Increase In Potassium Permeability Research Articles

The effect of valinomycin in fibroblasts from patients with fatty acid oxidation disorders

Disorders of the carnitine cycle and of the beta oxidation spiral impair the ability to obtain energy from fats at time of fasting and stress. This can result in hypoketotic hypoglycemia, cardiomyopathy, cardiac arrhythmia and other chronic medical problems. The in vitro study of fibroblasts from patients with these conditions is impaired by their limited oxidative capacity. Here we evaluate the capacity of valinomycin, a potassium ionophore that increases mitochondrial respiration, to increase the oxidation of fatty acids in cells from patients with inherited fatty acid oxidation defects. The addition of valinomycin to fibroblasts decreased the accumulation of the lipophilic cation tetraphenylphosphonium (TPP+) at low concentrations due to the dissipation of the mitochondrial membrane potential. At higher doses, valinomycin increased TPP+ accumulation due to the increased potassium permeability of the plasma membrane and subsequent cellular hyperpolarization. The incubation of normal fibroblasts with valinomycin increased [14C]-palmitate oxidation (measured as [14C]O2 release) in a dose-dependent manner. By contrast, valinomycin failed to increase palmitate oxidation in fibroblasts from patients with very long chain acyl CoA dehydrogenase (VLCAD) deficiency. This was not observed in fibroblasts from patients heterozygous for this condition. These results indicate that valinomycin can increase fatty acid oxidation in normal fibroblasts and could be useful to differentiate heterozygotes from patients affected with VLCAD deficiency.

The vagus and the heart: revisiting an early contribution to a still on-going dispute

The vagus and the heart: revisiting an early contribution to a still on-going dispute

Cerebellar Golgi cell inhibition gets slowly more complicated

The cellular architecture of the cerebellum that was so beautifully described by Ramon y Cajal (1899) offers one of the most complete systems for understanding how networks of excitatory and inhibitory neurons interact with one another to control the flow of sensory information in the mammalian brain. However, despite decades of anatomical and electrophysiological studies, this relatively simple and well-defined neuronal circuit still holds a few surprises. For example, in a recent issue of The Journal of Physiology, Holtzman and colleagues (2011) demonstrate some unusual features of cerebellar Golgi cell function. Our view of cerebellar Golgi cells has, in many respects, changed very little since Eccles first described the basic features of feed-forward and feed-back inhibition of granule cells by Golgi cell interneurons in the 1960s (Eccles, 1964). Cerebellar Golgi cells are one of the few cell types that can control granule cell firing within the cerebellar cortex due to their ability to release the inhibitory neurotransmitter γ-aminobutyric acid (GABA) onto both synaptic and extrasynaptic GABAA receptors that are expressed on granule cells (Brickley et al. 1996; Crowley et al. 2009). The current study is concerned with the excitatory drive onto Golgi cells that triggers the release of this GABA onto granule cells. Excitatory drive onto Golgi cells arises from glutamatergic mossy fibre inputs that convey sensory information into the cerebellar cortex (Kanichay & Silver, 2008), and from the glutamate released from granule cell axons that form the parallel fibre inputs onto Golgi cell dendrites (Dieudonne, 1998). It is this feedback loop involving the mossy-fibre → granule cell → Golgi cell → granule cell circuit that has been the focus of attention for the current study. Holtzmann et al. used in vivo recording techniques to monitor action potential firing patterns from Golgi cells during parallel fibre stimulation. In this way they confirmed an observation that was first made in an acute in vitro slice preparation of the cerebellum (Watanbe & Nakanishi, 2003) that the release of glutamate from granule cells leads to a decrease in Golgi cell firing through activation of mGluR2 receptors. This contradicts the classical view that granule cells serve only to excite Golgi cells. The decrease in Golgi firing that occurs following parallel fibre input lasts for hundreds of milliseconds, resulting in a conspicuous disinhibition of granule cells. This sustained depression of Golgi cell firing has been reported previously in vivo (Vos et al. 1999; Holtzman et al. 2006), but the authors have now confirmed the involvement of mGluR2 activation in this process. Moreover, the authors now show that it is possible to trigger this mGluR2-mediated disinhibition by modulating granule cell excitability through an enhancement of the tonic inhibition that is present on granule cells. In this study, tonic inhibition of granule cells was enhanced using the application of the GABA agonist gaboxadol (or THIP) that is known to have a selective action on extrasynaptic GABAA receptors that contain the delta subunit when used at sub-micromolar concentrations. Obviously, in an in vivo study of this type it is difficult to establish the effective concentration of the drug within the cerebellum, but it would appear that increased glutamate release from granule cells can lead to mGluR2 activation and, therefore, inhibition of Golgi cell firing. The authors further speculate that a spillover-type mechanism is involved in this glutamate activation of mGluR receptors on Golgi cells in an analogous manner to the GABA activation of extrasynaptic GABAA receptors. Spillover refers to the fact that glutamate diffuses out of the synaptic cleft to activate mGluR2 receptors located some distance away from the synapse. Binding of this ‘escaped’ glutamate to mGluR2 receptors is assumed to reduce Golgi cell firing due to an increased potassium permeability associated with activation of this G-protein-coupled receptor. However, an involvement of other mechanisms such as electrical coupling via gap-junctions cannot be ruled out at this stage (e.g. Vervaeke et al. 2010). Nevertheless, it is interesting to speculate that this form of slow intercellular signalling may support oscillations in granule cell firing such that modulation of granule cell activity levels over such slow time-scales are matched to incoming sensory inputs of the type observed during sleep and certain types of behaviour (Dugueet al. 2009; Roset al. 2009). Given the importance of the cerebellar cortex in the acquisition of new motor skills, an important challenge for the future will be to determine how these types of intercellular signalling mechanisms may impact upon granule cell firing during motor learning tasks.

Cardiac Arrest during Gamete Release in Chum Salmon Regulated by the Parasympathetic Nerve System

Cardiac arrest caused by startling stimuli, such as visual and vibration stimuli, has been reported in some animals and could be considered as an extraordinary case of bradycardia and defined as reversible missed heart beats. Variability of the heart rate is established as a balance between an autonomic system, namely cholinergic vagus inhibition, and excitatory adrenergic stimulation of neural and hormonal action in teleost. However, the cardiac arrest and its regulating nervous mechanism remain poorly understood. We show, by using electrocardiogram (ECG) data loggers, that cardiac arrest occurs in chum salmon (Oncorhynchus keta) at the moment of gamete release for 7.39±1.61 s in females and for 5.20±0.97 s in males. The increase in heart rate during spawning behavior relative to the background rate during the resting period suggests that cardiac arrest is a characteristic physiological phenomenon of the extraordinarily high heart rate during spawning behavior. The ECG morphological analysis showed a peaked and tall T-wave adjacent to the cardiac arrest, indicating an increase in potassium permeability in cardiac muscle cells, which would function to retard the cardiac action potential. Pharmacological studies showed that the cardiac arrest was abolished by injection of atropine, a muscarinic receptor antagonist, revealing that the cardiac arrest is a reflex response of the parasympathetic nerve system, although injection of sotalol, a β-adrenergic antagonist, did not affect the cardiac arrest. We conclude that cardiac arrest during gamete release in spawning release in spawning chum salmon is a physiological reflex response controlled by the parasympathetic nervous system. This cardiac arrest represents a response to the gaping behavior that occurs at the moment of gamete release.

The use of nicorandil in cardioplegia solution

EDITOR: We would like to comment on the article by Chinnan and co-workers [1] written on myocardial protection by nicorandil during open-heart surgery under cardiopulmonary bypass (CPB). In relation to cardioplegia, coronary artery spasm (CAS) is frequently underestimated during CPB. Until now, CAS was not detectable with current monitoring techniques during CPB and its impact on cardiac surgery has not been elucidated in previous studies. During CAS the blood flow to the myocardium decreases depending on the degree of coronary vasoconstriction, thereby increasing anaerobic metabolites and myocardial ischaemia. Consequently, CAS could be responsible for right and left ventricular failure after leaving CPB. Therefore, apart from being a potassium channel opening drug preventing intracellular Ca2+ loading [2], the fact that nicorandil is a coronary vasodilator might have contributed to favourable results in your study. The link between vasospastic angina and vulnerability for ventricular tachycardia or ventricular fibrillation outside the setting of cardiac surgery has already been described in several case reports [3-5]. Iida and colleagues [5] reported that ventricular fibrillation improved and disappeared with the start of treatment with nitrates and suggested that CAS should be considered as a differential diagnosis in the presence of ST-segment changes and/or intractable ventricular arrhythmias. Extending this conclusion into CPB surgery, this could imply that rather than giving an anti-arrhythmic drug, a coronary vasodilator should be used to prevent and to treat arrhythmias secondary to ischaemic episodes caused by CAS. As your results show, a link between CAS and ventricular fibrillation seems to be apparent. After cross-clamp removal in coronary artery bypass grafting (CABG) patients, two patients in the nicorandil group developed significant cardiac dysrythmias vs. six patients in the placebo group. Unfortunately, the number of your patients is limited and it would be interesting to pursue your study as a multicentre trial on a larger scale. The relationship between nicorandil and coronary vasodilatation also explains the enhanced distribution of cardioplegia solution, a faster time until electromechanical arrest in the nicorandil group and a faster return of electromechanical activity after aortic cross clamp removal in patients under nicorandil in the mitral valve replacement group. Interestingly, although not significantly different, in CABG patients four nicorandil patients compared to two placebo patients had creatine kinase (CK-MB) levels higher than 75 IU L−1. Within the patients treated with nicorandil in the CABG group, as far as it can be concluded from Table 3, the rise in CK-MB mainly occurred between 6 and 24 h postoperatively, which is delayed in respect to the nicorandil patients for mitral valve replacement where the peak is at 6 h postoperatively. You conclude that these patients receiving nicorandil had a lower incidence of significant elevation of postoperative myocardial damage, suggesting that ongoing myocardial injury in the postoperative period might possibly be due to reperfusion injury and that nicorandil may have attenuated this due to its anti-inflammatory property. This could be correct but is in contrast to the finding that four nicorandil CABG patients compared to two placebo CABG patients had significantly elevated CK-MB levels. Theoretically, the incidence of significantly elevated CK-MB levels should be lower in the nicorandil group. Nonetheless, this could mean that there are multifactorial causes for postoperative myocardial infarction in your patients. When did the myocardial infarction actually happen in both groups and could other factors have been responsible such as the quality of coronary vessels? In addition, could postoperative factors rather than perioperative management have influenced the occurrence of postoperative infarction in the cases you mentioned? Unfortunately, the limited number of patients in your study precludes any conclusion on this issue. Ideally, troponin I should also have been measured. Nevertheless, the general outcome in your publication is favourable for the use of nicorandil during cardioplegia. Just as a remark, because enzymes of the potassium channel openers work more efficiently during normothermia, warm cardioplegia could have improved results due to increased potassium permeability leading to rapid sinus node arrest. In addition, Cohen and colleagues [6] concluded that terminal infusion of warm blood cardioplegia repleted myocardial ATP levels and improved postoperative myocardial function. In general, hypothermic cardioplegia should be avoided because it can trigger CAS, thereby inducing ventricular fibrillation leading to depletion of myocardial ATP. Warm cardioplegia was not applied in your study. On the other hand, CK-MB levels under cold cardioplegia were improved in mitral valve replacement patients with nicorandil, which make your results even more interesting because CAS is more enhanced by hypothermia. In spite of above remarks, we want to congratulate Chinnan and co-workers for their excellent paper which brings us one step further ahead to optimize coronary blood flow during CPB and thereby decreasing events of myocardial ischaemia.

Simplified interpretation of the pacemaker potential as a tool for teaching membrane potentials.

Most courses of physiology start by teaching about membrane potentials in different cells. Many of our students find these ideas difficult to understand. Often they try to memorize facts rather than understand mechanisms. The most difficult task may be interpretation of the pacemaker potential

Blockade of myocardial ATP-sensitive potassium channels by ketamine.

The adenosine triphosphate (ATP)-sensitive potassium (KATP) channel underlies the increase in potassium permeability during hypoxia and ischemia. The increased outward potassium current during ischemia may be an endogenous cardioprotective mechanism. This study was designed to determine the effects of ketamine on KATP channel in rat hearts. Inside-out and cell-attached configurations of patch-clamp techniques and 3 M potassium chloride-filled conventional microelectrodes were used to investigate the effect of ketamine on KATP channel currents in single rat ventricular myocytes and on the action potential duration of rat papillary muscles, respectively. Ketamine inhibited KATP channel activity in rat ventricular myocytes in a concentration-dependent manner. In the inside-out patches, the concentration of ketamine for half-maximal inhibition and the Hill coefficient were 62.9 microM and 0.54, respectively. In a concentration-dependent manner, ketamine inhibited pinacidil- and 2,4-dinitrophenol-activated KATP channels in cell-attached patches. The application of ketamine to the intracellular side of membrane patches did not affect the conduction of single-channel currents of KATP channels. Ketamine increased the action potential duration, which was then shortened by pinacidil in a concentration-dependent manner. Ketamine inhibited KATP channel activity in a concentration-dependent manner. These results suggest that ketamine may attenuate the cardioprotective effects of the KATP channel during ischemia and reperfusion in the rat myocardium.

Modulation of the isoprenaline‐induced membrane hyperpolarization of mouse skeletal muscle cells

1. The hyperpolarization of the resting membrane potential, Vm, induced by isoprenaline in the lumbrical muscle fibres of the mouse, was investigated by use of intracellular microelectrodes. 2. In normal Krebs-Henseleit solution (potassium concentration: K+o = 5.7 mM, 'control'), Vm was -7.40 +/- 0.2 mV; lowering K+o to 0.76 mM ('low K+o') resulted in either a hyperpolarization (Vm = -95.7 +/- 2.9 mV), or a depolarization (Vm = -52.0 +/- 0.3 mV). 3. Isoprenaline (> or = 200 nM) induced a hyperpolarization of Vm by delta Vm = -5.6 +/- 0.4 mV in control solution. 4. When Vm hyperpolarized after switching to low K+o, the addition of isoprenaline resulted in increased hyperpolarization Vm: delta Vm = -16.3 +/- 3.2 mV to a final Vm = -110.1 +/- 3.4 mV. Adding iso-prenaline when Vm depolarized in low K+o, leads to a hyperpolarization of either by -11.6 +/- 0.5 mV to -63.6 +/- 0.8 mV or by -51.7 +/- 2.7 mV to -106.9 +/- 3.9 mV. 5. Ouabain (0.1 to 1 mM) did not suppress the hyperpolarization by isoprenaline in 5.7 mM K+o (delta Vm = -6.7 +/- 0.4 mV) or the hyperpolarization of the depolarized cells in low K+- (delta Vm = -9.7 +/- 1.5 mV). 6. The hyperpolarization is a logarithmically decreasing function of K+o in the range between 2 and 20 mM (12 mV/decade). 7.IBMX and 8Br-cyclic AMP mimicked the response to isoprenaline whereas forskolin (FSK) induced in low K+o a hyperpolarization of -7.0 +/- 0.7 mV that could be augmented by addition of isoprenaline (delta Vm = -8.2 +/- 1.8 mV). 8. In control and low K+o, Ba2+ (0.6 mM) inhibited the hyperpolarization induced by isoprenaline, IBMX or 8Br-cyclic AMP. Other blockers of the potassium conductance such as TEA (5 mM) and apamin (0.4 microM) had no effect. 9. We conclude that in the lumbrical muscle of the mouse the isoprenaline-induced hyperpolarization is primarily due to an increase in potassium permeability.

The effect of a phorbol ester upon the cholinergic regulation of potassium permeability in the rat submandibular gland.

Acetylcholine releases calcium from cytoplasmic stores and permits an influx of calcium in salivary acinar cells. The resultant rise in [Ca2+]i causes an increase in potassium permeability which is an important part of the secretory response. We have investigated the effects of 12-O-tetradecanoyl phorbol-13-acetate, a potent activator of protein kinase C, upon this regulation of potassium permeability in superfused pieces of rat submandibular salivary gland. This compound inhibited the initial [Ca2+]O-independent component of the response of acetylcholine but had no effect upon the subsequent [Ca2+]O-dependent phase. This compound does not, therefore, appear to inhibit receptor-regulated calcium influx.

Pinacidil-induced electrical heterogeneity and extrasystolic activity in canine ventricular tissues. Does activation of ATP-regulated potassium current promote phase 2 reentry?

Pinacidil is known to augment a time-independent outward current in cardiac tissues by activating the ATP-regulated potassium channels. Activation of this current, IK-ATP, is thought to be responsible for increased potassium permeability in ischemia. The contribution of IK-ATP activation to arrhythmogenesis and the role of activation of this current in suppression of arrhythmias are areas of great interest and debate. Because electrical depression attending myocardial ischemia is more accentuated in ventricular epicardium than in endocardium, we endeavored to contrast the effects of pinacidil-induced IK-ATP activation on the electrophysiology of canine ventricular epicardium and endocardium. Standard microelectrode techniques were used. Pinacidil (1 to 5 mumol/L) produced a marked dispersion of repolarization and refractoriness in isolated canine ventricular epicardium as well as between epicardium and endocardium. In endocardium, pinacidil abbreviated action potential duration (APD90) and refractoriness by 8.0 +/- 2.3%. In epicardium, the effects of pinacidil were nonhomogeneous. At some sites, pinacidil induced an all-or-none repolarization at the end of phase 1 of the action potential, resulting in 55.5 +/- 8.7% abbreviation of APD90 and refractoriness. Adjacent to these were sites at which the dome was maintained with only minor changes in APD and refractoriness. Extrasystolic activity displaying features of reentry was observed in isolated sheets of epicardium (63.2%) after exposure to pinacidil (1 to 5 mumol/L) but never in its absence. Dispersion of repolarization and ectopic activity was most readily induced in epicardium by a slowing of the stimulation rate in the presence of pinacidil. Electrical homogeneity was restored and arrhythmias abolished after washout of pinacidil or addition of either a transient outward current blocker, 4-aminopyridine, or a blocker of the ATP-regulated potassium channels, glybenclamide. Our data suggest that the activation of IK-ATP can produce a marked dispersion of repolarization and refractoriness in epicardium as well as between epicardium and endocardium, leading to the development of extrasystolic activity via a mechanism that we have called phase 2 reentry. The available data also suggest that blockade of the transient outward current and/or the ATP-regulated potassium channels may be useful antiarrhythmic interventions under ischemic or "ATP depleted" conditions.

Adenosine as adjunct to potassium cardioplegia: Effect on function, energy metabolism, and electrophysiology

Adenosine is known to induce rapid cardioplegic arrest and to improve postischemic recovery in the isolated rat heart. Long exposures to high doses of adenosine impair postischemic recovery, however. In this paper we tested the combination of low-dose adenosine (1 mmol/L) with potassium (26 mmol/L), with the aim of achieving rapid arrest (as with high-dose adenosine) but eliminating the need for postarrest washout of adenosine. Cardioplegic solutions studied were (1) Krebs-Henseleit potassium (26 mmol/L) (K); (2) K plus adenosine (1 mmol/L) (KA); (3) K plus an adenosine deaminase inhibitor [erythro-9-(2-hydroxy-3-nonyl)adenine] (0.1 mmol/L) (KE); and as control (4) Krebs-Henseleit potassium (6 mmol/L) (C). We induced cardiac arrest in Langendorff-perfused rat hearts by infusing the cardioplegic solution for 3 minutes at 3 ml/min. Total ischemia lasted 20 minutes at 37 degrees C, followed by reperfusion for 30 minutes. High potassium decreased the arrest time from 260 +/- 16 seconds (group C, mean values +/- standard error of the mean) to 22 +/- 4 seconds (group K). A further decrease to 10 +/- 2 seconds was observed with KA (p = 0.016 versus K). KE, which increased endogenous adenosine, gave intermediate effects. All hearts recovered during reperfusion; the product of developed tension and heart rate (grams per minute) was superior in KA hearts (6250 +/- 740 versus K hearts 4380 +/- 390; p = 0.050). KE gave an intermediate result (5290 +/- 900), while C showed the worst recovery (3180 +/- 830). Our electrophysiologic studies with sinus node and atrial tissue suggest that adenosine induced hyperpolarization and an increase in potassium permeability, thereby arresting the sinus node before depolarization of the membrane by potassium (26 mmol/L). We conclude that low-dose adenosine as an adjunct to potassium shortens the arrest time in this model and improves postischemic recovery.

Analysis of the hyperpolarizing effect of catecholamines on canine cardiac Purkinje fibres.

1. The hyperpolarization induced by catecholamines on barium-depolarized (0.2-0.8 mM BaCl) canine cardiac Purkinje fibres, in vitro, was studied by use of conventional microelectrode recordings of transmembrane electrical potentials. 2. Noradrenaline, adrenaline and isoprenaline hyperpolarized Purkinje fibres in a concentration-dependent manner from a threshold concentration around 5 nM. The three catecholamines were shown to be approximately equipotent. Tachyphylaxis was observed when the interval between catecholamine applications was less than 15 min. 3. Atenolol (10 microM) blocked the hyperpolarization reversibly and theophylline (0.5 mM) potentiated it. 4. Tetrodotoxin (5 microM) did not affect the hyperpolarization induced by isoprenaline. Acetylcholine and histamine, up to 10 microM, were not effective in hyperpolarizing Purkinje fibres. 5. Low extracellular potassium concentrations (zero and 1 mM) did not affect the hyperpolarization, but high extracellular potassium concentrations (10-20 mM), markedly reduced the effect of isoprenaline (100 nM). 6. Reduction of the extracellular sodium concentration produced a roughly proportional reduction in the isoprenaline-induced hyperpolarization. The hyperpolarization was reversibly blocked in 34 mM sodium Tris-Tyrode solution. 7. The hyperpolarization was not reduced in Tyrode solution containing 0.6 mM calcium, but was drastically reduced in zero-calcium Tyrode solution. This effect was reversible. 8. Addition of verapamil (5-10 microM) diminished the hyperpolarization, in a concentration-dependent manner. This effect was partially reversed after washing. 9. Ouabain (0.7-1 microM) significantly reduced the isoprenaline-induced hyperpolarization, but 2,4-dinitrophenol (0.2 mM) did not affect it. 10. Caesium chloride (20 mM) abolished the hyperpolarization. The blockade was only partially reversed upon washing. 11. It is suggested that the hyperpolarization induced by a short exposure to catecholamines is mainly due to an increase in potassium permeability (PK). A mechanism involving calciumdependent potassium channels might underlie the increase in PK.

Ionic currents in neurones cultured from embryonic cockroach (Periplaneta americana) brains.

Neurones isolated from embryonic cockroach brains were maintained in culture for up to 8 weeks. A single patch electrode was used to record voltage changes in response to injected current, membrane ionic currents under whole-cell voltage-clamp conditions or single-channel currents from isolated membrane patches. The voltage changes in response to injected current that depolarized the cell indicated increases in membrane permeability to calcium and potassium. These observations were confirmed using a voltage clamp. The potassium current observed in the youngest cultures turned on with a delay and was blocked by tetraethylammonium (TEA) and 4-aminopyridine (4-AP). Two kinds of decrease in the outward potassium current were observed. One may be associated with extracellular potassium accumulation, inactivation of the potassium channel or inactivation of a calcium channel. The other appears to be a voltage-dependent inactivation. The magnitude of the calcium permeability appeared to increase as the cultures developed, being most prominent in cultures more than 2 weeks old. Single-channel conductance measured from an analysis of records from six isolated membrane patches ranged from 15 to 110 pS. Except for one channel, the probability of the channels being open did not change appreciably with membrane potential. Our results suggest that much of the increase in potassium permeability may be due an increase in intracellular calcium level.

Pharmacologically distinct actions of serotonin on single pyramidal neurones of the rat hippocampus recorded in vitro.

1. The actions of serotonin (5-HT) on pyramidal cells of the CA1 region of the rat hippocampus were characterized using intracellular recording in in vitro brain slices. 2. 5-HT typically evokes a biphasic response consisting of a hyperpolarization which is followed by a longer-lasting depolarization. These effects on membrane potential are accompanied by a decrease in the calcium-activated after-hyperpolarization (a.h.p). 3. Detailed analysis using 5-HT antagonists and agonists indicates that the hyperpolarization is mediated by a 5-HT1A receptor. Spiperone is the most effective antagonist of the response and the selective 5-HT1A agonist, 8-OHDPAT, behaves as a partial agonist at this receptor. In agreement with the distribution of 5-HT1A binding sites, responses to 5-HT were most prominent in the stratum radiatum. 4. The hyperpolarizing response is associated with a decrease in input resistance, is blocked by extracellular barium and intracellular caesium, is unaffected by the chloride gradient, and its reversal potential shifts with the extracellular concentration of potassium as predicted for a response mediated by a selective increase in potassium permeability. 5. The depolarizing response and reduction in the a.h.p. could be studied in isolation by blocking the hyperpolarizing response with either pertussis toxin or spiperone. The pharmacology of these responses did not correspond to that of any of the 5-HT binding sites reported in C.N.S. tissue. Although the depolarization and blockade of the a.h.p. have the same time course it is unclear if they are mediated by the same or different receptors. 6. The depolarization most likely results from a decrease in resting potassium conductance. However, neither a blockade of the M current nor the a.h.p. current can account for the depolarization. 7. Blockade of phosphodiesterase activity by 3-isobutyl-1-methylxanthine (IBMX) did not enhance the depressant action of 5-HT on the a.h.p., making it unlikely that this action is mediated by cyclic AMP. 8. Blockade of the a.h.p. by 5-HT reduces spike frequency adaptation and counteracts the inhibitory action of 5-HT on 5-HT1A receptors. This excitatory action outlasts the hyperpolarizing action. 9. In summary 5-HT acts on at least two distinct receptors on hippocampal pyramidal cells, one coupled to the opening of potassium channels and a second coupled to a decrease in a resting potassium conductance and a decrease in the a.h.p.

Increased Potassium Permeability in Erythrocytes from Patients with Hyperparathyroidism

Parathyroid hormone (PTH) has been shown to modify Ca2+ and Na+ transport in several epithelia. The molecular mechanisms of these effects are poorly understood. We investigated here whether PTH may modify Na+ and K+ transport across the human red blood cell membrane in vitro and ex vivo. Fourteen patients with severe primary or secondary hyperparathyroidism and hypercalcemia were studied before and 5-7 days after surgical parathyroidectomy. Erythrocyte ouabain-sensitive as well as furosemide-sensitive Na+ efflux rates of the patients were comparable to that of healthy volunteers and remained unchanged after parathyroidectomy. Moreover, erythrocyte Na+ fluxes of control subjects remained unchanged when red blood cells were incubated in the presence of 1.0 IU/ml of bovine PTH (1-85). However, erythrocytes from hyperparathyroid patients showed a significant increase in passive K+ permeability when compared to that of healthy controls (p less than 0.05). This abnormality could be corrected in vivo after parathyroidectomy and in vitro using quinine, respectively. It is concluded that hyperparathyroidism induces a moderate increase in Ca2+ dependent K+ permeability of erythrocytes ("Gardos effect") which is reversible after parathyroidectomy.

Increased arterial potassium transport in reduced renal mass hypertension of the rat.

Aortic potassium turnover was studied during the development of hypertension induced by salt load in male rats after 70-75% of total renal mass was removed. Systolic blood pressure in the saline-drinking experimental reduced renal mass (RRM) rats steadily increased until the fourth week after surgery and thereafter stayed at the same level. Control RRM rats given tap water for drinking, and unilaterally nephrectomized saline-drinking control rats maintained normal blood pressure. Compared to controls, experimental RRM rats exhibited increased plasma aldosterone concentration while plasma renin activity was low in all groups with no significant difference. Aortic hypertrophy, greater 42K turnover, and elevated 42K exchange were observed with experimental RRM hypertension. Sensitivity to the effect of norepinephrine (NE) on aortic 42K turnover was increased four- to ninefold in the experimental RRM group as compared to controls. These results indicate that reduced renal mass hypertension is associated with increased potassium permeability and NE supersensitivity in vascular smooth muscle.

Calcium movements accompanying the transport of sugar or amino acid by rabbit enterocytes

Isolated rabbit enterocytes can be loaded with radioactive calcium most of which is presumably in intracellular stores. In the presence of sugar or amino acid there is a transient loss of calcium, followed by replenishment. It is suggested that this movement might be related in the signalling leading to increased potassium permeability observed in enterocytes transporting sugar and amino acids.

Chronic exposure to lead causes persistent alterations in the electric membrane properties of neurons in cell culture.

The effects of chronic lead (Pb) exposure on neuronal electric membrane properties (EMP) were determined using neural cell cultures of adult mouse dorsal root ganglia (DRG). Cultures were exposed to Pb concentrations ranging from 0 to 100 microM for 12 days (8 DIV to 20 DIV). EMP were determined in Pb-free medium either immediately after withdrawal (IWD), or 6 days after withdrawal (6WD) from Pb. For IWD, regression analysis indicated that a number of EMP varied significantly with increasing Pb concentration. The largest such change occurred for electrical excitability which decreased significantly with increasing Pb (P = 0.000), being reduced by approximately two-thirds for neurons exposed to 100 microM Pb; resting membrane potential increased with Pb (P = 0.000); membrane time constant decreased with Pb (P = 0.007); action potential afterhyperpolarization decreased with Pb (P = 0.023). There was also evidence that the time course of action potentials was accelerated with increasing Pb concentrations, the rate of fall of neurons with biphasic falling phases being particularly increased (P = 0.047). This general pattern of altered EMP was observed for the 6WD condition also, indicating that chronic exposure to Pb caused persistent abnormalities in neuronal membranes even after 6 days of cultivation in Pb-free medium. The patterns of alterations in EMP suggested that chronic Pb exposure caused a prolonged increase in potassium permeability. It was proposed that the latter was mediated through a Pb-induced increase in intracellular ionic calcium and the associated disruption of calcium homeostasis.

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 EC 50 of 50 n m. 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 IC 50 of 15 n m. 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 +.

Volume regulation in lymphoid leukemia cells and assignment of cell lineage.

Normal T lymphocytes readjust their volume after swelling in hypotonic medium, whereas B lymphocytes remain swollen. The difference between the two cell types appears to lie in the inability of B cells to increase their permeability to potassium in response to swelling. We have studied mononuclear cells from the bone marrow of 65 patients with lymphocytic leukemia, to determine whether the response to a hypotonic environment could be used to assign cell lineage. In contrast to leukemic cells of B-cell lineage (as in B-cell acute or chronic lymphocytic leukemia and non-T-cell, non-B-cell acute lymphocytic leukemia), T-cell-leukemia cells responded within five minutes to hypotonic swelling with an increase in potassium permeability and restoration toward isotonic volume. We conclude that the response to hypotonic shock may distinguish leukemic cells of T-cell origin from those derived from B-cell precursors and provides a simple, rapid, and reliable means for assigning cell lineage to lymphoid leukemia cells.