Na-free Contracture Research Articles

Caffeine stimulates the reverse mode of Na/Ca2+ exchanger in ferret ventricular muscle.

This study investigated the effect of caffeine on the sarcolemmal mechanisms involved in intracellular calcium control. Ferret cardiac preparations were treated with ryanodine and thapsigargin in order to eliminate the sarcoplasmic reticulum (SR) function. This treatment abolished caffeine contracture irreversibly in normal solution. The perfusion with K-free medium that blocked the Na+--K+ pump resulted in a recovery of slow relaxing caffeine contractures similar to Na-free contractures. The amplitude of caffeine contractures was dependent on the bathing [caffeine]o and [Ca2+]o. Divalent cations Ni2+ and Cd2+, which have an inhibitory effect on the Na+/Ca2+ exchanger, produced dose-dependent inhibition of caffeine responses with apparent Ki of 780 +/- 19 and 132 +/- 5 microM, respectively. Caffeine also caused dose-dependent inhibition of Na-free contractures (Ki=4.62 +/- 1.5 mM), and the reduction or removal of [Na+]o exerted an inhibitory effect on caffeine contractures (Ki=73.5 +/- 17.12 mM). These experiments indicate that the increase in resting tension following exposure to caffeine was mediated by Na+/Ca2+ exchanger, which represents an additional element of complexity in caffeine action on cardiac muscle.

O-526 - Depression in Na-free contracture in aorta isolated from SHR chronically treated with SM-6586

O-526 - Depression in Na-free contracture in aorta isolated from SHR chronically treated with SM-6586

Are Ba 2+ and Sr 2+ ions transported by the Na +Ca 2+ exchanger in frog atrial cells?

Ba 2+ and Sr 2+ ions are widely used to replace Ca 2+ ions for the study of Ca 2+ channel currents in electrophysiological experiments. Using the double sucrose gap technique, we investigated the effects of Sr 2+ and Ba 2+ ions on the Na +Ca 2+ exchange activity in frog atrial fibres where it is the major relaxation mechanism. With either Sr 2+ or Ba 2+ ions instead of Ca 2+ in the extracellular bath, Na-free contractures reversibly developed but with different kinetics. Voltage clamp experiments showed that the tonic tension recorded in the presence of Sr 2+ or Ba 2+ was markedly increased following the addition of monensin, a Na + ionophore known to increase the intracellular Na + activity. In Na-free solutions (Li-substituted), it was possible to induce contractures by substituting Sr 2+ or Ba 2+ ions for extracellular Ca 2+. These contractures could be relaxed by reintroducing Na + or Ca 2+ ions in the extracellular medium. Taken together, these results suggest that Sr 2+ and Ba 2+ ions can interact with the Na +Ca 2+ exchange mechanism and potentially participate not only in Na +-cation but also in Ca 2+-cation exchanges on either side of the sarcolemmal membrane.

Contracture caused by sodium removal in the pregnant rat myometrium.

The contracture produced by removal of the external Na was studied in the pregnant rat myometrium and this was compared with that of the non-pregnant myometrium. The Na-free contracture was small compared with spontaneous phasic contractions in the circular muscle of non-pregnant rat, but the contracture increased during pregnancy. On the other hand, in the longitudinal muscle, the contracture was large in both non-pregnant and pregnant myometria. The change in circular muscle did not seem to result from an increased sensitivity to the external Ca. Since the contracture induced by 40 mM K had properties similar to those of the Na-free contracture, it is likely that weak tension development in non-pregnant circular muscle is either due to a limited Ca influx or due to a low efficiency of the excitation-contraction coupling for the sustained contraction compared with the phasic contraction. A small contraction which was independent of the external Ca was observed in Na-free solution in the both layers of non-pregnant myometrium, but this was not observed in the longitudinal muscle of pregnant myometrium.

Characterization of oscillations of intracellular calcium concentration in ferret ventricular muscle.

The photoprotein aequorin was injected into superficial cells of ferret papillary muscles. Tension and aequorin light (a function of intracellular [Ca2+]) were monitored. Increasing intracellular Ca concentration ([Ca2+]i), either by decreasing extracellular Na, or by inhibiting the Na pump with strophanthidin, produced spontaneous oscillations of [Ca2+]i and tension. Fourier analysis showed that these oscillations had frequencies of up to 3-4 Hz. If the muscle was stimulated in these conditions the Ca transient associated with the twitch was followed by a series of damped oscillations of [Ca2+]i which were accompanied by after-contractions. Under a given set of conditions the frequency of the stimulated oscillations was similar to that of the spontaneous oscillations. Manoeuvres which increase [Ca2+]i increased the frequency of both spontaneous and stimulated oscillations. Drugs which inhibit the function of the sarcoplasmic reticulum (caffeine and ryanodine) abolished both stimulated and spontaneous oscillations. The spontaneous oscillations during a Na-free contracture were unaffected by the Ca channel blocker D-600. When repetitive stimulation was begun the frequency and magnitude of the stimulated oscillations increased over several minutes. Increasing the frequency of stimulation increased the magnitude of the stimulated oscillations. It is concluded that the spontaneous oscillations of [Ca2+]i may be due to oscillatory Ca release from the sarcoplasmic reticulum. The similar properties of the spontaneous and stimulated oscillations suggest that the latter may be due to a synchronization of the former.

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)

Potential and tension changes induced by sodium removal in dog Purkinje fibres: role of an electrogenic sodium-calcium exchange.

1. Isolated dog Purkinje fibres were bathed in K-free media or in the presence of ouabain 10(-4) M in order to depress the electrogenic sodium pump activity. Membrane potential and mechanical tension were recorded in the presence of normal external sodium concentration and during lowering or removal of external Na. 2. Lowering or removal of external Na (Na being replaced by choline, Tris, sucrose or Li) induced a hyperpolarization and a contracture which reached a maximum after 1 or 2 min and then decreased progressively. Using Tris, Em increased from -40 +/- 3 to -72 +/- 10 mV (n = 39). The Na-free contracture and hyperpolarization did not occur in the absence of Na pump depression. 3. Tetrodotoxin (1.2 x 10(-5)M), Mn (4 mM), verapamil (1-4 x 10(-5) M) tetraethylammonium (5 mM), 4-aminopyridine (5 mM) and Cs (20 mM, in the presence of ouabain) did not alter the Na-free contracture and hyperpolarization. On the other hand Mn (20 mM), acid media (external pH less than 6.0) and low temperatures depressed or suppressed both the hyperpolarization and contracture. Lanthanum (0.4 mM) did not suppress the hyperpolarization and the contracture. On the contrary the Na-free contracture was generally increased in the presence of La. 4. Caffeine (10 mM) induced strong contractures with no changes in Em, thus demonstrating the possibility for the Purkinje fibers of developing contractures without concomitant hyperpolarizations. 5. It can be concluded that the Na-free contracture and hyperpolarization are not due to changes in passive conductances but are related to the functioning of an electrogenic Na-Ca exchange mechanism which carries inwardly 1 Ca and outwardly 3 or more Na.

Sodium-calcium exchange in regulation of cardiac contractility. Evidence for an electrogenic, voltage-dependent mechanism.

The origin and regulatory mechanisms of tonic tension (Ca current-independent component of contractility) were investigated in frog atrial muscle under voltage-clamp conditions. Tonic tension was elicited by depolarizing pulses of 160 mV (Em = +90 mV, i.e., close to E ca) and 400--600 ms long. An application of Na-free (LiCl) or Ca-free Ringer's solutions resulted in a fast (less than 120 s), almost complete abolition of tonic tension. When [Na]o was reduced (with LiCl or sucrose as the substitutes), the peak tonic tension increased transiently and then decreased below the control level. The transient changes in tonic tension were prevented by using low-Na, low-Ca solutions where the ratios [Ca]0/[Na]40 to [Ca]o/[Na]4o were kept constant (1.1 X 10(-8) mM-3 to 8.7 X 10(-13) mM-5). Na-free (LiCl) solution elicited contractures accompanied by a membrane hyperpolarization or by an outward current even when the Na-K pump was inhibited. 15 mM MnCl2 (or 3 mM LaCl3) inhibited the development of the Na-free contracture and the related part of hyperpolarization or the outward current. In conclusion, our results indicate that tonic tension is regulated by a Na-Ca exchange mechanism. Furthermore, they suggest that this exchange could be electrogenic (exchanging three or more Na ions for one Ca ion) and thus voltage dependent. The possible contribution of an electrogenic Na-Ca exchange in the maintenance of cardiac membrane potential is discussed.

The effect of sodium and magnesium and their interaction with quinine and lanthanum on spontaneous activity and related calcium movements of rat ileal smooth muscle

1. Na-free media, with or without atropine, induced tonic contractures and enhanced spontaneous contractions of the rat ileum. 2. Quinine and lanthanum did not significantly inhibit these Na-free contractures, and neither agent, nor Na-free saline itself, caused any significant net gain of tissue calcium. 3. Na-free media enormously stimulated calcium efflux from the ileum whether atropine was present or not, suggesting a high tissue calcium turnover due to stimulation of surface transport sites. Quinine and lanthanum had no significant effect on this calcium efflux. 4. Mg-free media increased tension in the ileum and enhanced spontaneous contractions while high-Mg media inhibited contractility. 5. Mg-free media were without significant effect on calcium efflux from the ileum but they did induce a very small rise in tissue calcium. 6. These results are not consistent with the concept of a simple Na/Ca membrane counter exchange system in the ileum to regulate tissue calcium. On present evidence, a linked counter exchange system of Ca with Mg and Na would seem more likely.

Enhancement of Na-free contractures in mammalian cardiac fibres by ouabain

Enhancement of Na-free contractures in mammalian cardiac fibres by ouabain

A study of the contractures induced in frog atrial trabeculae by a reduction of the bathing sodium concentration.

1. The relationship between the [Ca](o), the [Na](o) and the strength of the contracture evoked when the [Na](o) is reduced has been investigated in isolated frog atrial trabeculae.2. The strength of the contracture varies by the [Ca](o) (2) and by 4 radical([Na](o)) over the lower tension range.3. The contracture induced by reduction of [Na](o) is not sustained, but relaxes spontaneously. The rate of this relaxation is only dependent on the [Na](o) is the presence of strophanthidin.4. After the spontaneous relaxation of an Na-free contracture, the ability of the trabecula to develop tension upon a second challenge with Na-free solution returns in about 3 min if the muscle is perfused with Na-containing fluid. This recovery process is slowed if the [Na](o) is low during the recovery period, but the recovery is hastened by electrical stimulation of the preparation or by perfusion with K-free or strophanthidin containing sodium-Ringer.5. It is suggested that the influx of Ca(2+) which induces the Na-free contracture depends on the presence of Na(+) inside the cells. When the intracellular Na concentration falls, the Ca influx falls, and the muscle relaxes as a result of the activity of an intracellular relaxing structure.

The effect of adrenaline on the tension developed in contractures and twitches of the ventricle of the frog.

1. The effect of adrenaline on contracture and twitch tension in frog's ventricle has been examined, using the superfused preparation.2. In 1 mM-Ca Ringer, contractures induced with excess KCl concentrations from 50 to 200 mM, are reduced by 1 x 10(-6) g/ml. adrenaline to an average of 0.62 of control values, in marked contrast to the well known positive inotropic effect of adrenaline on the heart twitch. This effect of adrenaline is directly dose dependent. Increasing [Ca](o) diminishes the effect of adrenaline on contracture tension, and on the twitch tension.3. Adrenaline has a significantly greater effect on the KCl contracture tension than noradrenaline or isoprenaline.4. In 1 mM-Ca Ringer, Na-free contractures are reduced to 0.72 of controls by 1 x 10(-6) g/ml. adrenaline. Adrenaline also significantly reduces tension in contractures induced by 50 c/s alternating current.5. The action of adrenaline on contracture tension is largely complete in 1-2 min at various rates of stimulation and calcium concentrations. A similar time course has been found for the effect of adrenaline on membrane potential.6. Pronethalol blocks the action of adrenaline on both twitch and contracture. The action on the contracture can also be blocked by ouabain (1 x 10(-5)M), and exposure of the tissue to K-free or Na-free Ringer solution.7. Adrenaline hyperpolarizes the membrane potential with a range of [K](o) from 0 to 200 mM. This effect is blocked by pronethalol and ouabain. After exposure to ouabain, adrenaline causes a significant decrease in the membrane potential. This may be due to an increase in the sodium permeability.8. At low values of the [Ca]/[Na](2) ratio, adrenaline takes a relatively constant number of beats for full action, but at high values of the ratio the development of full effect is largely time dependent.9. The time course of the effect on the twitch of changing from 0.5 to 2 mM-Ca Ringer has been studied at various rates of stimulation. The equilibration time has been found to depend on the heart rate.10. The effect on the contracture suggests that adrenaline decreases the calcium permeability. It is further suggested that the development of twitch tension is not due to direct Ca entry but is due to the release of Ca from a local store within or between the cells. The inotropic action of adrenaline is explained in terms of this store.