Electrical properties of oligodendrocytes in culture

The electrical properties of immunocytologically identified oligondendrocytes from embryonic mouse spinal cord maintained in culture for 3 to 6 weeks were studied by passing current and recording potential changes with two separate intracellular electrodes. The average input resistance was 3.3 M and ranged from 0.7 to 16 M (n=35). The input resistance increased by 19% with depolarization and decreased by 9% with hyperpolarization of 25 mV. The membrane time constant determined from the slope of the late exponential tail was 3.45±2.5 ms SD (n=15). The specific membrane resistance of three cells was determined by a simplified square pulse analysis combined with measurement of membrane area. Membrane area was estimated from photomicrographs of cells injected with Lucifer Yellow CH and stained with the cell surface-reactive antibody 04 and from electron micrographs. An average specific membrane resistance of 1.3×10³ cm² and specific capacitance of 1.7 F/cm² were calculated. Increasing [K⁺]_o depolarized the cells and decreased the input resistance and the time constant.     

Amiloride-sensitive Na+ transport across cultured renal (A6) epithelium: evidence for large currents and high NaK selectivity

Electrical techniques were used to determine the NaK selectivity of the amiloride-sensitive pathway and to characterize cellular and paracellular properties of A6 epithelium. Under control conditions, the mean transepithelial voltage (V _T) was –57±5 mV, the short-circuit current (I _sc) averaged 23±2 A/cm² and the transepithelial resistance (R _T) was 2.8±0.3 kcm² (n=13). V _T and I _sc were larger than reported in previous studies and were increased by aldosterone. The conductance of the amiloride-sensitive pathway (G _amil) was assessed before and after replacement of Na⁺ in the mucosal bath by K⁺, using two independent measurements: (1) the slope conductance (G _T), determined from current-voltage (I-V) relationships for control and amiloride-treated tissues and (2) the maximum amiloride-sensitive conductance (G _max) calculated from the amiloride dose-response relationship. The ratio of G _amil in mucosal Na⁺ solutions to G _amil for mucosal K⁺ solutions was 22±6 for G _T measurements and 15±2 for G _max data. Serosal ion replacements in tissues treated with mucosal nystatin indicated a potassium conductance in the basolateral membrane. Equivalent circuit analyses of nystatin and amiloride data were used to resolve the cellular (E _c) and paracellular (R _j) resistances (5 kcm² and 8–9 kcm², respectively). Analysis I-V relationships for tissues depolarized with serosal K⁺ solutions revealed that the amiloride-sensitive pathway could be described as a Na⁺ conductance with a permeability coefficient (P _Na)=1.5±0.2× 10^–6 cm/s and the intracellular Na⁺ concentration (Na_i)=5±1 mM (n=5), similar to values from other tight epithelia. We conclude that A6 epithelia are capable of expressing large amiloride-sensitive currents which are highly Na⁺ selective.     

Transcellular bicarbonate transport in rabbit gallbladder epithelium: mechanisms and effects of cyclic AMP

HCO₃ permeation through rabbit gallbladder epithelium has been investigated in vitro using voltage-clamp, pH-stat and microelectrode techniques. Mucosa-to-serosa flux of HCO₃ (4.9 mol cm^–2h^–1) was dependent on luminal Na and inhibited by amiloride (1 mmol/l, luminal bath), methazolamide (0.1 mmol/l, both sides), and ouabain (30 mol/l, serosal bath). Maximal rates of serosa-to-mucosa flux of HCO₃ (2.8 mol cm^–2h^–1) required serosal Na and mucosal Cl. This flux was inhibited by ouabain, 4-acetamido-4-isothiocyanato-stilbene-2,2-disulfonic acid (1 mmol/l, serosal bath), and 5-nitro-2-(3-phenylpropylamino)-benzole acid (0.1 mmol/l, luminal bath). Ineffective were methazolamide (0.1 mmol/l, both sides) and amiloride (1 mmol/l, serosal bath). 8-Br-cAMP (1 mmol/l, serosal bath) largely inhibited the absorptive and moderately stimulated the secretory flux. In tissue conductance, short-circuit current, and transmural voltage prostaglandin E₁ (1 mol/l, serosal bath) and 8-Br-cAMP caused moderate to negligible increases. No significant alterations of apical membrane potential ( –65 mV) and the apparent ratio of membrane resistances (R_a/R_b;1.9) were found. Cell membranes responded to luminal Cl removal mostly with a slow hyperpolarization that was mitigated by 8-Br-cAMP or, in some cases, converted into a small, transient depolarization. Our results are best explained by transcellular HCO₃ transport in both directions. In secretion, basolateral HCO₃ entry occurs by some form of co-transport with Na, and apical exit by Cl/HCO₃ exchange. cAMP opens no major electrodiffusive pathway for apical anion efflux. In absorption, HCO₃ import from the lumen into the cell is secondary to cAMP-sensitive Na/H exchange.     

Influence of glucose absorption on ion activities in cells and submucosal space in goldfish intestine

Mucosal glucose addition evokes in goldfish intestinal epithelium a fast depolarization of the mucosal membrane potential (_mc = 12 mV) followed by a slower repolarization (_mc = –7 mV). The intracellular sodium activity, a_iNa⁺, rises from 13.2±2.4 meq/l by 6.7±0.5 meq/l within 5 min, a_iCl^– rises about 3 meq/l above the control value of 37.7±2.2 meq/l, while a_iK is constant (97.7 ±7.4 meq/l). The potassium activity measured in the submucosal interstitium near the basal side of the cells (a_iK⁺) is 5.2±0.2 meq/l in non-absorbing tissue compared to 4.2 meq/l in the bathing solution and shows a transient increase due to glucose absorption (1.1±0.1 meq/l).In chloride-free media a_sK⁺=4.2±0.1 meq/l and _mc hyperpolarizes by –13 mV. The depolarization due to glucose absorption increases (_mc = 14.1 ± 1.4 mV) and the repolarization ( _mc ^repol ) disappears. In addition, a_iNa⁺ rises from 16.3±2.4 meq/l by 9.9±1.5 meq/l within 5 min, a_iK⁺ remains constant and equal to the value in chloride containing solutions (88.5±2.8 meq/l); a_sK⁺ increases transiently (1.1±0.1 meq/l).Serosal Ba²⁺ (5 mM) depolarizes _mc (+14.2±1.0 mV) and abolishes the repolarization. Increased serosal or mucosal potassium activity depolarizes _mc and abolishes the repolarization.These effects are discussed in terms of changes of ion activities, the basolateral potassium conductance, the influence of intracellular Ca²⁺, the functional state of the Na/K-pump, and modulation of membrane permeabilities by extracellular potassium.     

Passive electrical properties of the atrio-ventricular node

Summary The passive electrical properties of the atrioventricular node and of the atrial muscle of the rabbit heart were determined. To polarize the cells, a small suction electrode was used. The space constant of the atrio-ventricular node was found to be small (430 ) compared to other cardiac fibers. The small value of in nodal cells is due to a high intracellular resistance (r_a, 40.9 ± 9 M/cm) which is higher than in atrial muscle (9.6 ± 2.2 M/cm). The input resistance of cells of the N layer was found equals to 880 K while in the right atrium was 320 K. The time constant of nodal cells in the AN layer was 3.4 ms, in the N layer 9 ms and in the atrium 5 ms. Assuming a specific membrane capacity (C _m) of nodal and atrial fibers of 1 F/cm²,R _m was found equals to 9.000 cm² in N layer, 3.400 cm² in AN layer and 3.800 cm² in the right atrium.Acetylcholine (5 g/ml) reduced the space constant of the atrio-ventricular node by 38% and the time constant of nodal cells by 33%.The delay of impulse conduction in the A-V node semms then related to a high intracellular resistance along the pathway of conduction.This work was supported by Grant # HL-10897 from the NHI, Bethesda, Maryland and in part by a Grant from the P. R. Heart Association     

Glucose dependent K+-channels in pancreaticβ-cells are regulated by intracellular ATP

The resting conductance of cultured-cells from murine pancreases was investigated using the whole-cell, cell-attached and isolated patch modes of the patch-clamp technique. Whole-cell experiments revealed a high input resistance of the cells (>20 G per cell or>100 k·cm²), if the medium dialysing the cell interior contained 3 mM ATP. The absence of ATP evoked a large additional K⁺ conductance. In cell-attached patches single K⁺-channels were observed in the absence of glucose. Adition of glucose (20 mM) to the bath suppressed the channel activity and initiated action potentials. Similar single-channel currents were recorded from isolated patches. In this case the channels were reversibly blocked by adding ATP (3 mM) to the solution at the intracellular side of the membrane. The conductances (51 pS and 56 pS for [K⁺]₀=145 mM, T=21° C) and kinetics (at –70 mV: _open=2.2 ms and _closed=0.38 ms and 0.33 ms) of the glucose- and ATP-dependent channels were found to be very similar. It is concluded that both channels are identical. The result suggests that glucose could depolarize the-cell by increasing the cytoplasmic concentration of ATP.     

Effect of bicarbonate on potassium conductance of isolated perfused rat pancreatic ducts

The aim of this study was to investigate the role of the K⁺ conductance in unstimulated and stimulated pancreatic ducts and to see how it is affected by provision of exogenous HCO ₃ ^– /CO₂. For this purpose we have applied electrophysiological techniques to perfused pancreatic ducts, which were dissected from rat pancreas. The basolateral membrane potential PD_bl of unstimulated duct cells was between –60mV and –70mV, and the cells had a relatively large K⁺ conductance in the basolateral membrane as demonstrated by (a) 20–22 mV depolarization of PD_bl in response to increase in bath K⁺ concentration from 5 mmol/l to 20mmol/l and (b) the effect of a K⁺ channel blocker, Ba²⁺ (5 mmol/l), which depolarized PD_bl by 30–40mV. These effects on unstimulated ducts were relatively independent of bath HCO ₃ ^– /CO₂. The luminal membrane seemed to have no significant K⁺ conductance. Upon stimulation with secretin or dibutyryl cyclic AMP, PD_bl depolarized to about –35 mV in the presence of HCO ₃ ^– /CO₂. Notably, the K⁺ conductance in the stimulated ducts was now only apparent in the presence of exogenous HCO ₃ ^– /CO₂ in the bath solutions. Upon addition of Ba₂₊, PD_bl depolarized by 13±1 mV (n=7), the fractional resistance of the basolateral membrane, FR_bl increased from 0.66 to 0.78 (n=6), the specific transepithelial resistance, R _te, increased from 52±13 cm² to 59±15 cm² (n=11), and the whole-cell input resistance, R _c, measured with double-barrelled electrodes, increased from 20 M to 26 M (n=3). These results are consistent with Ba²⁺ inhibition of the K⁺ conductance. Following removal of exogenous HCO ₃ ^– /CO₂ in the same ducts, stimulation led to a larger depolarization on PD_bl to about –25 mV, and Ba²⁺ had a smaller effect on PD_bl and no significant effect on the resistances. The individual resistances in the duct epithelium were estimated from equivalent circuit analysis. The luminal membrane resistance, R ₁ decreased from about 2000 cm² to 80 cm² upon stimulation. The basolateral membrane resistance, R _bl, remained at 90–120 cm², and the paracellular shunt resistance, R _s, at 50–80 cm². Ba²⁺ increased R _bl of stimulated ducts to about 200 cm², an effect present only if the ducts were provided with exogenous HCO ₃ ^– /CO₂. Taken together, the present results indicate that the basolateral K⁺ conductance of pancreatic ducts is sensitive to exogenous HCO ₃ ^– /CO₂, i.e. without HCO ₃ ^– /CO₂ the conductance becomes very low although the ducts are undergoing stimulation.A preliminary report of the present study has been presented at the XXXI International Congress of Physiological Sciences, Helsiniki, Finland, July 1989     

Ouabain-induced cell swelling in rabbit connecting tubule: evidence for thiazide-sensitive Na+-Cl− cotransport

Mechanisms of Na⁺ entry across the luminal membrane of the rabbit connecting tubule (CNT) and cortical collecting duct (CCD) were investigated in vitro by analyzing factors that block the ouabain-induced tubular swelling. In the CNT and CCD, cell swelling caused by 100 M ouabain added to the bath was completely blocked by luminal Na⁺ removal, suggesting that the main factor inducing cell swelling is Na⁺ entry through the luminal membrane. Trichlormethiazide (100 M) and amiloride (10 M) inhibited the swelling in CNT when applied in combination to the lumen, but not when given separately. The swelling was also inhibited by Cl^– omission from the lumen in the presence of amiloride. By contrast, no effect was noted when furosemide (100 M), 4-acetamide-4-isothiocyanatostilben-2,2-disulfonic acid (1 mM) or 5-(N-ethyl-N-isopropyl)amiloride (100 M) was added to the lumen in the presence of amiloride, indicating the absence of any influence of the Na⁺-K⁺-2Cl^– cotransporter and the parallel Na⁺/H⁺, Cl^–/HCO ₃ ^– exchanger. The cell swelling in the CCD was blocked by luminal addition of amiloride alone with no effect from trichlormethiazide. In CNT, when the ouabain-induced cell swelling was prevented by both diuretics, addition of parathyroid hormone (PTH, 3 nM) to the bath induced cell swelling, suggesting that another Na⁺ entry pathway is newly generated by PTH. These results demonstrate that ouabain-induced cell swelling depends on Na⁺ entry across the luminal membrane. In the CNT, the pathways include an amiloride-sensitive Na⁺ channel, thiazide-sensitive Na⁺-Cl^– cotransport and a PTH-stimulated Na⁺ pathway, whereas the CCD has only the amiloride-sensitive Na⁺ channel.     

Taurocholate depolarizes rat hepatocytes in primary culture by increasing cell membrane Na+ conductance

Rat hepatocytes in primary culture were impaled with conventional microelectrodes. Addition of 5–100 mol/l taurocholate led to a slowly developing depolarization that was maximal at 50 mol/l (10.5±1.5 mV, n=15) and not reversible. The effect was Na⁺ dependent and decreased in cells preincubated with 1 mol/l taurocholate. Increasing external K⁺ tenfold depolarized the cells by 12.3±2.3 mV under control conditions and by 6.3±1.2 mV with 50 mol/l taurocholate present (n=7). Depolarization by 1 mmol/l Ba²⁺ was 7.6±0.8 mV and 6.0±0.7 mV (n=9) before and after addition of taurocholate, respectively. Cable analysis and Na⁺ substitution experiments reveal that this apparent decrease in K⁺ conductance reflects an actual increase in Na⁺ conductance: in the presence of taurocholate, specific cell membrane resistance decreased from 2.8 to 2.3 k · cm² · Na⁺ substitution by 95% depolarized cell membranes by 8.9±2.9 mV (n=9), probably due to indirect effects on K⁺ conductance via changes in cell pH. With taurocholate present, the same manoeuvre changed membrane voltages by –0.8±2.6 mV. When Na⁺ concentration was restored to 100% from solutions containing 5% Na⁺, cells hyperpolarized by 3.5±3.6 mV (n=7) under control conditions and depolarized by 4.4±2.9 mV in the presence of taurocholate, respectively. In Cl^– substitution experiments, there was no evidence for changes in Cl^– conductance by taurocholate. These results show that taurocholate-induced membrane depolarization is due to an increase in Na⁺ conductance probably via uptake of the bile acid.     

Bewertung von Kontaktvermittlern für Bioelektroden bei Langzeituntersuchungen

Zusammenfassung An 18 Kontaktvermittlern (KV) wurden untersucht: Elektrische LeitfÄhigkeit, Ionengehalt, pH-Wert, HautvertrÄglichkeit sowie hautimpedanzreduierende Wirksamkeit bei unterschiedlicher HautprÄparation, Bewegungsstörspannung, Rauschspannung bei unterschiedlichem Elektrodenmaterial, Austrocknungsneigung, Dosierbarkeit und Kosten. Es ergaben sich signifikante Unterschiede zwischen den KV hinsichtlich der Leistungsmerkmale. Die HautvertrÄglichkeit bei achtstündiger Applikation reichte von gut vertrÄglich bis zu stark reizend, wobei die Irritationswahrscheinlichkeit bei dekornifizierender HautprÄparation am grö\ten war. Vierundzwanzig Minuten nach Applikationsbeginn lag die spezifische Hautimpedanz zwischen 15 k·cm² und 125 k·cm². Die Bewegungsstörspannungen wiesen Unterschiede bis zu 250% auf. Alle KV hatten bei Ag/AgCl- und Edelstahlelektroden eine vernachlÄssigbare RauschaktivitÄt, bei Gold- und Silberelektroden kamen Werte von 19 bzw. 23 V vor. Der NaCl-Gehalt der KV korrelierte mit der Hautreizung und mit der Impedanzreduktion. Die Impedanzreduktion korrelierte gegenlÄufig mit der HautvertrÄglichkeit und der Bewegungsstörspannung. Die AbhÄngigkeit der KV-Bewertung von den übrigen Systemkomponenten wird dargelegt und es werden Richtlinien zur KV-Auswahl bei Langzeitableitungen gegeben.Diese Arbeit wurde durch die Deutsche Forschungsgemeinschaft im Rahmen des Sonderforschungsbereichs 152 finanziell gefördert     

An anatomical basis for the resistance and capacitance in series with the excitable membrane of the squid giant axon

Summary To correlate periaxonal tissue layer resistance with Schwann cell layer anatomy, cross and longitudinal sections of giant axons ofLoligo pealei were examined by transmission electron microscopy. Measurements were made of the width and frequency of mesaxonal clefts entering the Schwann cell layer from the periaxonal space and leaving the cell layer adjacent to the basal lamina. The average mesaxonal cleft width is 10.5 nm. One cm² of the giant axon surface is enclosed by a single cell layer containing about 690 000 Schwann cells. One cm² of axon surface has a sheath mesaxonal area of 0.002 cm² at the periaxonal surface and 0.016 cm² at the basal lamina, the mesaxons branching frequently as they cross the sheath. The volume of the Schwann cell layer extracellular space was estimated to be roughly 1% of the Schwann cell layer volume. Several models were used to predict the resistance,R, across the Schwann cell layer. Assuming the mesaxonal clefts contain seawater, and can be lumped into volume conductors having simple geometries, then (normalized for one cm² of axon surface)R was estimated to be between 0.4 and 0.9 cm². This value compares favourably with electrophysiological estimates of the periaxonal tissue resistance (current clamp value = 0.9 cm² and the voltage clamp value = 1.4 cm²) as these electrically measured values include the resistance across the outer connective tissue layer as well as the Schwann cell layer. The value of the Schwann cell membrane capacity was estimated to be approximately 0.7 F/cm².     

Facilitated giga-seal formation with a just originated glass surface

A simple technique of tip preparation in patch pipettes is described, which facilitates giga-seal formation. The pipettes were fabricated from thick-walled borosilicate glass tubing (external diameter 2.0 mm, internal diameter 0.5 mm) and the tips could be repeatedly broken in the bath. The pipette resistance correspondingly fell in steps of 3–20 M from about 80 M to about 2 M (double concentrated Tyrode). Scanning electron microscopy showed that the tip obtained after breaking was fairly plain. These broken tips were especially appropriate for patch-clamping. In cardiac myocytes in 11 out of 26 patches with Na⁺ channel activity, giga-seals developed spontaneously, i.e. without suction. In these patches the amplitude of the mean current with depolarizing pulses to –40 mV was significantly higher in comparison with patches formed under negative pressure. It is concluded that spontaneously sealed patches are most likely of planar configuration and the Na⁺ channel activity exceeds that in suction-induced patches.     

Effect of external sodium on intracellular chloride activity in the surface cells of frog gastric mucosa

The intracellular chloride activity and its dependence on ionic substitutions in the bathing media was studied in individual surface cells of resting gastric mucosa using conventional and Cl^– selective microelectrodes. When the tissue was perfused with control NaCl-Ringer the cell membrane p.d.'s, cell-lumen (_cm) and cell-serosa (_cs) were –40.9±0.6 mV and –66.8±0.5 mV (n=175) respectively specitively and the p.d. measured by the Cl^– selective microelectrodes across the serosal membrane ( _cs ^Cl- ) averaged –32.4±0.7 mV (n=138). From these values an intracellular Cl^– activity (a _c ^Cl– ) of 15.3 mmol/l can be estimated. The data indicate that chloride ion is distributed close to equilibrium at the luminal membrane while it is accumulated by an energy requiring step at the serosal membrane. Reduction (2 mmol/l) or absence of chloride from the luminal bath did not result in any detectable change ofa _c ^Cl– ; on the other hand, after removal of Cl^– from the serosal bath the intracellular Cl^– activity fell to 7.1 mmol/l.When the tissue was exposed to serosal Na⁺-free Ringer (Na⁺ replaced by choline or TMA), although thea _c ^Cl– remained unaffected, a marked reduction of the electrochemical gradient for Cl^– at the serosal membrane was observed.These data indicate that: (i) chloride is accumulated in the surface cells against its electrochemical potential difference at the serosal membrane; (ii) the luminal membrane has a negligible conductance to Cl^–, while the serosal membrane represents a conductive pathway to chloride; (iii) the uphill entry of chloride at the serosal membrane seems to be, at least partially, Na⁺-dependent.Parts of this work were presented at the International Symposium on the Theory and Application of Ion-Selective Electrodes in Physiology and Medicine, Erlangen, 1983, and at the SIBS-SIFSINU Congress, Saint Vincent, 1983. The study was supported by C.N.R. (Rome)-Grants CT 81.00173. 04 and CT 82.00147.04     

The AC impedance of necturus gallbladder epithelium

The impedance of Necturus gallbladder epithelium was determined using sine wave currents of 1 Hz to 30 kHz. In control Ringer's solution the impedance locus exhibited a simple semicircle with minute shift of the high frequency end along the real axis and a minute depression of the center below the real axis (average 0.9±0.7°). Neglecting the slight suppression, the impedance of 1 cm² of epithelium can be represented by an electrical analogue consisting of a parallel RC element of 115±26 and 5.16±0.9 F in series with a small resistor of 5.3±1.3. In agreement with experimental results obtained under ionic or osmotic substitutions, the applicability of this simple RC analogue to gallbladder epithelium under control conditions can be explained by the influence of the paracellular shunt and by assuming the time constants of the apical and basal cell membranes to be comparable. Based on these data and on voltage divider measurements obtained with microelectrodes the capacitances of the apical and basal cell membrane can be estimated to be 7 and 18 F/cm². The latter value agrees well with estimates of the surface folding obtained from electronmicrographs, if the specific cell membrane capacitance is assumed to be 1 F/cm² as in other cell membranes.     

Effects of ouabain and temperature on cell membrane potentials in isolated perfused straight proximal tubules of the mouse kidney

In isolated perfused segments of the mouse proximal tubule, the potential difference across the basolateral cell membrane (PDbl) was determined with conventional microelectrodes. Under control conditions with symmetrical solutions it amounted to –62±1 mV (n=118). The potential difference across the epithelium (PDte) was –1.7±0.1 mV (n=45). Transepithelial resistance amounted to 1.82±0.09 k cm (n=28), corresponding to 11.4±0.6 cm². Increasing bath potassium concentration from 5 to 20 mmol/l depolarized PDbl by +24±1 mV (n=103), and PDte by +1.6±0.1 mV (n=19). Thus, the basolateral cell membrane is preferably conductive to potassium. Rapid cooling of the bath perfusate from 38°C to 10°C led to a transient hyperpolarization of PDbl from –60±1 to –65±1 mV (n=21) within 40 s followed by gradual depolarization by +18±1% (n=14) within 5 min. The transepithelial resistance increased significantly from 1.78±0.11 k cm to 2.20±0.21 k cm (n=15). Rapid rewarming of the bath to 38°C caused a depolarization from –61±2 mV (n=17) to –43±2 mV (n=16) within 15 s followed by a repolarization to –59±2 mV (n=10) within 40 s. Ouabain invariably depolarized PDbl. During both, sustained cooling or application of ouabain, the sensitivity of PDbl to bath potassium concentration decreased in parallel to PDbl pointing to a gradual decrease of potassium conductance. Phlorizin hyperpolarized the cell membrane from –59±2 to –66±1 mV (n=13), virtually abolished the transient hyperpolarization under cooling, and significantly reduced the depolarization after rewarming from +17±2 mV (n=16) to +9±3 mV (n=9).The present data indicate that the contribution of peritubular potassium conductance to the cell membrane conductance decreases following inhibition of basolateral (Na⁺+K⁺)-ATPase. Apparently, cooling from 37° to 10°C does not only reduce (Na^–+K⁺)-ATPase activity but in addition luminal sodium uptake mechanisms such as the sodium glucose cotransporter. As a result, cooling leads to an initial hyperpolarization of the cell followed by depolarization only after some delay.Parts of this study have been presented at the 60th and 61th Meeting of the Deutsche Physiologische Gesellschaft, Dortmund 1984 and Berlin 1985     

The force-velocity relationship of arm flexion in untrained males and females and arm-trained athletes

Summary The force-velocity curve (FVC) of arm flexion was established in 123 untrained males and 110 untrained females aged from 15 to 36 years, and 48 arm-trained athletes competing in different sport disciplines. The FVC was described by Hill's equation and defined by the parameters: maximal static moment (M₀), maximal angular velocity (₀), maximal power (P₀) and the concavity of the FVC (H). Within the given age range the level of the curve parameters of both untrained men and women was independent of age.On average, H was the same in all three groups. As compared to M₀ of the untrained males, M₀ of the athletes was 33% higher and M₀ of the females was 38% lower; with regard to P₀ these differences were +30% and –43% respectively. ₀ was the same for trained and untrained males, wherease ₀ of the women was 10% lower than ₀ of the men.     

Spatial EEG Synchronisation Over Sensorimotor Hand Areas in Brisk and Slow Self-Paced Index Finger Movements

The changes of spatial EEG synchronisation during brisk and slow voluntary self-paced movements of the right and left index finger were analysed in 12 right-handed and 11 left-handed subjects. EEG was recorded from the left and right sensorimotor area using 24 closely spaced electrodes. A novel measure of spatial EEG synchronisation, -complexity, was computed separately for the left and right sensorimotor area in 64 overlapping one-second epochs representing 4.5 s of the pre-movement and 3.5 s of the post-movement period. -complexity was higher, hence spatial synchronisation was lower, in slow than in brisk movements, especially in the right-handed. A sustained increase of -complexity was observed during execution of a slow movement. A decrease of -complexity which was often associated with a brief burst of spatially synchronised 10-Hz oscillations occurred at the onset of extensor muscle contraction. We suggest that increased spatial EEG synchronisation at movement onset may prevent spillover of excitation from the sensorimotor hand area to other cortical regions. During movement, the cortical neuronal assemblies subserve distinct, specialised functions manifesting in increased -complexity.     

A new oil-gap method for internal perfusion and voltage clamp of single cardiac cells

(1.) We designed a new technique to achieve fast voltage clamp, combined with internal perfusion. The single guinea-pig cardiac cell, dissociated by collagenase treatment, was stretched across an oil-gap (30–40 m wide) from a pool of Tyrode solution to a pool of internal solution. Part of the cell membrane was disrupted in the internal solution by crushing on the cell, a tapered tip of a glass capillary. Through the open end, the intracellular medium was equilibrated with test solutions and electrical current was injected for the voltage clamp of the membrane in the Tyrode pool. (2.) The capacitive transient on stepping the membrane potential decayed with a time constant of 10–60 s, depending on the capacitive area (20–80 pF). The time course was a single exponential in 46% of the atrial cells and in 66% of the ventricular cells. In these tissues the series resistance, approximated by a ratio of the time constant andC _m, was 686±180 k (n=37) in the ventricular cells or 812±143 k (n=18) in the atrial cells. The stable seal resistance (R _seal) established in the oil-gap was around 33 M in the ventricular cells and 100 M in the atrial cells. (3.) A rapid increase in the inward current followed by a slow decay was observed on repolarization over the range negative to the potassium equilibrium potential. From the inward rectification of both peak and late currents and suppressive effects of Cs⁺ on the current, the current changes were atrributed to activation and inactivation of the inward rectifier K channel. (4.) The Na current was activated by depolarization from a holding potential of –100 mV across a threshold of about –60 mV. In normal external (145 mM Na⁺) and internal (15 mM Na⁺) solutions, peak amplitude was obtained around –25 mV. The maximum chord conductance was 6.2±1.6 mS/F in 15 ventricular cells and 3.0±0.90 mS/F in 9 atrial cells in normal Tyrode solution. The process of inactivation was fitted with a sum of two exponential functions. (5.) The reversal potential of the Na current agreed well with that predicted from the Nernst equation during perfusion of 15 and 100 mM Na⁺ internal solutions in the presence of external 140 mM Na⁺. The shift of the reversal potential was completed within 30 s of switching the internal solution. (6.) This oil-gap voltage clamp technique facilitates control of the composition of both the intra- and extra-cellular media and should prove suitable for use in studies of intracellular mechanisms controlling the membrane current of enzymatically dissociated elongated cells.     

Time-dependent effects of aldosterone on sodium transport and cell membrane resistances in rabbit distal colon

Aldosterone stimulates Na⁺-absorption in rabbit distal colon. Due to circadian variations in plasma aldosterone level, Na⁺-transport varies in this epithelium. In vitro measurements (Ussing-chambers) yielded a transepithelial voltage (V _t ) of 13±1.6 mV for low-transporting epithelia (LT) and 25.7±2 mV for high-transporting epithelia (HT). However, the comparison of transepithelial conductance (G _t ) in LT epithelia (2.73±0.21 mS/cm²) and HT epithelia (2.96±0.41 mS/cm²) revealed no difference. Colons from both groups were stimulated by exogenous aldosterone (4 h prior to experiment). The transepithelial values changed as follows: LT epithelia showed a significantly increased V _t (26.1±4 mV) and G _t (3.74±0.23 mS/cm²), whereas in HT epithelia both parameters remained unchanged. Transepithelial amiloride-sensitive conductance was higher in HT than in LT. However, only in LT epithelia aldosterone increased this conductance. To get a more detailed view of the action of aldosterone, we used intracellular microelectrodes to calculate the resistances of apical (R _a ), basolateral (R _bl ) and paracellular (R _p ) pathway. The calculation of the resistances was based on a lumped equivalent circuit model and changes in R _a were induced by 50 M/l mucosal amiloride. Comparison of the control tissues revealed a lower R _bl only in HT tissues. In both groups stimulation by exogenous aldosterone led to a marked decrease of R _a . Furthermore R _bl was reduced to the same value as in HT control tissues. A leak resistance (R _l) was found, which was modulated by aldosterone in LT- and in HT epithelia. Differences in amiloride-sensitive transepithelial conductance between both epithelia groups could be explained by a regulation of r _l . Along with the regulation of the R _p the results indicate that the effects of exogenous aldosterone depended on the transport state of the rabbit distal colon. In LT epithelia aldosterone only influenced the resistances of the transcellular pathway. In HT epithelia aldosterone decreased cellular resistances and increased the paracellular resistance. Possible reasons of the augmented R _p are discussed.Abbreviations V _t transepithelial potential difference (mV) - R _r transepithelial resistance ( · cm²) - G _t transepithelial conductance (mS/cm²) - I _sc calculated short circuit current (A/cm²) - V _a apical membrane potential difference (mV) - V _bl basolateral membrane potential difference (mV) - voltage divider ratio ( = R _a /R _bl ) - voltage divider ratio ( = R _a /R _bl ) after amiloride - R _a apical membrane resistance ( · cm²) R _a = R _l · R _a ^/Na /(R _l + R _a ^/Na ) - R _a ^/Na apical membrane resistance to Na, ( · cm²) - R _bl basolateral membrane resistance ( · cm²) - R _c cellular resistance ( of apical and basolateral resistance) ( · cm²) - R _l leak resistance located in the apical membrane ( · cm²) - R _p resistance of the paracellular pathway ( · cm²) - G _a apical membrane conductance (mS/cm²) - G _bl basolateral membrane conductance (mS/cm²) - G _l apical leak conductance (compare R _l ) (mS/cm²) - G _p paracellular conductance (mS/cm²) - G _t transepithelial conductance (mS/cm²) - HF hard feces period - SF soft feces period - HT_contr high transporting control epithelia - LT_contr low transporting control epithelia - HT_aldo aldosterone pretreated high transporting epithelia - LT_aldo aldosterone pretreated low transporting epithelia; the prime designates data obtained after addition of amiloride     

Direct measurement of Na influx by23Na NMR during secretion with acetylcholine in perfused rat mandibular gland

Intracellular Na content (Na_in) in the perfused rat mandibular gland was measured by using a²³Na NMR spectroscopy at 24°C. An aqueous chemical shift reagent, dysprosium triethylenetetramine-N,N,N,N,NN-hexaacetic acid [Dy(TTHA)] was used in order to discriminate between the intracellular and the extracellular Na signal. The mandibular gland of rat was perfused arterially with a modified Krebs solution containing 10 mM Dy(TTHA). At rest, Na_in was not changed by blocking the Na⁺/K⁺ ATPase with ouabain (1 mM) and atropine (3 M), implying that, in the absence of stimulation, the spontaneous Na influx across the plasma membrane must have been negligibly small. Following onset of stimulation with acetylcholine (1 M), Na_in increased by 9.1±1.5 mmol/l intracellular fluid (mean±SEM,n=13), and remained at this level during stimulation. In the initial phase of secretion (0–5 min), about 50 mmol/min/l intracellular fluid of Na was secreted into the luminal space (estimated from the secretory rate by assuming an isotonic primary secretion) but, in spite of the higher secretion rate, Na_in increased only at an initial rate of 4.1 mmol/min/l intracellular fluid. During the steady phase of secretion (15–30 min) evoked by acetylcholine (1 M), ouabain (1 mM) caused an increment of Na_in of 44±8 mmol/l intracellular fluid (mean±SEM,n=4). From the rate of Na_in increment, the Na influx rate at the steady phase was estimated as 4.5 mmol/min/l intracellular fluid. These results suggest that the influx of Na is caused by stimulation with acetylcholine. The observed Na influx rate was about 50% of the Na secretory rate at the steady phase of secretion, estimated from the secretory rate by assuming an isotonic primary secretion. This is fully compatible with the operation of Na–K-2Cl contransport system for which one would expect a Na influx rate exactly half of the rate of Na and Cl secretion.