K+ depolarization and phospholipid metabolism in frog sartorius muscle

K+ depolarization evokes phosphatidylinositol response, i.e. the increased 32P orthophosphate labelling of phosphatidylinositol in frog sartorii muscles. The phosphatidylinositol response seems to be closely related to K+ depolarization and not to the transient Ca2+ release at the beginning of depolarization. It ceases as soon as the muscles depolarized by 90 mmol/l KCl for a short period of time are repolarized, while it continues when the depolarization is maintained. When the muscles are depolarized with 20 mmol/l KCl, the phosphatidylinositol response is also observed. This response is not suppressed by drugs that block Ca2+ mobilization. Other agents like caffeine, azide or EGTA which induce some effects similar to that of K+ depolarization, do not evoke phosphatidylinositol response. Rather, they simply cause a decrease in the labelling of phospholipids, phosphatidylinositol being the least affected. In muscles derived from frogs maintained under healthy conditions Ca2+ release in the early phase of K+ depolarization does not cause significant changes in phospholipid labelling. However, in muscles from frogs starving for many months, a large decrease in the labelling of phospholipids is observed in the early phase of K+ depolarization. It is postulated that the changes in the physicochemical state of the membrane and not Ca2+ gating mechanism or free cell Ca2+ level are crucial in the phosphatidylinositol response in the frog sartorii muscles depolarized by high K+.     

The effectiveness of the longitudinal field,coupled with depolarization in activating frog twitch muscles

EFFECTIVE EXCITATION, PRECEDING THE MECHANICAL RESPONSE IN FROG TWITCH MUSCLES, INVOLVES TWO DISTINCT EVENTS: depolarization of the excitable membrane and the flow of internal currents. To distinguish between the effects of these two potential factors in activation of the contractile machinery, experiments ought to be conducted in which one or the other is excluded. Our experiments are designed to distinguish between these effects by indirect methods. Depolarization in a longitudinal electric field can be expected to be greatest at the ends where current leaves the muscle fibers (space constant at [K] = 16 mM/liter is >1 mm.), whereas the internal longitudinal current is known to be greatest in the middle portion. Depolarization, therefore, should affect the ends more strongly and internal current the middle portion. Our experiments show that non-propagating frog twitch muscles shorten, during isotonic work, along their whole length and not only at their ends, when effectively stimulated in a longitudinal A.C. field. At a field strength about twice the new threshold value (at [K](o) = 16 mM) shortening is distinctly greater in the middle portion of the muscle than at the ends. The muscles, although temporarily non-propagating, remain intact throughout the experiment, as demonstrated by complete recovery after repolarization. These findings may be taken as an indication that internal currents are more directly linked to activation than is depolarization, but the latter is an essential priming step, which must precede or coincide with effective current flow.     

Caffeine contracture and iodoacetate rigor in frog skeletal muscle. A comparison

Frog sartorius muscle treated with 5.0 mM or greater caffeine exhibits stiffness similar to that obtained from muscle in iodoacetate rigor. The data provide quantitative evidence that suggests that caffeine at irreversible contracture-producing concentrations somehow induces a rigor or rigorlike state in skeletal muscle.     

The physostigmine depolarization potentiating effect of salicylate in frog skeletal muscle.

1) The frog's sartorius muscle was depolarized depending on the degree of concentration 2--4 times more intensely by physostigmine salicylate than by physostigmine sulphate. 2) In normal Ringer's solution, 1 mM physostigmine salicylate decreased the sensitivity of the membrane to potassium depolarization by about 90%. Under similar experimental conditions, physostigmine sulphate and Na salicylate, respectively, decrease the sensitivity of the membrane to potassium depolarization by about 30%. 3) The difference manifested in the depolarizing effect of salicylate and other physostigmine salts (chloride, sulphate, phosphate, formiate, acetate, monochloracetate, benzoate and para-oxy-benzoate) is expressed already at 1 mM concentration (about 10-fold), if the muscle had been equilibrated in chloride-free glucuronate or sulphate milieu. 4) The depolarization develops slowly. It takes 30--60 minutes for the new steady state to develop even in the superficial sartorius fibres. If depolarization has reached its maximum on an average 100 mV, the membrane potential remains unchanged for hours. 5) Depolarization ensues at an unchanged degree in the presence of Na-free (choline) Ringer as well as in the presence of 2X10(-8) g/ml tetrodotoxin; therefore, it is not a Na-dependent process. 6) Under the influence of 1 mM physostigmine salicylate the membrane's resistance to the inward potassium current increased about twofold, while the increase was only 15% to the outward potassium current. It is assumed that the salicylate anion is characteristically capable of potentiating the decreasing effect of physostigmine on potassium permeability, though the role of the metabolic effect of salicylate cannot be excluded.     

Effects of K+-induced depolarization and purinergic receptor activation on elemental content in insulin-producing RINm5F-cells.

X-ray microanalysis was used to detect elemental changes in the insulin-producing tumor cell-line RINm5F. To improve discrimination between mobile ions and ions bound to macromolecules a new approach was employed, consisting of multivariate statistical analysis of correlations between the concentrations of Na, Mg, P, S, Cl, K, and Ca. RINm5F cells, cultured on Formvar-coated titanium grids, were stimulated with high K⁺ or ATP, that are both known to stimulate insulin release. The buffers used contained Ca²⁺ or one of the Ca²⁺-analogues Sr²⁺ and Ba²⁺, to represent Ca²⁺ uptake in response to stimulation. After stimulation the cells were shock-frozen and freeze-dried overnight. Incubation for 10-20 seconds in a Ca²⁺-containing buffer did not significantly affect elemental composition, whereas cellular Mg, P and K decreased in a Sr²⁺-containing buffer. Depolarization with high K⁺ concentration caused an increase in the cellular Na content, both in Ca²⁺- and Sr²⁺-containing buffers, but not in the buffer where Ca²⁺ had been replaced by Ba²⁺. X-ray microanalysis is useful for detection of elemental changes subsequent to stimulation of cultured cells. Moreover, multivariate statistical analysis strengthens the idea that stimulation of RINm5F cells causes redistribution of ions possibly due to changes in the state of binding of some elements to cellular proteins.     

Intracellular pH of frog sartorius muscle.

A weak base, morpholine, has been labelled with 3H and tested for its suitability as an indicator for intracellular pH, by distribution in the tissue water of frog sartorius muscle in the species Hyla litoria. Its pK'a at 20 degrees C in a solution of the same ionic strength as frog Ringer was found to be 8.45 +/- 0.02, which is in the range of maximal sensitivity. Morpholine equilibrated with the tissue in 17 h; it was shown that it was not bound to intracellular constituents, that it was not metabolised nor toxic in the concentrations used; it was therefore judged suitable as a pH indcator. Intracellular pH was then measured by distribution of morpholine (6.985 +/- 0.08), nicotine (6.915 +/- 0.03) and the weak acid 5,5'-dimethyl-2,4-oxazolidinedione (7.10 +/- 0.05) and the pH-sensitive microelectrodes (5.9, the equilibrium value). It was shown that the four significantly different values could not be reconciled in terms of experimental error, heterogeneity of intracellular pH, liquid junction potential differences, or binding of indicator molecules inside the fibre. They could, however, be reconciled if the fibre water had different structure and solvent properties from the extracellular water and all ions were distributed across the membrane as between two liquid phases containing different solvents. Then the H+ would be in equilibrium, as shown by the microelectrode measurement, but intracellular pH would be indeterminable and probably greater than 6.     

A comparative analysis of the contracture responses induced by acetylcholine and choline in the twitch and tonic fibers of frog skeletal muscles

A comparative analysis of the contractile responses induced by acetylcholine and replacement of the external Na⁺ ions with choline ions in the isolated twitch and tonic fibers of frog skeletal muscles was performed. The effects of extracellular Ca²⁺ concentration and several pharmacological agents modulating the activity of various systems maintaining Ca²⁺ level in the myoplasm (dantrolene, cresol, d-tubocurarine, and tetrodotoxin) were studied. It has been found that a voltage-dependent Ca²⁺ release from the sarcoplasmic reticulum depot is the main mechanism inducing the acetylcholine contracture in the fibers of both types. However, the twitch and tonic fibers differ in the properties of the α-isoform and(or) the ratio of α- to β-isoforms of ryanodine-sensitive channels. In the fibers of both types, the replacement of over 25% of Na⁺ ions with choline induces long-term contracture responses, which are also mediated by activation of acetylcholine receptors. It is assumed that an additional mechanism—accumulation of choline ions in the myoplasm and their direct action on the ryanodine-sensitive channels—is involved in the development of such contractile responses.     

Effects on sodium efflux of treating frog sartorius muscles with hypertonic glycerol solutions

Summary Efflux of sodium from frog sartorius muscles was measured during and after exposure to Ringer's fluid made hypertonic by addition of 400mm glycerol. Effects of strophanthidin, removal of external Na, and variation of external K were determined. During exposure to glycerol-containing solutions, Na efflux increased. Upon return to Ringer's fluid, Na efflux at first increased further. After the initial increase, Na efflux gradually declined; for the first two hours the efflux of Na from treated muscles was higher than that from untreated muscles. In the second hour, the strophanthidin-sensitive fractions of Na efflux were slightly increased while the strophanthidin-insensitive fractions were slightly decreased when compared with untreated muscles. The responses of Na efflux to removal of external sodium and to varying external K were comparable in both treated and untreated muscles. This shows that, at first, the membranes which remained after glycerol treatment exhibited the normal characteristics of Na extrusion. For at least eight hours after glycerol withdrawal the Na efflux from treated muscles declined relative to that of untreated muscles. The decline was largely due to reduction in strophanthidinsensitive fractions of efflux. Six to eight hours after glycerol withdrawal the Na efflux in treated muscles was less responsive to alterations in external K and Na than it was in untreated muscles. This indicates that aged glycerol-treated sartorii lost a substantial part of their capacity to actively transport sodium.     

Effects on Mitochondrial K Flux of pH,K Concentration,and N-Ethyl Maleimide

Based on published evidence that cation transport in mitochondria is not significantly dependent on a membrane potential, it is suggested that the process of mitochondrial cation transport may be nonelectrogenic. These experiments focused on the possibility that K⁺ flux into rat liver mitochondria may be directly coupled, via an energy-linked carrier mechanism, to OH^? influx or H⁺ efflux. The dependence of the unidirectional K⁺ influx on the external K⁺ concentration indicates involvement of a saturable mechanism. Increasing the external pH from 7.0 to 8.0 increases the apparent V_max of the K⁺ influx without significantly altering the apparent K_m for K⁺. The pH dependence is greater in the presence of N-ethyl maleimide, a known inhibitor of the mitochondrial P_i/OH^? exchange mechanism. N-Ethyl maleimide decreases the apparent V_max at pH 7.0 and increases it at pH 8.0. Evidence indicates that both N-ethyl maleimide and a high external P_i concentration may stimulate the K⁺ influx at alkaline external pH (8.0) by preventing net exchanges between endogenous P_i and external OH^?. An apparent first-order dependence of the K⁺ influx on the external OH^? concentration is observed in the presence of N-ethyl maleimide. These results are consistent with a possible role of external OH^? as a cosubstrate of the K⁺ transport mechanism.     

Ba2+ ions block K+-induced contractures by antagonizing K+-induced membrane depolarization in frog skeletal muscle fibres

The effects of Ba2+ ions on twitches, K+-induced contractures, and on intracellularly recorded membrane potentials (Em) and depolarizations of frog skeletal muscle fibres were investigated. Exposure of toe muscles to choline--Ringer's solution with 10(-3) M Ba2+ with Ca2+ (1.08 mM) eliminated or very greatly reduced contractures produced by 60 mM K+. In contrast, not only did the same concentration of Ba2+ ions fail to depress the twitch tension of isolated semitendinosus fibres when added to Ringer's with Ca2+, but it even restored twitches that had been eliminated in a zero Ca2+ Ringer's solution. The resting Em of sartorius muscle fibres in choline--Ringer's solution was reduced about 20 mV by 10(-3) M Ba2+. This Ba2+ ion concentration also antagonized the K+-induced depolarization. Thus in the presence of 1 mM Ba2+, 20 mM K+ hyperpolarized rather than depolarized the fibres and 60 or 123 mM K+ produced only very slowly developing, small depolarizations. These results suggest that the loss of the K+-induced contracture in choline-Ringer's caused by Ba2+ ions is due to an inhibition of the K+-induced depolarization. The latter result is consistent with previous findings of other workers that Ba2+ ions block membrane K+ channels.     

植物性雌激素genistein对哇巴因所致豚鼠乳头状肌迟后去极化及触发活动的效应

应用标准玻璃微电极技术,研究了植物性雌激素genistein(GST)对哇巴因所引起的豚鼠乳头状肌迟后去极化（DAD）及触发活动（TA）的效应,结果如下：（1）预先给予是GST（10,50,100umol/L)剂量依赖性地抑制哇巴因（1umol/L)所引起的鼠乳头状肌DAD及TA;（2）预先应用一氧化氮合酶抑制剂L－NAME（1mmol/L),不影响GST（50μmol/L）对DAD及TA的效应;（3）单独应用17β－雌二醇（E2,5μmol/L）或GST（10μmol/L）对DAD及TA无明显影响,而联合应用相同剂量的GST和E2则产生明显儿应,以上结果提示,GST可能通过抑制钙离子内流从而具有抗心律失常作用,这对于心血管系统的保护有一定意义。