Interpretation of relevance of sodium–calcium exchange in action potential of diabetic rat heart by mathematical model

Sarcolemmal Na⁺–Ca²⁺ exchange plays a central role in ion transport of the myocardium and the current carried with it contributes to the late phase of the action potential (AP) besides the contribution of outward K⁺-currents. In this study, the mathematical model for AP of the diabetic rat ventricular myocytes [34] was modified and used for the diabetic rat papillary muscle. We used our experimentally measured values of two K⁺-currents; transient outward current, I_to and steady-state outward current, I_ss, as well as L-type Ca²⁺-current, I_CaL, then compared with the simulated values. We have demonstrated that the prolongation in the AP of the papillary muscle of the diabetic rats are not due to the alteration of I_CaL but mainly due to the inhibition of the K⁺-currents and also the Na⁺–Ca²⁺ exchanger current, I_Na–Ca. In combination with our experimental data on sodium-selenite-treated diabetic rats, our simulation results provide new information concerning plausible ionic mechanisms, and second a possible positive effect of selenium treatment on the altered I_Na–Ca for the observed changes in the AP duration of streptozotocin-induced diabetic rat heart. (Mol Cell Biochem 269: 121–129, 2005)     

A mathematical model of action potential heterogeneity in adult rat left ventricular myocytes.

Mathematical models were developed to reconstruct the action potentials (AP) recorded in epicardial and endocardial myocytes isolated from the adult rat left ventricle. The main goal was to obtain additional insight into the ionic mechanisms responsible for the transmural AP heterogeneity. The simulation results support the hypothesis that the smaller density and the slower reactivation kinetics of the Ca(2+)-independent transient outward K(+) current (I(t)) in the endocardial myocytes can account for the longer action potential duration (APD), and more prominent rate dependence in that cell type. The larger density of the Na(+) current (I(Na)) in the endocardial myocytes results in a faster upstroke (dV/dt(max)). This, in addition to the smaller magnitude of I(t), is responsible for the larger peak overshoot of the simulated endocardial AP. The prolonged APD in the endocardial cell also leads to an enhanced amplitude of the sustained K(+) current (I(ss)), and a larger influx of Ca(2+) ions via the L-type Ca(2+) current (I(CaL)). The latter results in an increased sarcoplasmic reticulum (SR) load, which is mainly responsible for the higher peak systolic value of the Ca(2+) transient [Ca(2+)](i), and the resultant increase in the Na(+)-Ca(2+) exchanger (I(NaCa)) activity, associated with the simulated endocardial AP. In combination, these calculations provide novel, quantitative insights into the repolarization process and its naturally occurring transmural variations in the rat left ventricle.     

Na+-Ca2+ exchange activity in rat skeletal myotubes: effect of lithium ions

The whole-cell patch-clamp technique coupled with intracellular [Ca2+] measurements was used to investigate the sodium-calcium exchange mechanism in rat skeletal muscle cells in primary culture. Replacing external Na+ ions with Li+ or N-methyl-D-glucamine (NMDG+) ions generated outward currents which were correlated with significant increases of free cytosolic-calcium concentration. These results strongly argue for a functional Na+-Ca2+ exchange mechanism working in its reverse mode. Moreover, the outward currents were sensitive to the new compound KB-R7943 (10 microM), which has been shown to be a potent inhibitor of the sodium-calcium exchanger. Outward Na+-Ca2+ exchange current densities were reduced in the presence of external Li+ as compared to those measured in the presence of NMDG+. After replacing internal sodium by lithium ions, rapid changes of external lithium concentrations generated sarcolemmal currents which were accompanied by subsequent variations of intracellular calcium activity. The currents were dependent on extracellular Li+ with a half-maximal activation at 67 mM and a Hill coefficient of 2.9. This work shows that the Na+-Ca2+ exchanger is able to significantly influence the myoplasmic calcium concentration of cultured rat myotubes. On the other hand, our results suggest that Li+ ions may substitute Na+ ions to catalyse an electrogenic Li+/Ca2+ counter transport.     

Simulation of the undiseased human cardiac ventricular action potential: model formulation and experimental validation

Cellular electrophysiology experiments, important for understanding cardiac arrhythmia mechanisms, are usually performed with channels expressed in non myocytes, or with non-human myocytes. Differences between cell types and species affect results. Thus, an accurate model for the undiseased human ventricular action potential (AP) which reproduces a broad range of physiological behaviors is needed. Such a model requires extensive experimental data, but essential elements have been unavailable. Here, we develop a human ventricular AP model using new undiseased human ventricular data: Ca(2+) versus voltage dependent inactivation of L-type Ca(2+) current (I(CaL)); kinetics for the transient outward, rapid delayed rectifier (I(Kr)), Na(+)/Ca(2+) exchange (I(NaCa)), and inward rectifier currents; AP recordings at all physiological cycle lengths; and rate dependence and restitution of AP duration (APD) with and without a variety of specific channel blockers. Simulated APs reproduced the experimental AP morphology, APD rate dependence, and restitution. Using undiseased human mRNA and protein data, models for different transmural cell types were developed. Experiments for rate dependence of Ca(2+) (including peak and decay) and intracellular sodium ([Na(+)](i)) in undiseased human myocytes were quantitatively reproduced by the model. Early afterdepolarizations were induced by I(Kr) block during slow pacing, and AP and Ca(2+) alternans appeared at rates >200 bpm, as observed in the nonfailing human ventricle. Ca(2+)/calmodulin-dependent protein kinase II (CaMK) modulated rate dependence of Ca(2+) cycling. I(NaCa) linked Ca(2+) alternation to AP alternans. CaMK suppression or SERCA upregulation eliminated alternans. Steady state APD rate dependence was caused primarily by changes in [Na(+)](i), via its modulation of the electrogenic Na(+)/K(+) ATPase current. At fast pacing rates, late Na(+) current and I(CaL) were also contributors. APD shortening during restitution was primarily dependent on reduced late Na(+) and I(CaL) currents due to inactivation at short diastolic intervals, with additional contribution from elevated I(Kr) due to incomplete deactivation.     

Effect of reduced Na gradient on electrical activity in isolated bovine and feline ventricular myocytes

We have studied changes in electrical activity resulting from abrupt alterations of the Na gradient, using ventricular myocytes isolated from feline and bovine hearts. Attempting to investigate the ionic current possibly generated by Na-Ca exchange, we studied the effects of the changes in [Na]o in the presence of 20 mM CsCl to inhibit K currents. To facilitate the effect of Cs, we also used a K-free solution and a patch electrode filled with 150 mM cesium glutamate. The application of 20 mM Nao resulted in hyperpolarization and the action potential duration was reduced. Under voltage clamp, 20 of 45 mM Nao generated an outward current at all membrane potentials investigated. The initial part (100-200 ms) of this current was only partially inhibited by 5 mM NiCl2 which is known to fully block the Ca inward current. However, the outward current generated by the reduced [Na]o was fully inhibited by 20 mM MnCl2 (which presumably inhibits Na-Ca exchange). Our observations extend the work on multicellular cardiac preparations indicating that the outward current elicited by a sudden decrease in Na gradient could be generated by Na-Ca exchange. Although the characteristics of this outward current support certain concepts of the Na-Ca exchange in cardiac muscle, we cannot at present exclude a contribution of other membrane current(s).     

Effects of selenium on altered mechanical and electrical cardiac activities of diabetic rat

Since selenium compounds can restore some metabolic parameters and structural alterations of diabetic rat heart, we were tempted to investigate whether these beneficial effects extend to the diabetic rat cardiac dysfunctions. Diabetes was induced by streptozotocin (50mg/kg body weight) and rats were then treated with sodium selenite (5 micromol/kg body weight/day) for four weeks. Electrically stimulated isometric contraction and intracellular action potential in isolated papillary muscle strips and transient (I(to)) and steady state (I(ss)) outward K(+) currents in isolated cardiomyocytes were recorded. Sodium selenite treatment could reverse the prolongation in both action potential duration and twitch duration of the diabetic rats, and also cause significant increases in the diminished amplitudes of the two K(+) currents. Treatment of rats with sodium selenite also markedly increased the depressed acid-soluble sulfhydryl levels of the hearts. Our data suggest that the beneficial effects of sodium selenite treatment on the mechanical and electrical activities of the diabetic rat heart appear to be due to the restoration of the diminished K(+) currents, partially, related to the restoration of the cell glutathione redox cycle.     

Increased excitability of acidified skeletal muscle: role of chloride conductance

Generation of the action potentials (AP) necessary to activate skeletal muscle fibers requires that inward membrane currents exceed outward currents and thereby depolarize the fibers to the voltage threshold for AP generation. Excitability therefore depends on both excitatory Na+ currents and inhibitory K+ and Cl- currents. During intensive exercise, active muscle loses K+ and extracellular K+ ([K+]o) increases. Since high [K+]o leads to depolarization and ensuing inactivation of voltage-gated Na+ channels and loss of excitability in isolated muscles, exercise-induced loss of K+ is likely to reduce muscle excitability and thereby contribute to muscle fatigue in vivo. Intensive exercise, however, also leads to muscle acidification, which recently was shown to recover excitability in isolated K(+)-depressed muscles of the rat. Here we show that in rat soleus muscles at 11 mM K+, the almost complete recovery of compound action potentials and force with muscle acidification (CO2 changed from 5 to 24%) was associated with reduced chloride conductance (1731 +/- 151 to 938 +/- 64 microS/cm2, P < 0.01) but not with changes in potassium conductance (405 +/- 20 to 455 +/- 30 microS/cm2, P < 0.16). Furthermore, acidification reduced the rheobase current by 26% at 4 mM K+ and increased the number of excitable fibers at elevated [K+]o. At 11 mM K+ and normal pH, a recovery of excitability and force similar to the observations with muscle acidification could be induced by reducing extracellular Cl- or by blocking the major muscle Cl- channel, ClC-1, with 30 microM 9-AC. It is concluded that recovery of excitability in K(+)-depressed muscles induced by muscle acidification is related to reduction in the inhibitory Cl- currents, possibly through inhibition of ClC-1 channels, and acidosis thereby reduces the Na+ current needed to generate and propagate an AP. Thus short term regulation of Cl- channels is important for maintenance of excitability in working muscle.     

Bethanidine increased Na+ and Ca2+ currents and caused a positive inotropic effect in heart cells

The effects of bethanidine sulphate, a pharmacological analog of the cardiac antibrillatory drug, bretylium tosylate, were studied on action potentials (APs) and K+, Na+, and Ca2+ currents of single cultured embryonic chick heart cells using the whole-cell current clamp and voltage clamp technique. Extracellular application of bethanidine (3 X 10(-4) M) increased the overshoot and the duration of the APs and greatly decreased the outward K+ current (IK) and potentiated the inward fast Na+ currents (INa) and the inward slow calcium current (ICa). However, intracellular introduction of bethanidine (10(-4) M) blocked INa. In isolated atria of rat, bethanidine increased the force of contraction in a dose-dependent manner. These findings suggest that when applied extracellularly, bethanidine exerts a potentiating effect on the myocardial fast Na+ current and slow Ca2+ current and an inhibitory effect of IK. The positive inotropic effect of bethanidine could be due, at least in part, to an increase of Ca2+ influx via the slow Ca2+ channel and the Na-Ca exchange. It is suggested that the decrease of IK by bethanidine may account for its antifibrillatory action.     

增强Na+-Ca2+交换对大鼠心脏的正性变力作用及对哇巴因效应的加强

本文采用大鼠乳头肌张力测定及离体心脏灌流技术,研究大鼠心肌Na -Ca2 交换对乳头肌及离体灌流心肌变力性的影响。采用大鼠特异性Na -Ca2 交换激动剂E-4031能剂量依赖性地增加大鼠乳头肌的发展张力(P<0.05,n=6)及离体心脏的心泵功能(P<0.05,n=4);特异性Na -Ca2 交换抑制剂KB-R7943具有相反的效应,并可完全消除E-4031引起的正性变力作用。哇巴因(ouabain,0.5μmol/L)与E-4031(3μmol/L)联合使用,可使乳头肌发展张力由单独使用哇巴因时的0.25±0.03 g升高至0.29±0.04g(P<0.05,n=6);联合用药对大鼠离体心脏心泵功能的影响也强于哇巴因单独作用的效果。本研究结果证实,E-4031通过增强心肌Na -Ca2 交换,对大鼠乳头肌和离体心脏产生正性变力作用;与哇巴因合用时,它们的正性变力作用有相加作用。     

Tamoxifen inhibits Na+ and K+ currents in rat ventricular myocytes

Tamoxifen is an estrogen receptor antagonist used in the treatment of breast cancer. However, tamoxifen has been shown to induce QT prolongation of the electrocardiogram, thereby potentially causing life-threatening polymorphic ventricular arrhythmias. The purpose of the present study was to elucidate the electrophysiological mechanism(s) that underlie the arrhythmogenic effects of tamoxifen. We used standard ruptured whole cell and perforated patch-clamping techniques on rat ventricular myocytes to investigate the effects of tamoxifen on cardiac action potential (AP) waveforms and the underlying K+ currents. Tamoxifen (3 micromol/l) markedly prolonged AP duration, decreased maximal rate of depolarization, and decreased resting membrane potential. At this concentration, tamoxifen significantly depressed the Ca2+-independent transient outward K+ current (Ito), sustained outward delayed rectifier K+ current (Isus), inward rectifier K+ current (IK1), and Na+ current (INa) in the myocytes. Lower concentrations of tamoxifen (1 micromol/l) also decreased the resting membrane potential and significantly depressed IK1 to 79 +/- 5% (n = 5; at -120 mV) of pretreatment values. The results of this study indicate that inhibition of Ito, Isus, and IK1 by tamoxifen may underlie AP prolongation in cardiac myocytes and thereby contribute to prolonged QT interval observed in patients.     

DITPA restores the repolarizing potassium currents Itof and Iss in cardiac ventricular myocytes of diabetic rats

Diabetes Mellitus (DM) can produce an increase in the cardiac action potential duration and QT interval that can be associated with sudden death. These cardiac effects are due to a region-specific decrease in repolarizing outward K(+) currents. Some authors have suggested that the proarrhythmic effects of diabetes can be due to diabetes-induced hypothyroidism. Thus, we have examined the effect of the thyroid hormone analog diiodothyropropionic acid (DITPA) on calcium-independent outward potassium currents in ventricular myocytes from diabetic rats. Sustained (I(ss)) and fast transient outward (I(tof)) K(+) currents were recorded using the whole-cell configuration of the patch-clamp technique. Myocytes were enzymatically isolated from the free wall of the right ventricle, and the epicardial and endocardial layers of the left ventricle of healthy, diabetic and DITPA-treated diabetic rats. Circulating thyroid hormones were measured by electrochemiluminescence. DITPA-treatment of diabetic rats restored I(tof) and I(ss) current densities in cardiac myocytes from the three regions studied, but did not alter current densities in myocytes of control rats. T(3) and T(4) levels were reduced by diabetes, and DITPA-treatment increased circulating T(3) levels. T(3)-treatment of diabetic rats also restored current densities to control values. However, direct incubation of diabetic myocytes with DITPA did not restore current densities. In summary, DITPA-treatment of diabetic rats restored the potassium current (I(tof) and I(ss)) densities in myocytes from all ventricular regions.     

Rapid charge translocation by the cardiac Na(+)-Ca2+ exchanger after a Ca2+ concentration jump.

The kinetics of Na(+)-Ca2+ exchange current after a cytoplasmic Ca2+ concentration jump (achieved by photolysis of DM-nitrophen) was measured in excised giant membrane patches from guinea pig or rat heart. Increasing the cytoplasmic Ca2+ concentration from 0.5 microM in the presence of 100 mM extracellular Na+ elicits an inward current that rises with a time constant tau 1 < 50 microseconds and decays to a plateau with a time constant tau 2 = 0.65 +/- 0.18 ms (n = 101) at 21 degrees C. These current signals are suppressed by Ni2+ and dichlorobenzamil. No stationary current, but a transient inward current that rises with tau 1 < 50 microseconds and decays with tau 2 = 0.28 +/- 0.06 ms (n = 53, T = 21 degrees C) is observed if the Ca2+ concentration jump is performed under conditions that promote Ca(2+)-Ca2+ exchange (i.e., no extracellular Na+, 5 mM extracellular Ca2+). The transient and stationary inward current is not observed in the absence of extracellular Ca2+ and Na+. The application of alpha-chymotrypsin reveals the influence of the cytoplasmic regulatory Ca2+ binding site on Ca(2+)-Ca2+ and forward Na(+)-Ca2+ exchange and shows that this site regulates both the transient and stationary current. The temperature dependence of the stationary current exhibits an activation energy of 70 kj/mol for temperatures between 21 degrees C and 38 degrees C, and 138 kj/mol between 10 degrees C and 21 degrees C. For the decay time constant an activation energy of 70 kj/mol is observed in the Na(+)-Ca2+ and the Ca(2+)-Ca2+ exchange mode between 13 degrees C and 35 degrees C. The data indicate that partial reactions of the Na(+)-Ca2+ exchanger associated with Ca2+ binding and translocation are very fast at 35 degrees C, with relaxation time constants of about 6700 s-1 in the forward Na(+)-Ca2+ exchange and about 12,500 s-1 in the Ca(2+)-Ca2+ exchange mode and that net negative charge is moved during Ca2+ translocation. According to model calculations, the turnover number, however, has to be at least 2-4 times smaller than the decay rate of the transient current, and Na+ inward translocation appears to be slower than Ca2+ outward movement.     

Inhibitory effects of purified antibody against α-1 repeat (117-137) on Na(+)-Ca(2+) exchange and L-type Ca(2+) currents in rat cardiomyocytes

Considering that α-1 repeat region may be involved in the ion binding and translocation of Na(+)-Ca(2+) exchanger (NCX), it is possible that the antibodies against NCX α-1 repeat may have a crucial action on NCX activity. The aim of the present study is to investigate the effect of antibody against α-1 repeat (117-137), designated as α-1(117-137), on NCX activity. The antibody against the synthesized α-1(117-137) was prepared and affinity-purified. Whole-cell patch clamp technique was used to study the change of Na(+)-Ca(2+) exchange current (I(Na/Ca)) in adult rat cardiomyocytes. To evaluate the functional specificity of this antibody, its effects on L-type Ca(2+) current (I(Ca,L)), voltage-gated Na(+) current (I(Na)) and delayed rectifier K(+) current (I(K)) were also observed. The amino acid sequences of α-1(117-137) in NCX and residues 1?076-1?096 within L-type Ca(2+) channel were compared using EMBOSS Pairwise Alignment Algorithms. The results showed that outward and inward I(Na/Ca) were decreased by the antibody against α-1(117-137) dose-dependently in the concentration range from 10 to 160 nmol/L, with IC(50) values of 18.9 nmol/L and 22.4 nmol/L, respectively. Meanwhile, the antibody also decreased I(Ca,L) in a concentration-dependent manner with IC(50) of 22.7 nmol/L. No obvious effects of the antibody on I(Na) and I(K) were observed. Moreover, comparison of the amino acid sequences showed there was 23.8% sequence similarity between NCX α-1(117-137) and residues 1?076-1?096 within L-type Ca(2+) channel. These results suggest that antibody against α-1(117-137) is a blocking antibody to NCX and can also decrease I(Ca,L) in a concentration-dependent manner, while it does not have obvious effects on I(Na) and I(K).     

Na+-Ca2+ exchange in plasma membranes of crayfish striated muscle

Na+-Ca2+ exchange rates and some physico-chemical properties of the exchanger were studied in crayfish striated muscle membranes enriched in plasma membranes prepared by differential centrifugation of muscle microsomal fraction on discontinuous sucrose density gradient. The lightest subfraction with the highest Na+, K+-ATPase and Mg2+-ATPase activities also showed the highest Na+-Ca2+ exchange rates. A number of physico-chemical characteristics of the Na+-Ca2+ exchanger found in the present experiments were similar to those reported for excitable membranes of mammals, except for the temperature optimum (20 degrees C for the crayfish).     

Mechanisms of impaired calcium handling underlying subclinical diastolic dysfunction in diabetes

Isolated diastolic dysfunction is found in almost half of asymptomatic patients with well-controlled diabetes and may precede diastolic heart failure. However, mechanisms that underlie diastolic dysfunction during diabetes are not well understood. We tested the hypothesis that isolated diastolic dysfunction is associated with impaired myocardial Ca(2+) handling during type 1 diabetes. Streptozotocin-induced diabetic rats were compared with age-matched placebo-treated rats. Global left ventricular myocardial performance and systolic function were preserved in diabetic animals. Diabetes-induced diastolic dysfunction was evident on Doppler flow imaging, based on the altered patterns of mitral inflow and pulmonary venous flows. In isolated ventricular myocytes, diabetes resulted in significant prolongation of action potential duration compared with controls, with afterdepolarizations occurring in diabetic myocytes (P < 0.05). Sustained outward K(+) current and peak outward component of the inward rectifier were reduced in diabetic myocytes, while transient outward current was increased. There was no significant change in L-type Ca(2+) current; however, Ca(2+) transient amplitude was reduced and transient decay was prolonged by 38% in diabetic compared with control myocytes (P < 0.05). Sarcoplasmic reticulum Ca(2+) load (estimated by measuring the integral of caffeine-evoked Na(+)-Ca(2+) exchanger current and Ca(2+) transient amplitudes) was reduced by approximately 50% in diabetic myocytes (P < 0.05). In permeabilized myocytes, Ca(2+) spark amplitude and frequency were reduced by 34 and 20%, respectively, in diabetic compared with control myocytes (P < 0.05). Sarco(endo)plasmic reticulum Ca(2+)-ATPase-2a protein levels were decreased during diabetes. These data suggest that in vitro impairment of Ca(2+) reuptake during myocyte relaxation contributes to in vivo diastolic dysfunction, with preserved global systolic function, during diabetes.     

Unidirectional Na+, Ca2+, and K+ fluxes through the bovine rod outer segment Na-Ca-K exchanger

The properties of the Na-Ca exchanger in the plasma membrane of rod outer segments isolated from bovine retinas (ROS) were studied. Unidirectional Ca2+, Na+, and K+ fluxes were measured with radioisotopes and atomic absorption spectroscopy. We measured K+ fluxes associated with the Ca-Ca self-exchange mode of the Na-Ca exchanger to corroborate our previous conclusion that the ROS Na-Ca exchanger differs from Na-Ca exchangers in other tissues by its ability to transport K+ (Schnetkamp, P. P. M., Basu, D. K. & Szerencsei, R. T. (1989) Am. J. Physiol. 257, C153-C157). The Na-Ca-K exchanger was the only functional cation transporter in the plasma membrane of bovine ROS with an upper limit of a flux of 10(5) cations/ROS/s or a current of 0.01 pA contributed by other cation channels, pumps, or carriers; cation fluxes via the Na-Ca-K exchanger amounted to 5 x 10(6) cations/ROS/s or a current of 1 pA. Ca2+ efflux via the forward mode of the Na-Ca-K exchanger did not operate with a fixed single stoichiometry. 1) The Na/Ca coupling ratio was increased from three to four when ionophores were added that could provide electrical compensation for the inward Na-Ca exchange current. 2) The K/Ca coupling ratio could vary by at least 2-fold as a function of the external Na+ and K+ concentration. The results are interpreted in terms of a model that can account for the variable Ca/K coupling ratio: we conclude that the Ca2+ site of the exchanger can translocate independent of translocation of the K+ site, whereas translocation of the K+ site requires occupation of the Ca2+ site, but not its translocation. The results are discussed with respect to the physiological role of Na-Ca-K exchange in rod photoreceptors.     

Physiological basis of a steady endogenous current in rat lumbrical muscle

In an attempt to determine the mechanism by which rat skeletal muscle endplates generate a steady outward current, we measured the effects of several drugs (furosemide, bumetanide, 9-anthracene carboxylic acid [9-AC]) and changes in external ion concentration (Na+, K+, Cl-, Ba++) on resting membrane potential (Vm) and on the steady outward current. Each of the following treatments caused a 10-15-mV hyperpolarization of the membrane: replacement of extracellular Cl- with isethionate, addition of furosemide or bumetanide, and addition of 9-AC. These results suggest that Cl- is actively accumulated by the muscle fibers and that the equilibrium potential of Cl- is more positive than the membrane potential. Removal of external Na+ also caused a large hyperpolarization and is consistent with evidence in other tissues that active Cl- accumulation requires external Na+. The same treatments greatly reduced or abolished the steady outward current, with a time course that paralleled the changes in Vm. These results cannot be explained by a model in which the steady outward current is assumed to arise as a result of a nonuniform distribution of Na+ conductance, but they are consistent with models in which the steady current is produced by a nonuniform distribution of GCl or GK. Other treatments (Na+-free and K+-free solutions, and 50 microM BaCl2) caused a temporary reversal of the steady current. Parallel measurements of Vm suggested that in none of these cases did the electrochemical driving force for K+ change sign, which makes it unlikely that the steady current arises as a result of a nonuniform distribution of GK. All of the results, however, are consistent with a model in which the steady outward current arises as a result of a nonuniform distribution of Cl- conductance, with GCl lower near the endplate than in extrajunctional regions.     

Coexisting independent sodium-sensitive and sodium-insensitive mechanisms of genetic hypertension in spontaneously hypertensive rats (SHR)

Some essential hypertensive patients and genetic hypertensive rat strains have less than the normal levels of Mg2+ tightly bound to the plasma membranes of their erythrocytes and other cells, i.e., the magnesium binding defect (MgBD). This binding defect appears to cause increased passive permeability of the membrane to Na+ and thereby its increased intracellular concentration, particularly if the Na+-extrusion enzyme systems of the cell are also defective. The Na+-Ca2+ exchange system in the cell membrane exports Na+ and imports Ca2+, increasing the tone of the smooth muscle cell and thus producing hypertension (HTn). This HTn is Na+-sensitive. Evidence supporting this postulate was obtained by determining the intraerythrocyte total concentrations of Na+, Ca2+, K+, and Mg2+ in two strains of spontaneously hypertensive rats (SHR and SS/Jr rats, having the MgBD together with the other requisites of the Na+-sensitive pathway) and their respective controls (WKY and SR/Jr rats, in which this complete pathway is absent). The Na+ and Ca2+ concentrations in the hypertensive rats were increased, and that of K+ was decreased. The concentrations of these cations were very similar in the two hypertensive strains. The level of membrane tightly bound Ca2+ in SHR erythrocyte membranes was significantly higher than those in the other three rat strains, which were not statistically different from each other. These results support previously reported evidence of the existence of a novel HTn-generating mechanism in the SHR rat, in which the intracellular Ca2+ concentration is increased as the result of the enhanced diffusion of this ion into the cell and the accompanying deficiency of the Ca2+ extrusion enzyme systems. This pathway is therefore Na+-insensitive, i.e., Ca2+-sensitive.     

Ionic currents of smooth muscle cells isolated from the ctenophore Mnemiopsis

The ionic currents of smooth muscle cells isolated from the ctenophore Mnemiopsis were examined by using conventional two-electrode voltage clamp and whole-cell patch clamping methods. Several separable currents were identified. These include: (1) a transient and (2) a steady-state voltage-activated inward current; both are tetrodotoxin (TTX) and saxitoxin (STX) insensitive, partly reduced by decreasing external Ca2+ or Na+ or by addition of 5 mM Co2+, D-600 or verapamil and are totally blocked with 5 mM Cd2+; (3) an early, transient, cation-dependent, outward K+ current (IKCa/Na); (4) a transient, voltage-activated, outward K+ current provisionally identified as IA; (5) a delayed, steady-state, voltage-activated outward K+ current (IK) and (6) a late, transient, outward K+ current which is blocked by Cd2+ and evident only during long voltage pulses. Despite their phylogenic origin, most of these currents are similar to currents identified in many vertebrate smooth and cardiac muscle preparations, and other excitable cells in higher animals.