Intracellular Na+ measurements in smooth muscle using SBFI – changes in [Na+], Ca2+ and force in normal and Na+-loaded ureter

 Our understanding of the control and effects of intracellular [Na⁺] ([Na⁺]_i) in intact smooth muscle is limited by the lack of data concerning [Na⁺]_i. The initial aim of this work was therefore to investigate the suitability of using the Na⁺-sensitive fluorophore SBFI in intact smooth muscle. We find this to be a good method for measuring [Na⁺]_i in ureteric smooth muscle. Resting [Na⁺]_i was found to be around 10 mM and rose to 25 mM when the Na⁺-K⁺-ATPase was inhibited by ouabain. This relatively low [Na⁺]_i in the absence of Na⁺-K⁺-ATPase suggests that other cellular processes, such as Na⁺-Ca²⁺ exchange, play a role in maintaining [Na⁺]_i under these conditions. Simultaneous measurements of [Na⁺]_i or [Ca²⁺] _i and force showed that Na⁺-Ca²⁺ exchange can play a functional role in ureteric smooth muscle. We found that the greater the driving force for Na⁺ exit and hence Ca²⁺ entry, the larger the contraction. In addition the Na⁺-Ca²⁺ exchanger activity under these conditions was found to be pH sensitive: acidification reduced the contraction and concomitant changes in [Ca²⁺] and [Na⁺]_i. We conclude that SBFI is a useful method for monitoring [Na] in smooth muscle and that Na⁺-Ca²⁺ exchange may play a functional role in the ureter. Received: 26 August 1997 / Received after revision: 27 October 1997 / Accepted: 28 October 1997     

Amiloride enhances the secretion but not the synthesis of renin in renal juxtaglomerular cells

In this study we have examined a potential role of the sodium/proton exchange system in the regulation of renin secretion. We found that the inhibitors of the Na⁺/H⁺ antiport, amiloride (1 mM) and ethylisopro-pylamiloride (EIPA, 50 M), led to a 125% increase of renin secretion from cultured mouse juxtaglomerular cells. The stimulatory effect of EIPA on renin secretion was dependent on the extracellular concentrations of sodium and hydrogen ions. While lowering the extracellular pH from 7.3 to 7.0, and lowering [Na⁺]_e from 130 mM to 5 mM had no effect on basal renin release, it markedly attenuated or even blunted the effect of EIPA on renin secretion. The stimulatory effect of forskolin on renin secretion, however, was not altered by decreases of extracellular pH and of sodium. Inhibition of basal renin release was achieved with angiotensin II (1 M). In the presence of EIPA the inhibitory effect angiotensin II was markedly attenuated. Although effective on renin secretion, neither amiloride nor EIPA exerted a significant effect on the de novo synthesis of renin in cultured mouse JG cells. These findings are compatible with the idea that an amiloride-sensitive transport process, presumably the Na⁺/H⁺ exchanger, acts indirectly as an inhibitory signal transduction system for renin secretion from renal juxtaglomerular cells.     

Sodium conductance in calcium channels of guinea-pig ventricular cells induced by removal of external calcium ions

An inward current characterized by a slow inactivation, was induced when the extracellular Ca^2– concentration was reduced by EGTA. It was suppressed by replacing external Na^– with Tris⁺ or by D-600, increased by epinephrine, and was not affected by TTX. These findings suggest that this current is carried by Na⁺ ions through the Ca channels. The Na current decreased in amplitude as the concentration of external divalent cations was elevated. Blocking the Na current by divalent cations could be approximated by a bimolecular interaction between divalent cation and channel, with a dissociation constant of 1.2 M for Ca²⁺ and 60 M for Mg²⁺. Single channel currents were recorded in the cell-attached configuration. With a pipette solution of pCa=7.5 or pCa>8, the single channel I-V relationship was linear and the slope conductance was 70–75 pS. For 40 mV depolarizations from the resting potential, unitary currents were smaller at pCa=6 than at pCa=7.5. However, single channel events, which were observed after the repolarizing step to the resting potential, were much the same amplitude. The open time histogram was fitted with a single exponential having a time constant of 1.9 ms at around –40 mV (pCa>8, with 5 M Bay K 8644 in the bath solution), which was decreased with increasing the Ca²⁺ concentration in the pipette solution. Noise power spectra of patch currents at pCa=6 revealed a high-frequency component at around 1500 Hz. These results suggest that Ca binding to the sites with a high affinity for Ca²⁺ blocks the Na conductance in Ca channels. Reduction of the unitary current at higher concentrations of Ca²⁺ might be attributed to a rapid block by Ca²⁺.     

Kinetics and distribution of voltage-gated Ca,Na and K channels on the somata of rat cerebellar Purkinje cells

Voltage gated ion channels on the somatic membrane of rat cerebellar Purkinje cells were studied in dissociated cell culture with the combination of cell-attached and whole-cell variation of patch clamp technique. The method enables us to record local somatic membrane current under an improved space clamp condition. Transient (fast-inactivating) and steady (slow inactivating) Ca channel currents, Na current, transient (fast-inactivating) and steady (slow-inactivating) K currents, were observed. Transient and steady Ca channel currents were activated at test potentials more positive than –40 mV and –20 mV, respectively (in 50 mM external Ba). The transient current inactivated with a half-decay time of 10–30 ms during maintained depolarizing pulses, while the steady current showed relatively little inactivation. Na current was activated at more positive potentials than –60 mV, and inactivated with a half-decay time of less than 5 ms. Transient and steady K outward currents were recorded at more positive potential than –20 mV and –40 mV, respectively. The transient current inactivated with a half-decay time of 2–8 ms. Ca, Na and K channels showed different patterns of distribution on the somatic membrane. Steady Ca channels tended to cluster compared with Na or K channels.     

Potential-dependent action ofAnemonia sulcata toxins III and IV on sodium channels in crayfish giant axons

Effects of toxins III and IV (ATX III and IV) from the sea anemoneAnemonia sulcata on the Na current of crayfish giant axons were studied. Both toxins slowed the inactivation of Na channels, producing a maintained Na current during a depolarizing voltage pulse. Using the intensity of the toxin-induced maintained current as an index for the fraction of Na channels to which toxin is bound, the toxin association and dissociation kinetics were analyzed. The dissociation rate of ATX III was increased by two orders of magnitudes by depolarizing the membrane from –70 to –40mV. This increase of the dissociation rate caused a marked decrease in the binding rate of ATX III to Na channels in the same potential range. ATX IV exhibited association and dissociation kinetics that had a potential dependency quite similar to that of ATX III in spite of different ionic charge distribution in these two toxins. The results support the view that the potential-dependent kinetics of these toxins are not due to an electrostatic interaction between the ionic charges of toxins and the membrane potential but result from a modulation of the binding energy depending on the gate configuration of the Na channel.     

Development of pump electrogenesis in hypokalaemic rat muscle

In hypokalaemic rats maintained on a potassium deficient diets for 10–50 days, the isolated Na-loaded and K-depleted (Na-rich) muscle fibers showed the membrane potential less than –115 mV in fresh muscles of normal rats in K⁺-free Krebs solution. Upon adding 5 mM K⁺ to the K⁺-free medium bathing the soleus muscles, the measured potentials of Na-rich muscles always exceeded the membrane potentials of fresh muscles in 5 mM K⁺. The hyperpolarization was dependent on the amount of intracellular Na⁺ concentration ([Na]_i) accumulated during the potassium deficiency. The electrogenic Na-pump was activated by an increase of [Na]_i of less than 5 mM. Further increases in [Na]_i resulted in increases in membrane potential which appeared to approach a limit at [Na]_i levels higher than 65 mM.     

Temperature experiments on nerve and muscle membranes of frogs

The influence of temperature changes in the range of 25°C to –6°C on the time constants of Na activation (m) and inactivation (h) was studied in twitch muscle fibers and the node of Ranvier under voltage-clamp conditions. Arrhenius plots of m and h exhibit a change in activation enthalpy at temperatures below 10°C. Cooling and subsequent heating induce a hystersis in the temperature dependence of m and h Ni²⁺ and UO ₂ ²⁺ increase the hysteresis width. With fast temperature changes the gating kinetics relax to their new values more slowly than the temperature change. Hence, temperature must be changed more slowly than 5°C/min if an additional apparent hysteresis due simply to this relaxation is to be avoided. The data are explained by the hypothesis of a phase transition in the membrane lipids. This conception is favoured over a temperature-induced change in protein conformation, since the neutral local anaesthetic benzocaine shows use-dependent block as if low temperature restricted the access of the drug through the lipid phase to its receptor.Supported by grant NS 08174 from the U.S. Public Health Service and SFB 38 from Deutsche Forschungsgemeinschaft     

Magnesium restores high K-induced inactivation of the fast Na channel in guinea pig ventricular muscle

The effect of extracellular magnesium concentration (Mg_o) on the upstroke of the action potential was studied in guinea pig ventricular muscle under various K⁺ concentrations (2.7–19mM). Increased Mg_o shifted the steady state inactivation curve of the fast Na channel in the depolarizing direction and this effect was concentration-dependent (0–20mM). Such an effect could explain the Mg-induced increase in maximum rate of rise of the action potential which Späh and Fleckenstein (1979) proposed to be due to a Mg channel.