The effects of valinomycin on striated muscles of the frog and the crayfish

Effects of neutral ionophores on membrane electrical characteristics of NG108-15 cells

The effects of several K(+)-selective neutral ionophores on membrane electrical characteristics of differentiated NG108-15 (neuroblastoma X glioma hybrid) cells were examined. Specifically, alterations in membrane resting potential (V(m)), input resistance (R(in)) and electrically-induced action potential generation were determined upon bath application of enniatin (0.1-10 microg/ml), nonactin (0. 1-10 microM) and valinomycin (0.1-10 microM). Although some cells exhibited a slight hyperpolarization and/or reduced R(in), i.e. membrane electrical correlates of enhanced K(+) loss, neither V(m) nor R(in) were significantly altered by any of the ionophores. However, valinomycin and especially nonactin affected action potentials induced by electrical stimulation. This was apparent in the ablation of action potentials in some cells and in the occurrence of degenerative changes in action potential shape in others. The simultaneous administration of the neutral ionophores and the protonophore CCCP or the superfusion of enniatin, nonactin or valinomycin in high (50 mM) glucose-containing physiological solution did not yield more extensive alterations in V(m) or R(in). These data suggest that the neutral ionophores are unable to materially enhance K(+) flux above the relatively high resting level in NG108-15 cells. Thus, alterations in action potentials appear to be unrelated to K(+) transport activity.

Valinomycin and excitation-contraction coupling in skeletal muscle fibres of the frog

1. Experiments were carried out on intact frog skeletal muscle fibres to study the role of H+ and K+ as counter-ions during the release of Ca2+ from the sarcoplasmic reticulum (SR). A specific focus was to test whether valinomycin, a potassium ionophore, markedly reduces or abolishes H+ counter-ions fluxes across the SR membrane in response to electrical stimulation. 2. Single twitch fibres, mounted on an optical bench apparatus and stretch to long sarcomere length (3.6-4.0 microns), were activated by single action potentials (16 degrees C). Two optical signals related to excitation-contraction coupling were measured: (i) the 'second component' of the intrinsic birefringence signal, which is closely related to the myoplasmic free [Ca2+] transient, and (ii) the transient myoplasmic alkalization (delta pH) detectable from the pH indicator Phenol Red, a signal thought to reflect the movement of protons from the myoplasm into the SR in partial electrical exchange for released Ca2+. 3. Exposure of a fibre to 5 microM-valinomycin produced a slight, progressive decrease in the amplitude of the birefringence signal, approximately 5-6% per hour. This result suggests that, if anything, the peak rate at which Ca2+ is released from the sarcoplasmic reticulum is slightly decreased by valinomycin. 4. The amplitude of the Phenol Red delta pH signal, measured after exposure of a fibre to valinomycin for a period of at least 60 min, averaged 0.0020 +/- 0.0002 (+/- S.E.M.); this value is slightly smaller than, but not significantly different from (P greater than 0.05; two-tailed t test) that measured in fibres not exposed to valinomycin (0.0025 +/- 0.0002). This result does not support the idea that valinomycin, but virtue of increasing the flux of K+ into the SR, markedly reduces the flux of protons during Ca2+ release. 5. Our findings of minimal changes in the birefringence and delta pH signals are consistent with the idea that, at the time of Ca2+ release, the potassium conductance of the SR membrane is large and not substantially increased by the addition of valinomycin to Ringer solution.

The effects of valinomycin on ion movements across the sarcoplasmic reticulum in frog muscle

The effects of valinomycin on the elemental composition and the fractional volume of the terminal cisternae (t.c.) of the sarcoplasmic reticulum (s.r.) were determined in rapidly frozen frog semitendinosus muscles. The concentrations of valinomycin used for the electron probe studies (5 microM) had no effect on tetanus tension or t.c. volume (2.% of fibre volume). Mitochondria were markedly swollen and their K content was significantly increased in both the resting and the tetanized valinomycin-treated muscles. Valinomycin had no effect on the concentration of Na, Mg, P, Cl, K and Ca in the t.c. of resting muscles. In untreated, tetanized muscles, Ca2+ release was accompanied by the uptake of K and Mg into the t.c. in an amount that was significantly less than the positive charge removed through Ca2+ release, confirming previous observations showing an apparent charge deficit (Somlyo, Gonzalez-Serratos, Shuman, McClellan & Somlyo, 1981). Valinomycin abolished the apparent charge deficit: in tetanized, valinomycin-treated muscles, the uptake of K into the t.c. was significantly (P less than 0.001) greater than in the untreated muscles and Mg uptake also remained highly significant. It is suggested that Ca2+ release from activated muscle is an electrogenic process and that the K+ conductance of the s.r. in untreated frog muscles is insufficient to allow charge neutralization of the Ca2+ current during release. The increase in K+ permeability caused by valinomycin permits the greater counter movement of K+ under the combined influence of the electrical potential generated by outward Ca2+ movement and the acidic cation binding proteins in the lumen of the s.r. The results are consistent with the proposal (Somlyo et al. 1981) that in normal frog muscles not treated with valinomycin, the apparent positive charge deficit observed after a tetanus reflects the movement of protons and, possibly, organic cations.

A potassium ionophore (valinomycin) inhibits lymphocyte proliferation by its effects on the cell membrane

Valinomycin is a depsipeptide antibiotic which selectively translocates potassium across biologic membranes. This potassium ionophore was observed to inhibit phytohemagglutinin-stimulated blastogenesis and proliferation in human lymphocytes. The effect was not due to toxicity to the cells, nor appeared to be due to the effects of valinomycin as an uncoupler of oxidative phosphorylation. Furthermore, the inhibitory effect on phytohemagglutinin stimulated lymphocytes was prevented by increasing the potassium concentration of the external media. These results suggest that the interaction of mitogens with specific receptors at the cell membrane may involve mechanisms affecting cation fluxes and membrane potential. These ionic events may play a role in the transduction of membrane signals for lymphocyte stimulation.

Increase of potassium flux by valinomycin in embryonic chick heart

The effect of different concentrations of the antibiotic valinomycin, was determined on 42K efflux and Na, K content of embryonic chick hearts. Valinomycin produces an increase of K efflux which is progressive in time and markedly dependent on the concentration of external K (0-5 mM) and valinomycin (10(-8) to 10(-5) M). The changes in K efflux is not due to a reversal of the Na-K pump mechanism, secondary to ATP depletion: i) the increase of K efflux by valinomycin persists in the absence of external Na ions. ii) analysis of Na and K content and 42K influx measurements with and without valinomycin indicate that active K influx is not inhibited in a solution containing 0.5 mM K and only slightly decreased in a solution containing 5 mM K. Valinomycin, acting as a K carrier, presumably increases K conductance of the cell membrane resulting in a rise in K efflux.

Enhancement of the electrical excitability of neuroblastoma cells by valinomycin

Mouse neuroblastoma cells in stationary phase of growth display partially developed electrical properties. Addition of the K+ selective carrier valinomycin to these cells causes rapid enhancement of electrical excitability. We suggest that the appearance of molecules with properties similar to valinomycin is essential for the full expression of electrical excitability in differentiating neuroblastoma.

Transmembrane potentials in guinea-pig hepatocytes

1. In pieces of guinea-pig liver with an intact capsule, the mean resting potential (E) is -49.8 mV, close to the value for in vivo hepatocytes and significantly higher than a similar preparation of rat liver. E is considerably less than the calculated K equilibrium potential of about -90 mV.2. Changes in [K(+)](o) in the range of 0-10 mM produce almost no change in E. At [K(+)](o) above 20 mM, a tenfold increase produces only a 33 mV depolarization. The results with changing [K(+)](o) are the same in Cl(-) free solutions (isethionate substitution) or when the [K(+)](o)[Cl(-)](o) product is kept constant. E rapidly returned to normal in liver pieces returned to Ringer after prolonged soaking in high K(+) solution.3. Changes in [Na(+)](o), [Cl(-)](o), [H(+)](o), [Ca(2+)](o) or [Mg(2+)](o) have little effect on E. Simultaneous variation of [Na(+)](o) and [Cl(-)](o) in opposite directions to maximize predicted changes in E also has only a minimal effect on E.4. Cyanide (5 mM) or ouabain (10(-4)M) cause depolarization, so there is no reason to believe that pumps sensitive to these poisons are normally lowering resting potential. Part of the normal E may be produced by an electrogenic ouabain-sensitive ion pump.5. The data is interpreted by using a form of the constant field equation developed by Brading & Caldwell (1971). Application of this method yields P(Na)/P(K) less than 0.04, P(Cl)/P(K) approximately 0.3. Additional terms, x and y, are required to account for the behaviour of E. Their physical basis remains undetermined.6. Suggestions are presented for further study and for the application of the method of Brading & Caldwell to other non-excitable cells and to excitable cells in low [K(+)](o), in which E is poorly understood.