Acetylcholine and potassium movements in rabbit auricles

Mutant KCNJ3 and KCNJ5 Potassium Channels as Novel Molecular Targets in Bradyarrhythmias and Atrial Fibrillation

BACKGROUND

Bradyarrhythmia is a common clinical manifestation. Although the majority of cases are acquired, genetic analysis of families with bradyarrhythmia has identified a growing number of causative gene mutations. Because the only ultimate treatment for symptomatic bradyarrhythmia has been invasive surgical implantation of a pacemaker, the discovery of novel therapeutic molecular targets is necessary to improve prognosis and quality of life.

METHODS

We investigated a family containing 7 individuals with autosomal dominant bradyarrhythmias of sinus node dysfunction, atrial fibrillation with slow ventricular response, and atrioventricular block. To identify the causative mutation, we conducted the family-based whole exome sequencing and genome-wide linkage analysis. We characterized the mutation-related mechanisms based on the pathophysiology in vitro. After generating a transgenic animal model to confirm the human phenotypes of bradyarrhythmia, we also evaluated the efficacy of a newly identified molecular-targeted compound to upregulate heart rate in bradyarrhythmias by using the animal model.

RESULTS

We identified one heterozygous mutation, KCNJ3 c.247A>C, p.N83H, as a novel cause of hereditary bradyarrhythmias in this family. KCNJ3 encodes the inwardly rectifying potassium channel Kir3.1, which combines with Kir3.4 (encoded by KCNJ5) to form the acetylcholine-activated potassium channel ( I_KACh channel) with specific expression in the atrium. An additional study using a genome cohort of 2185 patients with sporadic atrial fibrillation revealed another 5 rare mutations in KCNJ3 and KCNJ5, suggesting the relevance of both genes to these arrhythmias. Cellular electrophysiological studies revealed that the KCNJ3 p.N83H mutation caused a gain of I_KACh channel function by increasing the basal current, even in the absence of m₂ muscarinic receptor stimulation. We generated transgenic zebrafish expressing mutant human KCNJ3 in the atrium specifically. It is interesting to note that the selective I_KACh channel blocker NIP-151 repressed the increased current and improved bradyarrhythmia phenotypes in the mutant zebrafish.

CONCLUSIONS

The I_KACh channel is associated with the pathophysiology of bradyarrhythmia and atrial fibrillation, and the mutant I_KACh channel ( KCNJ3 p.N83H) can be effectively inhibited by NIP-151, a selective I_KACh channel blocker. Thus, the I_KACh channel might be considered to be a suitable pharmacological target for patients who have bradyarrhythmia with a gain-of-function mutation in the I_KACh channel.

If Channel as an Emerging Therapeutic Target for Cardiovascular Diseases: A Review of Current Evidence and Controversies

In 2015, non-communicable diseases accounted for 39.5 million (70%) of the total 56.4 million deaths that occurred globally, of which 17.7 million (45%) were due to cardiovascular diseases. An elevated heart rate is considered to be one of the independent predictors and markers of future cardiovascular diseases. A variety of experimental and epidemiological studies have found that atherosclerosis, heart failure, coronary artery disease, stroke, and arrhythmia are linked to elevated heart rate. Although there are established drugs to reduce the heart rate, these drugs have undesirable side effects. Hence, the development of new drugs that selectively inhibit the heart rate is considered necessary. In the search for such drugs, almost four decades ago the I_f channel, also known as the “funny channel,” emerged as a novel site for the selective inhibition of heart rate. These I_f channels, with a mixed sodium and potassium inward current, have been identified in the sinoatrial node of the heart, which mediates the slow diastolic depolarization of the pacemaker of the spontaneous rhythmic cells. The hyperpolarization-activated cyclic nucleotide-gated (HCN) subfamily is primarily articulated in the heart and neurons that are encoded by a family of four genes (HCN1-4) and they identify the funny channel. Of these, HCN-4 is the principal protein in the sinoatrial node. Currently, funny channel inhibition is being targeted for the treatment and prevention of cardiovascular diseases such as atherosclerosis and stroke. A selective I_f channel inhibitor named ivabradine was discovered for clinical use in treating heart failure and coronary artery disease. However, inconsistencies regarding the clinical effects of ivabradine have been reported in the literature, suggesting the need for a rigorous analysis of the available evidence. The objective of this review is therefore to assess the current advances in targeting the I_f channel associated with ivabradine and related challenges.

The GIRK1 subunit potentiates G protein activation of cardiac GIRK1/4 hetero-tetramers

G protein gated inward rectifier potassium (GIRK) channels are gated by direct binding of G protein beta-gamma subunits (Gβγ), signaling lipids, and intracellular Na(+). In cardiac pacemaker cells, hetero-tetramer GIRK1/4 channels and homo-tetramer GIRK4 channels play a central role in parasympathetic slowing of heart rate. It is known that the Na(+) binding site of the GIRK1 subunit is defective, but the functional difference between GIRK1/4 hetero-tetramers and GIRK4 homo-tetramers remains unclear. Here, using purified proteins and the lipid bilayer system, we characterize Gβγ and Na(+) regulation of GIRK1/4 hetero-tetramers and GIRK4 homo-tetramers. We find in GIRK4 homo-tetramers that Na(+) binding increases Gβγ affinity and thereby increases the GIRK4 responsiveness to G protein stimulation. GIRK1/4 hetero-tetramers are not activated by Na(+), but rather are in a permanent state of high responsiveness to Gβγ, suggesting that the GIRK1 subunit functions like a GIRK4 subunit with Na(+) permanently bound.

Adrenaline, anti-adrenaline drugs and potassium movements in rabbits auricles

Adrenaline (2x10(-6) M) or isoprenaline (7.5x10(-8) M) increased the rate of (42)K uptake and the potassium content of right (spontaneously beating) auricles, but had no effect on potassium movements in quiescent left auricles. Although faster beating induced by electrical stimulation increased the rate of (42)K uptake, the actions of adrenaline were also apparent in auricles which were electrically stimulated so that they beat at a constant rate. The increase in (42)K uptake produced by adrenaline accounted entirely for the increase in potassium content of the tissue. Adrenaline, in concentrations ranging from 2x10(-6) M to 2x10(-4) M, had no effect on (42)K loss from electrically stimulated auricles. The action of adrenaline on (42)K uptake was blocked by dichloroisoprenaline (4x10(-6) M) but not by phenoxybenzamine (1.6x10(-6) M).

CALCIUM IN RELATION TO THE ACTIONS OF OUABAIN AND ADRENALINE ON THE HEART

Isolated pairs of rabbit auricles have been observed in media containing 6, 24 or 75 mM-potassium, with corresponding reductions in sodium concentration. In 24 mM-potassium, adrenaline restored beating and excitability, as did calcium chloride, but ouabain had no effect. In 75 mM-potassium, adrenaline had no effect; calcium chloride caused a contracture; ouabain had no direct effect, but auricles which had been beating in the presence of ouabain contracted promptly on transfer to 75 mM-potassium. Left auricles, which do not beat spontaneously, were less sensitive to calcium and to ouabain. The results showed a membrane stabilizing action of calcium and an action on muscular contraction, and suggested that cardiac glycosides acted by causing accumulation of calcium at the activator site in the tissue.

Effect of carbachol in the absence and presence of phenylephrine on Rb+ efflux and tension in rabbit left atria

The muscarinic agonist carbachol produced a concentration-dependent increase in 86Rb+ efflux and decrease in tension in isolated, electrically stimulated rabbit left atria. However, the lowest concentration of carbachol tested produced only a very small increase in 86Rb+ efflux, while it caused a relatively greater decrease in tension. 4-Aminopyridine and pertussis toxin attenuated the carbachol-stimulated 86Rb+ efflux and negative inotropic effect. However, 4-aminopyridine had a greater inhibitory effect on carbachol-stimulated 86Rb+ efflux than on carbachol-induced decreases in tension. Pre-treatment of rabbits with pertussis toxin completely abolished the increase in 86Rb+ efflux and decrease in tension produced by carbachol in the presence of the alpha-adrenoceptor agonist phenylephrine. 4-Aminopyridine attenuated the negative inotropic response to carbachol in the presence of phenylephrine, but had less effect on the carbachol-induced increase in 86Rb+ efflux under these conditions. These results suggest that carbachol-induced increases in K+ efflux may contribute at least in part to the negative inotropic responses to carbachol in the presence and absence of phenylephrine. However, this may not be sufficient to explain the direct negative inotropic response of left atria to carbachol.

Assay of muscarinic acetylcholine receptor function in cultured cardiac cells by stimulation of 86Rb+ efflux

An assay for the increase in potassium permeability mediated by muscarinic acetylcholine receptors (mAChR) in cultured cardiac cells is described, using the K+ ion substitute 86Rb+ as the tracer ion. Cardiac cells accumulate 86Rb+ from the extracellular medium in a Na+/K+ ATPase-dependent manner. Subsequent efflux of 86Rb+ in the absence and presence of muscarinic agonists follows kinetics similar to those previously reported for 42K+. The mAChR agonist carbamylcholine (carbachol) stimulated 86Rb+ efflux with an EC50 of 50 nM. The half-time for efflux is reduced by greater than 40% at maximally effective concentrations of agonist. Stimulation of 86Rb+ efflux by carbachol is blocked by the mAChR antagonist atropine with an IC50 of 15 nM. The stimulation of 86Rb+ efflux by carbachol is not affected by the presence of the Na+/K+ ATPase inhibitor ouabain. This assay provides a method for quantitating the mAChR-mediated increase in K+ permeability in cardiac cells without the use of 42K+.

Intra- and extracellular potassium activities, acetylcholine and resting potential in guinea pig atria

Intracellular potassium activity in guinea pig left atria was measured using potassium ion-selective microelectrodes and conventional microelectrodes. The effects of extracellular potassium concentration and acetylcholine on both intracellular potassium activity and the relationship between the resting membrane potential and the potassium equilibrium potential were investigated. Intracellular potassium activity was 102.1 mM in bathing media with a potassium concentration of 5 mM. Neither increasing extracellular potassium concentration to 10 mM nor exposure to acetylcholine (2 x 10(-6) to 10(-3) M) significantly altered intracellular potassium activity. In contrast, intracellular potassium activity decreased to 92.9 mM in 2.5 mM potassium concentration solutions. Resting membrane potential was 18.6, 9.6, and 7.3 mV positive to the potassium equilibrium potential in 2.5, 5, and 10 mM potassium, respectively. Acetylcholine caused a significant hyperpolarization at each extracellular potassium activity, confirming that resting membrane potential was positive to the potassium equilibrium potential. Even after exposure to 10(-3) M acetylcholine, the resting membrane potential apparently remained positive to the potassium equilibrium potential. If potassium accumulates in extracellular clefts during acetylcholine exposure, the calculated potassium equilibrium potentials are too negative, and the resting membrane potential might closely approximate the potassium equilibrium potential under these conditions. Fading of the acetylcholine-induced hyperpolarization and overshoot of the resting membrane potential on washout of acetylcholine were observed and are consistent with an accumulation of potassium during exposure to acetylcholine. In 5.0 mM potassium bathing solution, preparation-to-preparation variability of resting membrane potential can largely be explained by variability of intracellular potassium activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholine and potassium-42 movements in right atrial muscle of the guinea pig

The effect of acetylcholine (ACh) on potassium (K-42) movements has been studied in right atrial tissue of the guinea pig. When quiescent fibers were stimulated at 108 beats/min, potassium uptake was increased to a small but significant extent. Administration of acetylcholine (2.7 X 10(-6) M) induced an increase in potassium uptake that was greater in fibers that were quiescent than in the same fibers when stimulated. The ACh induced increase in potassium uptake was abolished by atropine (5.2 X 10(-7) M) and enhanced in the presence of nicotine (10(-5) M). In addition, ACh induced an increase in potassium efflux that was greater in fibers that were quiescent than in the same fibers when stimulated. Atropine blocked the effect of ACh on potassium efflux. It is concluded that activity of atrial tissue increases potassium uptake and modifies the action of ACh on potassium uptake and efflux. Muscarinic receptors mediate the ACh induced increase in potassium movements. Nicotinic receptors may mediate an ACh induced decrease in potassium uptake in atrial muscle.

Relaxation of the ACh-induced potassium current in the rabbit sinoatrial node cell

Voltage clamp experiments were carried out in order to study the mechanism of the ACh action in the rabbit S-A node cell. The following results were obtained: 1. The reversal potential of the ACh-induced current behaved like a potassium electrode, confirming that the ACh-operated channels pass potassium ions selectively. 2. On depolarizing voltage jumps the ACh-induced current showed an instantaneous peak from which the current decayed to a new steady level (relaxation). On hyperpolarizing voltage jumps the initial step change in current was followed by a gradual increase. 3. The time course of the current change on voltage jumps was well fitted by a single exponential and the time constant became longer as the membrane potential was increased. 4. The instantaneous I-V curve was linear while in the steady state the curve became flatter at low negative membrane potentials and steeper at high negative membrane potentials. The results suggest that ACh opens a specific potassium channel when the drug is bound to the muscarinic receptor. The opening and closing rate constants for this potassium channel depend on the membrane potential in such a way that on depolarizing voltage jumps the fraction of open channels gradually decreases and on hyperpolarization the fraction increases.

Effects of acetylcholine and parasympathetic nerve stimulation on membrane potential in quiescent guinea-pig atria

1. In quiescent preparations of guinea-pig right atria the action of ACh applied in the superfusion medium or released from parasympathetic nerve fibres was investigated by membrane potential measurements. 2. ACh-containing solutions induced hyperpolarizations which did not show desensitization. 3. The relationship between hyperpolarization amplitude (corrected for non-linear summation) and ACh concentration could formally be described by simple saturation kinetics with an apparent dissociation constant KACh = 1.3 x 10(-6) M. 4. Stimulation with impulses subthreshold for myocardial excitation induced a membrane hyperpolarization by releasing ACh from post-ganglionic parasympathetic nerve fibres. 5. The hyperpolarization reached maximum about 1 sec after the onset of stimulation. This slow development of hyperpolarization cannot be accounted for simply by diffusion from the site of release to the receptor site. 6. The hyperpolarization declined exponentially with a time constant of about 3 sec. In the presence of neostigmine (3 x 10(-6) M) the hyperpolarization lasted for 2 min or more. It is concluded that the action of ACh is primarily terminated by enzymic hydrolysis.

A study of the composition of pericardial fluid, with special reference to the probable mechanism of fluid formation

The composition of pericardial fluid and simultaneously withdrawn plasma have been measured in rabbits and greyhounds. 1. Sodium and chloride distributions were found to be not markedly different from the ratio predicted for a passive distribution. The small deviation found in greyhounds could be largely corrected by the in vitro dialysis of plasma against pericardial fluid. 2. Calcium and magnesium were distributed in a manner expected from a passive ultrafiltrate of plasma. 3. Pericardial fluid was found to contain between one quarter and one third of the protein of plasma. 4. Separation of the protein constituents demonstrated a far higher proportion of albumin to other proteins in the pericardial fluid. 5. The osmolality of plasma was slightly higher than that of pericardial fluid, as would be expected from a plasma ultrafiltrate. 6. The potassium concentration of pericardial fluid was higher than the plasma concentration in all animals studied. This difference could be abolished, and an expected distribution obtained in the samples from greyhounds, by the in vitro dialysis of plasma against pericardial fluid. This observation for potassium cannot be attributed to haemolysis of blood in pericardial fluid samples or to the use of any inappropriate references. It is suggested that the elevated potassium concentration of pericardial fluid may reflect the lability of the cardiac intracellular potassium during cardiac contraction. 7. The results obtained in this study do not support the concept of an active secretion of pericardial fluid as has been claimed by others. The distribution of ions would appear to be passive and to follow the values predicted by the Gibbs-Donnan relationship.

Compartmental analysis of potassium efflux from growth-oriented heart cells

Radioisotopic flux studies were initiated with a new preparation of growth-oriented heart cells to determine the contribution of heterogeneous cell types and the limitations of extracellular diffusion in quantitating the passive movement of potassium ions. The efflux of potassium-42 from contractile preparations, which contain two populations of cells, cardiac muscle and fibroblastlike, could be resolved into two components similar to that described for naturally occurring preparations of cardiac muscle. Compartmental analysis of the efflux data, using analog and digital computational methods, resolved the tracer kinetics into a slow compartment (k=0.015 min-1) associated with fibroblastlike cells and a fast compartment (k=0.067 min-1) associated with the cardiac muscle cells. The rate constants derived from compartmental analysis were independent of tracer equilibration and preparation dry weight. Analytical measurements of the preparations provided a quantitative basis for determing the transmembrane potassium fluxes from the tracer kinetics. Cardiac muscle cells stimulated at a rate of 150 min-1 in the presence of 5.4 mM external potassium were found to have a potassium efflux of 15.7 pmoles cm-2sec-1 whereas the value obtained for the fibroblastlike cells was 1.88 pmoles cm-2sec-1. Diffusional limitations of 42K efflux were analyzed for several important variables which can affect isotopic reflux, namely, transmembrane flux, cell volume-to-surface area and cell packing fraction.

Changes in membrane currents in bullfrog atrium produced by acetylcholine

1. A double sucrose-gap voltage-clamp technique has been used to study the effects of acetylcholine on the membrane currents in atrial trabeculae of the bullfrog, Rana catesbeiana. 2. The second, or slow inward (Ca2+/Na+) current, was found to be markedly reduced by concentrations of acetylcholine greater than approximately 2-0 X 10(-8)M. The resulting decrease in net calcium entry provides a straightforward explanation for the negative inotropic action of acetylcholine in atrial muscle. 3. Measurements of membrane resistance near the resting potential showed that relatively high doses of acetylcholine (approximately 10(-7) M) decrease membrane resistance by about twofold. This effect is shown to be the result of an increase in a time-independent background current which appears to be carried mainly by potassium ions. 4. Using appropriate pharmacological techniques, it has been possible to demonstrate: (i) that the peak slow inward current is reduced to about half its initial value before any significant increase in background current occurs; (ii) that even when a sufficient dose of acetylcholine to produce an increase in background current is used, the background current shows inward-going rectification and cannot account for the observed reduction in the slow inward current. 5. No consistent change was observed in the degree of activation of the time-dependent outward membrane currents after application of concentrations of acetylcholine which produced large decreases in the peak slow inward current. 6. These results are discussed in relation to previous electro-physiological and radioisotope studies of the mechanism of the negative inotropic effect of acetylcholine in cardiac muscle.

On the mechanism of the negative inotropic effect of acetylcholine

The negative inotropic action of ACh was investigated by voltage clamping mammalian atrial myocardium with the single sucrose gap method. Acetylcholine (ACh) affected the outward current, slow inward current and clamp tension in a concentration dependent way. 1. Concentrations of ACh which reduced action potential twitch tensions by up to 30 or 40% (ED-30-ED-40) increased steady state outward currents but had no effect on the time dependent outward current, the slow inward current or voltage clamp tension. This indicates that in this dose range the negative inotropy during normal activity can be completely explained by an "indirect" effect on the slow inward current, i.e. increased outward current shortens the action potential and prevents the slow inward current from running its normal time course. 2. Higher concentration of ACh (ED-70-ED-90) greatly increased the steady state outward currents and abolished anomalous rectification without affecting delayed rectification. The slow inward current and voltage clamp tension were reduced indicating that in this concentration range a 'direct' effect of ACh on the slow inward current and tension may be expected to add to the 'indirect' effect mentioned above.

The effect of isolation on myocardial properties

Denervation deprives the heart of its normal adrenergic and cholinergic control via the sympathetic and parasympathetic pathways. In a heart which is blood supplied by a donor animal of the same species, normal contractility is maintained, probably by blood borne catecholamines or possibly by unknown inotropic agents of the donor. A heart receiving blood oxygenated by isolated lungs is in a state of failure. Substitution of blood by a cell and protein free solution diminishes oxygen availability in cardiac muscle, both in the perfused and bathed preparation. In the unphysiological environment, myocardial cells lose K+ and gain Na+. Under best possible conditions of oxygen supply but in a later stage of perfusion, contractility during rhythmical stimulation is depressed more at lower than at higher rates. Frequency potentiation and the inotropic effectiveness of noradrenaline is more pronounced in vitro than in situ. In excised papillary muscles and ventricular and atrial strips, the disarrangement and a more or less severe lesion of individual fibres accelerates the decay in mechanical performance. The role of endogenous catecholamines for the maintenance of normal contractility in situ and in vitro is still a matter of discussion.

Prinzmetal's variant form of angina pectoris. Re-evaluation of mechanisms

Thirty-five patients with typical Prinzmetal's variant angina were studied by coronary cineangiography. There was no demonstrable stenosis of the major coronary arteries in 19 patients. Nine patients with single coronary stenosis underwent aortocoronary bypass and had recurrence of the symptoms postoperatively. Administration of nifedipine effected complete cessation of the symptoms among patients formerly treated medically. Although surgical treatment did not effect permanent relief of pain, all patients initially treated surgically experienced relief of pain when nifedipine was administered. The pathophysiology of variant angina remains obscure. Our results suggest that neurohumoral factors exert more of an effect on the myocardial cell than on the coronary vessels.

Actions of adrenaline on the potassium balance of the isolated heart

1. The action of adrenaline on the K(+) balance of the isolated heart was found to depend on the ionic composition and the temperature of the perfusion fluids used.2. When the perfusion fluid contained 145 mM Na(+), 1.7 mM Ca(++) and 3-9 mM K(+), adrenaline caused the hearts to gain K(+); when the K(+) concentration was reduced to 1.2 mM, adrenaline caused a loss of K(+). Both were actions on beta receptors.3. When the Ca(++) or the Na(+) concentration in the perfusion fluid was reduced, together with a reduction in K(+), adrenaline no longer produced K(+) loss from the hearts, but produced a gain of K(+).4. When the temperature of the perfusion fluid was reduced to 25 degrees C, adrenaline still produced a gain of K(+) by hearts perfused with fluid containing 3.2 mM K(+), but did not produce a loss of K(+) from hearts perfused with fluid containing 1.2 mM K(+).

The efflux of potassium, sodium, chloride, calcium and sulphate ions and of sorbitol and glycerol during the cardiac cycle in frog's ventricle

1. The exchanges of potassium and various other substances have been measured in beating frog's ventricles, using both superfused and distended preparations. In both preparations the high fluid flow rates used (1 ml./sec) cleared the ventricular cavity with a half-time (T(1/2)) of about 130 msec.2. Histological sections show that the modal strand radius in the relaxed or contracted distended ventricle is 17.5 mu, and in the relaxed and contracted superfused ventricle is 17.5 and 27.5 mu respectively.3. In quiescent ventricles the resting potassium influx and efflux are approximately equal at about 16 p-mole/cm(2).sec. This figure is computed from Niedergerke's (1963b) estimate of a cell size of 3.5 mu taken from electron-micrographs. If the older figure of 9.2 mu from single isolated cells is used (Skramlik, 1921) then the fluxes are about 44 p-mole/cm(2).sec. To allow for some cell damage in these preparations a further increase in flux of about 30% may be necessary.4. Contraction leads to a diminution of both potassium influx and efflux. Measurements made at 100 msec intervals throughout the cardiac cycle have demonstrated (a) that this decreased K efflux occurs at the same time as the mechanical twitch, and (b) that the size of the decrease is dependent on the external calcium concentration. Other experiments show that a similar decrease can be obtained by inducing a contracture at a constant membrane potential. It is concluded that the decreased K efflux during contraction is due to mechanical distortion of the tissue. This leads to a further slowing of the K diffusion and allows considerable reabsorption of K to occur into the cells.5. Efflux analysis suggests that normal K diffusion in the extracellular space may be about 1/10 of that in free solution. If this is correct the true membrane fluxes may be x 5 those measured.6. Phasic efflux measurements of Na, Ca, K, Cl, SO(4), sorbitol and erythritol show that a peak of efflux occurs just after the point of maximum rate of contraction of the ventricle. The peak efflux of K is least but all the other substances show similar patterns. In calcium-free solutions these phasic changes are absent. It is concluded that these effects are mechanical.7. Net K and Na changes were measured in ventricles poisoned by ouabain. The computed net changes for quiescent ventricles were a gain of 2.8 p-mole/cm(2).sec of Na and a loss of 5.3 p-mole/cm(2).sec of K. On stimulation a further increase in Na uptake of 8 p-mole/cm(2) occurred with no further loss of potassium. These results are computed for a cell diameter of 3.5 mu, for the larger diameter of 9.2 mu appropriate values of Na and K are 7.4 and 13.4 p-mole/cm(2).sec respectively for quiescent ventricles and an extra Na uptake of 21 p-mole/cm(2) per action potential. These results: (a) show that no large degree of single-file interaction occurs on the K movements, (b) are in agreement with the hypothesis that the membrane K fluxes are underestimated and (c) show that sufficient Na enters the cells per action potential to discharge a capacity of about 4 muF/cm(2).8. A general conclusion reached in these experiments is that ion movements during the long cardiac action potential cannot easily be measured because of mechanical artifacts.

Ions and the action of digitalis

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Potassium in Dog Ventricular Muscle: Kinetic Studies of Distribution and Effects of Varying Frequency of Contraction and Potassium Concentration of Perfusate

Four phases (0 to 3) were defined kinetically for potassium in the dog papillary muscle when perfused arterially for four to five hours with solution containing 4 mM/liter K + . Total tissue K + fell from 91.1±1.99 to 41.0±1.06 mmoles/liter after three hours perfusion. It then remained stable during the period when kinetic studies were done. The mean rate constant λ (min -1 ), potassium content (mmoles/liter tissue water) and suggested origin of each phase are respectively: (phase 0) λ 0 = 3.2, 0.3, vascular; (phase 1) λ 1 = 0.65, 2.06, interstitial; (phase 2) λ 2 = 0.0139, 39.3, intracellular; (phase 3) λ 3 < 0.004, <0.5, origin unknown.

Alteration of K + concentration in perfusing fluid produced significant changes in the intracellular exchange rate of K + . This was in marked contrast to increments in frequency of contraction which had no effect on the overall exchangeability of intracellular potassium. Increases in rate, however, were associated with a transient net loss of intracellular K + . This loss continued if the active tension of the muscle declined and if contracture progressed. The loss ceased if muscle function remained stable during continued increased frequency of contraction. A positive tension staircase was approximately proportional to the net K + loss. The net K + loss was 0.93 mmole/liter tissue water in nine muscles in which a mean 27 beats/min rate increment was introduced for a mean of 16 minutes. This represented 2.4% of intracellular K + .

A significant time lag was found before the net K + loss reached a maximum rate and began to decline. This is compared with the previously demonstrated transient net increment in Ca ++ uptake that accompanies increased frequency of contraction. These ionic movements are consistent with the theory that Na + movements in and out of a "specialized membrane region" are related to Ca ++ movements and thereby influence the control of myocardial contractility.

POTASSIUM MOVEMENTS IN RAT UTERUS STUDIED IN VITRO: I. EFFECTS OF TEMPERATURE

1. Potassium movements have been studied in vitro in uteri of estrogenpretreated rats with42K as a tracer. At 37 °C the uterus was nearly in potassium balance in Krebs–Ringer bicarbonate and the exchange of potassium was adequately described by a single exchange constant, aside from a small fast fraction (17%) which probably contains potassium located superficially on cells as well as the extracellular potassium. No difference could be detected in their rates of exchange of potassium between two portions of the uterine horn, one containing only the longitudinal muscle layer and the other containing the remainder of the wall. The potassium exchange before or after flux correction for diffusion delay was about 5 or 9 moles cm−2sec−1, using a value of v/a of 1.8. There was a slow gain of sodium and water unrelated to potassium loss, attributed to expansion of the extracellular fluid.2. When the temperature of the Ringer fluid was reduced, the uterus remained in potassium balance at 27° and 17 °C. At 7 °C there was a net loss of potassium and exchange could no longer be described by one constant. On going from 37 to 7 °C the uterine horns shortened and the suggestion was made that muscle cells were depolarized initially by cold, or exuded water accounting for the rapidly exchanging fraction of potassium observed at this temperature. Loss of radioactive potassium from the myometrium owing to depolarization and associated with contraction appeared to account for the inhomogeneity on going to 7 °C. The Q10for influx of potassium between 27 and 7 °C was about 3 while that for efflux was about 1.6, excluding the fast fraction present at low temperatures. The Q10for efflux was diminished by depolarization and that for influx increased so that both may have been about 2. When uterine horns were stored overnight in the cold, they lost potassium and gained sodium, chloride, and water, but these ion changes were reversed on rewarming.

POTASSIUM MOVEMENTS IN RAT UTERUS STUDIED IN VITRO: I. EFFECTS OF TEMPERATURE

1. Potassium movements have been studied in vitro in uteri of estrogenpretreated rats with42K as a tracer. At 37 °C the uterus was nearly in potassium balance in Krebs–Ringer bicarbonate and the exchange of potassium was adequately described by a single exchange constant, aside from a small fast fraction (17%) which probably contains potassium located superficially on cells as well as the extracellular potassium. No difference could be detected in their rates of exchange of potassium between two portions of the uterine horn, one containing only the longitudinal muscle layer and the other containing the remainder of the wall. The potassium exchange before or after flux correction for diffusion delay was about 5 or 9 moles cm−2sec−1, using a value of v/a of 1.8. There was a slow gain of sodium and water unrelated to potassium loss, attributed to expansion of the extracellular fluid.2. When the temperature of the Ringer fluid was reduced, the uterus remained in potassium balance at 27° and 17 °C. At 7 °C there was a net loss of potassium and exchange could no longer be described by one constant. On going from 37 to 7 °C the uterine horns shortened and the suggestion was made that muscle cells were depolarized initially by cold, or exuded water accounting for the rapidly exchanging fraction of potassium observed at this temperature. Loss of radioactive potassium from the myometrium owing to depolarization and associated with contraction appeared to account for the inhomogeneity on going to 7 °C. The Q10for influx of potassium between 27 and 7 °C was about 3 while that for efflux was about 1.6, excluding the fast fraction present at low temperatures. The Q10for efflux was diminished by depolarization and that for influx increased so that both may have been about 2. When uterine horns were stored overnight in the cold, they lost potassium and gained sodium, chloride, and water, but these ion changes were reversed on rewarming.

Quantitative analysis of movements of potassium in rabbit auricles

When rabbit auricles are isolated in physiological saline solution at 29 or 37 °C, the influx or efflux of 42 K follows a course which can be described by the sum of two exponential terms. The presence of an initial fast component is more evident in beating than in spontaneously quiescent auricles, but the fast component is not due to beating because it occurs also in auricles in which beating is stopped by carbachol (2 x 10 -6 M). A small loss of potassium occurs in untreated auricles and a larger one under the influence of certain drugs, such as ouabain (10 -5 M) or dinitrophenol (10 -4 M). Several model systems are considered, especially those with three compartments all or partially communicating with each other, both with regard to exchange of tracer and to net changes in the total ionic content of the compartments. The simplest model which describes the observations had 79 ± 2 (S. E.) % of the tissue potassium free to exchange with the medium and the rest exchanging more slowly with the main fraction. In a steady state at 29 °C, in quiescent left auricles 1⋅07 % of the main fraction exchanged with the medium and 0⋅37 % with the slow fraction per minute. The corresponding rates for beating right auricles were 2⋅25 and 0⋅20 % per minute. In the presence of ouabain (10 -5 M) uptake from the medium to the main fraction was reduced to about 40% of the normal rate, with corresponding net loss of potassium. Dinitrophenol reduced or stopped uptake to the slow fraction and also accelerated loss from the main fraction to the medium.