Myoplasmic fixed charges of the barnacle muscle fiber (author's transl)]

The experimental model used to study diffusion and electrical conduction in the cytoplasm of large muscle fibers was adapted to evaluate the myoplasmic density of fixed charges. Membranes of myoplasm were prepared and phi X, the myoplasmic thermodynamically effective charge density, was calculated from the membrane potential (Kamo, N., Toyoshima, Y. and Kobatake, Y. (1971) Kolloid Z.u.Z. Polymère 1061--1068) when these membranes were used as the partition between two electrolyte solutions. The dilution of KCl in the external solutions reduced phi X, which increases with the reduction of the water content in the membrane of myoplasm. With a water content of 73.0 ml/100 g KCl concentration in the external medium equal to 0.15 M, phi X was evaluated to 0.058 equiv/l. The substitution of KCl by NaCl introduces a reduction in phi X of 20--50% depending on E1KCl] in the external solutions. The addition of ATP, Mg2+, and Ca2+ also causes a reduction of phi B by 30--50% according to the experimental conditions. These results indicate that the fraction of counterions dissociated from the myoplasmic macromolecules is reduced when the concentration of the counterions is diminished or when CKl is replaced by Nal. It also suggests a reduction of phi X during muscular contraction.     

Direct measurement of intracellular free magnesium in frog skeletal muscle using magnesium-selective microelectrodes

Mg²⁺-selective microelectrodes have been used to measure the intracellular free Mg²⁺ concentration in frog skeletal muscle fibers. Glass capillaries with a tip diameter of less than 0.4 μm were backfilled with the Mg²⁺ sensor, ETH 1117. In the absence of interfering ions, they gave Nernstian responses between 1 and 10 mM free Mg²⁺. In the presence of an ionic environment resembling the myoplasm, the microelectrode response was sub Nernstian (18–24 mV) but still useful. The electrodes were calibrated before and after muscle-fiber impalements. In quiescent fibers from sartorius muscle (Rana pipiens), with resting membrane potentials not less than ?82 mV, the intracellular free Mg²⁺ concentration was 3.8±0.41 (S.E.) mM (n=58) at 22°C. No significant change in the intracellular free Mg²⁺ was observed following extensive (approx. 6 h) incubation in Mg²⁺-free media. Increasing the external concentration of magnesium from 4 to 20 mM (approx. 15 min) produced a slow and small enhancement (1.8 mM) of [Mg²⁺]_i, which was fully reverted when the divalent cation was removed from the bathing solution. No change in ionic magnesium resting concentration was observed when the muscle fibers were treated either with caffeine 3 mM or with Na⁺-free solutions. In depolarized muscle fibers (?23±2.7 mV) treated with 100 mM K⁺, the myoplasmic [Mg²⁺] was 3.7±0.45 (S.E.) mM, n=6, immediately after the spontaneous relaxation of the contracture. Similar determinations in muscle fibers during stimulation at low frequency (5 Hz), and after fatigue development, showed no changes in the concentration of free cytosolic Mg²⁺. These results point out that [Mg²⁺]_i is not modified under these three different experimental conditions.     

Myoplasmic impedance of the barnacle muscle fiber.

Myoplasmic impedance was measured on a barnacle (Balanus nubilus) single muscle fiber that was placed in a cylindrical cavity to limit the volume and prevent the hydration of the myoplasm. At both ends of the cavity, the myoplasm was in direct contact with an electrolyte solution. When equilibrium with the external medium was reached, the myoplasmic impedance was measured at 10 degrees C with an impedance bridge at 1000 Hz. The results indicated that the myoplasmic impedance of the muscle fiber is mainly resistive. Treating the myoplasm as a suspension of small conductive particles, we deduced the specific conductivity of the contractile filaments kf and their volume fraction rho (kf = 2.78 X 10(-3) omega-1cm-1, and rho = 0.48). The experimental technique permits an estimate of the specific myoplasmic conductivity in vivo (6.27 X 10(-3) omega-1cm-1). Finally, a decrease in the pH of the external solution from 10.1 to 4.0 lowered the myoplasmic conductivity by 16%. This may be considered as indirect evidence that the conductivity of the contractile filaments is associated with the protein counter-ions, since Hinke et al. (1973. Ann. N.Y. Acad. Sci. 204, 274-296.) reported evidence that a lowering of pH decreases the number of counter-ions.     

Bimodal response in a chemotactic behaviour of Drosophila larvae to monovalent salts

A chemotactic behaviour in Drosophila larvae to monovalent salts was investigated. Larvae showed clear-cut bimodal responses; they preferred low concentration of salts and rejected high. For example, they were attracted to solutions over the range of concentrations of NaCl from 10^?4 M to 10^?1 M but avoided concentrations above 2 × 10^?1 M.The responses to KCl, KBr and Kl showed differences in attraction whereas avoidance thresholds were similar. On the other hand, when the responses to ChCl and ChBr were compared with those to KCl and KBr respectively, only the avoidance responses were depressed. Furthermore, from choice experiments between KCl, NaCl and ChCl, it was shown that cations did not affect the attraction responses.The results observed strongly suggest the existence of two salt receptors (or receptor sites) which act antagonistically on the chemotaxis.     

Ecosystem changes in Galápagos highlands by the invasive tree Cinchona pubescens

Background and aims

Salinity is an increasing problem for agricultural production worldwide. Understanding how Na⁺ enters plants is important if reducing Na⁺ influx, a key component of the regulation of Na⁺ accumulation in plants and improving salt tolerance of crop plants, is to be achieved. Our previous work indicated that two distinct low-affinity Na⁺ uptake pathways exist in the halophyte Suaeda maritima. Here, we report the external NaCl concentration at which uptake switches from pathway 1 to pathway 2 and the kinetics of the interaction between external K⁺ concentration and Na⁺ uptake and accumulation in S. maritima in order to determine the roles of K⁺ transporters or channels in low-affinity Na⁺ uptake.

Methods

Na⁺ influx, Na⁺ and K⁺ accumulations in S. maritima exposed to various concentrations of NaCl (0–200 mM) were analyzed in the absence and presence of the inhibitors TEA and Ba⁺ (5 mM TEA or 3 mM Ba²⁺) or KCl (0, 10 or 50 mM).

Results

Our earlier proposal was confirmed and extended that there are two distinct low-affinity Na⁺ uptake pathways in S. maritima: pathway 1 might be mediated by a HKT-type transporter under low salinity conditions and pathway 2 by an AKT1-type channel or a KUP/HAK/KT type transporter under high salinity conditions. The external NaCl concentration at which two distinct low-affinity Na⁺ uptake switches from pathway 1 to pathway 2, the ‘turning point’, is between 90 and 95 mM. Over a short period (12 h) of Na⁺ and K⁺ treatments, a low concentration of K⁺ (10 mM) facilitated Na⁺ uptake by S. maritima under high salinity (100–200 mM NaCl), whether or not the plants had been subjected to a longer (3 d) period of K⁺ starvation. The kinetics suggests that low concentration of K⁺ (10 mM) might activate AKT1-type channels or KUP/HAK/KT-type transporters under high salinity (100–200 mM NaCl).

Conclusions

The turning-point of external NaCl concentrations for the two low-affinity Na⁺ uptake pathways in Suaeda maritima is between 90 and 95 mM. A low concentration of K⁺ (10 mM) might activate AKT1 or KUP/HAK/KT and facilitate Na⁺ uptake under high salinity (100–200 mM NaCl). The kinetics of K⁺ on Na⁺ uptake and accumulation in S maritima are also consistent with there being two low-affinity Na⁺ uptake pathways.     

Using KCl to determine size at competence for larvae of the marine gastropod Crepidula fornicata (L.)

Competent larvae of the marine gastropod Crepidula fornicata (L.) were induced to metamorphose (i.e., lose the velum) by elevating sea-water [KCl] by 5–50 mM. The response was optimal at 15–20-mM elevations, at which 50% metamorphosis was obtained in <4 h. Larvae that did not metamorphose during brief exposures (1–5 h) to elevated [KCl] generally maintained the larval form following transfer to control sea water, suggesting that competent larvae must be continuously immersed in the test solutions for metamorphosis to occur. The smallest larvae to respond to elevated [KCl] had shell lengths of ≈700–800 μm, the range of shell lengths within which larvae of this species become responsive to natural inducers. All larvae >≈1125 μm shell length metamorphosed in response to increased [KCl]. Rearing temperature may affect the size at which larvae of this species become responsive to K⁺. CaCl₂ (20-mM concentration elevations), GABA (4×10⁻⁷, 4×10⁻⁶ M), and NaCl (10–20-mM concentration elevations) generally failed to trigger metamorphosis. Twenty-mM elevations of [RbCl] and [CsCl] induced 100% metamorphosis but the juveniles were immobile and died after several days. Elevating [KCl] appears to be a reliable way to assess competence and trigger metamorphosis in larvae of C. fornicata.     

Optimization of Microbial Flocculant-Producing Medium for <Emphasis Type="Italic">Bacillus subtilis</Emphasis>

This study aimed to improve microbial flocculant production by optimizing the components of a Bacillus subtilis CZ1003 culture medium. Using the flocculation rate as the dependent variable, single-factor experiments were performed and beef extract at a concentration of 9 g/L was found to be the optimal nitrogen source, while glucose at a concentration of 20 g/L was the optimal carbon source. KCl, MgCl₂, NaCl, and CaCl₂ at concentrations of 0.75, 2.5, 0.5, and 5.0 g/L, respectively, were the optimum inorganic salts, in order of flocculant production activity. Orthogonal experimental demonstrated that KCl played a dominant role for Bacillus subtilis production of bioflocculants, followed by NaCl and CaCl₂. Optimization experiments demonstrated that the optimal combination of the two salts was 0.75 g/L KCl and 0.5 g/L NaCl, resulting in a flocculation rate of 36.2% when included together at these concentrations. The final optimized medium consisted of 20 g/L glucose, 9 g/L beef extract, 0.75 g/L KCl, and 0.5 g/L NaCl. Compared with the initial medium, the optimized medium enhanced the flocculation activity from 12.1 to 36.2%, which equates to an increase of 199.17%. Meanwhile, the flocculant yield was increased from 0.058 g/L to 0.134/L, an increase of 131.03%. The optimized medium could be used to improve microbial flocculant production and provides a basis for further exploration.     

Na+ uptake and extrusion in the cyanobacterium SynechocystisPCC6714 in response to hypersaline treatment. Evidence for transient changes in plasmalemma Na+ permeability

The kinetics of uptake and loss of Na⁺ have been studied using radioisotopic tracer techniques in cells of the cyanobacterium Synechocystis PCC6714 exposed to hyperosmotic stress. Cells transferred from a fresh-water-based medium to NaCl at 300–1000 mmol·dm⁻³ showed net Na⁺ uptake during the first 2 min following transfer, with the intracellular Na⁺ level at 2 min increasing as a direct function of the external NaCl concentration. Further incubation of cells in low-level hypersaline media (350–500 mmol · dm⁻³ NaCl) led to a marked reduction in cell Na⁺ within 20 min, indicating an efficient active Na⁺ extrusion system. In contrast, cells maintained in more extreme hypersaline media showed little (750 mmol · dm⁻³ NaCl) or no (1000 mmol · dm⁻³ NaCl) net Na⁺ extrusion following upshock. Cells grown in a saline medium (with NaCl at 500 mmol · dm⁻³ showed a greatly reduced net Na⁺ uptake after 2 min in media containing higher levels of NaCl. However, net Na⁺ uptake was also observed when these cells were downshocked to media containing 50–200 mmol · dm⁻³ NaCl. This is the first demonstration of downshock-induced Na⁺ accumulation in a microbial cell. Time-courses for Na⁺ extrusion in cells downshocked from 500 mmol · dm⁻³ to 100 mmol · dm⁻³ NaCl were similar to those for cells upshocked from freshwater to 500 mmol · dm⁻³ NaCl, requiring approximately 30 min to reach their lowest values. Net Na⁺ extrusion in upshocked cells was found to be markedly sensitive to the external K⁺ concentration, with limited net Na⁺ extrusion in the absence of external K⁺ and maximal reductions in cell Na⁺ in media containing K⁺ at 1–10 mmol · dm⁻³. Temperature was also shown to affect uptake and loss of cell Na⁺ during upshock: cells maintained at 5°C showed no capacity for net Na⁺ extrusion, while higher temperatures (up to 35°C) led to a progressive reduction in the amount of cell Na⁺ at 2 min following upshock and also in the rate of net Na⁺ extrusion after this time.     

Physiological and proteomic analysis of salinity tolerance of the halotolerant cyanobacterium <Emphasis Type="Italic">Anabaena</Emphasis> sp

The halotolerant cyanobacterium Anabaena sp was grown under NaCl concentration of 0, 170 and 515 mM and physiological and proteomic analysis was performed. At 515 mM NaCl the cyanobacterium showed reduced photosynthetic activities and significant increase in soluble sugar content, proline and SOD activity. On the other hand Anabaena sp grown at 170 mM NaCl showed optimal growth, photosynthetic activities and comparatively low soluble sugar content, proline accumulation and SOD activity. The intracellular Na⁺ content of the cells increased both at 170 and 515 mM NaCl. In contrast, the K⁺ content of the cyanobacterium Anabaena sp remained stable in response to growth at identical concentration of NaCl. While cells grown at 170 mM NaCl showed highest intracellular K⁺/Na⁺ ratio, salinity level of 515 mM NaCl resulted in reduced ratio of K⁺/Na⁺. Proteomic analysis revealed 50 salt-responsive proteins in the cyanobacterium Anabaena sp under salt treatment compared with control. Ten protein spots were subjected to MALDI-TOF–MS/MS analysis and the identified proteins are involved in photosynthesis, protein folding, cell organization and energy metabolism. Differential expression of proteins related to photosynthesis, energy metabolism was observed in Anabaena sp grown at 170 mM NaCl. At 170 mM NaCl increased expression of photosynthesis related proteins and effective osmotic adjustment through increased antioxidant enzymes and modulation of intracellular ions contributed to better salinity tolerance and optimal growth. On the contrary, increased intracellular Na⁺ content coupled with down regulation of photosynthetic and energy related proteins resulted in reduced growth at 515 mM NaCl. Therefore reduced growth at 515 mM NaCl could be due to accumulation of Na⁺ ions and requirement to maintain higher organic osmolytes and antioxidants which is energy intensive. The results thus show that the basis of salt tolerance is different when the halotolerant cyanobacterium Anabaena sp is grown under low and high salinity levels.     

Effect of inorganic salt stress on the thermotolerance and ethanol production at high temperature of <Emphasis Type="Italic">Pichia kudriavzevii</Emphasis>

Application of cross-protection is expected to improve the thermotolerance of yeasts to enhance their ethanol production at high temperature. In this study, the effects of eight kinds of inorganic salts on the thermotolerance and ethanol production at high temperature in Pichia kudriavzevii were investigated. P. kudriavzevii showed strong thermotolerance and the ability to produce ethanol at high temperature, and higher ethanol production of P. kudriavzevii was observed at high temperature (37–42 °C) compared with that at 30 °C. Inorganic salt stresses induced obvious cross-protection of thermotolerance in P. kudriavzevii. The presence of 0.1 mol/L KNO₃ or Na₂SO₄ or 0.2 mol/L NaCl, KCl, NaNO₃, K₂SO₄ or MgCl₂ increased the yeast biomass in YEPD medium at 44 °C to 2.72–3.46 g/L, obviously higher than that in the absence of salt stress (2.17 g/L). The addition of NaCl, KCl, NaNO₃, KNO₃, Na₂SO₄, K₂SO₄, CaCl₂ and MgCl₂ significantly increased the ethanol production of P. kudriavzevii in YEPD fermentation medium at 44 °C by 37–58%. KCl and MgCl₂ exhibited the best performance on improving the thermotolerance and ethanol production, respectively, of P. kudriavzevii. A highly significant correlation (P?<?0.01) was obtained among ethanol production, biomass and glucose consumption, suggesting the important role of thermotolerance and glucose consumption in enhanced ethanol production. The combination of NaCl, KCl and MgCl₂ had a synergistic effect on the improvement of thermotolerance and ethanol production at high temperature in P. kudriavzevii. This study provides some important clues for improving ethanol production of thermotolerant yeasts at high temperature.     

PTH release stimulated by high extracellular potassium is associated with a decrease in cytosolic calcium in bovine parathyroid cells

We employed the calcium (Ca⁺⁺)-sensitive, intracellular dye QUIN-2 to examine the role of cytosolic Ca⁺⁺ in the stimulation of PTH release by high extracellular potassium (K⁺) concentrations. Addition of 55 mM KCl to cells incubated with 115 mM NaCl and 5 mM KCl lowered cytosolic Ca⁺⁺ at either low (0.5 mM) extracellular Ca⁺⁺ (from 194±14 to 159±9 nM, p<.01, N=6) or high (1.5 mM) extracellular calcium (from 465±38 to 293±20 nM, p<.01, N=10). This reduction in cytosolic Ca⁺⁺ was due to high K⁺$\text{per}\text{se}$ and not to changes in tonicity since addition of 55 mM NaCl was without effect while a similar decrease in cytosolic Ca⁺⁺ occurred when cells were resuspended in 60 mM NaCl and 60 mM KCl. PTH release was significantly (p<.01) greater at 0.5 and 1.5 mM Ca⁺⁺ in QUIN-2-loaded cells incubated with 60 mM NaCl and 60 mM KCl than in those exposed to 115 mM NaCl and 5 mM KCl. In contrast to most secretory cells, therefore, stimulation of PTH release by high K⁺ is associated with a decrease rather than an increase in cytosolic Ca⁺⁺.     

Apparent violations of the all-or-none law in relation to potassium in the protoplasm

The protoplasm of Nitella forms a thin layer surrounding a large central vacuole filled with sap. At the inner surface of the protoplasm is a non-aqueous layer called Y and at the outer surface is a similar layer called X. At each of these layers there is a potential due to the diffusion of KCl in contact with the layer. We thus have P = P_X + P_Y in which P is the total potential, P_X is the potential at X, and P_Y the potential at Y. We assume that when stimulation occurs P_Y disappears and P_X remains unaltered. The loss of part of the potential therefore involves no violation of the all-or-none law since the Y layer loses all of its potential and the X layer loses nothing. If the concentration of KCl in the external solution and in the sap is known the concentration in the protoplasm can be calculated at each stimulation by measuring the height of the spike.     

Mean activity coefficients for the simple electrolyte in aqueous mixtures of polyelectrolyte and simple electrolyte. V. common counterion mixtures of alkali-metal dextransulfates with alkali metal chlorides

Mean molal activity coefficients of simple electrolyte in aqueous solutions of Li, Na, K or Cs salts of dextransulfate (DS) with added LjCl, NaCl, KCl or CsCl are reported. The measurements were carried out by means of an electrochemical cell method using a cation exchange membrane as cation selective electrode and Ag/AgCl electrodes. For LiDS-LiCl, NaDS-NaCl and CsDS-CsCl systems the polymer concentration, m_p, was varied from 0.0088 to 0.113 m and at a given m_p the ratio X of the polymer to salt concentration was varied from 0.5 to 16. Due to the insolubility of KDS in high concentration of KCl, the measurements on KDS-KCl system were performed in the m_p range of 0.0088–0.089 m and some of the smaller X values were omitted. The activity coefficient results are compared to Manning's limiting laws, the additivity rule, and to new limiting laws. The additivity rule can give an excellent representation of the data for all m_p values when γ_p is used as an adjustable parameter.     

Structures located at the levels of the Z bands in mouse ventricular myocardial cells

Within ventricular myocardial cells of the mouse, the myoplasmic regions located immediately adjacent to the Z lines of the sarcomeres contain a variety of structures. These include: (1) transversely oriented 10 nm (‘intermediate’) filaments that apparently contribute to the cytoskeleton of the myocardial cell; (2) the majority of the transverse elements of the T-axial tubular system; (3) specialized segments of the sarcoplasmic reticulum (SR) that are closely apposed to the sarcolemma or T-axial tubules (junctional SR); (4) ‘extended junctional SR’ (‘corbular SR’) that exists free of association with the cell membrane; (5) ‘Z tubules’ of SR that are intimately apposed to the Z line substance; and (6) leptofibrils. In addition, fasciae adherentes supplant Z lines where myofibrils insert into the transverse borders (intercalated discs) of the cells. The concentration of these myocardial components at the level of the Z lines suggests that a particular specialization of structural and physiological activities exists in the Z-level regions of the myoplasm. In particular, it appears that the combination of intermediate filaments, T tubules, and Z-level SR elements forms a series of parallel planar bodies that extend across each myocardial cell to impart transverse rigidity. The movement and compartmentation of calcium ion (Ca²⁺) would seem especially active near the Z lines of the myofibrils, in view of the preferential location there of Ca²⁺-sequestering myocardial structures such as T tubules, junctional SR, extended junctional SR and Z tubules.     

Effect of the cations sodium,potassium and calcium on the interaction of hyaluronate chains: a light scattering and viscometric study

Light scattering and viscometric studies have been carried out on two preparations, A and B, of rooster comb hyaluronate. Sedimentation rate studies have also been performed with A. Light scattering measurements in 0.2 m KCl for preparation A gave a molecular weight of 3.3 × 10⁶ and for B, 1.0 × 10⁶. In (0.1–0.3) M NaCl similar measurements gave a particle weight for A of (4.4–6.4 × 10⁶ and for B (1.7–2.8 × 10⁶. In 0.066 m CaCl₂ molecular weight values of 9.5 × 10⁶ for A and 1.7 × 10⁶ for B were obtained. Thus in the presence of Na⁺ and Ca²⁺ ions aggregates of chains persisted into dilute solution. Measurements by light scattering on A and B in 4 m guanidinium chloride gave values in the same range as those obtained in 0.2 m KCl. Sedimentation rate studies on A gave values of 10.3 Svedbergs in 0.2 m KCl and 12.2 Svedbergs in 0.2 m NaCl and 0.066m CaCl₂. The shear dependence of the viscosity was studied using a conicylindrical viscometer at shear rates between 0.5 and 20 s^?1. Preparation A in 0.2 m KCl and NaCl yielded values for (ν_sp/cc→0 of 5000 and 7100 ml g^?1 respectively in keeping with the tendency to aggregate. The behaviour for preparation B was similar. In 0.066 m CaCl₂ there was a marked dependence of viscosity on shear speed below 10 s^?1 for all concentrations and the value of (ν_sp/c)→0 at 0 s^?1 for preparation A was 7700 ml g^?1 while at a shear rate of 8 s^?1 (ν_sp/c)c→0 ? 5000 ml g ^?1. Similar effects were found for preparation B and the data suggest associations of chains disruptable by weak shear forces. The increase in viscosity with concentration in the presence of 0.066 m CaCl₂ was much less than in the presence of KCl or NaCl, suggesting that the Ca²⁺ had a marked effect on the ”rigidity’ of the molecules in solution. A viscometric titration experiment with Ca^2? showed that a level of 0.02 m CaCl₂ in 0.2 m NaCl was sufficient to produce the change in viscosity presented above and that significant perturbations of the viscosity were present at 0.005?0.01 m CaCl₂.     

Conformational change of poly-N epsilon-glutaryl-L-lysine and poly-N epsilon-succinyl-L-lysine in aqueous salt solutions

The conformational changes of poly-N^ε-glutaryl-L -lysine (PGL) and poly-N^ε-succinyl-L -lysine (PSL) in various salt solutions were studied by use of ORD and potentiometric titration measurements. The addition of alkali metal salts to the fully ionized PGL or PSL solution caused helix formation. The helical content of the polymers increases in the following sequences: at salt concentration 0–2 M, CsCl < KCl < LiCl < NaCl; and at 2–3 M, LiCl < CsCl < KCl ～ NaCl. The preferential binding of the solvent components with various alkali metal salts of PGL or PSL was measured in LiCl, NaCl, and KCl solutions by means of equilibrium dialysis and differential refractometry. It was found that with increasing salt concentration, the polymers were preferentially hydrated in NaCl and KCl soultions; however the salt was preferentially bound to the polymers in LiCl solution. Such preferential binding was suggested to be closely related to conformational change. The addition of CaCl₂ to polymer solutions led to the stabilization of the helical structure of PGL or PSL.     

Mechanical Properties of the Sarcolemma and Myoplasm in Frog Muscle as a Function of Sarcomere Length

The elastimeter method was applied to the single muscle fiber of the frog semitendinosus to obtain the elastic moduli of the sarcolemma and myoplasm, as well as their relative contributions to resting fiber tension at different extensions. A bleb which was sucked into a flat-mouthed pipette at the fiber surface separated into an external sarcolemmal membrane and a thick inner myoplasmic region. Measurements showed that the sarcolemma does not contribute to intact fiber tension at sarcomere lengths below 3 µ. It was estimated that the sarcolemma contributed on the order of 10% to intact fiber tension at sarcomere lengths between 3 and 3.75 µ, and more so with further extension. Between these sarcomere lengths, the sarcolemma can be linearly extended and has a longitudinal elastic modulus of 5 x 10⁶ dyne/cm² (assuming a thickness of 0.1 µ). Resistance to deformation of the inner bleb region is due to myoplasmic elasticity. The myoplasmic elastic modulus was estimated by use of a model and was used to predict a fiber length-tension curve which agreed approximately with observations.     

Electron Spin Resonance Studies of Ionic Permeability Properties of Thylakoid Membranes of Beta vulgaris and Avicennia germinans

Measurement of intrathylakoid aqueous volumes by electron spin resonance spectroscopy was used to study ionic permeability properties of thylakoid membranes isolated from Beta vulgaris L. and Avicennia germinans L. The thylakoids behaved as perfect osmometers in the presence of sorbitol and betaine. Thylakoids exposed to hypertonic solutions of NaCl and KCl shrank and subsequently swelled, requiring 10 minutes to regain their original volume. The initial influx rate calculated from the kinetics of changes in intrathylakoid volume in response to 450 millimolar gradients of NaCl and KCl was 2.3 × 10⁻¹³ moles per square centimeter per second. These data show that the passive permeability to NaCl and KCl was low.     

Exposure of the Isolated Frog Skin to High Potassium Concentrations at the Internal Surface : II. Changes in epithelial cell volume,resistance and response to antidiuretic hormone

Isolated frog skin epithelia undergo marked, but reversible swelling when the external skin surface is bathed with conventional NaCl Ringer''s and the internal surface with KCl Ringer''s solutions. In 2 hours, epithelial thickness increased by over twofold. When NaCl Ringer''s was replaced on both sides of the skin, volume returned to control levels in less than 1 hour. When sulfate, rather than chloride, was the predominant anion, exposure of the internal surface to high potassium concentrations did not evoke changes in epithelial cell volume. With both KCl and K₂SO₄ Ringer''s, an immediate drop in DC resistance across the skin occurred. This was followed by partial recovery. Both the immediate drop and partial recovery were unrelated to changes in volume. A slow, sustained secondary drop in resistance was observed with KCl but not K₂SO₄ Ringer''s. This slower drop was associated temporally with swelling. When epithelial cell swelling occurred (i.e. with KCl Ringer''s), the characteristic response of the skin to vasopressin was abolished. However, with sulfate as anion, vasopressin elicited an increase in short-circuit current and/or in cell volume despite high internal potassium concentrations. It is concluded that epithelial swelling increased the permeability of the sodium-selective barrier at the external surface of the cells; and the possibility exists that stretching of cell membranes altered dimensions of pathways through which Na and water move, thereby mimicking the effects of vasopressin.