Influence of potassium ions and osmolality on the resting membrane potential of rabbit ventricular papillary muscle with estimation of the activity and the activity coefficient of internal potassium

Evaluation of electrocardiographic parameters in patients with diabetes insipidus

BACKGROUND

There is limited data regarding the effect of altered serum osmolality on cardiac electrical activity. The aim of the present study is to evaluate the electrocardiographic (ECG) effects of diabetes insipidus (DI) and any related hyperosmolality in a population of young patients with DI and without any known cardiovascular disease or risk factors.

METHODS

Twelve-lead ECG's of 44 consecutive untreated young male patients (age: 21.8 ± 2.9 years) who had been referred to endocrinology clinic and diagnosed as DI based on water deprivation test were retrospectively evaluated. A total of 30 age-matched (21.9 ± 2.4 years) healthy males were selected as control group and ECG's of these controls were obtained for comparison with ECG's of DI patients. All ECG parameters were measured and compared.

RESULTS

Duration of QRS complex was significantly shorter in patients with DI compared with controls (85.2 ± 12.0 vs. 94.0 ± 10.6 ms, p: 0.001). P wave dispersion (PWD) of patients with DI was significantly higher compared with controls (31.9 ± 9.9 vs. 26.5 ± 10.6 ms, p: 0.03) and it was significantly correlated with serum osmolality and serum sodium level (r = - 0.36, p: 0.02 and r: - 0.35, p: 0.02, respectively).

CONCLUSIONS

DI patients without any cardiovascular disease or risk factors displayed significantly shorter QRS duration and increased p wave dispersion compared with controls.

Low-affinity spermine block mediating outward currents through Kir2.1 and Kir2.2 inward rectifier potassium channels

The outward component of the strong inward rectifier K(+) current (I(Kir)) plays a pivotal role in polarizing the membranes of excitable and non-excitable cells and is regulated by voltage-dependent channel block by internal cations. Using the Kir2.1 channel, we previously showed that a small fraction of the conductance susceptible only to a low-affinity mode of block likely carries a large portion of the outward current. To further examine the relevance of the low-affinity block to outward I(Kir) and to explore its molecular mechanism, we studied the block of the Kir2.1 and Kir2.2 channels by spermine, which is the principal Kir2 channel blocker. Current-voltage relations of outward Kir2.2 currents showed a peak, a plateau and two peaks in the presence of 10, 1 and 0.1 microm spermine, respectively, which was explained by the presence of two conductances that differ in their susceptibility to spermine block. When the current-voltage relations showed one peak, like those of native I(Kir), outward Kir2.2 currents were mediated mostly by the conductance susceptible to the low-affinity block. They also flowed in a narrower range than the corresponding Kir2.1 currents, because of 3- to 4-fold greater susceptibility to the low-affinity block than in Kir2.1. Reducing external [K(+)] shifted the voltage dependences of both the high- and low-affinity block of Kir2.1 in parallel with the shift in the reversal potential, confirming the importance of the low-affinity block in mediating outward I(Kir). When Kir2.1 mutants known to have reduced sensitivity to internal blockers were examined, the D172N mutation in the transmembrane pore region made almost all of the conductance susceptible only to low-affinity block, while the E224G mutation in the cytoplasmic pore region reduced the sensitivity to low-affinity block without markedly altering that to the high-affinity block or the high/low conductance ratio. The effects of these mutations support the hypothesis that Kir2 channels exist in two states having different susceptibilities to internal cationic blockers.

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.

DCPIB is a novel selective blocker of I(Cl,swell) and prevents swelling-induced shortening of guinea-pig atrial action potential duration

1. We identified the ethacrynic-acid derivative DCPIB as a potent inhibitor of I(Cl,swell), which blocks native I(Cl,swell) of calf bovine pulmonary artery endothelial (CPAE) cells with an IC(50) of 4.1 microM. Similarly, 10 microM DCPIB almost completely inhibited the swelling-induced chloride conductance in Xenopus oocytes and in guinea-pig atrial cardiomyocytes. Block of I(Cl,swell) by DCPIB was fully reversible and voltage independent. 2. DCPIB (10 microM) showed selectivity for I(Cl,swell) and had no significant inhibitory effects on I(Cl,Ca) in CPAE cells, on chloride currents elicited by several members of the CLC-chloride channel family or on the human cystic fibrosis transmembrane conductance regulator (hCFTR) after heterologous expression in Xenopus oocytes. DCPIB (10 microM) also showed no significant inhibition of several native anion and cation currents of guinea pig heart like I(Cl,PKA), I(Kr), I(Ks), I(K1), I(Na) and I(Ca). 3. In all atrial cardiomyocytes (n=7), osmotic swelling produced an increase in chloride current and a strong shortening of the action potential duration (APD). Both swelling-induced chloride conductance and AP shortening were inhibited by treatment of swollen cells with DCPIB (10 microM). In agreement with the selectivity for I(Cl,swell), DCPIB did not affect atrial APD under isoosmotic conditions. 4. Preincubation of atrial cardiomyocytes with DCPIB (10 microM) completely prevented both the swelling-induced chloride currents and the AP shortening but not the hypotonic cell swelling. 5. We conclude that swelling-induced AP shortening in isolated atrial cells is mainly caused by activation of I(Cl,swell). DCPIB therefore is a valuable pharmacological tool to study the role of I(Cl,swell) in cardiac excitability under pathophysiological conditions leading to cell swelling.

Biphasic effects of hyposmotic challenge on excitation-contraction coupling in rat ventricular myocytes

The effects of short (1 min) and long (7-10 min) exposure to hyposmotic solution on excitation-contraction coupling in rat ventricular myocytes were studied. After short exposure, the action potential duration at 90% repolarization (APD(90)), the intracellular Ca(2+) concentration ([Ca(2+)](i)) transient amplitude, and contraction increased, whereas the L-type Ca(2+) current (I(Ca, L)) amplitude decreased. Fractional sarcoplasmic reticulum (SR) Ca(2+) release increased but SR Ca(2+) load did not. After a long exposure, I(Ca,L), APD(90), [Ca(2+)](i) transient amplitude, and contraction decreased. The abbreviation of APD(90) was partially reversed by 50 microM DIDS, which is consistent with the participation of Cl(-) current activated by swelling. After 10-min exposure to hyposmotic solution in cells labeled with di-8-aminonaphthylethenylpyridinium, t-tubule patterning remained intact, suggesting the loss of de-t-tubulation was not responsible for the fall in I(Ca,L). After long exposure, Ca(2+) load of the SR was not increased, and swelling had no effect on the site-specific phosphorylation of phospholamban, but fractional SR Ca(2+) release was depressed. The initial positive inotropic response to hyposmotic challenge may be accounted for by enhanced coupling between Ca(2+) entry and release. The negative inotropic effect of prolonged exposure can be accounted for by shortening of the action potential duration and a fall in the I(Ca,L) amplitude.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

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.

Contribution of a swelling-activated chloride current to changes in the cardiac action potential

The purpose of this investigation was to determine to what extent the swelling-activated Cl- current (ICl,swell) contributes to swelling-induced changes in the resting membrane potential and action potential duration (APD) in ventricular myocytes. Action potentials were recorded from guinea pig ventricular myocytes using conventional whole cell recording techniques. Cell swelling caused initial lengthening followed by a variable shortening of APD. In 59% of cells this secondary APD shortening had a 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-sensitive component, consistent with a contribution from ICl,swell. Furthermore, DIDS partially antagonized the depolarization of the resting membrane potential that occurred during cell swelling. We have modeled the ICl,swell using the Oxsoft Heart computer program. Action potential changes predicted by the model agree well with the observed DIDS-sensitive component of the change in the action potential during cell swelling. We conclude that activation of ICl,swell contributes to shortening of APD and depolarization of the resting membrane potential during cell swelling in cardiac myocytes.

The effect of sodium concentration on myocardial viability in donor heart preservation using a nondepolarizing solution

This study examined the effect of different sodium concentrations in a nondepolarizing solution on myocardial viability and functional recovery of the canine donor heart. Isolated canine hearts were preserved for 6 h at 5 degrees C, followed by normothermic reperfusion for 2 h. Dogs were divided into two groups of nine dogs each: group 1 received a nondepolarizing solution with 70 mM Na+ and group 2 with 30 mM Na+. The myocardial Ca2+ concentration at the end of preservation was significantly higher in group 1 than in group 2 and increased after reperfusion in both groups without any intergroup difference. Myocardial concentrations of ATP, ADP, and total adenine nucleotide at the end of reperfusion were significantly higher in group 1 than in group 2. Myocardial cyclic adenosine monophosphate concentration was significantly higher in group 1 than in group 2 at the end of both preservation and reperfusion. The myocardial cyclic guanosine monophosphate concentration in group 1 increased and was higher than in group 2 at the end of preservation, but had returned to normal levels by the end of reperfusion. However, it remained unchanged through preservation and reperfusion in group 2. The left ventricular systolic and diastolic function, assessed by pressure-volume relationship, was better in group 1 than in group 2. Mitochondrial ultrastructural changes were similar. These results suggest that a nondepolarizing solution containing 70 mM Na+ provides better myocardial protection than a solution containing 30 mM Na+.

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+.

An electrogenic component of resting potential in rabbit ventricular muscle?

The resting potential and the intracellular Na and K concentrations (Nai, Ki) were determined at several extracellular K concentrations (Ko) between 0.5 and 18 mM and after inhibition of the sodium pump with 0.5 microM ouabain. Exposure to low Ko (0.5 mM) produced a transient hyperpolarization (from -80 to -100 mV) followed by a depolarization that led to a stable potential of -60 mV within 25 min. Similar potential levels were observed in the presence of ouabain regardless of the Ko/Ki ratio. Intracellular sodium increased at Ko < 5 mM, whereas Ki rose at Ko less than or equal to 1.0 mM. Because of the large decrease of Ki at Ko = 0.5 mM, Ko/Ki was the same at 0.5 and 1 mM. However, the resting potentials at the steady state differed by 50 mV at these concentrations. A PNa/PK of 0.032 for the control conditions was obtained with the Mullins-Noda equation using 2.5 as the Na-K coupling ratio. This PNa/PK value yielded a Goldman potential of -69 mV; so we estimated that electrogenic sodium extrusion contributed -10 mV to the resting potential. The size of the electrogenic potential increased as Ko was lowered from 5 to 1 mM. This finding suggests that the control of the Na-K coupling ratio may be independent of the mechanism that controls the pumping rate.

Ionic basis of transient inward current induced by strophanthidin in cardiac Purkinje fibres

1. Voltage clamp experiements studied the ionic basis of the strophathidin-induced transient inward current (TI) in cardiac Purkinje fibres. 2. The reversal potential of TI (Erev) was determined in the presence of various bathing solutions. Erev averaged --5 m V in the standard modified Tyrode solution (Kass, Lederer, Tsien & Weingart, 1978). Erev was displaced toward more negative potentials when the external Na concentration (NaO) was reduced by replacement of NaCl with Tris Cl, choline Cl or sucrose. 3. A sudden reduction of NaO evoked a temporary increase in TI, followed after a few minutes by a sustained diminution. The initial increase was closely paralleled by an enhanced aftercontraction and could be explained by an indirect effect of NaO on internal Ca. The subsequent fall in TI amplitude could be accounted for by the reduced driving force, E--Erev. 4. Erev was not significantly changed by replacing extracellular Cl with methyl-sulphate, or by limited variations in external Ca (2.7--16.2 mM) or external K (1--8 MM). 5. These results are consistent with an increase in membrane permeability to Na and perhaps K. 6. TI was not directly affected by TTX, which blocks excitatory Na channels, or by Cs, which inhibits inwardly rectifying K channels. TI may be distinguished from the slow inward current by its kinetic, pharmacological and ionic properties. 7. TI might be carried by a pre-existing ionic pathway such as the 'leak' channel which provides inward current underlying normal pace-maker depolarization. Another possibility is that TI reflects Ca extrusion by an electrogenic Ca--Na exchange.

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.