Influence of changes in external potassium and chloride ions on membrane potential and intracellular potassium ion activity in rabbit ventricular muscle

Regulation of ion gradients across myocardial ischemic border zones: a biophysical modelling analysis

The myocardial ischemic border zone is associated with the initiation and sustenance of arrhythmias. The profile of ionic concentrations across the border zone play a significant role in determining cellular electrophysiology and conductivity, yet their spatial-temporal evolution and regulation are not well understood. To investigate the changes in ion concentrations that regulate cellular electrophysiology, a mathematical model of ion movement in the intra and extracellular space in the presence of ionic, potential and material property heterogeneities was developed. The model simulates the spatial and temporal evolution of concentrations of potassium, sodium, chloride, calcium, hydrogen and bicarbonate ions and carbon dioxide across an ischemic border zone. Ischemia was simulated by sodium-potassium pump inhibition, potassium channel activation and respiratory and metabolic acidosis. The model predicted significant disparities in the width of the border zone for each ionic species, with intracellular sodium and extracellular potassium having discordant gradients, facilitating multiple gradients in cellular properties across the border zone. Extracellular potassium was found to have the largest border zone and this was attributed to the voltage dependence of the potassium channels. The model also predicted the efflux of [Formula: see text] from the ischemic region due to electrogenic drift and diffusion within the intra and extracellular space, respectively, which contributed to [Formula: see text] depletion in the ischemic region.

Infection by Trypanosoma cruzi enhances anion conductance in rat neonatal ventricular cardiomyocytes

Recent studies on malaria-infected erythrocytes have shown increased anion channel activity in the host cell membrane, increasing the exchange of solutes between the cytoplasm and exterior. In the present work, we addressed the question of whether another intracellular protozoan parasite, Trypanosoma cruzi, alters membrane transport systems in the host cardiac cell. Neonatal rat cardiomyocytes were cultured and infected with T. cruzi in vitro. Ion currents were measured by patch-clamp technique in the whole-cell configuration. Two small-magnitude instantaneous anion currents, outward- and inward-rectifying, were recorded in all noninfected cardiomyocytes. In addition, ~10% of cardiomyocytes expressed a large anion-preferable, time-dependent current activated at positive membrane potentials. Hypotonic (230 mOsm) treatment resulted in the disappearance of the time-dependent current but provoked a dramatic increase of the instantaneous outward-rectifying one. Both instantaneous currents were suppressed by intracellular Mg(2+). T. cruzi infection did not provoke new anion currents in the host cells but caused an increase of the density of intrinsic swelling-activated outward current, up to twice in heavily infected cells. The occurrence of a time-dependent current dramatically increased in infected cells in the presence of Mg(2+) in the intracellular solution, from ~10 to ~80%, without a significant change of the current density. Our findings represent one further, besides the known Plasmodium falciparum, example of an intracellular parasite which upregulates the anionic currents expressed in the host cell.

Measuring and modeling chloride-hydroxyl exchange in the Guinea-pig ventricular myocyte

Protons are powerful modulators of cardiac function. Their intracellular concentration is regulated by sarcolemmal ion transporters that export or import H+-ions (or their ionic equivalent: HCO3-, OH-). One such transporter, which imports H+-equivalents, is a putative Cl-/OH- exchanger (CHE). A strong candidate for CHE is SLC26A6 protein, a product of the SLC26A gene family of anion transporters, which has been detected in murine heart. SLC26A6 protein is suggested to be an electrogenic 1Cl-/2OH-(2HCO3-) exchanger. Unfortunately, there is insufficient characterization of cardiac CHE against which the properties of heterologously expressed SLC26A6 can be matched. We therefore investigated the proton, Cl-, and voltage dependence of CHE activity in guinea-pig ventricular myocytes, using voltage-clamp, intracellular pH fluorescence, and mathematical modeling techniques. We find that CHE activity is tightly regulated by intracellular and extracellular pH, is voltage-insensitive over a wide range (+/-80 mV), and displays substrate dependence suggestive of electroneutral 1Cl-/1OH- exchange. These properties exclude electrogenic SLC26A6 as sole contributor to CHE. Either the SLC26A6 product in heart is electroneutral, or CHE comprises at least two transporters with oppositely balanced voltage sensitivity. Alternatively, CHE may comprise an H+-Cl- coinflux system, which cannot be distinguished kinetically from an exchanger. Irrespective of ionic mechanism, CHE's pH sensitivity helps to define resting intracellular pH, and hence basal function in the heart.

Inward-rectifier K+ current in guinea-pig ventricular myocytes exposed to hyperosmotic solutions

Superfusion of heart cells with hyperosmotic solution causes cell shrinkage and inhibition of membrane ionic currents, including delayed-rectifer K+ currents. To determine whether osmotic shrinkage also inhibits inwardly-rectifying K+ current (I(K1)), guinea-pig ventricular myocytes in the perforated-patch or ruptured-patch configuration were superfused with a Tyrode's solution whose osmolarity (T) relative to isosmotic (1T) solution was increased to 1.3-2.2T by addition of sucrose. Hyperosmotic superfusate caused a rapid shrinkage that was accompanied by a negative shift in the reversal potential of Ba(2+)-sensitive I(K1), an increase in the amplitude of outward I(K1), and a steepening of the slope of the inward I(K1)-voltage (V) relation. The magnitude of these effects increased with external osmolarity. To evaluate the underlying changes in chord conductance (G(K1)) and rectification, G(K1)-V data were fitted with Boltzmann functions to determine maximal G(K1) (G(K1)max) and voltage at one-half G(K1)max (V(0.5)). Superfusion with hyperosmotic sucrose solutions led to significant increases in G(K1)max (e.g., 28 +/- 2% with 1.8T), and significant negative shifts in V(0.5) (e.g., -6.7 +/- 0.6 mV with 1.8T). Data from myocytes investigated under hyperosmotic conditions that do not induce shrinkage indicate that G(K1)max and V(0.5) were insensitive to hyperosmotic stress per se but sensitive to elevation of intracellular K+. We conclude that the effects of hyperosmotic sucrose solutions on I(K1) are related to shrinkage-induced concentrating of intracellular K+.

Hyperosmolar Solutions Selectively Block Action Potentials in Rat Myelinated Sensory Fibers: Implications for Diabetic Neuropathy

Diabetic neuropathy is a common complication of diabetes mellitus patients. It is a wide range of abnormalities affecting proximal and distal peripheral sensory and motor nerves. Although plasma hyperosmolality is a common finding in diabetes mellitus, the effects of hyperosmolality on conduction of various sensory signal components have not been addressed in detail. Here we show that in rat dorsal root ganglion (DRG) preparations from normal rats, hyperosmolar solutions (360 mmol/kg, containing increased glucose, sucrose, NaCl, or mannitol) produce a selective block of signal propagation in myelinated sensory A-fibers. In compound action potential (CAP) recordings with suction electrodes, peak A-fiber CAP amplitude was selectively decreased (20%), while the C-fiber peak remained intact or was slightly increased. Hyperosmolar solutions had smaller effects on conduction velocity (CV) of both A- and C-fibers (approximately 5% decrease). Hyperosmolality-induced CAP changes could not be observed during recordings from isolated spinal nerves but were evident during recordings from desheathed spinal nerves. In intracellular recordings, hyperosmolar solutions produced a block of spinal nerve-evoked action potential invasion into the somata of some A-fiber neurons. Removal of extracellular calcium completely prevented the hyperosmolality-induced CAP decreases. Based on these data, we propose that the decreased CAP amplitudes recorded in human patients and in animal models of diabetes are in part due to the effects of hyperosmolality and would depend on the extracellular osmolality at the time of sensory testing. We also hypothesize that hyperosmolality may contribute to both the sensory abnormalities (paresthesias) and the chronic pain symptoms of diabetic neuropathy.

Osmosensitive properties of rapid and slow delayed rectifier K+ currents in guinea-pig heart cells

1. Changes in cell volume affect a variety of sarcolemmal transport processes in the heart. To study whether osmotically induced cell volume shrinkage has functional consequences for K+ channel activity, guinea-pig cardiac preparations were superfused with hyperosmotic Tyrode's solution (1.2-2-fold normal osmolality). Membrane currents and cell surface dimensions were measured from whole-cell patch-clamped ventricular myocytes and membrane potentials were recorded from isolated ventricular muscles and non-patched myocytes. 2. Hyperosmotic treatment of myocytes quickly (< 3 min to steady state) shrank cell volume (approximately 20% reduction in 1.5-fold hyperosmotic solution) and depressed the delayed rectifier K+ current (IK). Analysis using different activation protocols and a selective inhibitor (5 micro mol/L E4031) indicated that the IK inhibition was due to osmolality and cell volume-dependent changes in the two subtypes of the classical cardiac IK (rapidly activating IKr and slowly activating IKs); 1.5-fold hyperosmotic treatment depressed the amplitudes of IKr and IKs by approximately 30 and 50%, respectively. 3. Superfusion of muscles and myocytes with 1.5-fold hyperosmotic solution lengthened the action potentials by 14-17%. Hyperosmotic treatment also caused 6-7 mV hyperpolarization that is most likely due to a concentrating of intracellular K+. 4. The inhibition of IK helps explain the lengthening of action potentials observed in osmotically stressed heart cells. These results, together with the reported IK stimulation by hyposmotic cell swelling, provide further support for cell volume-sensitive properties of cardiac electrical activity.

Swelling-activated chloride channels in cardiac physiology and pathophysiology

Characteristics and functions of the cardiac swelling-activated Cl current (I(Cl,swell)) are considered in physiologic and pathophysiologic settings. I(Cl,swell) is broadly distributed throughout the heart and is stimulated not only by osmotic and hydrostatic increases in cell volume, but also by agents that alter membrane tension and direct mechanical stretch. The current is outwardly rectifying, reverses between the plateau and resting potentials (E(m)), and is time-independent over the physiologic voltage range. Consequently, I(Cl,swell) shortens action potential duration, depolarizes E(m), and acts to decrease cell volume. Because it is activated by stimuli that also activate cation stretch-activated channels, I(Cl,swell) should be considered as a potential effector of mechanoelectrical feedback. I(Cl,swell) is activated in ischemic and non-ischemic dilated cardiomyopathies and perhaps during ischemia and reperfusion. I(Cl,swell) plays a role in arrhythmogenesis, myocardial injury, preconditioning, and apoptosis of myocytes. As a result, I(Cl,swell) potentially is a novel therapeutic target.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

Swelling-induced decrease in spontaneous pacemaker activity of rabbit isolated sino-atrial node cells

The heart responds to an increase in sino-atrial node wall stress with an augmentation in rate of contraction. It has been suggested that swelling-activated ion channels may play a key role in that response. This paper investigates directly the effects of cell swelling on spontaneous activity of rabbit isolated sino-atrial node pacemaker cells. The main finding is that sino-atrial node cells, studied in current clamp mode using amphotericin-permeabilized patches, decrease their spontaneous pacemaker rate by 24.2 +/- 7.8% (P < 0.01, n = 9) during 75% hyposmotic swelling. This response is opposite to the predicted impact of volume-activation of sarcolemmal ion conductances. Computer modelling (OXSOFT Heart v4.8) suggests that swelling-induced dilution of the cytosol, reduction in intracellular potassium concentration, and decrease in the delayed rectifier potassium current, IK, are leading mechanisms in the response. This is supported by voltage-clamp data that show a swelling-induced positive shift in the reversal potential of IK by between 5 and 10 mV (n = 7) and a reduction in amplitude of its rapidly activating component, IKr, (n = 6). Thus, spontaneously active sino-atrial node cells reduce pacemaking rate during swelling. This response cannot be explained by the known volume-activated sarcolemmal ion conductances, but appears to be dictated by other mechanisms including dilution of the cytosol and reduction in IK. The results re-enforce the view that cardiac responses to cell volume changes may be quite different from those to longitudinal stretch.

Membrane currents underlying the modified electrical activity of guinea-pig ventricular myocytes exposed to hyperosmotic solution

1. Guinea-pig ventricular myocytes were superfused with hyperosmotic (sucrose) Tyrode solution (1.2-2.8 times (T) normal osmolality) for up to 40 min. Action potentials were recorded with microelectrodes, and membrane currents with the perforated- or ruptured-patch technique. 2. Hyperosmotic treatment for 20 min shrunk cell volume and hyperpolarized the membrane. Moderate (1.2-1.5 T) treatment caused biphasic changes in action potential configuration (rapid minor shortening quickly followed by lengthening to a stable 110% control duration). Severe (2.2-2.8 T) treatment caused triphasic changes (marked early shortening, strong rebound lengthening and subsequent pronounced shortening). At peak lengthening (6-10 min) action potentials (165% control duration) had a hump near -30 mV and slowed terminal repolarization. 3. In accordance with previous studies, hyperosmotic solution inhibited the delayed rectifier K+ current, and enhanced the outward Na(+)-Ca2+ exchange current (INaCa) at plateau potentials. A novel finding was that hyperosmolality reduced the amplitude of L-type Ca2+ current (ICa,L) and slowed its rate of inactivation. Experiments on myocytes loaded with indo-1 suggest that the reduction in ICa,L is due to a rapid elevation of [Ca2+]i. 4. When impaled myocytes were preloaded with EGTA, severe hyperosmotic treatment induced a rapid monotonic shortening of the action potential to a stable 20% of control duration. Addition of external K+ quickly nulled the hyperpolarization and slowly lengthened the action potential. 5. The results suggest that modified electrical activity in osmotically shrunken myocytes is primarily caused by increases in [K+]i, [Na+]i and [Ca2+]i: (i) elevated [K+]i hyperpolarizes the membrane (which may contribute to increased [Na+]i); (ii) elevated [Na+.]i shortens all phases of the action potential (increased outward-directed INaCa); and (iii) elevated [Ca2+]i has antagonistic plateau shortening (inhibition of inward ICa,L) and plateau lengthening (reduced outward INaCa) influences, as well as a strong subplateau lengthening effect (enhanced inward INaCa).

Swelling-activated Gd3+-sensitive cation current and cell volume regulation in rabbit ventricular myocytes

The role of swelling-activated currents in cell volume regulation is unclear. Currents elicited by swelling rabbit ventricular myocytes in solutions with 0.6-0.9x normal osmolarity were studied using amphotericin perforated patch clamp techniques, and cell volume was examined concurrently by digital video microscopy. Graded swelling caused graded activation of an inwardly rectifying, time-independent cation current (ICir,swell) that was reversibly blocked by Gd3+, but ICir,swell was not detected in isotonic or hypertonic media. This current was not related to IK1 because it was insensitive to Ba2+. The PK/PNa ratio for ICir,swell was 5.9 +/- 0.3, implying that inward current is largely Na+ under physiological conditions. Increasing bath K+ increased gCir,swell but decreased rectification. Gd3+ block was fitted with a K0.5 of 1.7 +/- 0.3 microM and Hill coefficient, n, of 1.7 +/- 0.4. Exposure to Gd3+ also reduced hypotonic swelling by up to approximately 30%, and block of current preceded the volume change by approximately 1 min. Gd3+-induced cell shrinkage was proportional to ICir,swell when ICir,swell was varied by graded swelling or Gd3+ concentration and was voltage dependent, reflecting the voltage dependence of ICir,swell. Integrating the blocked ion flux and calculating the resulting change in osmolarity suggested that ICir,swell was sufficient to explain the majority of the volume change at -80 mV. In addition, swelling activated an outwardly rectifying Cl- current, ICl,swell. This current was absent after Cl- replacement, reversed at ECl, and was blocked by 1 mM 9-anthracene carboxylic acid. Block of ICl,swell provoked a 28% increase in swelling in hypotonic media. Thus, both cation and anion swelling-activated currents modulated the volume of ventricular myocytes. Besides its effects on cell volume, ICir,swell is expected to cause diastolic depolarization. Activation of ICir, swell also is likely to affect contraction and other physiological processes in myocytes.

Electrophysiological studies of the interaction between ventricular myocardium and coronary artery in monkeys

1. The electrical influence of the coronary arteries on ventricular muscle was investigated using strips of ventricle that included a section of coronary artery (cardiac preparation) and isolated coronary arteries dissected from the ventricle (arterial preparation). 2. In cardiac preparations, a hyperpolarizing response was recorded from the epicardial surface of the ventricular muscle when acetylcholine (ACh) was added to the organ bath, on condition that the internal diameter of the coronary artery was between 0.15 and 0.6 mm, that the vessel ran at a depth of 0.2 mm or less below the surface of the preparation, and that the recording microelectrode was immediately adjacent to the artery. 3. ACh-induced hyperpolarization was not detected in cardiac preparations which had no detectable arteries, or at sites distant from visible arteries. 4. In arterial preparations, a similar hyperpolarizing response was evoked by ACh in all vessels with an i.d. of 0.15-1.2 mm. 5. In a preparation combining ventricular muscle and a strip of coronary artery (with the vascular endothelium in direct contact with the epicardial surface of the ventricular myocardium), hyperpolarization was also observed from the ventricular muscle after application of ACh. 6. The hyperpolarizing response of the ventricular myocardium in the cardiac preparation and in combined preparations of ventricular muscle and coronary artery was weakened or abolished by removal of the arterial endothelium. 7. These results indicate that some substance released from the coronary arterial endothelium after stimulation by ACh induces hyperpolarization of the ventricular myocardium.

Effect of osmotic swelling and shrinkage on Na(+)-K+ pump activity in mammalian cardiac myocytes

The effect on the sarcolemmal Na(+)-K+ pump of exposure to anisosmolar solutions was examined using whole cell patch clamping and ion-selective microelectrodes. Na(+)-K+ pump currents were measured in single ventricular myocytes by using pipette Na+ concentrations ([Na]pip) of 0-70 mM. The relationship between [Na]pip and pump current was well described by the Hill equation. The [Na]pip for half-maximal pump current (K0.5) was 21.4 mM in isosmolar (310 mosM) solution. K0.5 was 12.8 mM during cell swelling in hyposmolar solution (240 mosM) and 39.0 mM during cell shrinkage in hyperosmolar solution (464 mosM). The maximal pump currents, derived from the best fit of the Hill equation, and the Hill coefficients were similar in isosmolar, hyposmolar, and hyperosmolar solutions. A sustained (> 20 min) decrease in the intracellular Na+ activity developed during exposure of intact papillary muscles to hyposmolar solutions, and a sustained increase developed during exposure to hyperosmolar solutions. We conclude that osmotic myocyte swelling stimulates the sarcolemmal Na(+)-K+ pump at near-physiological levels of intracellular Na+, whereas shrinkage inhibits the pump. These changes are due to increases and decreases, respectively, in the apparent affinity of the pump for Na+.

Sodium-hydrogen exchange in guinea-pig ventricular muscle during exposure to hyperosmolar solutions

1. The effect on intracellular pH (pHi) and intracellular Na+ activity (aNai) of exposure to hyperosmolar solutions was investigated in guinea-pig ventricular muscle using ion-sensitive microelectrodes. 2. Exposure of tissue to solution made hyperosmolar by the addition of 100 mM-sucrose produced an intracellular alkalinization of 0.10 pH units and hyperpolarization of the membrane potential. 3. When extracellular Na+ was reduced to 15 mM by substitution of NaCl with choline chloride, exposure to hyperosmolar solutions caused a decrease in pHi. Identical experiments using LiCl as the sodium substitute resulted in an increase in pHi of a magnitude similar to that seen at physiological Na+ levels. 4. In the presence of 50 microM-5-(N,N-dimethyl)amiloride (DMA), an inhibitor of Na(+)-H+ exchange, pHi decreased upon exposure to hyperosmolar solution. 5. The recovery of pHi from an intracellular acidosis (induced by brief exposure to NH4Cl) was enhanced in hyperosmolar solution when compared to recovery in isosmolar solution. This enhancement was observed even when aNai was markedly elevated (greater than 25 mM) by inhibition of the Na(+)-K+ pump. 6. There was an increase in aNai during exposure to hyperosmolar solutions. When the Na(+)-K+ pump was inhibited with dihydro-ouabain a component of this increase in aNai was sensitive to DMA. 7. We conclude that exposure of cardiac tissue to hyperosmolar solutions results in an intracellular alkalosis due to activation of the sarcolemmal Na(+)-H+ exchanger. Such changes should be considered when exposure to hyperosmolar solutions is used in the study of excitation-contraction coupling and cardiac muscle mechanics.

The changes in CiNa of sheep cardiac Purkinje fibres in hyperosmotic solutions

1. The changes in intracellular sodium ion concentration (CiNa) of sheep cardiac Purkinje fibres in hyperosmotic solutions were studied using Na-sensitive liquid ion-exchanger microelectrodes. 2. CiNa was increased in hyperosmotic solutions containing different concentrations of sucrose from 0 to 300 mM. 3. The changes in resting membrane potential (RMP) in hyperosmotic solutions had no regularity. In most of the experiments there was hyperpolarization of the membrane but in a few cases a depolarization or no change of RMP were also observed. 4. The N-shape of I-V relations of the fibres became more pronounced in hyperosomotic solutions.

Ca2+ pumping ATPase of cardiac sarcolemma is insensitive to membrane potential produced by K+ and Cl- gradients but requires a source of counter-transportable H+

The sensitivity of the Ca2+ pumping ATPase of bovine cardiac sarcolemma (SL) to changes in membrane potential was studied in a preparation of sealed SL vesicles. Membrane potential was imposed by preincubating the vesicles in media of defined ion composition (K+, Cl-, choline+ and gluconate-) and diluting into media of differing ion composition. The durations of the ion gradients and relative ion permeabilities were determined in separate experiments by the dependence of the half time for net K+ (or choline+) movement coupled with these anions (Cl- or gluconate-), registered by the fluorescence of 1-anilino-8-naphthalene sulfonate (Chiu, V.C.K., Haynes, D.H. 1980. J. Membrane Biol. 56:203-218). Relative permeabilities were: 1.0, K+; greater than or equal to 10.0, 1 microM valinomycin-K+; 4.0, Cl-; 0.66, choline+; 0.38, gluconate-. Durations of the gradients ranged between 17 sec (KCl, valinomycin) to 195 sec (K(+)-gluconate-). In separate experiments, active Ca2+ uptake was monitored using chlorotetracycline (CTC) fluorescence, a technique validated by 45-Ca2+ measurements (Dixon, D., Brandt, N., Haynes, D.H. 1984. J. Biol. Chem. 259:13737-13741). Active Ca2+ uptake was initiated in the presence of monovalent ion gradients. The values of the membrane potentials (Em) imposed by the monovalent ion gradients were calculated using the ion concentrations, their relative permeabilities and the Goldman-Hodgkin-Katz equation. No effect of membrane potential on transport rate was observed (less than or equal to 4%, for 5-7% SD) for imposed potentials as extreme as greater than or equal to +71 and less than or equal to -67 mV. Formal analysis shows that the above observations are not compatible with models in which the Ca2+ pumping ATPase functions in an electrogenic or charge-uncompensated fashion. Further experimentation showed that the pump rate is slowed when uptake is measured at less-than-adequate concentrations of buffer (5 vs. 25 mM HEPES/Tris). This, together with further control experiments using nigericin and FCCP, gave evidence that the pump requires a source of counter-transportable H+ in the vesicle lumen. The above experimentation also underlines the need for control of internal pH to obviate erroneous interpretation of ion perturbation experiments. The results are compared with results obtained with the Ca2+ ATPase pump of skeletal sarcoplasmic reticulum.

A simple device for rapidly exchanging solution surrounding a single cardiac cell

This study describes the design and various physiological applications of a simple device to rapidly change the solution surrounding a single intact cardiac cell. It consists of a short length of double-barreled glass tubing (theta-tubing) attached to a miniature solenoid. A cell is positioned in one of two parallel streams of solution that simultaneously flow from each barrel. Rapid solution switching is achieved by activation of the solenoid that directs the adjacent stream over the cell, changing the bulk solution within 7 ms. Approximately 150 ms were required to change the solution at the membrane surface of guinea pig ventricular cells, judging from potassium-induced changes in resting membrane potential. This delayed response was probably due to, in part, restricted diffusion in the transverse tubular system. The switching speed of this device makes it possible to change extracellular solutions during action potentials and voltage-clamp pulses.

Role of the sodium pump and the background K+ channel in passive K+(Rb+) uptake by isolated cardiac sarcolemmal vesicles

A simple procedure was developed for the isolation of a sarcolemma-enriched membrane preparation from homogenates of bullfrog (Rana catesbeiana) heart. Crude microsomes obtained by differential centrifugation were fractionated in Hypaque density gradients. The fraction enriched in surface membrane markers consisted of 87% tightly sealed vesicles. The uptake of 86Rb+ by the preparation was measured in the presence of an opposing K+ gradient using a rapid ion exchange technique. At low extravesicular Rb+ concentrations, at least 50% of the uptake was blocked by addition of 1 mM ouabain to the assay medium. Orthovanadate (50 microM), ADP (2.5 mM) or Mg (1 mM) were also partial inhibitors of Rb+ uptake under these conditions, and produced a complete block of Rb+ influx in the presence of 1 mM ouabain. When 86Rb+ was used as a tracer of extravesicular K+ (Rb+0 less than or equal to 40 microM, K+0 = 0.1-5 mM) a distinct uptake pathway emerged, as detected by its inhibition by 1 mM Ba2+ (K0.5 = 20 microM). At a constant internal K+ concentration (K+in = 50 mM), the magnitude of the Ba2+-sensitive K+ uptake was found to depend on K+0 in a manner that closely resembles the K+ concentration dependence of the background K+ conductance (IK1) observed electrophysiologically in intact cardiac cells. We conclude that K+ permeates passively this preparation through two distinct pathways, the sodium pump and a system identifiable as the background potassium channel.

The effects of hypertonicity on tension and intracellular calcium concentration in ferret ventricular muscle

1. Tension and intracellular calcium concentration ([Ca2+]i) were measured in isolated ferret papillary muscles exposed to hypertonic solutions. [Ca2+]i was measured with aequorin which was microinjected into surface cells of the preparation. Correction was made for the effects of ionic strength on aequorin sensitivity to Ca2+. 2. Application of 100 mM-mannitol increased both developed tension and the intracellular Ca2+ signals on contraction (the Ca2+ transients). 300 mM-mannitol increased the Ca2+ transients further but led to a decrease in developed tension. 3. Mannitol caused a concentration-dependent slowing in the time course of a stimulated contraction but had no effect on that of the Ca2+ transient. 4. As the mannitol concentration was increased, the muscles exhibited increased viscosity which was demonstrated by measuring the tension response to a sudden stretch during diastole. This is probably a consequence of cell shrinkage and may cause the slower time course of the contraction. 5. In the presence of 300 mM-mannitol, oscillations of diastolic [Ca2+]i were detectable in both stimulated and quiescent preparations. However, in stimulated preparations the oscillations in mannitol were smaller than when a Ca2+ transient of similar amplitude was achieved by other means. 6. Immediately after the application or removal of mannitol large spontaneous Ca2+ signals were often observed. These signals were even larger in Na+-free solutions, suggesting that they cannot be attributed to Na+-Ca2+ exchange. 7. The increase in developed tension in 100 mM-mannitol can be accounted for by the increased Ca2+ transients in combination with the inhibitory effects of ionic strength on myofibrillar tension production (Kentish, 1984). The decrease in developed tension at 300 mM-mannitol is dominated by the inhibitory effect of increased ionic strength on maximum Ca2+-activated tension.

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.

Single chloride-permeable channels of large conductance in cultured cardiac cells of new-born rats

Large conductance channels were observed in the membrane of cultured cardiac cells of newborn rats studied with the patch-clamp technique in cell-attached and inside-out configurations. These channels were observed in approximately equal to 4% of the patches. In the cell-attached configuration they exhibited outward rectification and partial inactivation. In the inside-out configuration no rectification occurred but inactivation was present, mainly during hyperpolarizations. Two channels with large single unit conductances (400-450 pS) and one with a smaller conductance (200-250 pS) were frequently observed in the same patch. The two large channels generally had different kinetics. Under steady-state conditions the opening probability of the faster channel appeared to be voltage-independent. The slower channel was activated by depolarization. In asymmetrical solutions the permeability ratios PNa/PCl were 0.03 and 0.24 for the larger and smaller channels, respectively; corresponding values for PBa/PCl were 0.04 and 0.09. It is proposed that in cardiac membranes the chloride permeability system is composed of widely dispersed microclusters forming grouped channels of different types and sizes.

An investigation of chloride-bicarbonate exchange in the sheep cardiac Purkinje fibre

Intracellular Cl activity (aiCl), and intracellular pH (pHi) were measured in isolated sheep cardiac Purkinje fibres using a liquid ion exchanger Cl-selective micro-electrode and a glass recessed-tip, pH-selective micro-electrode. Removal of external Cl (glucuronate substituted) produced a fall in aiCl from about 20 to about 4 mmol/l: the residual level is probably caused by intracellular interference on the Cl-sensitive electrode. Re-exposure of the fibre to increased levels of external Cl produced, in the steady state, increased levels of aiCl. The dependence of steady-state aiCl upon external Cl activity, aoCl, was roughly hyperbolic with 50% recovery occurring at an aoCl of about 9.5 mmol/l. At all levels of external Cl tested, Cl was accumulated to a level much higher than that predicted for passive electrochemical equilibrium. Exposure of a Cl-depleted fibre to various levels of external Cl produced an exponential rise with time in aiCl. The initial rate-of-rise in aiCl was estimated to be a saturating function of aoCl, with a half-maximal effect occurring at an aoCl of about 33 mmol/l. The rate-of-rise was about 10-fold greater than that predicted from constant-field theory using published values for PCl, the Cl permeability coefficient. Steady-state aiCl was essentially insensitive to changes in external HCO3 concentration, [HCO3]o, if these changes were made at a constant external pH, pHo, i.e. when a reduction in [HCO3]o was accompanied by a simultaneous reduction in the partial pressure of CO2, PCO2. In contrast, if PCO2 was maintained constant, then a change in [HCO3]o (thus producing a change in pHo) resulted in an inverse change in aiCl. This change in aiCl was also accompanied by a change in pHi: when aiCl increased, pHi decreased and vice versa. The anion-exchange inhibitor, DIDS (4,4-diisothiocyanato-stilbene disulphonic acid) abolished the effect on aiCl of changes in [HCO3]o and pHo (at constant PCO2). Furthermore DIDS reduced the influence of pHo upon pHi. Both the fall of aiCl in Cl-free solution and the subsequent reuptake of Cl following re-exposure to Cl-containing solution were slowed by a reduction in [HCO3]o (constant pHo, reduced PCO2). Both reuptake and wash-out of Cl were saturating functions of [HCO3]o with half-maximal effect occurring at an [HCO3]o of 1-1.3 mmol/l. The reuptake of Cl was little affected by removal of external Na (bis,2-hydroxy ethyl, dimethyl ammonium substituted). The reuptake of Cl was unaffected by amiloride (1 mmol/l) but slowed by piretanide (1 mmol/l).(ABSTRACT TRUNCATED AT 400 WORDS)

Potassium-chloride cotransport in cultured chick heart cells

The polystrand preparation of cultured chick heart cells has a unidirectional transmembrane Cl- efflux that is twice K+ efflux. However, Cl- conductance of this heart cell membrane is low [regardless of extracellular K+ (K+o)], suggesting the existence of electroneutral Cl--dependent transport mechanisms. Furosemide (10(-3) M) decreases the 36Cl tracer efflux rate constant from a control value of 0.67 to 0.33 min-1. Extracellular Na+--free solution, which depletes intracellular Na+ within 1 min, has no significant effect on 36Cl efflux. K+o-free solution plus 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS; 10(-4) M) promotes the loss of Cl- against the Cl- electrochemical gradient; Cl- loss is furosemide sensitive in a dose-dependent manner. Incubating polystrands in 133 mM K+o, normal extracellular Cl- (Cl-o) solution causes net K+ and Cl- uptake in a 1:1 stoichiometry as well as a furosemide-sensitive volume increase; 130 mM extracellular choline or Li+ cannot mimic this high-K+o-induced volume increase. Removal of Cl-o from 133 mM K+o solution prevents K+ uptake and causes a Cl- loss as well as a furosemide-sensitive volume decrease. Adjusting Cl-o concentrations in high-K+o solution plus DIDS, so that the Cl- chemical gradient equally opposes the K+ chemical gradient, prevents high-K+o-induced volume changes. These data suggest that the cardiac cell membrane contains a furosemide-sensitive K+-Cl- cotransport mechanism.

Effects of tonicity on tension and intracellular sodium and calcium activities in sheep heart

We have measured the effects of changing tonicity of the bathing solution on intracellular sodium and calcium activities and tension of sheep cardiac Purkinje strands and ventricular muscle. For Purkinje strands in solutions of normal tonicity, resting membrane potential was -77.4 +/- 0.4 mV (mean +/- SE), sodium activity was 7.9 +/- 0.4 mM, and calcium activity was 98 +/- 9 nM. For ventricular muscle in solutions of normal tonicity, resting membrane potential was -86.4 +/- 1.2 mV, sodium activity was 6.9 +/- 0.5 mM, and calcium activity was 70 +/- 4 nM. Reduction of tonicity to 75% of normal in both tissues produced depolarization of a few millivolts, and sodium activity fell almost to the level predicted for simple osmotic dilution. In Purkinje strands, calcium activity fell much more than that predicted for simple osmotic dilution. Twitch contraction was reduced in the hypotonic solution. Increase of tonicity to 150% and 200% caused the resting membrane potential to become more negative. In both tissues, sodium activity increased somewhat less than predicted from simple water movement, and calcium activity increased proportionately much more than sodium activity. The much larger change of calcium activity in both hypo- and hypertonic solutions could be explained by water movement plus the effect of sodium-calcium exchange. In hypertonic solutions, tonic tension was increased, along with the rise in calcium activity; however, the twitch tension was reduced. This reduction of twitch tension may be due to a direct effect of hypertonicity on cross-bridge behavior, as has been reported for skeletal muscle.

Conductance properties of single inwardly rectifying potassium channels in ventricular cells from guinea-pig heart

Single ventricular cells were enzymatically isolated from adult guinea-pig hearts (Isenberg & Klöckner, 1982). The patch-clamp technique (Hamill, Marty, Neher, Sakmann & Sigworth, 1981) was used to examine the conductance properties of an inward-rectifying K+ channel present in their sarcolemmal membrane. When the K+ concentration on the extracellular side of the patch was between 10.8 and 300 mM, inward current steps were observed at potentials more negative than the K+ equilibrium potential (EK). At more positive potentials no current steps were detectable, demonstrating the strong rectification of the channel. The zero-current potential extrapolated from the voltage dependence of the inward currents depends on the external K4 concentration [K+]o in a fashion expected for a predominantly K+-selective ion channel. It is shifted by 49 mV for a tenfold change in [K+]o. The conductance of the channel depends on the square root of [K+]o. In approximately symmetrical transmembrane K+ concentrations (145 mM-external K+), the single-channel conductance is 27 pS (at 19-23 degrees C). In normal Tyrode solution (5.4 mM-external K+) we calculate a single-channel conductance of 3.6 pS. The size of inward current steps at a fixed negative membrane potential V increases with [K+]o. The relation between step size and [K+]o shows saturation. Assuming a Michaelis-Menten scheme for binding of permeating K+ to the channel, an apparent binding constant of 210 mM is calculated for a membrane potential of -100 mV. For this potential the current at saturating [K+]o is estimated as 6.5 pA. The rectification of the single-channel conductance at membrane potentials positive to EK occurs within 1.5 ms of stepping the membrane potential from a potential of high conductance to one of low conductance. In addition to the main conductance state, the channel can adopt several substates of conductance. The main state could be the result of the simultaneous opening of four conducting subunits, each of which has a conductance of about 7 pS in 145 mM-external K+. The density of the inward-rectifying K+ channels in the ventricular sarcolemma is 0-10 channel/10 micron2 of surface membrane; the average of twenty-eight patches was 1 channel/1.8 micron2. It is concluded that the inward-rectifying K+ channels mediate the resting K+ conductance of ventricular heart muscle and the current termed IK1 in conventional voltage-clamp experiments.

Transmembrane chloride flux in tissue-cultured chick heart cells

To evaluate the transmembrane movement of chloride in a preparation of cardiac muscle lacking the extracellular diffusion limitations of natural specimens, intracellular chloride concentration ( [Cl] i) and transmembrane 36Cl efflux have been determined in growth-oriented embryonic chick heart cells in tissue culture. Using the method of isotopic equilibrium, [Cl]i was 25.1 +/- 7.3 mmol x (liter cell water)-1, comparable to the value of 24.9 +/- 5.4 mmol x (liter cell water)-1 determined by coulometric titration. Two cellular 36Cl compartments were found; one exchanged with a rate constant of 0.67 +/- 0.12 min-1 and was associated with the cardiac muscle cells; the other, attributed to the fibroblasts, exchanged with a rate constant of 0.18 +/- 0.05 min-1. At 37 degrees C, transmembrane Cl flux of cardiac muscle under steady-state conditions was 30 pmol x cm-2 x s-1. In K-free, normal, or high-Ko solutions, the responses of the membrane potential to changes in external Cl concentration suggested that chloride conductance was low. These results indicate that Cl transport across the myocardial cell membrane is more rapid than K transport and is largely electrically silent.

Towards an estimate of chloride permeability in the smooth muscle of guinea-pig vas deferens

Cl movements across the cell membranes of smooth muscle from the guinea-pig vas deferens were measured using Cl-sensitive micro-electrodes and 36Cl fluxes. The rate constants for the loss of Cl ions measured by both methods under a variety of conditions were used to calculate the apparent Cl permeability (PCl). If it is assumed that the initial rate of decline of the intracellular Cl activity (aiCl) on removal of extracellular Cl (Clo) represents net transmembrane Cl movement, the apparent PCl was 3-6 X 10(-8) cm s-1. This value is in good agreement with those calculated from the rate constant of 36Cl efflux into both normal Krebs solution (steady-state) and Cl-free solution. Such a value for PCl predicts a large depolarization on removal of Clo, but only a minimal change was recorded. It also predicts that changes in membrane potential (Em) would affect aiCl; furthermore that removal of Clo would increase membrane resistance and thus the hyperpolarization observed on reactivation of the electrogenic Na pump. Neither of these was observed. The PCl/PK ratio obtained from changes in Em on rapid changes in Clo and Ko gives a value for PCl which is an order of magnitude lower: 4 X 10(-9) cm s-1, using Casteels' (1969 b) value for PK. These observations can be reconciled by a substantial proportion of the measured Cl movements being carrier-mediated. The presence of the stilbene derivative DIDS greatly slowed both the steady-state efflux and uptake of 36Cl, as has previously been shown for the loss and reaccumulation of Cl ions on removal and replacement of Clo. PCl calculated in the presence of DIDS was about 5 X 10(-9) cm s-1. The nominal absence of CO2 and HCO3, which slows the reaccumulation and loss of Cl, had no effect on the steady-state fluxes. This indicates that the carrier operates in the self-exchange mode in the steady state.

The passive electrical properties of guinea-pig ventricular muscle as examined with a voltage-clamp technique

1. A voltage-clamp technique was developed for stable recording of small currents in guinea-pig ventricular muscle. Small cylindrical preparations were impaled with three micro-electrodes, one for measuring the feed-back potential and two for injecting current. 2. The longitudinal potential profile resulting from current injection at one point was measured. It agreed well with the theoretical predictions for a linear cable which is sealed at both ends ('healing over'), with a length constant (lambda) of 580 +/- 145 micron. 3. When the clamp current was injected symmetrically into each half of the preparation via two electronic current pumps a spatially homogeneous clamp could be achieved in preparations with a diameter of less than or equal to 250 micron and a length of less than or equal to 2 lambda. 4. The membrane capacity and the membrane resistance of the preparations at the resting potential were measured with small voltage-clamp pulses. Assuming a specific membrane capacity (Cm) of 1 microF/cm2 a specific membrane resistance (Rm) of 6.7 +/- 1.8 k omega cm2 was obtained in Tyrode solution containing 3 mM-K. 5. The total surface area was calculated from the measured capacity of the preparation assuming a Cm of 1 microF/cm2. The total cellular volume was estimated from optical measurement of the external dimensions of the preparation assuming an extracellular space of 25%. From these data the average surface/volume ratio of individual cells was calculated to be 7200 cm2/cm3. 6. From the measured electrical constants the specific resistance of the intracellular space (Ri) was calculated to be 200-250 omega cm. With small constant current pulses a membrane time constant of 6.6 +/- 1.3 ms was measured. 7. The influence of the extracellular potassium concentration ([K]o) on Rm was studied in the range 1.5-6 mM-[K]o. Rm was found to depend on [K]o less than predicted by the constant field theory.

Calcium- and length-dependent force production in rat ventricular muscle

1. Trabeculae from the right ventricles of rat hearts were ;skinned' by immersion for 30 min in a solution containing the non-ionic detergent Brij-58 at a concentration of 1%.2. The average sarcomere length in the central region of the relaxed preparation was estimated by laser diffraction and set at pre-determined values within the range of 1.9-2.4 mum by adjustment of muscle length. Isometric contractions were then induced by raising the Ca(2+) concentration under carefully controlled chemical conditions.3. The dependence of Ca(2+)-activated force production on sarcomere length over the ascending limb of the length-force relation was examined at Ca(2+) concentrations giving partial and full activation of the contractile system of the muscle.4. The dependence of Ca(2+)-activated force on Ca(2+) concentration was compared at sarcomere lengths on the ascending limb and plateau of the length-force relation.5. The results obtained from both kinds of experiment showed that the sensitivity of the contractile system to Ca(2+) increases with sarcomere length over the ascending limb of the length-force relation.6. Possible explanations for this observation have been discussed.

Intracellular chloride activity in mammalian ventricular muscle

The intracellular chloride activity (formula, see text) of quiescent rabbit right papillary muscle was measured with ion-selective microelectrodes (ISE) made with Corning 477315 liquid ion-exchange resin. The (formula, see text) was 9.8 +/- 2.4 (SD) mM, a value significantly greater than the 6.1 +/- 0.6 mM expected from passive distribution. The chloride equilibrium potential (ECl) was -64.4 +/- 6.6 mV, while membrane potential was -75.9 +/- 2.2 mV and significantly negative to ECl. These values are corrected for a nonchloride signal detected by the ISE. An apparent (formula, see text) of 4.8 +/- 0.6 mM was measured after exposure to Cl-free media for 1 h. Since isotopic chloride was totally washed out by this time, the apparent (formula, see text) in Cl-free media was interpreted as interference and subtracted from the (formula, see text) measured in other media. The conclusion that chloride is not passively distributed is supported by the observation that the (formula, see text) increase in high potassium media was smaller than predicted. In contrast to findings in papillary muscle, (formula, see text) of frog sartorius muscle was consistent with passive distribution, if it is assumed that interference was less than 0.5 mM.

Intra- and extracellular K+ and Na+ activities and resting membrane potential in sheep cardiac purkinje strands

K+- and Na+-selective liquid ion-exchanger microelectrodes were used to measure intracellular K+ activity (aK i) and intracellular Na+ activity (aNa i) of sheep cardiac Purkinje strands in different solutions. In Tyrode's solution with an extracellular K+ concentration ([K+]o) of 5.4 mM, aK i was between 80 and 140 mM and averaged 109.6 +/- 4.0 mM (mean +/- SE, 20 strands). The measured aK i was closely correlated with the resting membrane potential, so that the K+ equilibrium potential was always about 10 mV more negative. When [K+]o was lower than 5.4 mM, aK i fell, and when [K+]o was higher than 5.4 mM it increased, aNa i was between 4 and 12 mM, and averaged 6.6 +/- 0.6 mM (14 strands). Its variation was also correlated with resting potential. Over a wide range of [K+]o and extracellular Na+ concentrations ([Na+]o), the aiNa changes were such that Na+ equilibrium potential remained between +70 and +80 mV. The quiescent membrane behaved as a K+-electrode when [K+]o was higher than 5.4 mM. When [K+]o was low and [Na+]o was zero, then Ca2+ and perhaps Cl- contributed to the resting potential.

The effects of extracellular potassium, ouabain, and prostaglandins on intracellular potassium activity in sheep cardica Purkinje fibers

(1) Intracellular K activity (aKi) of sheep heart Purkinje fibers was measured using K-selective microelectrodes (liquid ion exchanger). aKi in the resting state with an extracellular K of 4 mmol . l-1 EK was 112.9 +/- 6.1 mmol . l-1 (n = 47) for a membrane potential (VM) of -73.3 +/- 0.9 mV. VM deviated from the calculated potassium equilibrium potential (EK = -93mV). (2) When extracellular K was decreased to 2 mmol . l-1 or increased to 6 and 10 mmol . l-1 EK changed from -114 to -84 and -73 mV, with little change in aKi. (3) aKi and VM significantly decreased after administration of 10(-6) mol . l-1 ouabain. (4) Prostaglandins (PGI2 10-100 micrograms . l-1 and PGE2 0.01-1 microgram . l-1) decreased aKi without greatly changing VM. The differences between VM and EK became smaller. These effects indicate an increase in K permeability and may explain the antiarrhythmic action of prostaglandins.

Sodium-calcium exchange in rabbit heart muscle cells: direct measurement of sarcoplasmic Ca2+ activity

Calcium ion-selective microelectrodes made with Simon's neutral carrier were used to measure simultaneously sarcoplasmic Ca2+ activity (aiCa) and resting tension (Tr) of rabbit ventricular muscle during reduction and restoration of external sodium ion concentration, [Na]0. Under the same experimental conditions the change in contractile tension (Ta) also measured. In resting muscle the aiCa was 38 +/- 17 nanomolar (mean +/- standard deviation; N = 10). The reduction of [Na]O from 153 to 20 millimolar led to about a threefold increase in aiCa with parallel increases in Tr and Ta. The time course of the change in aiCa was similar to that of the changes in Tr and Ta. The results are consistent with an important role of the sodium-calcium exchange system for regulating sarcoplasmic Ca2+ activity.

Chloride distribution and exchange in rat ventricle

The cellular Cl content and concentration ([Cl]cell) and the cellular uptake of 36Cl have been measured in the rat left ventricle in vivo. The in vitro efflux of 36Cl from perfused contracting ventricles preequilibrated with 36Cl in vivo was also determined at 22, 30, and 38 degrees C. [Cl]cell was 8.2 +/- 0.5 mmol/kg cell water, corresponding to a calculated equilibrium potential of Cl of -70 to -80 mV. This figure for [Cl]cell is significantly lower than previous estimates in the literature, which were subject to an analytical error leading to overestimation of muscle Cl content obtained coulometrically. At 38 degrees C, Cl exchange under quasi-steady-state conditions was 31.2 mumol . (g dry ventricle . min)-1 or 42.5 pmol . (cm2 plasma membrane . s).-1 Apparent activation energy of the flux was 10.4 kcal/mol. At 22 degrees C, no dependence of the exchange on contraction frequency was detectable over a range of 80-160 contractions/min. The Cl exchange flux is among the fastest, if not the fastest. known for myocardial ion transport.

Relation between intracellular Na ion activity and tension of sheep cardiac Purkinje fibers exposed to dihydro-ouabain

The intracellular Na ion activity (aiNa) and the contractile tension (T) of sheep cardiac Purkinje fibers were simultaneously measured employing recessed-tip Na+-selective glass microelectrodes and a mechano-electric transducer. The aiNa of 6.4 +/- 1.6 mM (mean +/- SD, n = 56) was obtained in fibers perfused with normal Tyrode's solution. The changes in aiNa and T were measured during and after the exposure of fibers to a cardiac glycoside, dihydro-ouabain (DHO) in concentrations between 5 X 10(-8) M and 10(-5) M. The exposure time to DHO was 15 min. Both aiNa and T did not change in fibers exposed to 5 X 10(-8) M DHO, and the threshold concentration for the effect of DHO appeared to be around 10(-7) M. In DHO concentrations greater than the threshold, the increases in aiNa and T strongly correlated during the onset of DHO effects. The recoveries of aiNa and T were variable and slow, being dependent on the DHO concentration. In those fibers which recovered from the effects of DHO, the time-course of aiNa recovery was similar to that of T recovery. In fibers exposed to DHO of 5 X 10(-6) M or greater, the apparent toxic effects were observed in both action potential and contraction after an initial increase in T. The fibers manifesting the apparent toxic effects has a aiNa of approximately 30 mM or greater. The results of this study indicate that the increase in aiNa is associated with the positive inotropic action of the cardiac glycoside.

Changes in liquid-junction potential following chloride replacement in cat papillary muscle

The transient depolarizations in cat papillary muscle produced by chloride substitution with low mobility anions are largely due to changes in liquid-junction potential at the tissue level.

Regulation of chloride in quiescent sheep-heart Purkinje fibres studied using intracellular chloride and pH-sensitive micro-electrodes

1. The intracellular Cl activity, alpha iCl was measured inside quiescent sheep cardiac Purkinje fibres, bathed in normal Tyrode at pH 7.40, buffered with approximately 22 mM-bicarbonate/approximately 5% CO2 + 95% O2. The measurements were made using liquid ion-exchanger Cl-sensitive micro-electrodes. 2. After internal Cl levels had been depleted by prolonged exposure to Cl-free media (glururonate-substituted) when external Cl was restored, there was a rapid re-accumulation of Cl inside the fibres to levels that were much higher than those expected for a passive Cl distribution. Such a process can be conveniently defined as an active inward Cl pump. 3. The inward-pumping was noticeably temperature-sensitive (Q10 approximately 2.6), its rate was reduced about eighteenfold in the nominal absence of external bicarbonate/CO2 and it was substantially inhibited by the drug SITS (4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulphonic acid). 4. The fall of alpha iCl in Cl-free solution was slow and was also equally temperature-sensitive and substantially inhibited by SITS, but was only slightly impaired in the nominal absence of external bicarbonate/CO2. 5. pHi was measured using recessed-tip pH-sensitive micro-electrodes, and in some experiments both pHi and alpha iCl were monitored simultaneously. When alpha iCl slowly declined in Cl-free solution then pHi slowly became alkaline. Upon restoring external Cl, then there was, as usual, a rapid recovery of a high alpha iCl and this was accompanied by a rapid re-acidification of pHi. Both the recovery of alpha iCl and pHi were exponential with virtually the same time constant. 6. Both the slow alkalinization of pHi in Cl-free solution and the rapid re-acidification upon restoring external Cl were substantially inhibited by the drug SITS. 7. When [k]O was raised to 45 mM or more (by removing equivalent amounts of [Na]O), there was a large depolarization of Em and a slow rise of alpha iCl, which was not accompanied by a large change of pHi. The rise of alpha iCl appeared to be unaffected by SITS. 8. It is suggested that a Cl/CHO-3 exchange mechanism can operate reversibly across the membrane of quiescent Purkinje fibres, and that it can account, at least in part, for the high levels of alpha iCl measured in the resting state. It is also concluded that Cl can cross the membrane in other ways, especially in high-K solution possibly by moving passively through conductance channels that are open under these conditions.

Stimulation of the Na+ pump by hypotonic solutions in skeletal muscle

The fractional loss of 22 Na+ from frog sartorius muscle is increased when the tonicity of the external solution is reduced. The effect, which is larger the lower the osmolarity, exhibits the following characteristics: (1) quick onset and reversibility, (2) is not reduced in the absence of external Na+, (3) is completely abolished by strophanthidin (3. 10-5 M), (4) is neither the result of membrane depolarization nor K+ accumulation in the extracellular space.

Reduction of potassium permeability by chloride substitution in cardiac cells

1. The efflux of radioactive K and Cl ions was measured in cow Purkinje fibres and ventricular preparations of cow, cat and frog. The effect of K and Cl was studied by changing the extracellular K concentration between zero and 54 mM, and by substituting Cl ions by acetylglycinate, isethionate, benzenesulphonate, propionate and nitrate. 2. In the absence of Cl the rate coefficient for 42K efflux showed a pronounced fall, which was more pronounced the higher the K concentration. This effect was not related to the change in membrane potential. The rate coefficient for 42K efflux increased in the presence of higher extracellular K concentrations. 3. 36Cl efflux increased in the presence of reduced as well as increased extracellular K concentrations. 4. The calculated permeability coefficient for K (PK) was maximal at 5-4 mM-K, decreased slightly at higher K concentrations, but fell markedly in K-free solutions, (to about 1/5 of the value in 5-4 mM-K). In Cl-free medium PK reduced to 0-67 of the value in the Cl medium, irrespective of the K concentration. 5. The calculated PCl was greater in K-free and 16-2 mM-K than in 5-4 mM-K. 6. The ratio PK/PCl showed important changes as a function of extracellular K concentration: the value was 5 in 5-4 mM-K and fell to 2 and 0-5 respectively in 16-2 and K-free solutions. 7. The results suggest that part of the changes in membrane resistance measured by electrical methods in Cl-free media is due to a simultaneous decrease in K conductance.

A study of the ion selectivity and the kinetic properties of the calcium dependent slow inward current in mammalian cardiac muscle

1. A voltage-clamp method combining a single surcose gap and two intracellular micro-electrodes was used to measure membrane currents in ventricullar myocardial fibres. 2. The adequacy of the voltage-clamp method is demonstrated by comparing the total current, It, across the gap with the voltage difference, delta V, between the two intracellular micro-electrodes, i.e. another independent way of measuring membrane currents. With both current measurements the slow inward current, Is, shows the same voltage- and time-dependences. 3. The sensitivity of the slow inward current to variation in external Ca and Na concentrations was investigated systematically. The reversal potential of the slow inward current was sensitive to variation of both ion species. 4. From the reversal potential measurements relative permeabilities of the conductance channels of the slow inward current were estimated as PCa/PNa approximately 1/0-01 and PCa/PK approximately 1/0-01 by means of the constant field equation. 5. The activation and inactivation kinetics of the slow inward current were explored in detail and related to the plateau of the action potential.