Analog circuit of theAcetabularia membrane

Summary The high membrane potential ofAcetabularia (E _m=–170 mV) is due to an electrogenic pump in parallel with the passive diffusion system (E _d=–80 mV) which could be studied separately in the cold, when the pump is blocked. Electrical measurements under normal conditions show that the pump pathway consists of its electromotive forceE _p with two elementsP ₁ andP ₂ in series;P ₂ is shunted by a large capacitance (C _p=3 mF cm^–2). The nonlinear current-voltage relationship ofP ₁ (light- and temperature-sensitive) could be determined separately; it reflects the properties of a carrier-mediated electrogenic pump. The value ofE _p (–190 mV) indicates a stoichiometry of 21 between electrogenically transported charges and ATP. The electrical energy, normally stored inC _p, compares well with the metabolic energy, stored in the ATP pool. The nonlinear current-voltage relationship ofP ₂ (attributed to phosphorylating reactions) is also sensitive to light and temperature and is responsible for the region of negative conductance of the overall current-voltage relationship. The power of the pump (1 W cm^–2) amounts to some percent of the total energy turnover. The high Cl^– fluxes (1 nmol cm^–2 sec^–1) and the electrical properties of the plasmalemma are not as closely related as assumed previously. For kinetic reasons, a direct and specific Cl^– pathway between the vacuole and outside is postulated to exist.     

Effects of 2-deoxy-d-glucose,amiloride, vasopressin,and ouabain on active conductance andE Na in the toad bladder

Summary The effects of various agents on active sodium transport were studied in the toad bladder in terms of the equivalent circuit comprising an active conductanceK ^a, an electromotive forceE _Na, and a parallel passive conductanceK ^p. For agents which affectK ^a, but notE _Na orK ^p, the inverse slope of the plot of total conductance against short-circuit currentI ₀ evaluatesE _Na, and the intercept representsK ^p. Studies employing 5×10^–7 m amiloride to depressK ^a indicate a changingE _Na, invalidating the use of the slope technique with this agent. An alternative suitable technique employs 10^–5 m amiloride, which reducesI ₀ reversibly to near zero without effect onK ^p. Despite curvilinearity of the -I₀ plot under these conditions,K ^p may therefore be estimated fairly precisely from the residual conductance. It then becomes possible to follow the dynamic behavior ofK ^a andE _Na (in the absence of 10^–5 m amiloride) by frequent measurements of andI ₀, utilizing the relationshipsK ^a=K-K ^p, andK _Na=I _O/(K-K ^p). 2-deoxy-d-glucose (7.5×10^–3 m) depressedK ^a without affectingE _Na. Amiloride (5×10^–7 m) depressedK ^a and enhancedE _Na. Vasopressin (100 mU/ml) enhancedK ^a markedly and depressedE _Na slightly. Ouabain (10^–4 m) depressed bothK ^a andE _Na. All of the above effects were noted promptly;K ^p was unaffected. The electromotive force of Na transportE _Na appears not to be a pure energetic parameter, but to reflect kinetic factors as well, in accordance with thermodynamic considerations.     

Electrophysiological properties ofDictyostelium derived from membrane potential measurements with microelectrodes

Summary Electrical membrane properties of the cellular slime moldDictyostelium discoideum were investigated with the use of intracellular microelectrodes. The rapid potential transients (1 msec) upon microelectrode penetration of normal cells had a negative-going peak-shaped time course. This indicates that penetration of a cell with a microelectrode causes a rapid depolarization, which can just be recorded by the microelectrode itself. Therefore, the initial (negative) peak potential transient valueE _p (–19 mV) should be used as an indicator of the resting membrane potentialE _m ofD. discoideum before impalement, rather than the subsequent semistationary depolarized valueE _n (–5 mV). Using enlarged cells such as giant mutant cells (E _p=–39 mV) and electrofused normal cells (E _p=–30 mV) improved the reliability ofE _p as an indicator ofE _m. From the data we concluded thatE _m ofD. discoideum cells bathed in (mm) 40 NaCl, 5 KCl and 1 CaCl₂ is at least –50 mV. This potential was shown to be dependent on extracellular potassium. The average input resistanceR _i of the impaled cells was 56 M for normalD. discoideum. However, our analysis indicates that the membrane resistance of these cells before impalement is >1 G. Specific membrane capacitance was 1–3 pF/cm². Long-term recording of the membrane potential showed the existence of a transient hyperpolarization following the rapid impalement transient. This hyperpolarization was associated with an increase inR _i of the impaled cell. It was followed by a depolarization, which was associated with a decrease inR _i. The depolarization time was dependent on the filling of the microelectrode. The present characterization of the electrical membrane properties ofDictyostelium cells is a first step in a membrane electrophysiological analysis of signal transduction in cellular slime molds.     

Cell-bound cholinesterase inTrichoderma harzianum

Cell-bound cholinesterase enzyme activity is reported for the first time in the mycelium ofTrichoderma harzianum. This enzyme hydrolyzes both the acetylcholine and the butyryl thiocholine esters. TheK _m andV _max for choline ester are 0.69 mM and 1.0 nmol acid released min^–1 g^–1 protein. However, the thiocholine ester has aK _m value of 2.2 mM andV _max value of 3.33 nmol product formed min.^–1 g^–1 protein. The enzyme is inhibited by eserine, a true classical cholinesterase inhibitor.     

Time- and dose-dependent effects of protein kinase C on proximal bicarbonate transport

Summary Activation of protein kinase C has been shown to cause both stimulation and inhibition of transport processes in the brush-border membrane and renal tubule. This study was designed to examine the dose-response nature and time-dependent effect of 4 -phorbol-12-myristate-13-acetate (PMA) on the rates of bicarbonate absorption (J _HCO3) and fluid absorption (J _v) in the proximal convoluted tubule (PCT) of rat kidney. Bicarbonate flux was determined by total CO₂ changes between the collected fluid and the original perfusate as analyzed by microcalorimetry. Luminal perfusion of PMA (10^–10 10^–5 M) within 10 min caused a significant increase ofJ _HCO3 andJ _v. A peaked curve of the dose response was observed with maximal effect at 10^–8 M PMA on both bicarbonate and fluid reabsorption, which could be blocked completely by amiloride (10^–3 m) and EIPA (10^–5 M). On the other hand, with an increase of perfusion time beyond 15 min, PMA (10^–8 and 10^–6 M) could inhibitJ _HCO3 andJ _v. Amiloride (10^–3 M) or EIPA (10^–5 M) significantly inhibitsJ _HCO3 andJ _v, while there is no additive effect of PMA and amiloride or EIPA on PCT transport. An inactive phorbol-ester, 4-phorbol, that does not activate protein kinase C, had no effects onJ _HCO3 andJ _v. Capillary perfusion of PMA (10^–8 M) significantly stimulate bothJ _HCO3 andJ _v; however, PMA did not affect glucose transport from either the luminal side or basolateral side of the PCT. These results indicate that activation of endogenous protein kinase C by PMA could either stimulate or inhibit both bicarbonate and fluid reabsorption in the PCT dependent on time and dose, and these effects are through the modulation of Na⁺/H^– exchange mechanism.     

Stochastic late effects after partial body irradiation in diagnostic radiology: Evaluation of approximate data

Summary The paper describes a method to estimate the risk of inducing a malignant disease by the highly nonuniform partial body X-irradiation as performed in diagnostic radiological examinations. The cumulative probability,p, for the development of a radiation-induced malignant neoplasm is obtained from the equationp =G _t E _s, whereE _s is the energy imparted to the soft tissues of trunk and head during a special radiological procedure.G _t is the mean integral incidence function for trunk and head, reflecting the cancer inducibility of organs and tissues in trunk and head,G _t 0.3 kJ^–1. The value ofG _t was obtained from mortality risk factors for the different tissues at risk, adopted in ICRP Publication 26, 1977.The energy,E, imparted to the body in typical radiographic procedures is in the range of 1–30 mJ, going up to about 1 J in an extensive fluoroscopic examination of the gastrointestinal tract. The corresponding values forp are about 10^–6 to 10^–5, in extensive examinations 10^–4. As to a radiograph of chest, the method described in this paper yields practically the same value forp as the Monte Carlo calculation, using the MIRD phantom and the relevant mortality risk factors.     

Role of lipids in theNeurospora crassa membrane

Summary The effect of doubling the saturated fatty acid content on the electrophysiology ofNeurospora crassa membranes was studied. Intracellular membrane input resistance (R _m ) and potential (E _m ) were measured for wild-type (w/t) andcel ^– (Tween 40) organisms as a function of temperature. Over the 0 to 40°C temperature range studied, meanE _m values of bothw/t andcel ^– (Tw 40) organisms increased from –160 to –210 mV. This difference is greater than that expected from Nernst potential considerations, indicating an active component ofE _m . This active component is insensitive to a doubling of the saturated fatty acid content.R _m exhibits a temperature dependence and hysteresis. Averaged data indicate an increase inR _m with decreased temperature. The slope of the temperature dependence varies among individual hyphae. Above 17.5°Ccel ^– (Tw 40) hyphae averaged greater than 70% higher values ofR _m thanw/t. Below 17.5°Cw/t R _m data divided into low and high temperature dependence groups, whilecel ^– data exhibited a low temperature dependence. The results are discussed in relation to gel-liquid crystal phase transitions, membrane fluidity, and the contribution of fatty acid structure to membrane electrical properties.     

Compartment analysis of plant cells by means of turgor pressure relaxation: II. Experimental results onChara corallina

Summary The three-compartment model (paper I) described turgor pressure relaxations as a sum of two exponential functions. The predicted shape of the curves could be confirmed inChara corallina by improving the recording and processing of data. An evaluation on the basis of the three-compartment model provided values for the hydraulic conductivity of the plasmalemma ofLp _p=2×10^–5 to 4×10^–5 cm sec^–1 bar^–1 and ofLp _i=3×10^–5 to 1×10^–4 cm sec^–1 bar^–1 for the tonoplast (assuming the area to be 90% of the plasmalemma area). The mean proportion of the total volume occupied by the cytoplasm was calculated to be 9%. This value is consistent with the concept of a highly vacuolated cell. Other explanations for the biphasic relaxation curves are discussed.     

Invertase and phosphatase of yeast in a phosphate-limited continuous culture

Summary Deficiency of inorganic phosphate caused the hyper production of invertase and the derepression of acid phosphatase in a continuous culture ofSaccharomyces carlsbergensis. The specific invertase activity was 40,000 enzyme units per g dry cell weight at a dilution rate lower than 0.05 h^–1 with a synthetic glucose medium of which the molecular ratio of KH₂PO₄ to glucose was less than 0.006. This activity is eight fold higher than in a batch growth and 1.5 fold as much as the highest enzyme activity observed so far in a glucose-limited continuous culture.For the hyper production of invertase, it is necessary to culture the yeast continuously by keeping the Nyholm's conservative inorganic phosphate concentration at less than 0.2 m mole per g dry weight cell. The derepression of acid phosphatase brought about by phosphate deficiency, was similar in both batch and continuous cultures.Nomenclature D dilution rate of continuous culture (h^–1) - E_i invertase concentration in culture (enzyme unit l^–1) - E_p acid phosphatase concentration in culture (enzyme unit l^–1) - P inorganic phosphate concentration in culture (mM) - S glucose concentration in culture (mM) - X cell concentration in culture (g dry weight cell l^–1) Greek Letter specific rate of growth (h^–1) Suffix f feed - 0 initial value     

Characteristics of the vacuolar membrane ofNitella

Summary Taking advantage of vacuolar perfusion, concentrations of K⁺, Cl^–, and H⁺ in the vacuole ofNitella pulchella were changed in a wide range. Both the potential difference (E _vo ) and specific resistance (R _vo ) between the vacuole and the external medium were scarcely affected by K⁺ in the vacuole, while they responded sensitively to K⁺ in the external medium. E _vo also responded to Cl^– in both internal (vacuolar) and external medium. However, the sign of the response was opposite to that expected from the constant field assumption.R _vo was almost independent of Cl^–-concentrations of both internal and external medium.The response ofE _vo to internal pH was similar to that of external pH. Between pH's 4 and 8,E _vo changed by about 10 mV for one unit change of both external and internal pH.E _vo responded very sensitively to internal pH in the strongly acid region (30–60 mV at pH 3–4) irrespective of the concentration of KCl in the vacuole. In the alkaline region, however,E _vo responded to vacuolar pH only when the KCl concentration in the vacuole was low (0.1 mM).R _vo increased significantly when the vacuolar pH was lowered to 4 or 3.Increase in tonicity of the vacuolar medium to twice normal caused no significant change in bothE _vo andR _vo , while it raised the threshold for excitation.Even when the chemical potential gradient between the internal and external medium was made zero by replacing the cell sap for the same solution used for the external medium, a significant amount ofE _vo was observed. The short-circuit current which was first outward decreased to zero or changed its direction with time. Light did not affect the current. These facts show that the possibility for the contribution of an ion pump toE _vo can be excluded.The results were discussed under the assumption that responses ofE _vo andR _vo to either internal or external ions reflect the passive property of either tonoplast or plasmalemma.     

Diffusion von45Ca,85Sr und32P in Hydroxylapatit

Zusammenfassung Es wurde der Transport von⁴⁵Ca,⁸⁵Sr und³²P in polykristallinen Sinterkörpern von synthetischem Hydroxylapatit im Temperaturbereich 1000 bis 1400 °C untersucht. Nach sorgfältiger Berücksichtigung von Korngrenzen-Diffusionseffekten ergaben sich für die Diffusion von⁴⁵Ca und⁸⁵Sr gleiche Werte für die Aktivierungsenthalpien und Frequenzfaktoren, und zwar beipH₂O<30 Torr:Q=3,50 ± 0,02 eV;D ₀=41 ± 5 cm²s^–1 und beipH₂O=230 Torr:Q=3,55 ± 0,02 eV;D ₀=20 ± 3 cm²s^–1 Die Abhängigkeit des Kationen-Diffusionskoeffizienten vom Wasserdampfpartialdruck ist vermutlich dadurch bedingt, daß im untersuchten Temperaturbereich feste Lösungen von Hydroxylapatit und Oxyapatit entstehen und Leerstellen im OH^–-Teilgitter den Kationentransport beschleunigen. Der³²P-Transport wurde nur bei 1360 °C undpH₂O < 30 Torr gemessen. Der Diffusionskoeffizient ist um einen Faktor 400 ± 50 kleiner als der entsprechende Diffusionskoeffizient der Kationen.Die Ergebnisse der Diffusionsuntersuchungen werden in Verbindung mit einer einfachen Modellvorstellung zum Retentionsmechanismus der Erdalkalien im Skelett diskutiert.

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Diffusion of⁴⁵Ca,⁸⁵Sr, and³²P in hydroxyapatite

Summary The transport of⁴⁵Ca,⁸⁵Sr, and³²P in polycrystalline sinter pellets of synthetic hydroxyapatite has been investigated in the temperature range 1000 to 1400 °C. After subtraction of activity transports by grain boundary diffusion processes, equal values of activation enthalpy and frequency factor were found for the lattice diffusion of⁴⁵Ca and⁸⁵Sr: atpH₂O<30 Torr:Q=3,50 ± 0,02 eV,D ₀=41 ± 5 cm²s^–1 and atpH₂O=230 Torr:Q=3,55 ± 0,02 eV,D ₀=20 ± 3 cm²s^–.The dependence of the cation diffusion coefficient on the partial vapour pressure is probably caused by formation of solid solutions of hydroxyapatite and oxyapatite where vacancies of the OH^– sublattice accelerate the cation transport. The diffusion of³²P was investigated only atT=1360 °C andpH₂O<30 Torr. The value obtained is smaller by a factor of 400 ± 50 then the cation diffusion coefficient.The results of the diffusion experiments are discussed in the light of a simple model for the retention mechanism of the alkaline earth metals in the skeleton.

     

Diversity of K+ channels in the basolateral membrane of restingnecturus oxyntic cells

Summary Patch-clamp techniques have been applied to characterize the channels in the basolateral membrane of resting (cimetidine-treated, nonacid secreting) oxyntic cells isolated from the gastric mucosa ofNecturus maculosa. In cell-attached patches with pipette solution containing 100mm KCl, four major classes of K⁺ channels can be distinguished on the basis of their kinetic behavior and conductance: (1) 40% of the patches contained either voltage-independent (a) or hyperpolarization-activated (b), inward-rectifying channels with short mean open times (16 msec fora, and 8 msec forb). Some channels showed subconductance levels. The maximal inward conductanceg _max was 31±5 pS (n=13) and the reversal potentialE _rev was atV _p=–34±6 mV (n=9). (2) 10% of the patches contained depolarization-activated and inward-rectifying channels withg _max=40 ±18 pS (n=3) andE _rev was atV _p=–31±5 mV (n=3). With hyperpolarization, the channels open in bursts with rapid flickerings within bursts. Addition of carbachol (1mm) to the bath solution in cell-attached patches increased the open probabilityP _o of these channels. (3) 10% of the patches contained voltage-independent inward-rectifying channels withg _max=21±3 pS (n=4) andE _rev was atV _p=–24±9 mV (n=4). These channels exhibited very high open probability (P _o=0.9) and long mean open time (1.6 sec) at the resting potential. (4) 20% of the patches contained voltage-independent channels with limiting inward conductance of 26±2 pS (n=3) andE _rev atV _p=–33±3 mV (n=3). The channels opened in bursts consisting of sequential activation of multiple channels with very brief mean open times (10 msec). In addition, channels with conductances less than 6 pS were observed in 20% of the patches. In all nine experiments with K⁺ in the pipette solution replaced by Na⁺, unitary currents were outward, and inward currents were observed only for large hyperpolarizing potentials. This indicates that the channels are more selective for K⁺ over Na⁺ and Cl^–. A variety of K⁺ channels contributes to the basolateral K⁺ conductance of resting oxyntic cells.     

Cyclic AMP and intracellular ionic activities innecturus gallbladder

Summary Open-tip and liquid ion-exchanger microelectrodes were used to study the effects of cAMP (6mm, added to the serosal medium) on apical membrane potential (E _m ) and intracellular sodium, potassium, and chloride activities (a _Na ⁱ ,a _K ⁱ ,a _Cl ⁱ ) inNecturus gallbladder under open-circuit conditions. Transepithelial potential difference (E _Tr ) was also measured. In the presence of cAMP,a _Cl ⁱ fell from about 1.5 times its equilibrium value to a level that corresponded to electrochemical equilibrium across the apical and basolateral cell membranes. Under these conditionsa _Na ⁱ decreased anda _K ⁱ increased,E _m was unchanged andE _Tr increased from virtually zero to a small but significant serosal positive value. The cAMP-induced increase ina _K ⁱ was abolished when Cl^–-free incubation media were used. Addition of the Ca⁺⁺-ionophore A23187 (0.5 g/ml) to the serosal medium had no effect onE _m ,E _Tr , ora _Cl ⁱ . When A23187 was added to the mucosal medium,E _m and the basolateral membrane potential hyperpolarized by about 20 mV and an increase in the outwardly directed electrochemical driving force for Cl^– was observed. These results indicate that cAMP inhibits coupled transapical Na–Cl entry into epithelial cells ofNecturus gallbladder and suggest that this inhibition may not be mediated by an increase in intracellular Ca⁺⁺ concentration.     

Mechanism of stimulation by epinephrine of active transepithelial Cl transport in isolated frog cornea

Summary In the isolated frog cornea, the effects of 0.1mm epinephrine were measured on both the transepithelial and intracellular electrical parameters. Epinephrine increased the short-circuit current (I _sc) and transepithelial electrical conductance (g _t) by 176 and 96%, respectively. The effective electromotive driving force for active transepithelial Cl transport (E _Cl) was 45 mV and agrees with the value forE _Cl calculated by a different technique in the isolated rabbit corneal epithelium (Klyce, S.D., Wong, R.K.S., 1977,J. Physiol. (London) 266: 777). With respect to the tear-side bathing solution, epinephrine caused the intracellular potential difference of shortcircuited frog corneas to decrease from –54 to –50 mV (P>0.05). The fractional resistance of the apical membrane {F(R _o)=(R_o/R_o+R_i)} whereR _o andR _i represent the resistances of the apical and basolateral membranes, respectively, decreased from 0.38±0.06 to 0.23±0.03. Using these values ofF(R _o) and the cellular conductances, the calculated Cl resistances ofR _o andR _i decreased 4.3- and 2.3-fold, respectively. However, the value forE _Cl calculated from the intracellular electrical measurements (48 mV) did not appear to change since this value was in close agreement with the value forE _Cl calculated from the effects of epinephrine on the transepithelial electrical parameters. Thus, the effects of epinephrine onI _sc andg _t can be accounted for by increases in the Cl conductance of both the apical and basolateral membranes. Epinephrine caused the potential difference across the basolateral membrane to hyperpolarize by 9 mV. All of these results are consistent with the notion that the steps in transepithelial Cl transport include uphill movement into the cell across the basolateral membrane followed by downhill movement across the apical membrane into the tear-side bathing solution.     

Cl− channels in basolateral renal medullary memnbranes: III. Determinants of single-channel activity

Summary We evaluated the effects of vawrying aqueous Cl^– concentrations, and of the arginyl- and lysyl-specific reagent phenylglyoxal (PGO), on the properties of Cl^– channels fused from basolaterally enriched renal medullary vesicles into planar lipid bilayers. The major channel properties studied were the anion selectivity sequence, anionic requirements for, channel activity. and the efects of varying Cl^– concentrations and/or PGO on the relation between holding voltageV _H -mV) and open-time probability (P _o).Reducingcis Cl^– concentrations, in the range 50–320mm, produced a linear reduction in fractional open time (P _v) with a half-maximal reduction inP _o atcis Cl^–170mM. Channel activity was sustained by equimolar replacement ofcis Cl^– with F^–, but not with impermeant isethionate. Fortrans solutions, the relation between Cl^– concentration andP ₀ at 10mm Cl^–. Reducingcis Cl^– had no effect on the gating charge (Z) for channel opening, but altered significantly the voltage-independent, energy (G) for channel opening.Phenylglyoxal (PGO) reducedZ and altered G for Cl^– channel activity when added tocis, but nottrans solutions, Furthermore, in the presence ofcis PGO, reducing thecis Cl^– concentration had no effect onZ but altered G. Thus we propose thatcis PGO and,cis Cl^– concentrations affect separate sites determining channel activity at the extracellular faces of, these Cl^– channels.     

Swimming performance and metabolic rate of three tropical fishes in relation to temperature

Oxygen consumption was measured for three tropical fishes,Exodon paradoxus, Leporinus fasciatus andLabeo erythrurus in relation to swimming speed and temperature. For each species the logarithm of oxygen consumption (mg 0₂ · g^–1 · h^–1) increased linearly with relative swimming speed (1 · s^–1) with the value of the regression coefficients varying inversely with temperature. Active metabolism and critical swimming speed ofE. paradoxus andL. fasciatus increased with temperature to a maximum at 30 and 35° C respectively. Basal metabolic rates ofE. paradoxus andL. fasciatus increased with temperature. Metabolic rates and critical swimming speed of the three fishes studied were consistent with values for polar, temperate and other tropical species over their respective thermal ranges of tolerance. Tropical fishes have lowered their metabolism and swimming performance from that expected for many temperate species at the same temperature.     

Impedance of the electrogenic Cl− pump inAcetabularia: Electrical frequency entrainements,voltage-sensitivity,and reaction kinetic interpretation

Summary Reaction kinetic analysis of the electrical properties of the electrogenic Cl^– pump inAcetabularia has been extended from steady-state to nonsteady-state conditions: electrical frequency responses of theAcetabularia membrane have been measured over the range from 1 Hz to 10 kHz at transmembrane potential differences across the plasmalemma (V _m ) between –70 and –240 mV using voltage-clamp techniques. The results are well described by an electrical equivalent circuit with three parallel limbs: a conventional membrane capacitancec _m , a steadystate conductanceg _o (predominantly of the pump pathway plus a minor passive ion conductance) and a conductanceg _s in series with a capacitancec _p which are peculiar to the temporal behavior of the pump. The absolute values and voltage sensitivities of these four elements have been determined:c _m of about 8 mF m^–2 turned out to be voltage insensitive; it is considered to be normal.g _o is voltage sensitive and displays a peak of about 80 S m^–2 around –180 mV. Voltage sensitivity ofg _s could not be documented due to large scatter ofg _s (around 80 S m^–2).c _p behaved voltage sensitive with a notch of about 20 mF m^–2 around –180 mV, a peak of about 40 mF m^–2 at –120 mV and vanishing at –70 mV. When these data are compared with the predictions of nonsteady-state electrical properties of charge transport systems (U.-P. Hansen, J. Tittor, D. Gradmann, 1983,J. Membrane Biol. in press), model A (redistribution of states within the reaction cycle) consistently provides magnitude and voltage sensitivity of the elementsg _o ,g _s andc _p of the equivalent circuit, when known kinetic parameters of the pump are used for the calculations. This analysis results in a density of pump elements in theAcetabularia plasmalemma of about 50 nmol m^–2. The dominating rate constants for the redistribution of the individual states of the pump in the electric field turn out to be in the range of 500 sec^–1, under normal conditions.     

Conditioning prepulses and kinetics of potassium conductance in the frog node

Summary The kinetics of potassium conductance were analyzed in response to voltage-clamp steps with holding potential (–75 mV) as initial condition and after a positive prepulse to-wards +45 mV of 10-msec duration. As the potassium reversal potentialE _K altered during potassium current flow, a method to obtain the conductance independent ofE _K was used. Conductance kinetics at 15°C were analyzed according to the Hodgkin-Huxley (HH) model. The time constant of potassium activation, with holding potential as initial condition, is a monotonous decreasing function of membrane potential. Its value ofca. 9 msec at –50 mV decreases to 1 msec at +30 mV. Changes inE _K did not affect the voltage dependency of this time constant. The time constant of potassium deactivation, i.e. the off-response following a 10-msec prepulse towards +45 mV, shows a completely different voltage dependency. At a membrane potential of –90 mV it is approximately 2 msec and gradually increases for more positive voltages towards a maximum value of about 6 msec, that is reached between –5 and 0 mV. At still larger values of membrane voltage this time constant starts to fall again. It is concluded that a HH-model, as applied for a single population of potassium channels, has to be rejected. Computer simulations indicate that an extension to two populations of independent potassium channels, each with HH-kinetics, is also inconsistent with the observed results.     

Echocardiographic Abnormalities in Normotensive Obese Patients: Relationship with Visceral Fat

Objective: To evaluate the relationship of echocardiographic characteristics and visceral adipose tissue (VAT) distribution in normotensive obese patients. Research Methods and Procedures: Echocardiographic parameters were assessed in 28 normotensive obese patients [7 men, 21 women, mean age, 43.2 years; mean body mass index (BMI), 37.2 kg/m²; 10 with impaired glucose tolerance (IGT); 6 with type 2 diabetes] and 18 sex‐ and age‐matched healthy, normal‐weight controls (4 men, 14 women; mean age, 45.8 years; mean BMI, 22.4 kg/m²) by an M‐mode, color‐doppler videofluoroscope. VAT in the obese patients was assessed by computed tomography (at L4 level). Results: The obese patients had a significantly larger internal diastolic left ventricular (LV) diameter (p < 0.05), a thicker end‐diastolic septum (p < 0.001) and posterior wall (p < 0.001), a greater indexed (g/m^2.7) LV mass (p < 0.001), a higher atrial diastolic filling wave velocity (p < 0.001), a lower ratio between early and atrial diastolic filling wave velocities (p < 0.01), and a prolonged isovolumic relaxation time (p < 0.05). End‐diastolic septum and posterior wall thickness and the LV mass were significantly greater in patients with a VAT area >130 cm² than with <130 cm². In the multivariate regression analysis, only VAT (p < 0.0001), waist‐to‐hip ratio (p < 0.001), and sex (p < 0.001) were associated with the most important echocardiographic alterations. Discussion: The morphological and functional echocardiographic alterations usually found in normotensive obese patients closely correlate with the amount of intra‐abdominal fat deposition, even in the presence of diabetes or IGT.