Intracellular calcium ions as regulators of renal tubular sodium transport

Summary This review addresses the putative role of intracellular calcium ions in the regulation of sodium transport by renal tubules. Cytoplasmic calcium-ion activities in proximal tubules of Necturus are less than 10^–7 M and can be increased by lowering the electrochemical potential gradient for sodium ions across the peritubular cell membrane, or by addition of quinidine or ionomycin to peritubular fluid. Whereas lowering of the peritubular Na concentration increases cytosolic [Ca⁺⁺] and [H⁺], ionomycin, a calcium ionophore, raises intracellular [Ca⁺⁺] without decreasing pH_i. The intracellular calcium-ion level is maintained by transport processes in the plasma membrane and membranes of intracellular organelles, as well as by calcium-binding proteins. Calcium ions inhibit net transport of sodium by reducing the rate of sodium entry across the luminal cell membrane. In the collecting tubule this inhibition is caused, at least in part, by an indirect reduction in the activity of the amiloride-sensitive sodium channel.Supported by NIH grant PHS-AM-11489 and a New York Heart Association Established Fellowship (G.F.). J.M. Yang's works was in partial fullfillment of his Ph.D. thesis requirements; he is supported by a fellowship from the Chung Shan Institute of Science and Technology, Taiwan     

Influence of Mitragyna Ciliata (MYTA) on the Microsomal Activity of ATPase Na+/K+ Dependent Extract on a Rabbit Heart

Mitragyna ciliata (MYTA) (Rubiaceae) inhibits plasmodia activity. MYTA induces a cardiotonicity of the digitalic type on rat''s isolated heart. In this work we studied the effect of MYTA on microsomal Na⁺/K⁺ dependant ATPase (Na⁺, K⁺ ATPase) extracted from the heart of a rabbit since digitalics inhibit Na⁺, K⁺ ATPase. Our results revealed that the Na⁺/K⁺ ATPase has an optimum pH of 7.4 and temperature of 37°C respectively. There is a linear relationship between the organic phosphate formed and the incubation time over 25 mins incubation period. The ATP hydrolysis rate in the presence of MYTA was 0.775 µM/min. LINEWEAVER and BURK plots showed that MYTA did not alter K_M (1.31 mM) but decreased V_MAX. This study shows that MYTA exerts a non-competitive inhibition on the microsomal Na⁺/K⁺ ATPase extracted from rabbit heart with a Ci₅₀ of 48 µg / ml. We conclude that the mechanism of action of MYTA is linked to the inhibition of the Na⁺/K⁺ ATPase like cardiotonics of the digitalic type.     

Two components of inward current in myocardial muscle fibers

Summary The membrane potential of trabecula of the sheep ventricle was clamped. In response to a depolarizing step a transient inward current lasting 100 ms was elicited. This transient current could be separated into: (1) A rapid component which was strongly dependent on the holding potential prior to depolarization. The reversal-potential agreed with the sodium equilibrium-potential. It is concluded that this component is carried by sodium ions. (2) A slower component which could not be inactivated by a variation in the holding potential and which could also be observed in sodium free solutions. The amplitude of the current depended on the extracellular Ca⁺⁺-concentration. The slower transient inward component is possibly carried by calcium and/or sodium ions. when the extracellular sodium was 25 mM/l, the flow of components (1) and (2) caused two distinct peaks in the net current. The second, slower current is related to the positive plateau level of the myocardial action potential.This work was supported by the Deutsche Forschungsgemeinschaft     

Effect of Cs+, Li+ and Na+ on the potassium conductance and gating kinetics in the frog node of Ranvier

The effects of monovalent internal cations Cs⁺, Li⁺ and Na⁺ on potassium channel conductance in the frog node of Ranvier were studied by means of the voltage clamp. As previously reported, when 10–80% of the internal K⁺ was replaced by one of the above cations, the steady-state current-voltage relationship was significantly modified. The main effect was a voltage-dependent attenuation of the currents. We demonstrate that the current attenuation is associated with a change in the channel gating kinetics. For small depolarizations the kinetics can be described by the usual potassium conductance activation time constant, τ _n . However, under certain experimental conditions (e.g. substitution of the intracellular K⁺ with 10% Cs⁺), during larger depolarizations, stepping the membrane potential to values above 40–60 mV, the conductance develops with two time constants: τ _n and a new, slower time constant that, in contrast to τ _n , grows with membrane potential. These results can be explained by assuming that the catins may occupy two different sites in the channel; when the first site is occupied the channel is blocked, while occupation of the second site results in slowing of the gating kinetics in the affected channels.     

Extracellular free calcium and potassium during paroxysmal activity in the cerebral cortex of the cat

Summary Extracellular calcium and potassium activities (a_Ca and a_K) as well as neuronal activity were simultaneously recorded with ion-sensitive electrodes in the somatosensory cortex of cats. Baseline a_Ca was 1.2–1.5 mM/1, baseline a _k 2.7–3.2 mM/1. Transient decreases in a_Ca and simultaneous increases in a_K were evoked by repetitive stimulation of the contralateral forepaw, the nucleus ventroposterolateralis thalami and the cortical surface. Considerable decreases in a_Ca (by up to 0.7 mM/1) were found during seizure activity. A fall in a_Ca preceded the onset of paroxysmal discharges and the rise in a_K after injection of pentylene tetrazol. The decrease in a_Ca led also the rise in a_K during cyclical spike driving in a penicillin focus. It is concluded that alterations of Ca⁺⁺ dependent mechanisms participate in the generation of epileptic activity.     

Activity of (Na+K+)-stimulated adenosintriphosphatase in the rat nephron

Summary In 17 male Wistar rats in antidiuresis 10 different nephron segments and arteries are identified with the aid of Lowry's technique, dissected and total-and (Mg⁺⁺)-adenosintriphosphatase (=ATPase) determined. (Na⁺K⁺)-activated ATPase in the distal tubule is four to five times (max. eight times) more active than in the proximal segment. This difference of activity may speak for a high pump mechanism mediated by the way of a (Na⁺K⁺)-activated enzyme system in the distal nephron and for a partially passive reabsorption of sodium from the proximal convolution.With the support of the Schweiz. Nationalfonds zur Förderung der wissenschaftlichen Forschung (Nr. 4256 and Nr. 4809.3)     

The slow inward current and the action of manganese ions in guinea-pig's myocardium

Summary In guinea-pig's myocardium the slow inward current, which seems to be related to the positive plateau of the myocardial action potential, was studied by voltage clamp technique. This current could be displayed when the initial rapid inward current was inactivated by shifting the holding potential to levels positive to the resting potential. The threshold potential for the slow inward current was about –40 mV and the current was maxium at depolarizations to between –10 mV and +10 mV. The reversal potential was about +70 mV. A similar slow inward current was obtained when the preparation was bathed either in a sodium-free solution which contained calcium ions or in a calcium-free solution which contained sodium ions. In the latter solution, the reversal potential was between +30 mV and +50 mV. It seems possible that not only calcium but also sodium ions contribute to the slow inward current in normal Tyrode's solution. Manganese ions in concentrations of 5–20 mM depressed the slow inward current, especially when the depolirizations were small. In a calcium-free solution, the amplitude of the slow inward current flowing during large depolarizations was increased by manganese ions. In a calcium and sodium-free solution, a slow inward current was obtained when manganese ions were present.Supported by the Deutsche Forschungsgemeinschaft.     

Das Problem des zellulären Herzglykosidrezeptors

Zusammenfassung Diese Übersicht beschreibt die Eigenschaften der Na⁺, K⁺-ATPase bzw. der Na⁺, K⁺-Pumpe in der intakten Membran und die hochspezifische Hemmung dieses Transportenzyms durch Herzglykoside.Es wird auf die Wechselwirkung zwischen Glykosid und Enzym eingegangen und da bei insbesondere auf die Steuerung der Glykosidbindung durch ATP, K⁺, Na⁺, Mg²⁺ und Ca²⁺.Die Bedeutung des Enzyms als wahrscheinlicher pharmakologischer Rezeptor für Herzglykoside wird herausgestellt. Zunächst erfolgt eine Beschreibung der Schwierigkeiten und Fortschritte bei der Herstellung eines Zusammenhanges zwischen positiv inotropem Effekt von Herzglykosiden und deren Bindung an die Na⁺, K⁺-Pumpen des Herzmuskels bzw. der Hemmung des Transportsystems durch Herzglykoside.Es werden neuere Erkenntnisse über die Verknüpfung von der Hemmung der Na⁺, K⁺-Pumpe, die zu einer Erhöhung der intrazellulären Na⁺-Konzentration führt, und der indirekten Erhöhung der intrazellulären Ca²⁺-Aktivität referiert.Mitteilungen über den Nachweis einer herzglykosidartigen endogenen Aktivität entsprechen der hohen Rezeptorspezifität für Herzglykoside.     

Mechanism of Ca++ Release from the Sarcoplasmic Reticulum: A Computer Model

The proposed model describes myocyte calcium (Ca⁺⁺) cycling, emphasizing the kinetics of sarcoplasmic reticulum (SR) Ca⁺⁺ release channels. The suggested SR channel regulating mechanism includes two types of Ca⁺⁺ binding sites: (1) low affinity sites with high binding rates, regulating the opening of Ca⁺⁺ channels and (2) high affinity sites with low binding rates, which regulate their closing. The amount of Ca⁺⁺ released from the SR and the peak value of Ca⁺⁺ ion concentration [Ca⁺⁺] in the cytoplasm were found to depend on the rate of the increase of [Ca⁺⁺], similar to Ca⁺⁺ induced Ca⁺⁺ release experiments. The model describes spontaneous release of Ca⁺⁺ from overloaded SR. The dependence of the control mechanism on the activating and inactivating sites is substantiated by simulations of ryanodine intervention, providing results similar to experimental results. Simulations under conditions of isolated SR vesicles produced Ca⁺⁺ release results similar to measured data. Consequently, it is suggested that the recovery of Ca⁺⁺ release channels represents the rate limiting factor in the process of mechanical restitution. © 1998 Biomedical Engineering Society. PAC98: 8722Fy, 8710+e     

c-src Locus determines increased rate of Na+, K+-cotransport and increased calcium content in (SHR×WKY)F2 hybrid erythrocytes

Central Research Laboratory, Ministry of Health of the USSR, Moscow. Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 108, No. 11, pp. 608–609, November, 1989.     

Molecular origin of Na/Li exchanger: Evidence against the involvement of major cloned erythrocyte transporters

Numerous studies have demonstrated heightened Na⁺/Li⁺ countertransport (NLCT) activity in erythrocytes of patients with essential hypertension or diabetic nephropathy. The same carrier also contributes to the therapeutic action of lithium salt, widely used in the treatment of psychiatric disorders. However, the molecular origin of NLCT remains unknown. This study examined the role of major ion transporters in NLCT by comparative analysis of its activity and that of ion transporters providing inwardly directed ⁸⁶Rb, ²²Na and ³²P fluxes. NLCT was below the detection limit in rat erythrocytes and ∼50-fold higher in rabbits compared to humans. Unlike NLCT, the activities of Na⁺,K⁺-ATPase, Na⁺,K⁺,2Cl⁻ cotransporter and anion exchanger were somewhat similar in the erythrocytes of these species, whereas Na⁺,P_i cotransport was in 1:2:6 proportion in rats, humans and rabbits, respectively. Loading of erythrocytes with Li⁺ for NLCT measurement did not affect the activity of Na⁺,P_i cotransporter. Keeping in mind that NLCT is much higher in rabbits vs humans and rats, we compared the set of membrane proteins in these species using 2-dimensional gel electrophoresis. This approach revealed 174 common spots, whereas 132 proteins were detected only in human and rabbit erythrocyte membranes. Among these proteins, we found 17 spots whose expression was higher by more than 5-fold in rabbit compared to human erythrocytes. Thus, our results argue against the involvement of major ion transporters in NLCT. They also show that comparative proteomics is a potent tool to identify the molecular origin of this carrier.     

Water and solute secretion by the choroid plexus

The cerebrospinal fluid (CSF) provides mechanical and chemical protection of the brain and spinal cord. This review focusses on the contribution of the choroid plexus epithelium to the water and salt homeostasis of the CSF, i.e. the secretory processes involved in CSF formation. The choroid plexus epithelium is situated in the ventricular system and is believed to be the major site of CSF production. Numerous studies have identified transport processes involved in this secretion, and recently, the underlying molecular background for some of the mechanisms have emerged. The nascent CSF consists mainly of NaCl and NaHCO₃, and the production rate is strictly coupled to the rate of Na⁺ secretion. In contrast to other secreting epithelia, Na⁺ is actively pumped across the luminal surface by the Na⁺,K⁺-ATPase with possible contributions by other Na⁺ transporters, e.g. the luminal Na⁺,K⁺,2Cl⁻ cotransporter. The Cl⁻ and HCO₃ ⁻ ions are likely transported by a luminal cAMP activated inward rectified anion conductance, although the responsible proteins have not been identified. Whereas Cl⁻ most likely enters the cells through anion exchange, the functional as well as the molecular basis for the basolateral Na⁺ entry are not yet well-defined. Water molecules follow across the epithelium mainly through the water channel, AQP1, driven by the created ionic gradient. In this article, the implications of the recent findings for the current model of CSF secretion are discussed. Finally, the clinical implications and the prospects of future advances in understanding CSF production are briefly outlined.     

Interspersed normoxia during live high, train low interventions reverses an early reduction in muscle Na+, K+ATPase activity in well-trained athletes

Hypoxia and exercise each modulate muscle Na⁺, K⁺ATPase activity. We investigated the effects on muscle Na⁺, K⁺ATPase activity of only 5 nights of live high, train low hypoxia (LHTL), 20 nights consecutive (LHTLc) versus intermittent LHTL (LHTLi), and acute sprint exercise. Thirty-three athletes were assigned to control (CON, n = 11), 20-nights LHTLc (n = 12) or 20-nights LHTLi (4 × 5-nights LHTL interspersed with 2-nights CON, n = 10) groups. LHTLc and LHTLi slept at a simulated altitude of 2,650 m (F_IO₂ 0.1627) and lived and trained by day under normoxic conditions; CON lived, trained, and slept in normoxia. A quadriceps muscle biopsy was taken at rest and immediately after standardised sprint exercise, before (Pre) and after 5-nights (d5) and 20-nights (Post) LHTL interventions and analysed for Na⁺, K⁺ATPase maximal activity (3-O-MFPase) and content ([³H]-ouabain binding). After only 5-nights LHTLc, muscle 3-O-MFPase activity declined by 2% (P < 0.05). In LHTLc, 3-O-MFPase activity remained below Pre after 20 nights. In contrast, in LHTLi, this small initial decrease was reversed after 20 nights, with restoration of 3-O-MFPase activity to Pre-intervention levels. Plasma [K⁺] was unaltered by any LHTL. After acute sprint exercise 3-O-MFPase activity was reduced (12.9 ± 4.0%, P < 0.05), but [³H]-ouabain binding was unchanged. In conclusion, maximal Na⁺, K⁺ATPase activity declined after only 5-nights LHTL, but the inclusion of additional interspersed normoxic nights reversed this effect, despite athletes receiving the same amount of hypoxic exposure. There were no effects of consecutive or intermittent nightly LHTL on the acute decrease in Na⁺, K⁺ATPase activity with sprint exercise effects or on plasma [K⁺] during exercise.     

Age differences in hormonal regulation of Na+, K+-ATPase activity in the rat renal cortex

Laboratory of Physiological Genetics, Institute of Cytology and Genetics, Siberian Brach, Russian Academy of Sciences, Novosibirsk. (Presented by Academician of the Russian Academy of Medical SciencesV. P. Lozov.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 114, No. 8, pp. 150–153, August, 1992.