Intracellular Sodium Ion Activity Research Articles

Action-Potential Duration and Contractility in Canine Cardiac Tissues: Action of Inotropic Drugs

1. Inotropic and electrophysiologic effects of veratrine, vesnarinone, d-sotalol and tetraethylammonium (TEA) were compared. Action-potential duration (APD) and contractility were measured in isolated canine Purkinje fiber and ventricular trabecular muscle preparations by using standard microelectrode techniques. Each drug significantly increased APD and force development in either tissue. 2. Drug-induced increases in force development were normalized to increases in APD. The order of efficacy was found to be vesnarinone>veratrine>TEA in ventricular myocardium, whereas it was veratrine>>vesnarinone= d-sotalol=TEA in Purkinje fibers. 3. The force–APD relation was linear for all drugs in the concentrations used. 4. Simultaneous measurements of APD, force development and intracellular sodium ion activity (a i Na) in the presence of either veratrine or lidocaine indicated a linear relation between force development and changes in a i Na. 5. The relation between APD and force development was different in ventricular and Purkinje fiber preparations. Differences in the veratrine sensitivity of the force–APD relation observed between Purkinje and ventricular preparations suggest that a i Na-dependent changes in Na +/Ca 2+ exchange may play a more important role in regulation of force generation in Purkinje fibers than in ventricular myocardium.

Effects of halothane on intracellular sodium ion activity under condition of sodium pump inhibition in dog Purkinje fibers

Effects of halothane on intracellular sodium ion activity under condition of sodium pump inhibition in dog Purkinje fibers

Acceleration of H+ extrusion via Na(+)-H+ exchange in guinea-pig ventricular papillary muscle under intracellular acidic condition.

We investigated mechanisms by which intracellular pH was regulated under intracellular acidic condition in resting guinea-pig ventricular papillary muscles in vitro. Intracellular sodium ion activity (aiNa), intracellular and surface pH (pHi and pHs) were measured with Na(+)- and H(+)-selective microelectrodes and resting tension was measured. By exposure to 0 mM K solution aiNa and resting tension increased progressively while pHi decreased but reached the steady level of pH 6.95. pHs which was lower than external bulk pH (pHo) decreased progressively by exposure to 0 mM K solution. In 4 mM K solution, amiloride (1 mM), an inhibitor of Na(+)-H+ exchange, induced a reversible decrease in both aiNa and pHi, and an increase in pHs. Changes in pHi and pHs induced by application of amiloride in 0 mM K solution were larger than those in 4 mM K solution. The rate of decrease in pHi induced by amiloride became larger at longer exposure to 0 mM K solution. Lowering pHo from 7.4 to 6.4 induced a larger decrease in pHi in 0 mM K solution than that in 4 mM K solution. Lowering pHo from 7.4 to 5.4 reversed the difference between pHs and pHo. These results suggest that in guinea-pig papillary muscle, Na(+)-H+ exchange is active to regulate intracellular H+ under resting condition and under intracellular acidic condition, H+ extrusion via the Na(+)-H+ exchange would be accelerated not only by the net thermodynamic driving force for Na+ and H+ but also by other factors.

Acidification and intracellular sodium ion activity during stimulated myocardial ischemia

With the use of microelectrodes, intracellular pH (pHi), surface pH (pHs), and intracellular Na+ activity (aiNa) were measured in isolated guinea pig papillary muscles during normal superfusion and during a reversible condition of simulated ischemia. Acid loading by NH+4 prepulse or by CO2-HCO3- addition during superfusion with pH 7.4 solutions caused internal acidification followed by a recovery of pHi, which could be inhibited by amiloride. pHi recovery was associated with an amiloride-sensitive peak rise of aiNa and membrane hyperpolarization, indicative of Na(+)-H+ exchange. Peak increase of aiNa was absent if the pH of the superfusion solution was concomitantly lowered. Imposed ischemia after control superfusion caused membrane depolarization and acidification of pHi and pHs. The change of pHs consistently was larger than that of pHi. aiNa decreased from 5.5 to 4.6 mM after 10-min ischemia. Enlarging the pHi (and pHs) decrease in ischemia by prior reduction of the tissue buffer capacity (CO2-HCO3(-)-free superfusion) was unable to induce a rise of aiNa during the subsequent ischemic period. Amiloride had no significant effect on aiNa during ischemia. It is concluded that the important acidification of pHs reduces the rate of pHi regulatory Na(+)-H+ exchange and thereby contributes to a longer maintenance of the Na+ electrochemical gradient in ischemic cardiac muscle.

Role of Na+/H+ exchange in the control of intracellular pH and cell membrane conductances in frog skin epithelium.

Ion-sensitive microelectrodes and current-voltage analysis were used to study intracellular pH (pHi) regulation and its effects on ionic conductances in the isolated epithelium of frog skin. We show that pHi recovery after an acid load is dependent on the operation of an amiloride-sensitive Na+/H+ exchanger localized at the basolateral cell membranes. The antiporter is not quiescent at physiological pHi (7.1-7.4) and, thus, contributes to the maintenance of steady state pHi. Moreover, intracellular sodium ion activity is also controlled in part by Na+ uptake via the exchanger. Intracellular acidification decreased transepithelial Na+ transport rate, apical Na+ permeability (PNa) and Na+ and K+ conductances. The recovery of these transport parameters after the removal of the acid load was found to be dependent on pHi regulation via Na+/H+ exchange. Conversely, variations in Na+ transport were accompanied by changes in pHi. Inhibition of Na+/K+ ATPase by ouabain produced covariant decreases in pHi and PNa, whereas increases in Na+ transport, occurring spontaneously or after aldosterone treatment, were highly correlated with intracellular alkalinization. We conclude that cytoplasmic H+ activity is regulated by a basolateral Na+/H+ exchanger and that transcellular coupling of ion flows at opposing cell membranes can be modulated by the pHi-regulating mechanism.

Intracellular sodium ion activity: reliable measurement and stimulation-induced change in cardiac Purkinje fibers.

Recently Na+-selective microelectrodes (NaSM) have been used to measure quantitatively small changes in intracellular sodium ion activity (aiNa) and to determine a precise time course of comparatively rapid change in aiNa. In such studies, accurate measurement of aiNa requires the following criteria: (i) NaSM should have a fast response time and (ii) an NaSM and a conventional voltage microelectrode should measure the same membrane potential. These criteria were evaluated by measuring aiNa when membrane potential of cardiac Purkinje fibers was suddenly hyperpolarized and depolarized by changing stimulation rate. The NaSM coated with a conductive silver paint had fast response times so that rapid changes in aiNa could be reliably measured. The cardiac Purkinje fibers stimulated at a constant rate generated uniform membrane voltage and the NaSM and conventional microelectrode measured virtually the same membrane potential. This result is somewhat different from that reported under voltage-clamp condition by other investigators. The aiNa of the fibers increased as the stimulation rate was increased over the range of 0.5-3 Hz. In fibers stimulated at 1 Hz, cessation of stimulation was immediately followed by an exponential decline of aiNa with an average time constant of 53 +/- 9 s (SD, n = 8), or rate constant of 0.020 +/- 0.004/s. Restimulation of the fibers produced an exponential rise of aiNa with an average time constant of 65 +/- 12 s (n = 8). Similar results were obtained in fibers stimulated at 2 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)

The ionic basis of the anti-ischemic and anti-arrhythmic properties of magnesium in the heart.

The role of magnesium (Mg) in the prevention of ischemia-induced injury during cardioplegic arrest and in the treatment of cardiac arrhythmias has been considered. Although Mg possesses negative inotropic properties, potassium (K) is more effective than Mg in inducing cardiac arrest. The rationale for the inclusion of Mg in cardioplegic solutions therefore lies not in its cardioplegic properties, but in its ability to influence other cellular events such as the loss of Mg and K and perhaps to counter the detrimental effects of ischemia by antagonizing calcium (Ca) overload. Most of the Mg in the cardiac cell is complexed with high energy phosphate compounds and the loss of Mg during ischemia may restrict the repletion of ATP upon reperfusion and so impair the return of normal contractile function. The ability of Mg to limit K efflux from the cell is of importance not only in the prevention of ischemia-induced K loss but also in the treatment of digitalis-induced arrhythmias. Elevation of extracellular Mg has been shown to reduce the intracellular sodium ion activity ([Na]i) and this decline in [Na]i can be related to the negative inotropic properties of Mg. Mg may therefore exert some of its antiarrhythmic and antiischemic effects by limiting [Na]i-stimulated Ca influx (or facilitating Ca efflux) and hence preventing cellular Ca overload.

Intracellular K+ activity, intracellular Na+ activity and maximum diastolic potential of canine subendocardial Purkinje cells from one-day-old infarcts.

The basis for the reduced maximum diastolic potential of canine cardiac subendocardial Purkinje fibers surviving one day after extensive transmural infarction was investigated, using double-barrel potassium and sodium ion-sensitive microelectrodes. The maximum diastolic potential of Purkinje fibers in infarct preparations from the left ventricular apex measured during the first hour of superfusion in a tissue bath was -50.1 +/- 13.7 mV, a value markedly reduced from the value in control Purkinje fibers from noninfarcted preparations (-85.0 +/- 4.5 mV). The intracellular potassium ion activity was reduced by 50.4 mM during this time (intracellular potassium ion activity equals 61.6 +/- 16.1 mM, as compared to control intracellular potassium ion activity of 112 +/- 19.8 mM). The potassium equilibrium potential was reduced by 16.0 mV (from -97.2 +/- 4.7 mV in controls to -81.2 +/- 6.9 mV), thus accounting for about one half of the reduction in the maximum diastolic potential. After 6 hours of superfusion, the maximum diastolic potential increased to -78.9 +/- 8.7 mV (still significantly less than control). The potassium equilibrium potential had largely recovered (-93.8 +/- 5.9 mV). The intracellular sodium ion activity of Purkinje fibers in the infarcts (15.6 +/- 6.9 mM) was elevated during the first hour of superfusion by 6.2 mM compared to control (9.4 +/- 2.6 mM), and this was only 12% as much as the initial intracellular potassium ion activity decrease. Sodium ion activity after 3-6 hours of superfusion was not significantly different than normal (12.1 +/- 4.9 mM). In conclusion, only a portion of the maximum diastolic potential changes can be explained by a reduction of the potassium equilibrium potential. It is likely that change(s) in the cell membrane sodium-potassium pump's function and in the membrane conductance are also involved. Furthermore, the lack of a compensatory increase in intracellular sodium ion activity accompanying the large reduction of intracellular potassium ion activity may be a consequence of the cellular acidosis, which is known to occur during myocardial ischemia.

O-145 - Effects of extracellular potassium and pH on intracellular sodium ion activity of guinea-pig cardiac muscle

O-145 - Effects of extracellular potassium and pH on intracellular sodium ion activity of guinea-pig cardiac muscle

Effect of norepinephrine and cyclic AMP on intracellular sodium ion activity and contractile force in canine cardiac Purkinje fibers.

The effect of norepinephrine on the Na+-K+ pump was investigated by simultaneously measuring intracellular sodium ion activity (aiNa) and contractile force of canine cardiac Purkinje fibers driven at 1.0 Hz in K+-free solution, high K+ solution, and in the presence of tetrodotoxin. In Tyrode solution containing 5.4mM [K+]o, 10(-6) M norepinephrine decreased aiNa, whereas in K+-free solution 10(-6) M norepinephrine did not lower aiNa. 16.2 mM [K+]o decreased aiNa from 8.8 +/- 0.9 mM to 6.5 +/- 0.5 mM (mean +/- SD, n = 5). Exposure to 10(-6) M norepinephrine in the presence of high [K+]o further decreased aiNa by 0.7 +/- 0.4 mM. This further decrease was prevented by exposure to 2.5 X 10(-6) M strophanthidin (n = 4). Blockade of the fast sodium channel with 5 X 10(-6) M tetrodotoxin lowered aiNa from 8.5 +/- 1.3 mM to 7.4 +/- 1.1 mM (n = 4). Exposure to 10(-6) M norepinephrine in the presence of tetrodotoxin further lowered aiNa by 0.9 +/- 0.2 mM. We also studied the effects of the analogues of adenosine 3':5'-cyclic monophosphate, N6, 2'-0-dibutyryladenosine 3':5'-cyclic monophosphate, and 8-(4-chlorophenylthiol)-adenosine 3':5'-cyclic monophosphate on aiNa and twitch tension. Both analogues lowered to aiNa and increased twitch tension mimicking the effects of norepinephrine. Our results support the hypothesis that norepinephrine lowers aiNa by stimulating the Na+-K+ pump in this tissue. This stimulation appears to be mediated by adenosine 3':5'-cyclic monophosphate and does not appear to be due to intercellular K+ accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of intracellular sodium and pH by the electrogenic H+ pump in frog skin

We have investigated the role of the electrogenic hydrogen ion pump in the regulation of intracellular sodium ion activity (aiNa) and intracellular pH (pHi) in frog skin epithelial cells using double-barreled ion sensitive microelectrodes. When Rana esculenta skin is mounted in an Ussing chamber and bathed in 1 mM Na2SO4 buffered to pH 7.34 with imidazole on the apical side and in normal Ringer on the serosal side, the apical addition of the carbonic anhydrase inhibitor ethoxzolamide (10(-4)M) blocks net H+ ion excretion and Na absorption, producing a depolarization of 25-30 mV of the apical membrane potential (psi mc). We demonstrate that these changes are accompanied by a fall in aiNa from 6.2 +/- 0.5 mmol/l to 3.4 +/- 0.6 mmol/l and an increase in pHi from 7.20 +/- 0.03 to 7.38 +/- 0.08 (n = 12 skins). Voltage clamping psi mc to its control value in the presence of ethoxzolamide restored aiNa but the pHi remained alkaline. Furthermore, the fall in aiNa produced by ethoxzolamide could be mimicked by voltage clamping psi mc towards the value of the Nernst potential for Na at the apical membrane. These results indicate that the maintenance of the cellular Na+ transport pool is dependent on a favourable electrical driving force and counter-current generated by an electrogenic H+ pump at the apical membrane. Addition of amiloride (10(-5) mol/l) or substitution of external Na+ by Mg2+ or K+ caused a hyperpolarization of psi mc and a fall in aiNa.2+ e

Sodium‐selective micro‐electrode study of apical permeability in frog skin: effects of sodium, amiloride and ouabain.

The intracellular sodium ion activity (aiNa), apical membrane potential (psi ac) and apical sodium electrochemical driving force (delta mu Na) in Rana temporaria skin were measured using double-barrelled sodium-sensitive micro-electrodes, in the presence of various apical sodium activities (aoNa), amiloride, ouabain, and during voltage clamp of psi ac. The permeability and specific conductance of the apical cell membrane to sodium entry (PaNa and GaNa respectively) were calculated from the Goldman-Hodgkin-Katz equation and the Nernst-Planck (electrodiffusion) permeability equations respectively. The roles of aoNa and aiNa in the control of apical sodium entry were studied. PaNa increased linearly with log decrease in aoNa between 79 and 0.01 mM. Under short-circuit conditions, aiNa remained constant over the aoNa range of 10-79 mM, but decreased when aoNa was lower than 10 mM, due to a fall in delta mu Na and GaNa. Amiloride decreased PaNa, GaNa and aiNa, a result analogous to that observed in spontaneous low-transporting skins. Ouabain inhibited sodium transport and increased aiNa before any changes in PaNa occurred. The latter decreased only when aiNa rose above 15 mM. Increasing delta mu Na by hyperpolarizing voltage clamp of the apical cell membrane elicited a saturable increase in aiNa. The opposite effect was elicited by depolarizing psi ac. Electrodiffusion appears to be the sole mode of apical sodium entry.

The effects of membrane potential, extracellular potassium, and tetrodotoxin on the intracellular sodium ion activity of sheep cardiac muscle.

The intracellular sodium ion activity was measured using liquid ion-exchange microelectrodes with rapid response times in sheep Purkinje fibers and ventricular muscle under voltage control. The mean sodium ion activity in quiescent Purkinje fibers was 8.5 mM at a holding potential of -80 mV. With maintained hyperpolarizing (-110 mV) or depolarizing (-40 and 0 mV) voltage steps, sodium ion activity increased or decreased, respectively. At 0 mV, the mean steady state value for the sodium ion activity was 3.8 mM. Following a voltage step to 0 mV, or back to -80 mV, the time course of the sodium ion activity change could be fitted by single exponentials, with similar half-times. Increasing the extracellular potassium ion concentration from 5.4 to 15 mM did not alter the steady state value of the sodium ion activity at clamped voltages of -80 or 0 mV, which suggests that the external potassium ion activating site of the Na-K pump was saturated. With the extracellular potassium concentration 0 mM (holding potential -80 mV), the sodium ion activity increased. When maintained depolarizing steps to 0 mV were applied, the sodium ion activity decreased by up to 20 mM. This large fall in sodium ion activity is assumed to represent partial reactivation of the Na-K pump due to potassium ion accumulation in clefts. We also studied the stimulation-dependent change in sodium ion activity. Trains of action potentials or short duration depolarizing voltage clamp steps caused a frequency dependent rise in sodium ion activity. The magnitude of the rise of sodium ion activity was not altered by lengthening the duration of each voltage clamp step, but was inhibited by tetrodotoxin or by holding the membrane potential at -50 mV between depolarizing steps. These results show that sodium ion activity is a complex function of membrane voltage, depolarization frequency, and time. The rise in sodium ion activity with stimulation appears to depend on sodium ion entry regulated by the sodium channel, and may be important in the modulation of intracellular calcium and tension through the Na+-Ca++ exchange mechanism.

The relationship among intracellular sodium activity, calcium, and strophanthidin inotropy in canine cardiac Purkinje fibers.

The role of sodium and calcium ions in strophanthidin inotropy was studied by measuring simultaneously the electrical, mechanical, and intracellular sodium ion activities in electrically driven cardiac Purkinje fibers under conditions that change the intracellular sodium or calcium level (tetrodotoxin, strophanthidin, high calcium, and norepinephrine). Tetrodotoxin (TTX; 1-5 X 10(-6)M) shifted the action potential plateau to more negative values, shortened the action potential duration, and decreased the contractile tension and the intracellular sodium ion activity (aiNa). The changes in tension and in aiNa caused by TTX appear to be related since they had similar time courses. Strophanthidin (2-5 X 10(-7)M) increased tension and aiNa less in the presence of TTX, and, for any given value of aiNa, tension was less than in the absence of TTX. Increasing extracellular calcium (from 1.8 to 3.3-3.6 mM) or adding norepinephrine (0.5-1 X 10(-6)M) increased tension and decreased aiNa less in the presence than in the absence of TTX. When two of the above procedures were combined, the results were different. Thus, during the increase in aiNa and tension caused by strophanthidin in the presence of TTX, increasing calcium or adding norepinephrine increased tension markedly but did not increase aiNa further. In a TTX-high calcium or TTX-norepinephrine solution, adding strophanthidin increased both tension and aiNa, and the increase in tension was far greater than in the presence of TTX alone. The results indicate that: (a) the contractile force in Purkinje fibers is affected by a change in aiNa; (b) a decrease in aiNa by TTX markedly reduces the inotropic effect of strophanthidin, possibly as a consequence of depletion of intracellular calcium; (c) increasing calcium influx with norepinephrine or high calcium in the TTX-strophanthidin solution produces a potentiation of tension development, even if aiNa does not increase further; and (d) when the calcium influx is already increased by high calcium or norepinephrine, strophanthidin has its usual inotropic effect even in the presence of TTX. In conclusion, the positive inotropic effect of strophanthidin requires that an increase in aiNa be associated with suitable calcium availability.

Relation between intracellular sodium and twitch tension in sheep cardiac Purkinje strands exposed to cardiac glycosides.

Changes in intracellular sodium ion activity (aiNa) produced by several cardiac glycosides were correlated with twitch tension in sheep cardiac Purkinje strands. Simultaneous measurements of aiNa and twitch tension were obtained through the use of Na-sensitive intracellular microelectrodes (ETH 227) in Purkinje preparations stimulated at a frequency of 1 Hz. All concentrations of ouabain, acetylstrophanthidin, and actodigin that were tested caused an increase in aiNa immediately before, or coincident with, a positive inotropic effect. No fall in aiNa was observed at any positive inotropic concentration of digitalis in these beating fibers. In all cases, the onset and washout of the positive inotropic effect were paralleled by the rise and fall in aiNa, respectively. No dissociation between changes in aiNa and twitch tension occurred at any concentration of any of the agents used. The relation between changes in aiNa and twitch tension was linear with 1 mM increase in aiNa producing about a 100% increase in the twitch magnitude. Propranolol did not significantly alter this relationship. The increase in aiNa with digitalis was also associated with a reduction in the maximum depolarization rate of the action potential, presumably as a consequence of a reduction in the transmembrane Na electrochemical gradient. These results indicate that the positive inotropic action of digitalis in sheep Purkinje strands is always associated with a rise in aiNa secondary to inhibition of the Na pump. This increase in aiNa could increase calcium available for contraction via the Na-Ca equilibrium exchange process. In addition, the increase in aiNa reduces Vmax, as a consequence of decreasing the electrochemical gradient for Na.

Increase in intracellular sodium ion activity during stimulation in mammalian cardiac muscle.

Changes in stimulation rate alter the electrical and mechanical characteristics of myocardial cells. We have investigated the possibility that intracellular sodium activity (aiNa) changes with stimulation and correlates with changes in contraction strength. Two kinds of liquid membrane Na+-selective microelectrodes were used to measure aiNa in guinea pig and sheep ventricular muscle and in sheep Purkinje strands. Stimulation produced a rate- and time-dependent elevation of aiNa. Small increases in aiNa were seen at stimulation rates as slow as 0.2 Hz, and faster rates of stimulation elevated aiNa by over 30%. The changes seen in Purkinje strands and ventricular muscle were similar. Following a period of stimulation, aiNa and Vm returned to their pre-stimulus levels with the same time courses. This is consistent with the suggestion that the post-stimulation hyperpolarization is the result of an increased rate of electrogenic Na+ extrusion. The effects of stimulation on aiNa and tension were compared with those of ouabain. The comparison suggests that rapid stimulation could produce increased contraction strength as the result of a substantial gain in intracellular calcium via a Na-Ca exchange mechanism, but that this is only one of several factors determining the force-frequency relationship.

Intracellular sodium ion activity and sodium transport in rabbit urinary bladder.

1. Intracellular potentials and the intracellular activities of Na+ and K+ were examined using conventional and ion-selective micro-electrodes. 2. In animals on a normal diet, the intracellular Na+ activity was 8.6 +/- 2.9 mM (mean +/- S.D.) with a mean short-circuit current of 2.8 +/- 0.9 microA/cm2. 3. In animals on a low-Na+ diet, the intracellular Na+ activity was 18.5 +/- 9.9 mM with a short-circuit current of 4.5 +/- 1.3 microA/cm2 (mean +/- S.D.). 4. There was a correlation between short-circuit current and intracellular Na+ activity which could be fitted by a saturating hyperbolic relationship. 5. Treatment of the issue with ouabain and amiloride produced an increase and a decrease, respectively, in the intracellular Na+ activity. 6. Treatment with aldosterone produced a large increase in short-circuit current with a substantial increase in intracellular Na+ activity. 7. Intracellular Na+ activity does not seem to affect apical membrane permeability directly.