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Biomedical subjects

C O Lee

Publications and source records attributed to C O Lee.

At least 19 recordsLinked to original sources
PubMed1991

The role of intracellular sodium in the control of cardiac contraction.

Intracellular sodium was estimated in ventricular myocytes using the new Na-sensitive fluorescent indicator SBFI. Membrane potential and contraction were also measured simultaneously. Using an in situ calibration method, we found that intracellular sodium activity (aiNa) was 2.9 mM in quiescent rabbit cells. When the digitalis analogue strophanthidin inhibited the Na-K pump of myocytes with action potentials (APs), changes of contraction and aiNa were dissociated in time. There was also marked hysteresis between contraction and aiNa. When strophanthidin was applied to the same myocytes under voltage-clamp conditions, temporal dissociation between contraction and aiNa was dramatically reduced. This suggests that much of the dissociation and hysteresis was due the change in AP shape with strophanthidin. A small amount of residual hysteresis still existed even with voltage-clamp, and this persisted when the pump was blocked by removal of external potassium as an alternative method. We suggest that a gradient of sodium concentration from the subsarcolemmal space to the bulk cytoplasm might be responsible for hysteresis. Whereas SBFI probably signals the average Na level of the cytoplasm, subsarcolemmal Na may control Ca influx and contraction via Na-Ca exchange.

Action Potentials↗
PubMed1990

Effect of norepinephrine on Na(+)-K+ pump and Na+ influx in sheep cardiac Purkinje fibers.

Effects of norepinephrine and Ca+ on Na(+)-K+ pump and pacemaker current were investigated by simultaneous measurement of intracellular Na+ activity (aiNa) and membrane potential in driven (1 Hz) and quiescent sheep cardiac Purkinje fibers. Concurrently, twitch force was measured in driven fibers, in which norepinephrine (NE) produced a decrease in aiNa, a prolongation in action potential duration, and a hyperpolarization in diastolic membrane potential, Vdm. In contrast, in quiescent fibers, NE produced an increase in aiNa and a depolarization in resting membrane potential, Vm. The decrease in aiNa, prolongation in action potential duration, and hyperpolarization in Vdm produced by NE were blocked by 5 x 10(-6) M strophanthidin, presumably through inhibition of the Na(+)-K+ pump. The increase in aiNa and membrane depolarization caused by NE were abolished by high [K+]o or Cs+, presumably through inhibition of the pacemaker current, if. These results indicate that in driven fibers NE stimulates predominantly the Na(+)-K+ pump, producing a decrease in aiNa and that in quiescent fibers it increases predominantly if, producing an increase in aiNa. The effect of NE on driven and quiescent fibers differs because of the voltage dependence of if and perhaps the Na(+)-K+ pump. Consequently, the relative magnitude of the two opposing effects of NE on aiNa appears to be dependent on membrane potential. In quiescent fibers, Cs+ monotonically decreased aiNa to a steady-state value, while Cs+ hyperpolarized membrane potential and then slowly depolarized to a steady-state level, producing a transient hyperpolarization. In driven fibers, Cs+ decreased aiNa, shortened action potential duration, and depolarized Vdm. Cs+ decreased aiNa more in quiescent fibers than in driven fibers. The decrease in aiNa and hyperpolarization in membrane potential produced by Cs+ in quiescent fibers were abolished by depolarization induced by high K+ extracellular concentration (25.4 mM) but were not abolished or reduced by 5 x 10(-6) M strophanthidin. These results suggest that the decrease in aiNa and hyperpolarization in membrane potential by Cs+ are caused by blockage of if but not by stimulation of the Na(+)-K+ pump and that if is an important source of Na+ loading into cells.

Action Potentials↗
PubMed1988

Na+-Ca2+ exchange in regulation of contractility in canine cardiac Purkinje fibers.

To study Na+-Ca2+ exchange, intracellular Na+ activity (aiNa), twitch tension, and transmembrane potential were simultaneously measured in canine cardiac Purkinje fibers driven at a constant rate (1 Hz) in the absence and presence of strophanthidin (5 X 10(-7) M) at normal, low, and high extracellular [Na+] ([Na+]o) or [Ca2+] ([Ca2+]o). Intracellular Ca2+ activity (aiCa) of the fibers was also measured in a normal Tyrode solution. Reductions of [Na+]o by 20, 40, and 60% decreased the ratio of extracellular Na+ activity (aoNa) and aiNa in the steady state but steeply increased twitch tension. This finding is consistent with the view that a decrease in aoNa/aiNa increases intracellular Ca2+ through Na+-Ca2+ exchange. In further agreement with this view, a Na+-free solution virtually depleted intracellular Na+ and increased the resting tension of the fibers. The slope of the relation of the logs of twitch tension and aiNa that was determined at normal [Na+]o and [Ca2+]o may reflect the properties of the Na+-Ca2+ exchange. Slope of log tension-aiNa relationship decreased when reducing [Na+]o or increasing [Ca2+]o had decreased the level of aiNa. On the other hand, the slope increased when a rise in [Na+]o or a reduction in [Ca2+]o had increased the level of aiNa. These results indicate that as the aiNa level increased, slope of tension-aiNa relation increased, which suggests that Na+-Ca2+ exchange may depend on level of aiNa.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1988

Role of aiNa in positive force-frequency staircase in guinea pig papillary muscle.

In the ventricular papillary muscle of guinea pig heart, membrane potential, intracellular sodium activity (aiNa), and twitch force were measured simultaneously and continuously for many hours at stimulation rates of 0, 0.5, 1, 2, 3, 4, 5, and 6 Hz to investigate the relation of aiNa to twitch force and membrane potential both in the steady state and during the changes in these variables. After an increase in stimulation rate, both aiNa and twitch force increased progressively, reaching steady-state levels. The relation between twitch force and aiNa in the steady state was generally sigmoidal over the range of 0.5-5 Hz and steep in the 1- to 4-Hz range. After either increase or decrease in stimulation rate, the time course of change in aiNa was exponential and similar to that of change in twitch force. Moreover, the force-aiNa relation observed after increase in stimulation rate from 0.5 to 3 Hz resembled that observed after decrease in the rate from 3 to 0.5 Hz, indicating an absence of hysteresis in the relation. The results suggest that an increase in aiNa is an important factor involved in the force staircase. As stimulation rate was increased from 0.5 to higher rates (5 or 6 Hz) and then decreased back to 0.5 Hz, a hysteresis phenomenon was observed in the relation between twitch force and aiNa. This suggests that some secondary factor may alter the relation between twitch force and aiNa. As stimulation rate increased and aiNa rose, the steady-state diastolic membrane potential hyperpolarized. This result is consistent with the view that an increase in aiNa enhances the electrogenic Na+-K+ pump and hyperpolarizes the cell membrane.

Action Potentials↗
PubMed1988

Regulation of cytosolic free calcium in isolated perfused proximal tubules of Necturus.

To study the role of intracellular Ca2+ in regulating renal tubular transport of ions and water, cytosolic calcium ion activity (aiCa), cytosolic sodium ion activity (aiNa), and intracellular pH (pHi) in cells of isolated perfused proximal tubules of Necturus kidney were measured with Ca2+-, Na+-, and H+-selective microelectrodes, respectively. In control conditions, i.e., HCO3-Ringer solution on both sides of the epithelium, aiCa was 82 +/- 7 (SE) nM (n = 54), aiNa averaged 12.8 +/- 0.4 mM (n = 53), and pHi was 7.33 +/- 0.03 (n = 27). When the Na-K pump was inhibited by nominally K-free Ringer circumfusion, aiCa increased from a control level of 75 +/- 13 to 237 +/- 40 nM (paired t test; n = 16; P less than 0.001); in a different set of tubules, aiNa rose from 11.3 +/- 0.6 to 51.5 +/- 5.8 mM (n = 11; P less than 0.001). When organic solutes were deleted in the luminal perfusate, aiCa decreased from 73 +/- 11 to 61 +/- 11 nM (n = 9; P less than 0.001) and aiNa decreased from 14.6 +/- 0.6 to 8.3 +/- 0.7 mM (n = 9; P less than 0.001). Depolarization of the peritubular cell membrane with high-K, low-Na Ringer decreased aiCa from 90 +/- 12 to 55 +/- 9 nM (n = 13; P less than 0.001) and reduced aiNa from 13.1 +/- 1.0 to 7.5 +/- 0.6 mM (n = 16; P less than 0.001). Ionomycin (2 X 10(-6) M) increased aiCa from 67 +/- 10 to 158 +/- 26 nM (n = 10; P less than 0.001) and pHi from 7.33 +/- 0.03 to 7.39 +/- 0.03 (n = 27; P less than 0.001) but reduced aiNa from 11.8 +/- 0.9 to 10.3 +/- 0.7 mM (n = 11; P less than 0.001). The data are consistent with the view that aiCa is determined, in part, by the magnitude of the electrochemical potential gradient for Na ions across the basolateral cell membrane.

Animals↗
PubMed1988

Measurement of cytosolic calcium: ion selective microelectrodes.

Since the Ca2+-selective neutral carrier was introduced in 1972, Ca2+-selective microelectrodes have been made and used for measurement of Ca2+ activity (effective Ca2+ concentration) in the cytoplasm. Over the last several years, application of the technique has provided valuable information about resting levels and changes of cytosolic Ca2+ activity. This article reviews technical aspects of the electrodes, including their construction, calibration, and determination of cytosolic Ca2+ activity. The Ca2+-selective microelectrodes successfully prepared show the voltage responses of about 20 mV between 10(-6) and 10(-7) M Ca2+, and about 10 mV between 10(-7) and 10(-8) M Ca2+ so that they could be used to monitor changes in cytosolic free Ca2+. However, the techniques of the electrodes are still difficult as compared with those of other ion-selective microelectrodes. Some advantages and disadvantages of the techniques are discussed.

Animals↗
PubMed1988

Potential role of cytoplasmic calcium ions in the regulation of sodium transport in renal tubules.

Experimental maneuvers that increase intracellular calcium ion levels inhibit sodium transport by renal tubules. In the isolated perfused renal tubule, intracellular calcium ion activity (aiCa) changes in response to alterations in the magnitude of the electrochemical potential gradient for sodium ions across the basolateral cell membrane. However, a potassium-induced depolarization of this cell boundary does not cause a rise but rather a fall in intracellular calcium ion levels. Ionomycin raises aiCa without causing intracellular acidification. This observation does not support the view that high cytosolic calcium produces intracellular acidification. At least in the case of ionomycin, the inhibition of sodium transport appears to be due to ionophore-induced increases in aiCa. The changes in intracellular calcium ion concentration found in the different experimental conditions studied were consistent with the notion that cytosolic calcium ions may mediate a feedback mechanism that links the luminal entry to the peritubular extrusion of sodium ions. The mechanisms by which cytosolic calcium alters entry is not yet clear but recent experiments suggest an indirect effect on sodium channel activity.

Animals↗
PubMed1988

Negative inotropic effect of platelet-activating factor: association with a decrease in intracellular sodium activity.

Platelet-activating factor (PAF) is an autacoid whose cardiovascular actions include a potent negative inotropic effect. The mechanism of this decrease in myocardial contractility is still at issue, as both a decrease and an increase in trans-sarcolemmal Ca++ influx have been reported. Because changes in intracellular sodium activity (aiNa) are known to influence myocardial contractility, we investigated whether PAF affects aiNa. Thus, we have measured contractile responses to PAF (1 nM-1 microM) in isolated guinea pig right ventricular papillary muscles paced at constant rate, and recorded transmembrane action potential and aiNa with conventional and sodium-selective microelectrodes, respectively. Our findings suggest that PAF does not affect slow inward Ca++ current, because PAF neither affected nor prevented histamine-induced restoration of contractile responses in K+-depolarized papillary muscles. On the other hand, we found the negative inotropic effect of PAF to be associated with a shortening of the action potential duration and with a decrease in aiNa. The specific PAF antagonist compound CV-3988 inhibited all three electro-mechanical responses. Our findings imply that the decrease in contractile force caused by PAF may depend on the reduction in aiNa; as aiNa falls, intracellular Ca++ may be lost via the Na+/Ca++ exchange and contractility decreases. The shortening of the action potential duration by PAF may reflect a decrease in Na+ influx and the consequent reduction in aiNa.

Action Potentials↗
PubMed1987

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)

Animals↗
PubMed1986

Intracellular calcium ions as regulators of renal tubular sodium transport.

This review addresses the putative role of intracellular calcium ions in the regulation of sodium transport by renal tubules. Cytoplasmic calcium-ion activities in proximal tubules of Necturus are less than 10(-7) M and can be increased by lowering the electrochemical potential gradient for sodium ions across the peritubular cell membrane, or by addition of quinidine or ionomycin to peritubular fluid. Whereas lowering of the peritubular Na concentration increases cytosolic [Ca++] and [H+], ionomycin, a calcium ionophore, raises intracellular [Ca++] without decreasing pHi. The intracellular calcium-ion level is maintained by transport processes in the plasma membrane and membranes of intracellular organelles, as well as by calcium-binding proteins. Calcium ions inhibit net transport of sodium by reducing the rate of sodium entry across the luminal cell membrane. In the collecting tubule this inhibition is caused, at least in part, by an indirect reduction in the activity of the amiloride-sensitive sodium channel.

Adenosine Triphosphate↗
PubMed1986

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)

Animals↗
PubMed1985

Strophanthidin inotropy: role of intracellular sodium ion activity and sodium-calcium exchange.

The relation among the ratio of extra- and intra-cellular sodium ion activities (aoNa/aiNa), contractile force and action of strophanthidin was studied in cardiac Purkinje fibers when transmembrane Na+ and Ca2+ gradients were changed. The aiNa, contractile force and action potential were simultaneously measured. Simultaneous reduction of [Na+]o and [Ca2+]o to 80.8 and 1.08 mM respectively, decreased aiNa from 8.0 +/- 1.1 mM (mean +/- S.D., n = 17) to 6.0 +/- 0.9 mM (n = 17) whereas contractile force transiently increased and then recovered toward the level similar to that in Tyrode solution. Reduction of [Ca2+]o alone increased aiNa by 1.7 +/- 0.4 mM (n = 5) and decreased contractile force by 87 +/- 5% (n = 5). Raising osmolarity of Tyrode solution with sucrose increased both aiNa and contractile force. Substitution of sucrose with Na+ (high [Na+] solution) increased aiNa by 1.2 +/- 0.3 mM (n = 5) and decreased contractile force by 31 +/- 9% (n = 5). Strophanthidin (2 X 10(-7) M) increased aiNa by 0.4 +/- 0.1 mM (n = 6) and contractile force by 24 +/- 8 (n = 6) in a low [Na+] - [Ca2+] solution. These changes were smaller than those in Tyrode solution (1.1 +/- 0.3 mM); 96 +/- 32%, n = 6). On the other hand, strophanthidin increased aiNa and contractile force more in a low [Ca2+] (2.7 +/- 0.5 mM; 220 +/- 24%, n = 5) or a high [Na+] (2.3 +/- 0.9 mM; 164 +/- 37%, n = 5) solution than in Tyrode solution. In the solutions containing the altered [Na+]o and/or [Ca2+]o, the increases in aiNa and force by strophanthidin were parallel. Therefore, the parallel increase in aiNa and contractile force due to strophanthidin depends on the initial level of aiNa, suggesting the dependence of digitalis inotropy on the rate of Na+ extrusion by the Na+ -K+ pump. The results also indicate that the ratio of aoNa/aiNa is an important and powerful factor in the control of contractile force. Presumably this is mediated through the Na+ -Ca2+ exchange.

Action Potentials↗
PubMed1985

Relation of sodium pump inhibition to positive inotropy at low concentrations of ouabain in rat heart muscle.

Low concentrations of ouabain which produce a positive inotropic effect on rat ventricular muscle do not inhibit the isolated Na+-K+-ATPase enzyme from this tissue, suggesting that these low-concentration inotropic effects are not related to sodium pump inhibition (Erdmann, Philipp & Scholz, 1980; Adams, Schwartz, Grupp, Grupp, Lee, Wallick, Powell, Twist & Gathiram, 1982). We tested this hypothesis by continuously measuring intracellular Na+ activity with Na+-selective micro-electrodes and, separately, twitch tension of rat ventricular muscle during exposure to and wash-out of ouabain. Intracellular Na+ activity (aiNa) and transmembrane potential of quiescent muscle cells averaged 8.5 +/- 2.6 mM (mean +/- S.D., n = 27) and -79.2 +/- 2.4 mV (n = 34) respectively. Low concentrations of ouabain (0.1, 0.5 and 1.0 microM) produced concentration-dependent increases in both aiNa and twitch tension. At lower concentrations of ouabain (0.01 and 0.05 microM), no detectable changes in aiNa and twitch tension were observed. The data strongly indicate that in rat ventricular muscle sodium pump inhibition is present at low concentrations of ouabain which produce positive inotropy. This is consistent with previous results in canine and sheep cardiac Purkinje fibres.

Action Potentials↗
PubMed1985

200 years of digitalis: the emerging central role of the sodium ion in the control of cardiac force.

Digitalis has been used therapeutically for two centuries, but the mechanism by which it enhances the ability of cardiac muscle to produce force (the positive inotropic effect) has not been fully elucidated. The major controversy concerns the question of whether the inhibition of the Na+-K+ pump by digitalis, particularly at low (therapeutic) concentrations, increases the intracellular Na+ concentration and thus is causally related to the positive inotropic effect. Na+-selective microelectrodes, introduced recently, have made it possible to measure small changes in intracellular Na+ activity (aiNa) in beating preparations of cardiac muscle and, in particular, to follow the exact time course of change in both aiNa and contractile force during the positive inotropic effect of digitalis. It has been demonstrated that digitalis at low and high concentrations produces a parallel increase in aiNa and in contractile force during the onset of its effect; washout of the drug results in a parallel and complete recovery of aiNa and contractile force. Additional strong evidence for a correlation between the pump inhibition and digitalis inotropy is the fact that the magnitude of increase in aiNa and contractile force produced by digitalis depends on the level of aiNa and therefore on the rate of Na+ extrusion by the Na+-K+ pump. The study on the quantitative relationship between aiNa and contractile force reveals that the force of contraction is a power function of aiNa, such that a small rise in aiNa produces a significant increase in contractile force. Direct measurements of aiNa and intracellular free Ca2+ during digitalis inotropy strongly support the hypothesis that an increase in aiNa raises intracellular Ca2+ via Na+-Ca2+ exchange, thus producing the positive inotropic effect. In conclusion, the recent data available from the simultaneous and continuous measurements of aiNa and contractile force strongly indicate that the inhibition of the Na+-K+ pump is causally related to the positive inotropic effect of digitalis on cardiac muscle.

Calcium↗
PubMed1979

Electrochemical properties of Na+- and K+-selective glass microelectrodes.

Electrochemical properties of Na+-selective glass microelectrodes were studied and compared with those of K+-selective glass microelectrodes. The selectivity of Na+-selective glass microelectrodes depended on the ion concentration of test solutions. With aging, resistance of Na+-selective microelectrodes increased and their selectivity for Na over K decreased. Na+-selective microelectrodes potential measured in NaCl solution remained constant with aging, while the potential measured in KCl solution decreased and became more positive. The changes in resistance and potential of Na+-selective microelectrodes may be due to the effects of the less mobile cation, i.e., H+ or K+ on the Na ion exchange in the Na-sensing region. The results indicate that Na+-selective microelectrodes must be used as soon after filling as possible. The selectivity of Na+-selective microelectrodes increased with increase of the sensitive exposed-tip length, whereas their response time became slow due to a large recessed volume, indicating requirement of an optimum exposed-tip length for intracellular applications. The changes in the properties of Na+-selective glass microelectrodes with aging contrasted with those of K+-selective glass microelectrodes in which resistance decreased and K+-selectivity increased. The K+-selective microelectrodes required aging before use for a high selectivity and low resistance. The K+-selective microelectrodes with low resistance after sufficient aging can be used without insulation to measure K+ and Na+ activities in aqueous solutions. The different properties between Na+- and K+-selective microelectrodes are understandable, because hydration of N+-selective glass is much less extensive than that of K+-selective glass.

Electric Conductivity↗
PubMed1979

Membrane permeability during low potassium depolarization in sheep cardiac Purkinje fibers.

Exposure of sheep Purkinje fibers to low [K]o leads to marked depolarization to a stable potential of about -40 mV. This level is equivalent to the plateau of the Purkinje fiber action potential. The low [K]o depolarization could be prevented by removal of [Na]o and was modified by tetrodotoxin. The membrane potential in the depolarized state was unresponsive to changes in [Cl]o or [Ca]o and it was poorly responsive to changes in [K]o between 0 and 2 mM. Repolarization was induced by decrease in [Na]o with a slope response of 30 mV/10-fold change in [Na]o. Average internal K activity (aK) in the resting state with a [K]o of 5 mM was 121.4 mM for a membrane potential of -80 mV. During low K depolarization aK was 119.7 mM with a membrane potential of -34 mV. The depolarization was therefore due to a change in membrane permeability, with little change in aK. Upon restoration of [K]o the fiber repolarized to values transiently more negative than the prior resting potential. These transient potentials were more negative than the K equilibrium potential (VK), if it is calculated assuming a uniform [K]o. The hyperpolarization was reduced by ouabain [10(-6)] or by low [Ca]o.

Animals↗
PubMed1976

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

1. The membrane responses of rabbit papillary muscles to rapid changes in [K](o) and [Cl](o) were measured with open-tipped micropipettes and with closed micropipettes made from K-selective glass.2. The muscle cells behaved primarily as a K electrode, and responses to changes in [K](o) with constant [Cl](o) or with constant [K](o) x [Cl](o) were substantially the same.3. When [Cl](o) was changed at a constant [K](o) the membrane potentials changed rapidly and symmetrically by a small value and remained constant for 30 min.4. Measurement of potential with K(+)-selective micro-electrodes in these experiments showed no change in intracellular K activity. In addition to permitting calculation of K permeability, these measurements reassured us that the K(+)-selective electrodes were well insulated and not influenced by electrical shunts at the impalement site.5. Although the membrane response to changes in [Cl](o) was small, it was possible to calculate that the permeability ratio (P(Cl)/P(K)), was 0.11. The Cl and K conductances were about 0.015 mmho/cm(2) and 0.09 mmho/cm(2) respectively, resulting in a conductance ratio (g(Cl)/g(K)) of about 0.17.6. The time course of depolarization by increase in [K](o) was rapid (half-time 5 sec), but repolarization on return to lower [K](o) was much slower (half-time 50 sec). The depolarization time course was easily fitted by the potential change calculated by assuming the need for K diffusion into the extracellular spaces and taking account of the logarithmic relation between membrane potential and [K](o). These calculations did not fit the time course of repolarization, which was slowed in the fashion expected from an inward-rectifying membrane.7. The influence of [K](i) on membrane potential was investigated by changes in tonicity of the external solution. Hypotonic solution produced a change in intracellular K activity close to that produced by ideal water movement. However, in hypertonic solution, intracellular K activity did not rise as much as predicted, suggesting a change in intracellular activity coefficient.

Animals↗