Rb+, Cs+ ions and the inwardly rectifying K+ channels in guinea-pig ventricular cells

Assessing the therapeutic and toxicological effects of cesium chloride following administration to nude mice bearing PC-3 or LNCaP prostate cancer xenografts

PURPOSE

The purpose of this study was to assess the therapeutic and toxicological effects of cesium chloride (CsCl) administration in mice bearing prostate cancer tumors.

METHODS

Three CsCl dose titration studies were completed in tumor-bearing and non-tumor-bearing athymic nude mice. All mice were administered either vehicle (controls), 150, 300, 600, 800, 1,000, or 1,200 mg/kg of CsCl once daily by oral gavage for 30 consecutive days. Body mass was measured daily, food and water consumption were measured every 2 days, and tumor volume was measured twice weekly. Histopathological analysis was conducted on tissues collected from each of the studies. Serum AST/ALT and creatinine were also measured.

RESULTS

Administration of 800-1,200 mg/kg CsCl reduced PC-3 tumor growth but had no effect on LNCaP tumors. Administration of 800-1,200 mg/kg CsCl also resulted in increased water consumption, bladder crystal development, and higher prevalence of cardiac fibrin clots. An observed loss in body mass was dependent on the xenograft type and concentration of CsCl administered. CsCl did not affect serum AST/ALT and creatinine levels.

CONCLUSIONS

CsCl may have a therapeutic effect against prostate cancer, but one cannot overlook the acute toxicities also described.

Comparison on plasma caesium kinetics in goats and horses with special emphasis on exercising horses

AIMS

Like potassium (K+), caesium (Cs+) tends to concentrate intracellularly. The aim here was to determine how moderate exercise affects the uptake of Cs+ from blood plasma.

METHODS

After an intravenous Cs+ dose of 5 micromol kg(-1), plasma Cs+ concentration was followed for 100 min in goats and for 60 min in horses. The latter were divided into two groups, one resting and the other trotting on a treadmill (inclination 3 degrees, speed 5 m s(-1)).

RESULTS

The plasma Cs+ concentration follows a multiphase exponential decay curve, which initially could be approximated with a two-phase curve. The initially high rate constant (approximately 10 h(-1)) decreased to around 1 h(-1) in 40 min. Exercise more than doubled the rate of removal of Cs+ from plasma between 20 and 40 min after the start of exercise. After exercise, the rate returned to resting levels within 10 min. Plasma K+, on the contrary, declined for at least 20 min after exercise had ended.

CONCLUSIONS

Moderate exercise significantly increases the rate of removal of Cs+ from the bloodstream. After exercise, the rate returns to the resting levels within 10 min. The increased rate of Cs+ removal during exercise is likely due to increased activity of Na+, K+-ATPase in working skeletal muscles.

Isolation and characterization of I(Kr) in cardiac myocytes by Cs+ permeation

Isolation of the rapidly activating delayed rectifier potassium current (I(Kr)) from other cardiac currents has been a difficult task for quantitative study of this current. The present study was designed to separate I(Kr) using Cs+ in cardiac myocytes. Cs+ have been known to block a variety of K+ channels, including many of those involved in the cardiac action potential such as inward rectifier potassium current I(K1) and the transient outward potassium current I(to). However, under isotonic Cs+ conditions (135 mM Cs+), a significant membrane current was recorded in isolated rabbit ventricular myocytes. This current displayed the voltage-dependent onset of and recovery from inactivation that are characteristic to I(Kr). Consistently, the current was selectively inhibited by the specific I(Kr) blockers. The biophysical and pharmacological properties of the Cs+-carried human ether-a-go-go-related gene (hERG) current were very similar to those of the Cs+-carried I(Kr) in ventricular myocytes. The primary sequence of the selectivity filter in hERG was in part responsible for the Cs+ permeability, which was lost when the sequence was changed from GFG to GYG, characteristic of other, Cs+-impermeable K+ channels. Thus the unique high Cs+ permeability in I(Kr) channels provides an effective way to isolate I(Kr) current. Although the biophysical and pharmacological properties of the Cs+-carried I(Kr) are different from those of the K+-carried I(Kr), such an assay enables I(Kr) current to be recorded at a level that is large enough and sufficiently robust to evaluate any I(Kr) alterations in native tissues in response to physiological or pathological changes. It is particularly useful for exploring the role of reduction of I(Kr) in arrhythmias associated with heart failure and long QT syndrome due to the reduced hERG channel membrane expression.

Transient outward current carried by inwardly rectifying K+channels in guinea pig ventricular myocytes dialyzed with low-K+solution

There have been periodic reports of nonclassic (4-aminopyridine insensitive) transient outward K+current in guinea pig ventricular myocytes, with the most recent one describing a novel voltage-gated inwardly rectifying type. In the present study, we have investigated a transient outward current that overlaps inward Ca2+current ( ICa,L) in myocytes dialyzed with 10 mM K+solution and superfused with Tyrode’s solution. Although depolarizations from holding potential ( Vhp) −40 to 0 mV elicited relatively small inward ICa,Lin these myocytes, removal of external K+or addition of 0.2 mM Ba2+more than doubled the amplitude of the current. The basis of the enhancement of ICa,Lwas the suppression of a large transient outward K+current. Similar enhancement was observed when Vhpwas moved to −80 mV and test depolarizations were preceded by short prepulses to −40 mV. Investigation of the time and voltage properties of the outward K+transient indicated that it was inwardly rectifying and unlikely to be carried by voltage-gated channels. The outward transient was attenuated in myocytes dialyzed with high-Mg2+solution, accelerated in myocytes dialyzed with 100 μM spermine solution, and abolished with time in myocytes dialyzed with ATP-free solution. These and other findings suggest that the outward transient is a component of classic “time-independent” inwardly rectifying K+current.

Effects of caffeine on potassium currents in isolated rat ventricular myocytes

Rapid exposure of cardiac muscle to high concentrations of caffeine releases Ca(2+) from the sarcoplasmic reticulum (SR). This Ca(2+) is then extruded from the cell by the Na(+)/Ca(2+) exchanger. Measurement of the current carried by the exchanger (I(Na/Ca)) can therefore be used to estimate of the Ca(2+) content of the SR. Previous studies have shown that caffeine, however, can also inhibit K(+) currents. We therefore investigated whether the inhibitory effects of caffeine on these currents could contaminate measurements of I(Na/Ca). Caffeine caused partial inhibition of the inward rectifier K(+) current (I(K1)): the outward current at -40 mV was 1.15+/-0.24 pA/pF in control and decreased to 0.34+/-0.15 pA/pF in the presence of 10 mmol/l caffeine (P<0.05, n=15). This was similar to the effect of caffeine on the holding current observed at -40 mV in the absence of K(+) channel block and could therefore account for the contaminating effects of caffeine observed during measurements of I(Na/Ca). Moreover, caffeine also partially inhibited the transient outward ( I(to)) and the delayed rectifier (I(K)) K(+) currents.

Characterization of inwardly rectifying K(+) conductance across the basolateral membrane of rat tracheal epithelia

The rat primary cultured-airway monolayer has been an excellent model for deciphering the ion channel after nystatin permeabilization of its basolateral or apical membrane. Inwardly rectifying K(+) currents were characterized across the basolateral membrane in symmetrical HCO(-)(3)-free high K(+) Ringer's solution (125 mM) in this study. The potency of K(+) channel inhibitors against K(+) conductance was Ba(2+) (IC(50) = 5 microM) > Cs(+) (IC(50) = 2 mM) >> glybenclamide (IC(50) > 5 mM) >> TEA (IC(50) >> 100 mM). The application of basolateral Cs(+) changed K(+) conductance into an oscillating current, and its frequency (holding voltage = -100 mV) increased with increase in concentration of basolateral Cs(+) (0.05-5 mM) and in degree of hyperpolarization. Addition of basolateral Cs(+) blocked inward current strongly at -100 mV and hardly at all at -60 mV, giving a sharp curvature to the I-V relation of the IRK current. RT-PCR, Western blotting, and immunohistochemical analyses showed that Kir2.1 might be present in basolateral membrane of tracheal epithelia and plasma membrane of pulmonary alveolar cells.

Kcnkø: single, cloned potassium leak channels are multi-ion pores

KCNKØ was the first clone to show attributes of a leak conductance: voltage-independent potassium currents that develop without delay. Its novel product is predicted to have two nonidentical P domains and four transmembrane segments and to assemble in pairs. Here, the mechanistic basis for leak is examined at the single-channel level. KCNKØ channels open at all voltages in bursts that last for minutes with open probability close to 1. The channels also enter a minutes-long closed state in a tightly regulated fashion. KCNKØ has a common relative permeability series (Eisenman type IV) but conducts only thallium and potassium readily. KCNKØ exhibits concentration-dependent unitary conductance, anomalous mole fraction behavior, and pore occlusion by barium. These observations argue for ion-channel and ion-ion interactions in a multi-ion pore and deny the operation of independence or constant-field current formulations. Despite their unique function and structure, leakage channels are observed to operate like classical potassium channels formed with one-P-domain subunits.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.