The electrical changes accompanying fertilization and cortical vesicle secretion in the medaka egg

The electrical properties of the egg of the medaka, Oryzias latipes, were studied before, during, and after fertilization. The resting potential of the unfertilized egg averaged ?39 ± 9 mV in Yamamoto's Ringers (Y. Ringers), but 20% of the values were between ?50 and ?60 mV. Fertilization triggers a small depolarization of 4 ± 3 mV in 10% Y. Ringers with an average duration of 20 ± 10 sec. The amplitude of this depolarization is independent of [Na⁺]_o, [Ca²⁺]_o, and [Cl^?]_o, so it appears to be due to a nonspecific leak triggered by sperm-egg fusion. The depolarization is followed by a longer hyperpolarizing phase with an average amplitude of 31 ± 12 mV. Recovery from this hyperpolarization has a fast phase lasting 155 ± 18 sec, followed by a slower phase which reaches a steady average membrane potential of ?19 ± 1 mV by 9 min after fertilization. The membrane resistance falls 10-fold during the first 2 min after fertilization, from 40 (1520 kΩ-cm²) to 3 MΩ. This is largely due to an increase in the K⁺ conductance. At the peak of the hyperpolarization, the membrane potential exhibits a 28 mV/decade [K⁺]_o dependence and a 6 mV/decade [Na⁺]_o dependence. The membrane resistance slowly recovers over the next 8 min to a value about 30% larger than before fertilization. The relation of current vs voltage was linear before, during, and after fertilization and indicated a reversal potential of ?98 ± 20 mV for the hyperpolarization peak. The egg's capacitance averaged 0.04 ± 0.01 μF (0.9 μF/cm²) before fertilization and approximately doubles within 90 sec after fertilization. It then decreases over a 9-min period, reaching a value 25% smaller than before fertilization.     

The second component of the fertilization potential in sea urchin (Pseudocentrotus depressus) eggs involves both Na+ and K+ permeability

The fertilization potential of the Pseudocentrotus depressus egg involved three transiently depolarizing components which had a different time course and a peak value. Three peaks were at less than 10 sec, 43 ± 4 sec (mean ± SD), and 182 ± 22 sec after the onset of the fertilization potential. Their peak values (mean ± SD) were 37 ± 4, 17 ± 3, and −31 ± 5 mV in standard artificial sea water. The effect of external ions on the membrane potential at the peak of the second component was measured with a conventional voltage-recording microelectrode. The peak value changed 51 mV with a 10-fold change in external Na⁺ concentration. However, it was about 65 mV more negative than the equilibrium potential of Na⁺, assuming that the internal Na⁺ concentration was 13.5 mM. H⁺, Ca²⁺, Mg²⁺, and Cl⁻ did not contribute to the peak value. The peak value was sensitive to the external K⁺ concentration. These data fitted a theoretical line obtained from the Goldman-Hodgkin-Katz equation, using a ratio of P_Na:P_K:P_Cl = 1.1:1.0:0. This means that the permeability to both Na⁺ and K⁺ is responsible for the second component of the fertilization potential. The fertilization potential was also measured in the artificial sea water containing Li⁺ or Cs⁺. The egg at the second component of the fertilization potential was almost equally permeable to Li⁺ as well as Na⁺ or K⁺ and somewhat permeable to Cs⁺. By contrast, the resting membrane potential before fertilization depended to a large extent upon K⁺ permeability.     

A long-lasting electrically mediated block, due to the egg membrane hyperpolarization at fertilization, ensures physiological monospermy in eggs of the crab Maia squinado

In response to fertilization, the membrane potential (Em) of the crab egg hyperpolarizes from about -50 mV to about -80 mV in 400 msec. To establish whether this fast hyperpolarization is correlated with physiological polyspermy or conversely mediates an electrical block to polyspermy, we examined the morphological and electrophysiological characteristics of eggs from the crab Maia squinado. Fertilized naturally spawned eggs were found to be physiologically monospermic and their average Em was constant at -77 +/- 0.5 mV. To examine a possible electrical block ensuring this monospermy, unfertilized eggs were voltage clamped at various Em values ranging from +20 to -90 mV, inseminated, and examined morphologically. All eggs clamped at +20 to -65 mV responded by developing a fertilization current, If. It consisted of an outwardly directed K+ current in one or several steps, each caused by a single spermatozoon interacting with the egg membrane. The percentage of eggs clamped at values more negative than -65 mV, which responded at insemination by developing an If, decreased and dropped to 0 at -80 mV. This indicated that the membrane processes occurring during the contact between gametes and eliciting an electrical response by the egg membrane are voltage dependent. Further, the spermatozoon never penetrated into eggs voltage clamped at a Em between +20 and -60 mV and at voltages more negative than -75 mV. Em values between -65 and -75 mV were required for spermatozoon incorporation into the egg, indicating that sperm entry is also voltage dependent. It is proposed that the hyperpolarization of the egg membrane in response to fertilization constitutes a long-lasting electrical block to polyspermy in crab eggs.     

Properties of single potassium channels in guinea pig hepatocytes

The patch-clamp technique of cell-attached and inside-out configurations was used to study the single potassium channels in isolated guinea pig hepatocytes. The single potassium channels in isolated guinea pig hepatocytes were recorded at different K⁺ concentrations. A linear single-channel current-voltage relationship was obtained at the voltage range of -80 to -20 mV with slope conductance of 70 ± 6 pS (n = 10). Under symmetrical high K⁺ concentration of 148 mM in the cell-attached patch membrane, the I-V curve exhibited a mild inward rectification at potentials positive to +20 mV. The values of reversal potential was +5 ± 2 mV (n = 10). When the external potassium concentration ([K⁺]₀) was decreased to 74 mM and 20 mM, the slope conductance was decreased to 48 ± 2 pS (n = 4) and 24 ± 3 pS (n = 3), respectively. The reversal potential was changed by 58 mV for a tenfold change in [K⁺]₀, indicating that this channel was highly selective for K⁺. Open probabilities (P₀) of the channel were 73-93% without apparent voltage dependence. The distributions of open time of the channels were fitted to two exponentials, while those of closed time were fitted to three exponentials, exhibiting no voltage dependence. The success rate of K⁺ channel activity to be recorded was 28% at room temperature, and there were no increases in the success rate nor in the channel opening probabilities at a temperature of 34-36°C. P₀ in inside-out patches was not changed by application of 1 μM Ca²⁺ nor 1 mM Mg²⁺ to the internal side of patch membranes. It is concluded that a novel type of the K⁺ channels in guinea pig hepatocytes had different properties of slope conductance, channel kinetics, and sensitivity to [Ca²⁺]_i, from those in other species. © 1994 Wiley-Liss, Inc.     

Changes in membrane potential during mouse egg development

The electrical membrane potential (E_m) was measured in the developing mouse egg with intracellular microelectrodes. The oocyte had a low negative E_m of ?8.3 ± 0.8 mV (mean ± SE) when immature, which decreased and reversed polarity to a small positive value (+1.9 ± 0.2 mV) in the mature ovulated oocyte. After fertilization E_m returned to a negative value (?9.2 ± 0.5 mV) similar in magnitude to that observed in immature oocytes and then increased significantly (P < 0.01) at both the two-cell (?10.7 ± 0.3 mV) and morula stage (?12.8 ± 0.7 mV) and leveled out at the blastocyst stage (?12.9 ± 0.7 mV). Average potential difference recorded across the blastocoele wall of not fully expanded blastocysts was ?5.0 ± 0.5 mV. These data represent the first report on membrane potentials of the mammalian egg during development. A striking similarity is seen in the relative changes in E_m throughout development of the mouse egg in comparison to those seen in other invertebrate and vertebrate eggs.     

External Mg2+ is required for hyperpolarization to occur in ovulated oocytes of the prawn Palaemon serratus

Immediately after dissection, the ovulated oocyte of the prawn Palaemon serratus had a resting potential E_m of −42 ± 2 mV and a membrane resistance R_m of 15 ± 5 MΩ; the membrane was more permeable to Cl⁻ than to K⁺. The oocyte spontaneously hyperpolarized and E_m gradually reached −70 mV 20–30 min after removal of the oocyte from the female, due to increased membrane permeability to K⁺. However, the hyperpolarization occured only if Mg²⁺ was present in the seawater; external Ca²⁺ was not required. Long-term incubation without external Mg²⁺ depolarized the membrane and increased membrane resistance. After preincubation in Mg²⁺-free ASW, oocytes transferred to standard artificial seawater (ASW) transiently hyperpolarized and then repolarized, before gradually hyperpolarizing to a sustained value of −62 ± mV. The respective roles of external Mg²⁺ and fertilization in eliciting the electrical response of the prawn egg at natural spawning are discussed.     

Decrease in the electrogenic Contribution of Na,K-ATPase and the resting membrane potential as a possible mechanism of Ca<Superscript>2+</Superscript> accumulation in rat soleus muscle in a short-term gravity unloading

The resting membrane potential and electrogenic contribution of α1- and α2-isoforms of Na⁺/K⁺-ATPase in the rat soleus muscle at early stages of gravity unloading were analyzed. The role of L-type calcium channels in accumulation of calcium ions in the myoplasm under these conditions was estimated. After 3-day antiorthostatic suspension, the resting membrane potential of the muscle fibers decreased from ?71.0 ± 0.5 to ?66.8 ± 0.7 mV, the muscle excitability reduced, and a trend of muscle fatigue acceleration appeared. The electrogenic contribution of ouabain-sensitive α2-isoform of Na⁺/K⁺-ATPase, determined as the depolarization caused by 1μM ouabain, decreased after suspension from 6.2 ± 0.6 to 0.5 ± 0.8 mV. The contribution of ouabain-resistant α1-isoform of Na⁺/K⁺-ATPase, determined as an additional depolarization after addition of 500 μM ouabain, decreased from 4.6 ± 0.6 to 2.6 ± 0.6 mV. The intensity of Fluo-4AM fluorescence in individual muscle fibers increased after suspension more than fourfold, which suggests an elevated calcium concentration in the myoplasm. A local delivery of nifedipine, a blocker of the L-type calcium channels, to the muscle removed this effect. The existence of a selective mechanism suppressing the electrogenic contribution of Na⁺/K⁺-ATPase α2-isoform, which is the main cause of the muscle fiber membrane depolarization after 3-day suspension, is postulated. The depolarization can activate part of potential-sensitive L-type Ca²⁺ channels, causing the accumulation of calcium ions in the muscle fiber myoplasm.     

Inward rectifier potassium channels in the HL‐1 cardiomyocyte‐derived cell line

HL‐1 is a line of immortalized cells of cardiomyocyte origin that are a useful complement to native cardiomyocytes in studies of cardiac gene regulation. Several types of ion channel have been identified in these cells, but not the physiologically important inward rectifier K⁺ channels. Our aim was to identify and characterize inward rectifier K⁺ channels in HL‐1 cells. External Ba²⁺ (100 µM) inhibited 44 ± 0.05% (mean ± s.e.m., n = 11) of inward current in whole‐cell patch‐clamp recordings. The reversal potential of the Ba²⁺‐sensitive current shifted with external [K⁺] as expected for K⁺‐selective channels. The slope conductance of the inward Ba²⁺‐sensitive current increased with external [K⁺]. The apparent Kd for Ba²⁺ was voltage dependent, ranging from 15 µM at ?150 mV to 148 µM at ?75 mV in 120 mM external K⁺. This current was insensitive to 10 µM glybenclamide. A component of whole‐cell current was sensitive to 150 µM 4,4′‐diisothiocyanatostilbene‐2,2′‐disulfonic acid (DIDS), although it did not correspond to the Ba²⁺‐sensitive component. The effect of external 1 mM Cs⁺ was similar to that of Ba²⁺. Polymerase chain reaction using HL‐1 cDNA as template and primers specific for the cardiac inward rectifier K_ir2.1 produced a fragment of the expected size that was confirmed to be K_ir2.1 by DNA sequencing. In conclusion, HL‐1 cells express a current that is characteristic of cardiac inward rectifier K⁺ channels, and express K_ir2.1 mRNA. This cell line may have use as a system for studying inward rectifier gene regulation in a cardiomyocyte phenotype. J. Cell. Physiol. 225: 751–756, 2010. © 2010 Wiley‐Liss, Inc.     

Maturation of Xenopus oocytes: II. Observations on membrane potential

Large, progesterone-responsive oocytes within their follicles have an average resting potential of about ?25 mV. When manually dissected out of their follicles, most of these oocytes undergo a hyperpolarization over the next 30–60 min to values of about ?60 to ?80 mV. The relatively high negative membrane potentials previously recorded on dissected amphibian oocytes may thus be an artifact in the sense that such measurements do not reflect the electrical characteristics of the oocyte within the follicle. The available evidence indicates that the hyperpolarization reflects the activation of an electrogenic Na⁺,K⁺-transport process. One of the terminal events of oogenesis appears to be a suppression of the generation of the Na⁺,K⁺-transport process when oocytes are ovulated artificially (by dissection) or naturally. Continuous, long-term recordings on dissected oocytes reveal that a rather pronounced depolarization of the membrane potential together with an inflection in the recorded potential around the time of germinal vesicle breakdown takes place in the presence of progesterone. Recordings of oocytes within the follicle reveal similar changes, although reduced in absolute magnitude. In both cases, final membrane potentials of ?10 to ?15 mV are achieved. The electrophysiological changes which accompany the normal maturation process thus do not appear to be as pronounced as previously indicated.     

Electrophysiology of phagocytic membranes: intracellular K+ activity and K+ equilibrium potential in macrophage polykaryons

The role of K⁺ as current carrier during the slow membrane hyperpolarizations (SH) elicited by iontophoretic Ca²⁺ injections into macrophage polykaryons is studied. The intracellular K⁺ activity (a_K) and the K⁺ equilibrium potential (E_K) are measured using ion-sensitive microelectrodes. The mean value of a_K is 84 ± 5 mM in a culture medium containing 5.3 mM K⁺, but increases to 100 ± 8 mM when the extracellular K⁺ concentration is raised to 30.3 mM. Under the same conditions the values of E_K obtained from the Nernst equation are −81 ± 2 mV and −40 ± 2 mV, respectively. The reversal potentials (E_R) of the SH are calculated from changes observed in transmembrane potential and input resistance, according to an equivalent model based only on passive ionic fluxes. The mean E_R values obtained are −74 ± 8 mV in the presence of low K⁺ concentration and −37 ± 3 mV for the high K⁺ medium. These values are significantly smaller than the estimated E_K for the corresponding situations. Evidence for the existence of an electrogenic (Na⁺ + K⁺)-ATPase activity is also presented. The evidence indicates that an increase in the membrane potassium permeability can account for about 90% of the total permeability change occurring during the SH.     

U.V. Irradiation Inhibits The Electrical Block to Polyspermy in Echinoderms*

Oocytes of the sea urchin Sphaerechinus granularis and the startish Marthasterias glacialis have been submitted to U.V. irradiation before fertilization. This treatment significantly increased the incidence and severity of polyspermy in the sea urchin and was also found effective on starfish oocytes. These were found more resistant to damage than sea urchin eggs and U.V. irradiation did not affect either their response to the hormone l-methyladenine or the rate of elevation of the fertilization envelope, which assures the late and definitive block to polyspermy. Electrophysiological measurements performed on M. glacialis oocytes definitively demonstrate that U.V. irradiation completely inactivates voltage-dependent sodium channels, without altering the other main conductances, Cl^?, K⁺ or Ca²⁺. After such a treatment, the relative permeability of the membrane to Na⁺ as compared to K⁺ shifted from 0.019±0.003 to 0.003±0.002 and only the calcium component of the action potentials could be observed. However, a fertilization potential, preceded by small sperm induced steps, is still present in these conditions, although its peak and plateau level are greatly reduced. These new findings are discussed, which confirm the electrical nature of the fast block to polyspermy but question about the specificity of those sperm-gated channels which are supposed to trigger the fertilization potential.     

Voltage dependent potassium fluxes and the significance of action potentials in Acetabularia

Membrane potential, V_m, and K⁺(⁸⁶Rb⁺) fluxes have been measured simultaneously on individual cells of Acetabularia mediterranea. During resting state (resting potential approx. ?170 mV) the K⁺ influx amounts to 0.24–0.6 pmol · cm^?2 · s^?1 and the K⁺ efflux to 0.2–1.5 pmol · cm^?2 s^?1. According to the K⁺ concentrations inside and outside the cell (40 : 1) the voltage dependent K⁺ flux (zero at V_m = E_K = ?90 mV) is stimulated approx. 40-fold for V_m more positive than E_K.It is calculated that during one action potential (temporary depolarization to V_m more positive than E_K) a cell looses the same amount of K⁺, which leaks in during 10–20 min in the resting state (V_m = ?170 mV). Since action potentials occur spontaneously in Acetabularia, they are therefore suggested to have a significant function for the K⁺ balance of this alga.     

Contributions of K+, Na+, and Cl− to the membrane potential of intact hamster vascular endothelial cells

The transmembrane potential (V_m) of vascular endothelial cells (EC) is an important property that may be involved in intra- and intercellular signal transduction for various vascular functions. In this study, V_m of intact aortic and vena caval EC from hamsters were measured using conventional microelectrodes. Vascular strips with the luminal surface upwards were suffused in a tissue chamber with krebs solution in physiological conditions. The resting V_m of aortic and vena caval EC was found to be ?40± 1 mV (n = 55) and ?43± 1 mV (n = 15), respectively. The V_m recordings were confirmed to have originated from EC by scanning and transmission electron microscopy combined with the comparison of electrical recordings between normal and endothelium-denuded aortic strips. The input resistance varied from 10–240 MΩ, which implied the presence of electrical coupling between vascular EC. Elevating the K⁺ level in the suffusate from 4.7 mM to 50 and 100 mM depolarized aortic EC by 19% and 29% and vena caval EC by 18% and 29%, respectively. These low percentages indicated a relatively small contribution of [K⁺] to the resting V_m of vascular EC. A positive correlation (r> 0.69) between the resting V_m and the magnitude of depolarization by the high [K⁺]₀ may be related to the involvement of voltage-dependent K⁺ channels. The hyperpolarization caused by lowering both [Na⁺]₀ and [CI^?]₀ suggested the disengagement of some electrogenic transport systems in the membrane, such as a Na⁺ -K⁺ -CI^? cotransporter. The transference number (t_ion), as an index of membrane conductance for specific ions, was calculated for K⁺ (15-20%), Na⁺ (16%), and Cl^? (9-15%), demonstrating that both Na⁺ and Cl^? as well as K⁺ contribute to the overall resting V_m. Our study documented some basic electrophysiology of the vascular EC when both structural and functional properties of the cell were maintained, thus furthering the understanding of the essential role of endothelial cells in mediating vascular functions. © 1993 Wiley-Liss, Inc.     

Potassium currents in cultured rabbit retinal pigment epithelial cells

Membrane potential and ionic currents were studied in cultured rabbit retinal pigment epithelial (RPE) cells using whole-cell patch clamp and perforated-patch recording techniques. RPE cells exhibited both outward and inward voltage-dependent currents and had a mean membrane capacitance of 26±12 pF (sd, n=92). The resting membrane potential averaged ?31±15 mV (n=37), but it was as high as ?60 mV in some cells. When K⁺ was the principal cation in the recording electrode, depolarization-activated outward currents were apparent in 91% of cells studied. Tail current analysis revealed that the outward currents were primarily K⁺ selective. The most frequently observed outward K⁺ current was a voltage- and time-dependent outward current (I _K) which resembled the delayed rectifier K⁺ current described in other cells. I _K was blocked by tetraethylammonium ions (TEA) and barium (Ba²⁺) and reduced by 4-aminopyridine (4-AP). In a few cells (3–4%), depolarization to ?50 mV or more negative potentials evoked an outwardly rectifying K⁺ current (I _Kt) which showed more rapid inactivation at depolarized potentials. Inwardly rectifying K⁺ current (I _KI) was also present in 41% of cells. I _KI was blocked by extracellular Ba²⁺ or Cs⁺ and exhibited time-dependent decay, due to Na⁺ blockade, at negative potentials. We conclude that cultured rabbit RPE cells exhibit at least three voltage-dependent K⁺ currents. The K⁺ conductances reported here may provide conductive pathways important in maintaining ion and fluid homeostasis in the subretinal space.     

Intracellular Na+, K+ and Cl− activities in Ehrlich ascites tumor cells

Ehrlich ascites tumor cell membrane potential (V_m) and intracellular Na⁺, K⁺ and Cl⁻ activities were measured under steady-state conditions in normal saline medium (Na⁺ = 154, K⁺ = 6, Cl⁻ = 150 mequiv./l). Membrane potential was estimated to be −23.3 ± 0.8 mV using glass microelectrodes. Intracellular ion activities were estimated with similar glass electrodes rendered ion-selective by incorporation of ion-specific ionophores. Measurements of V_m and ion-activity differences were made in the same populations of cells. Under these conditions the intracellular Na⁺, K⁺ and Cl⁻ activities are 4.6 ± 0.5; 68.3 ± 8.0; and 43.6 ± 2.1 mequiv./l, respectively. The apparent activity coefficients for Na⁺ and K⁺ are 0.18 ± 0.02 and 0.41 ± 0.05 respectively. These are significantly lower than the activity coefficients expected for the ions in physiological salt solutions (0.71 and 0.73, respectively). The activity coefficient for intracellular Cl⁻ (0.67 ± 0.03), however, is close to that of the medium (0.73), and the transmembrane electrochemical potential difference for Cl⁻ is not different from zero. The results establish that the energy available from the Na⁺ electrochemical gradient is much greater than previously estimated from chemical measurements.     

Maturation and fertilization of the sea urchin oocyte: An electrophysiological study

Some electrical properties of the sea urchin oocyte during germinal vesicle breakdown (GVBD) and fertilization have been studied using two intracellular electrodes. Oocytes with distinct germinal vesicles have resting potentials of ?70 to ?90 mV and the specific membrane resistance may range from 3 to 10 kΩ·cm². Around rest the I–V relationship is concave toward the axis origin and the membrane is K⁺ selective. A second electrical state, of lower potential and higher resistance, preexists in the membrane. Following GVBD, the K⁺-selective system is lost and the oocyte attains the characteristics of the second state with a resting potential of ?10 to ?50 mV and specific membrane resistance of 10–50 kΩ·cm². At rest the I–V relationship tends to be convex toward the axis origin. The majority of sea urchin eggs (which have undergone GVBD and completed meiosis) have a resting state of ?10 to ?30 mV; 10–50 kΩ·cm². The I–V relationship around rest is convex toward the axis origin and the resting potential is sensitive to changes of Na⁺, Cl^?, and K⁺ in the external medium. There is probably no major change in the electrical properties of the oocyte during the completion of meiosis. A small percentage of eggs from suboptimal animals have high resting potentials of ?70 to ?90 mV and specific membrane resistance of 5–50 kΩ·cm². Such eggs have predominantly K⁺-selective membranes and we suggest that they are either underripe or aged. The first electrical event across the egg plasma membrane during fertilization is a step-like depolarization which occurs about 2 sec after the attachment of the fertilizing spermatozoon to the vitelline layer. There is no change—at the level of the light microscope—either in the egg surface or in the behavior of the spermatozoon until the second event, the fertilization potential (FP), is initiated 11 sec later. The cortical reaction occurs simultaneously with the FP and during the rising phase of the FP the spermatozoon stops gyrating around its point of attachment. Oocytes, which do not have cortical granules, upon insemination exhibit step events but no FP; in contrast artificially activated eggs, either spontaneous or induced by the ionophore A23187, give rise to only the FP. We suggest that the FP is the electrical result of the modification of the egg plasma membrane during cortical exocytosis.     

Effect of various ionic compositions upon the membrane potentials during activation of sea urchin eggs

The membrane potentials of sea urchin (Hemicentrotus pulcherrimus) eggs before and after fertilization and their changes during the membrane elevation induced by intracellular electrical stimulation were recorded in solutions of various ionic compositions. Upon fertilization, the membrane potential (?10 mV) depolarized and reversed polarity by a few mV, then gradually returned to a new steady level ranging between ?50 and ?60 mV. The activation potential is closely associated with a transient increase in the membrane permeability. The potential of the unfertilized egg is hyperpolarized by monovalent anions (Br^?, Cl^? and NO₃^?) and depolarized slightly by K⁺. In contrast, the membrane of the fertilized egg is markedly depolarized by K⁺. Suppression of depolarization associated with an increase of the membrane permeability was recorded in Na-free medium (Tris-HCl). The selective increase in permeability to monovalent anions is thought to alternate with the selective increase in permeability to K⁺through the mediation of a transient increase of Na⁺-permeability at the time of fertilization. No causal relationship between the membrane elevation and the depolarization was established because the breakdown of the cortical granules occurs without depolarization or an increase in membrane permeability.     

Effects of static magnetic field on specific adenosine-5′-triphosphatase activities and bioelectrical and biomechanical properties in the rat diaphragm muscle

In this study, we aimed to clarify the effects of chronically applied static magnetic field (200 Gauss) on specific ATPase activities and bioelectrical and biomechanical responses in the isolated rat diaphragm muscle. The mean activities of Na⁺-K⁺ ATPase and Ca²⁺ ATPase determined from the diaphragm homogenates were significantly higher in the magnetic field exposed group (n = 20), but that of Mg²⁺ ATPase was nonsignificantly lower compared to the control group (n = 13). Resting membrane potential, amplitude of muscle action potential, and overshoot values (mean ± SE) in the control group were found to be ?76.5 ± 0.6, 100 ± 0.8, and 23.5 ± 0.6 mV, respectively; these values were determined to be ?72.8 ± 0.4, 90.3 ± 0.5, and 17.2 ± 0.4 mV in the magnetic field-exposed group, respectively. The latency was determined to increase in the experimental group, and all the above-mentioned bioelectrical differences between the groups were significant statistically. Force of muscle twitch was found to decrease significantly in the magnetic field-exposed group, and this finding was attributed to the augmenting effect of magnetic field on Ca²⁺ ATPase activity. These results suggest that magnetic field exposure changes specific ATPase activities and, thence, bioelectrical and biomechanical properties in the rat diaphragm muscle. © 1995 Wiley-Liss, Inc.