenaline. These effects were dependent upon external Ca2+ and were observed even in the presence of a Ca2+ channel blocker (10 microM-verapamil or 10 microM-diltiazem) and an alpha-receptor blocker (10 microM-phentolamine). 3. Reduction of external Na+ concentration, [Na+]o (replaced by N-methylglucamine, tetramethylammonium or Tris), also caused an external Ca2+-dependent increase in tonic tension and, in rat aorta, an increase in the response to caffeine. These effects were also observed in the presence of verapamil and phentolamine. 4. Caffeine relaxed the bovine tail artery, but increased the sensitivity of the rat aorta to reduced [Na+]o. The latter effect was presumably due to block of Ca2+ sequestration in the sarcoplasmic reticulum, so that entering Ca2+ was more effective in raising the intracellular free Ca2+ level, [Ca2+]i. 5. Relaxation from K+-free or low-Na+ contractions, in Ca2+-free media, depended upon [Na+]o. Reduction of [Na+]o to 1.2 or 7.5 mM slowed the relaxation of rat aorta (5 mM-caffeine present) 3- to 5-fold, and the relaxation of bovine tail artery (without caffeine) 5- to 10-fold. These effects were seen in the presence of verapamil and phentolamine. 6. These observations are all consistent with an Na-Ca exchange transport system that can move Ca2+ either into or out of the arterial smooth muscle cells. Ca2+ entry is enhanced by raising [Na+]i (by Na+ pump inhibition) and/or lowering [Na+]o. Ca2+ extrusion from the contracted muscles is largely dependent upon external Na+. The latter observation implies that, when [Ca2+] exceeds the contraction threshold, Ca2+ efflux is mediated primarily by the Na-Ca exchanger, rather than by the sarcolemmal ATP-driven Ca2+ pump. 7. When bovine tail artery was treated with verapamil and phentolamine, and [Na+]o was reduced from 139.2 to 43.9 mM, substitution of K+ for Na+ induced a larger external Ca2+-dependent contraction than did substitution of Tris for Na+. The amplitudes of these contractions were greatly increased when the Na+ pump was inhibited by 5 x 10(-5) M-strophanthidin, presumably because of the rise in [Na+]i.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for sodium-calcium exchange in the guinea-pig ureter

The effects of high-K and low-Na solutions on the smooth muscle of the guinea-pig ureter have been examined in both normal tissues, and tissues in which the Na pump had been blocked by exposure to K-free solutions or ouabain (high-Na tissues). Tension recording, membrane potential measurements and ion analysis were used. High-K solutions depolarize normal tissues, leading to action potential generation and phasic contractions followed, at concentrations greater than 20-30 mM, by cessation of action potentials and the development of a biphasic contracture which declines slowly during continuous exposure. The contracture is abolished by Ca-antagonist drugs, procaine and Ca-free solutions. Short exposures of normal tissues to Na-free solutions do not result in tension development. Longer exposures may initiate tension, depending on the Na substitute used. Sucrose causes depolarization of the cells and spike development associated with phasic contractions, superimposed on a small contracture; Li depolarizes the cells but causes no tension generation; Tris hyperpolarizes the cells and a small increase in basal tone may be seen. On exposure to K-free solutions or ouabain, the tissues do not develop significant tone but their response to short application of high-K solutions grows with time. The tissues also develop the ability to contract on short applications of low-Na solutions. The low-Na contractures are resistant to concentrations of Ca antagonists that abolish the K responses of normal tissues, but are abolished in Ca-free solutions. The ability of the tissues to contract in Na-free solutions is accompanied by an increase in intracellular Na and loss of intracellular K. Even after several hours' exposure to ouabain, however, the tissues still contain significant amounts of K and the membrane potential is the same as, or more negative than that in normal tissues. Therefore it appears that another mechanism, apart from the Na pump, can regulate intracellular Na. On continuous exposure to Na-free solutions, the contracture declines rapidly. The decline is associated with a loss of intracellular Na. The Na-free contracture is larger when K rather than Tris is used as the substitute. This difference persists in the presence of a concentration of Mn that abolishes the K contracture of normal tissues but is abolished by high concentrations (10 mM) of procaine.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence of a shared mechanism of vasoconstriction in pulmonary and systemic circulation in hypertension: a possible role of intracellular calcium

We investigated the hemodynamics of the greater and lesser circulation in 35 patients with primary hypertension, as well as the effects of calcium-channel blockade, to test whether a common factor may account for the excessive vascular resistance in the two circuits and whether intracellular calcium concentration ([Ca++]i) may be involved. We proved that (1) elevated pulmonary arteriolar resistance (PAR) is not related to pulmonary blood flow and volume, pleural pressure, arterial oxygen or carbon dioxide tension and pH, left ventricular filling pressure and function; (2) systemic vascular resistance (SVR) significantly correlates with PAR; (3) calcium-channel blockade with nifedipine reduces systemic and pulmonary arterial pressures toward normal and significantly lowers both SVR and PAR; (4) the percent decrease in vascular resistance after nifedipine is related to the baseline level of resistance in both the greater and the lesser circulations. Failure of the mechanisms currently indicated as responsible for pulmonary vasoconstriction to explain convincingly the increased PAR, the correlation between SVR and PAR, as well as the qualitatively similar response to calcium-channel blockade suggest that a common factor produces vasoconstriction in the two circuits. A pathogenetic role of a primary disorder in [Ca++]i cannot be excluded, but remains to be proved.

Effects of pentobarbital on contractile responses of feline cerebral arteries

The effects of pentobarbital on the contractile responses of isolated feline middle cerebral arteries have been examined. In the presence of pentobarbital (3 x 10(-4)M), the maximum contractions effected by potassium, noradrenaline, and prostaglandin F2 alpha were reduced by 37 +/- 3, 69 +/- 3, and 10 +/- 6%, respectively. The results caution against the use of pentobarbital as an anaesthetic agent in investigations of cerebrovascular reactivity.

Ca2+ movements in smooth muscle

We describe the Ca2+ movements in smooth muscle cells at rest and during activation and relaxation as deduced from transplasmalemmal Ca2+ fluxes and contractile respnses. The general picture which emerges is: the resting cell has a [Ca2+]cyt below 10(-7) M and large gradients are poised across both the cell membrane and intracellular membranes. Excitation opens up Ca2+ channels which are linked to receptors and, if depolarization occurs, to other channels not linked to specific receptors but capable of sensing the membrane potential. Receptor activation also leads to release of Ca2+ from a limited intracellular Ca2+ pool which is superficially located because it has to be refilled from the outside. Relaxation is effected by Ca2+ accumulation by another intracellular Ca2+ pool, very likely sarcoplasmic reticulum, which does not release Ca2+ during activation. The sarcoplasmic reticulum Ca2+ pump can also decrease initial activation of the myofilaments. Elevation of cAMP levels may inhibit contraction by stimulating the sarcoplasmic reticulum Ca-ATPase. An enormous amount of research is still required to prove the above scheme and to localize and quantitate the various intracellular Ca2+ pools.

Acetaldehyde on vascular smooth muscle: possible role in vasodilator action of ethanol

Previous studies on intact and isolated blood vessels indicate that ethanol can exert depressant actions on vascular smooth muscle. This study, using isolated rat aortic strips and portal veins, was designed to ascertain whether acetaldehyde (ACT), a major metabolite of ethanol, could exert similar effects. The results indicate that ACT can: (a) inhibit spontaneous mechanical activity and lower baseline tension in aortic strips; (b) depending upon concentration, enhance (abolished by phentolamine) or inhibit such spontaneous contractions in portal veins; (c) dose-dependently attenuate contractions induced by epinephrine, vasopressin, serotonin and KCl; (d) cause non-competitive displacement of the contraction--effect curves of these vasoactive compounds; (e) relax drug-induced contractions of aortic and venous smooth muscle, (f) attenuate Ca2+-induced contractions of K+-depolarized aortas and portal veins. These profound depressant actions of ACT are not attenuated, prevented or mimicked by alpha-adrenergic histaminergic, cholinergic, or serotonergic blocking drugs, nor are they attributable to actions on beta-adrenoreceptors, or release of prostaglandin-like substance. The direct vasodepressant actions of ACT on vascular smooth muscles may play significant roles in alcohol-induced peripheral vasodilatation and hypotension, and cardiovascular collapse noted in the alcohol-Antabuse reaction.

Possible role of endogenous catecholamines in the contractions induced in rabbit aorta by ouabain, sodium depletion and potassium depletion

Effects of ouabain, Na+ depletion and K+ depletion on the tension development of rabbit aorta were studied. Contraction in the strips was induced by ouabain and Na+-free solution substituted with Li+, tris or sucrose. The contractions were rarely seen under alpha-adrenergic blockade, in reserpinized aorta and mechanically denervated aorta. Contractions induced by norepinephrine were potentiated by Na+ depletion, K+ depletion and ouabain. Contractions induced by high K+ solution were also potentiated by Na+ depletion and ouabain. The potentiation in Li+ solution, K+-free solution and with ouabain was not seen under alpha-adrenergic blockade, in reserpinized aorta and denervated aorta although the potentiation by tris and sucrose solutions was still observed. These data suggest that Na+ depletion, K+ depletion and ouabain release endogenous catecholamines which affect the contraction of the vascular smooth muscle of rabbit aorta.

Sodium ions, calcium ions, blood pressure regulation, and hypertension: a reassessment and a hypothesis

An attempt is made to elucidate the cellular mechanisms which may account for the well-documented correlation between sodium metabolism and peripheral vascular resistance. As a starting point, the evidence that the Na electrochemical gradient across the vascular smooth muscle cell plasma membrane (sarcolemma) plays an important role in cell calcium regulation is reviewed. Because there is significant resting tension ("tone") in most resistance vessels, the ionized Ca2+ level ([Ca2+]1) in the smooth muscle fibers in these vessels must be maintained above the contraction threshold. Consequently, the Ca transport system in the sarcolemma, presumably an Na-Ca exchange mechanism, must be set so as to hold [Ca2+]1 at this suprathreshold level. Any change in the Na gradient will then be reflected as a change in [Ca2+]1 and, therefore, in steady vessel wall tension and peripheral resistance. The correlation between Na metabolism and hypertension could then be accounted for if a circulating agent, perhaps the "natriuretic hormone," affects the Na gradient (across the sarcolemma) and, therefore, [Ca2+]1 and tension.

Influences of aminophylline and reduction in external Na on the antispasmodic action of isoproterenol in the isolated rat rectum

Effects of aminophylline and the reduction in external Na on the antispasmodic action of isoproterenol were investigated in relation to the mobilization of Ca in the isolated rat rectum. The antispasmodic action of isoproterenol on the phasic contractions by acetylcholine and K both in Ca-free and in Ca-free and Na-poor media was potentiated by treatment with aminophylline, however the antispasmodic action was attenuated by reducing Na in the Ca-free medium. Dibutyryl cyclic AMP inhibited media and the inhibitory action was also potentiated by treatment with aminophylline, while the inhibitory action of dibutyryl cyclic AMP was attenuated by reducing Na in the Ca-free medium. From these findings, it appears that isoproterenol inhibits the release of Ca from storage sites induced by acetylcholine and K via the increase of intracellular cyclic AMP content and that the external Na may play an important role in the Ca release-inhibiting effect of cyclic AMP.

Relative contribution of superficially bound and extracellular calcium to activation of contraction in isolated rat portal vein

The spontaneous electrical and mechanical activity of the isolated rat portal vein is abolished after only 2-3 min in nominally Ca-free medium, and after 5-6 min there is no contractile response to depolarizing (122 mM K+), Ca-free solution. In the present study we have examined the electrical and mechanical responses of the portal vein to depolarization with simultaneous readministration of Ca2+ (2.5 mM) after periods of variable length in Ca-free standard solution. After 30 to 60 min of Ca depletion a slow contracture occurred in response to the high-K+ solution with 2.5 mM Ca2+. When the period in Ca-free medium was reduced below 30 min an early, faster phase appeared in the contracture response, and this phase was more rapid the shorter the time of Ca depletion. It is suggested that the slow contracture obtained after 30 min or more uses mainly extracellular Ca for activation and that the faster phase seen after shorter periods of Ca depletion is due to release of superficially bound Ca. This latter pool of tissue bound Ca does not alone produce contraction in response to depolarization, suggesting that extracellular Ca is required to trigger the release perhaps through a regenerative process.

Effects of adrenaline, angiotensin and calcium on spontaneously active and potassium-depolarized rat portal veins

1) Adrenaline and angiotensin increased both frequency and developed tension of spontaneously active rat isolated portal veins. Adrenaline 10(-7) M and angiotensin 10(-9) M, produced a maximal increment of the amplitude of twitch contraction. Greater concentrations elicited a tetanic state, which in all cases had the same amplitude as the maximal effects obtained on single twitches. This suggests that there exists a dissociation between membrane and electrical phenomena, so that the highest intensity of the contractile system activity is attained at agonist concentrations lower than those which result in maximal decrease of membrane stability. (2) Increasing K to a 106 mM final concentration caused a persistent contracture. Adrenaline and angiotensin exhibited pharmaco-mechanical properties, as later addition of both substances produced an increase of the contracture tension. (3) Ca dose-response curves were performed on previously Ca-depleted veins, after the addition of K 106 mM, K-plus-adrenaline or K-plus-angiotensin. Both substances made the Ca responses rise steeply, as evidenced by the leftward shift of ED50. This provides support to the belief that pharmacomechanical coupling is mediated through an increase of the membrane permeability to Ca.

Influences of sodium and calcium on the recovery process from potassium contracture in the guinea-pig taenia coli

1. In the guinea-pig taenia coli, influences of Na and Ca ions on the recovery process from the K contracture were investigated. In the absence of Na ion (sucrose-Krebs solution), the K contracture did not recover when the external K (143 mM) was returned to the normal concentration (5.9 mM), although the membrane was repolarized to normal resting potential.2. After reducing the external K concentration to normal, the addition of Na rapidly terminated the contracture. About 5 mM-Na was enough to produce the relaxation, but the rate of relaxation was slower the lower the Na concentration.3. Lithium could substitute for Na in the relaxation, but Tris-hydroxymethyl aminomethane could not. The possibility of a chloride contribution was excluded.4. Ouabain (2 x 10(-6) g/ml.) and K removal reduced the rate of relaxation by Na ion only slightly. Lowering the temperature also had a small effect, having a Q(10) of about 1.4. Therefore, the Na-K pump may not be involved in this process, but a physical process seems responsible.5. The contracture in K-Krebs solution and in sucrose-Krebs solution was dependent on the external Ca concentration suggesting a high Ca permeability of the membrane. When sucrose was isosmotically replaced with Mn, Mg, La or Ca ions the relaxation was produced with a relatively fast speed in the absence of external Na ions.6. These results may be explained by assuming that external Na ions are involved in decreasing the Ca permeability of the membrane and in reducing the intracellular Ca concentration by Na-Ca exchange, energy for which is supplied by Na influx. In the relaxation by polyvalent cations, suppression of the Ca permeability is probably the main factor.