Electrical excitability in the egg cell membrane of the tunicate

Ion channels and early development of neural cells

In this review we underscore the merits of using voltage-dependent ion channels as markers for neuronal differentiation from the early stages of uncommitted embryonic blastomeres. Furthermore, a fairly large part of the review is devoted to the descriptions of the establishment of a simple model system for neural induction derived from the cleavage-arrested eight-cell ascidian embryo by pairing a single ectodermal with a single vegetal blastomere as a competent and an inducer cell, respectively. The descriptions are focused particularly on the early developmental processes of various ion channels in neuronal and other excitable membranes observed in this extraordinarily simple system, and we compare these results with those in other significant and definable systems for neural differentiation. It is stressed that this simple system, for which most of the electronic and optical methods and various injection experiments are applicable, may be useful for future molecular physiological studies on the intracellular process of differentiation of the early embryonic cells. We have also highlighted the importance of suppressive mechanisms for cellular differentiation from the experimental results, such as epidermal commitment of the cleavage-arrested one-cell Halocynthia embryos or suppression of epidermal-specific transcription of inward rectifier channels by neural induction signals. It was suggested that reciprocal suppressive mechanisms at the transcriptional level may be one of the key processes for cellular differentiation, by which exclusivity of cell types is maintained.

Tetrodotoxin-resistant sodium channels

1. Tetrodotoxin (TTX) has been widely used as a chemical tool for blocking Na+ channels. However, reports are accumulating that some Na+ channels are resistant to TTX in various tissues and in different animal species. Studying the sensitivity of Na+ channels to TTX may provide us with an insight into the evolution of Na+ channels. 2. Na+ channels present in TTX-carrying animals such as pufferfish and some types of shellfish, frogs, salamanders, octopuses, etc., are resistant to TTX. 3. Denervation converts TTX-sensitive Na+ channels to TTX-resistant ones in skeletal muscle cells, i.e., reverting-back phenomenon. Also, undifferentiated skeletal muscle cells contain TTX-resistant Na+ channels. Cardiac muscle cells and some types of smooth muscle cells are considerably insensitive to TTX. 4. TTX-resistant Na+ channels have been found in cell bodies of many peripheral nervous system (PNS) neurons in both immature and mature animals. However, TTX-resistant Na+ channels have been reported in only a few types of central nervous system (CNS). Axons of PNS and CNS neurons are sensitive to TTX. However, some glial cells have TTX-resistant Na+ channels. 5. Properties of TTX-sensitive and TTX-resistant Na+ channels are different. Like Ca2+ channels, TTX-resistant Na+ channels can be blocked by inorganic (Co2+, Mn2+, Ni2+, Cd2+, Zn2+, La3+) and organic (D-600) Ca2+ channel blockers. Usually, TTX-resistant Na+ channels show smaller single-channel conductance, slower kinetics, and a more positive current-voltage relation than TTX-sensitive ones. 6. Molecular aspects of the TTX-resistant Na+ channel have been described. The structure of the channel has been revealed, and changing its amino acid(s) alters the sensitivity of the Na+ channel to TTX. 7. TTX-sensitive Na+ channels seem to be used preferentially in differentiated cells and in higher animals instead of TTX-resistant Na+ channels for rapid and effective processing of information. 8. Possible evolution courses for Na+ and Ca2+ channels are discussed with regard to ontogenesis and phylogenesis.

Developmental changes in delayed rectifier K+ currents in the muscular- and neural-type blastomere of ascidian embryos

1. Developmental changes in the amplitude, kinetic properties, tetraethyl-ammonium (TEA) sensitivity, and ion selectivity of the delayed rectifier K+ currents were investigated in differentiating muscular-type (M) and neural-type (N) blastomeres isolated from the early cleavage-arrested ascidian embryos, using conventional two-microelectrode voltage clamp techniques. 2. No voltage-sensitive outward K+ currents were found in either type of blastomere during the first 35 h of development at 9 degrees C. Thereafter the delayed rectifier K+ current became apparent. The peak amplitude of the K+ current in the M-blastomere increased abruptly from 50 to 60 h and tended to plateau after 60 h, while in the N-blastomere it continued to increase after initial emergence at around 35 h. 3. The threshold potential level of the K+ current in the M-blastomere was initially about -10 mV in a standard external solution (1 mM-K+ solution), but shifted towards the hyperpolarized direction until it reached a steady level at 45 h after fertilization. At the fully differentiated stages, the threshold was around -32 mV and -26 mV in the M- and N-blastomeres, respectively. 4. Throughout development, the reversal potential of the tail current changed with the external K+ concentration in both M- and N-blastomeres as expected for a K(+)-electrode. There was no significant difference in the selectivity ratios for the K+ channel between the two types of blastomeres. The relative selectivities were K+ (1.000): Rb+ (0.774): NH4+ (0.122): Na+ (0.074) and K+ (1.000): Rb+ (0.724): NH4+ (0.155): Na+ (0.074) in the M- and N-blastomeres, respectively. 5. Modified Scatchard plots of TEA-sensitivity data indicated a one-to-one reaction between TEA and the K+ channel. These plots revealed the presence of TEA-resistant K+ channels in addition to TEA-sensitive K+ channels in the M-blastomere, but revealed only TEA-sensitive K+ channels in the N-blastomere. The dissociation constant (Ki) values of these three types of K+ channel did not change during development. In the M-blastomere, the Ki of the TEA-sensitive K+ channel was 1.29 +/- 0.05 mM (mean +/- S.E.M., n = 31) and that of the TEA-resistant K+ channel was 1.4 +/- 0.1 M (mean +/- S.E.M., n = 31) at a test potential of 45 mV. The Ki value of the neural-type K+ current was 1.38 +/- 0.03 mM (mean +/- S.E.M., n = 20) at 45 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

A simple "neural induction" model with two interacting cleavage-arrested ascidian blastomeres

A single anterior-animal blastomere, which includes the presumptive neural region in the eight-cell embryo of the Halocynthia, a protochordate, when dissociated, cleavage-arrested with cytochalasin B, and cultured in isolation, differentiated exclusively to epidermal type judging from membrane excitability and immunoreactivity. However, when the same blastomere was cultured in contact with a single anterior-vegetal blastomere, which includes the presumptive notochordal region, it displayed Na spikes and showed no expression of the epidermal antigen, suggesting that "neural induction" resulted in a single cell during the interaction with a single neighboring cell. This simple two-cell system can be used for further studies on the induction mechanism.

Some properties of the membrane currents underlying the fertilization potential in sea urchin eggs

1. The ionic currents that underly the fertilization potential of sea urchin eggs were studied in Lytechinus pictus using a single-electrode voltage clamp technique. 2. In unfertilized eggs, a transient inward current was activated at membrane potentials more positive than -45 mV. The maximum amplitude of the current was 0.56 +/- 0.35 nA (mean +/- S.D., n = 33) at a membrane potential of -35 to -25 mV. 3. The amplitude of this transient inward current was decreased by reducing the external concentration of calcium ions and by substituting barium or strontium ions for calcium in the external medium. Cobalt (10-20 mM) and gadolinium (200-500 microM) ions reduced the amplitude of this current in the presence of calcium ions. 4. A transient outward current was activated in unfertilized eggs at membrane potentials more positive than -10 mV. This current inactivates with a time constant of 16 ms at a membrane potential of -9 mV and re-activates over a period of several seconds at a membrane potential of -72 mV. 5. When unfertilized eggs were treated with the calcium ionophore A23187 under voltage clamp conditions, an inward current developed. It reached a maximum 30 s after its onset and declined thereafter. By 90 s it had become constant at 10% of its peak value. 6. The inward current induced by A23187 was voltage dependent. It was maximal at -25 mV in the steady state. 7. When eggs were fertilized under voltage clamp conditions, the fertilization current, If, was recorded. At a holding potential of -50 or -70 mV If had the following characteristics: (a) an initial inward shoulder with a duration ranging from 12 to 30 s; (b) an inward current peak that was attained between 40 and 100 s after the onset of the shoulder current and declined over the next 60 s; (c) an outward current that appeared after the inward current had declined. 8. Current-voltage relations obtained during If showed that the late component of the inward current was voltage dependent. It was maximal at -25 mV in the steady state and resembled the late component of the inward current recorded in A23187-activated eggs. 9. These results indicate that the form of the action potential in unfertilized eggs is due to the activation of a transient inward current and an inactivating outward current. The sustained depolarization after fertilization is due to the activation of a voltage-dependent inward current by the increase in intracellular free calcium concentration that occurs at fertilization.

Bioelectric responses of sea urchin eggs inseminated with oyster spermatozoa: a sperm evoked potential without egg activation

Multiple oyster spermatozoa can enter sea urchin eggs with or often without fertilization membrane formation (Osanai and Kyozuka, 1982). In the present work, electrical responses of sea urchin (Temnopleurus hardwicki) eggs inseminated with oyster (Crassostrea gigas) sperm were examined and correlated to the failure of monospermy and egg activation. With diluted sperm, a transient depolarization of the membrane with a constant pattern appeared repeatedly and discretely, and the depolarizations (sperm evoked potentials, SEPs) were not associated with fertilization membrane elevation. With dense sperm, the SEPs occurred consecutively, and sometimes an assembled consecutive depolarization was followed by an activation potential associated with cortical granule discharge. When the membrane potential was artificially held at positive levels, the frequency of SEPs was strongly suppressed but not completely blocked. The present results indicate that an individual heterologous spermatozoon neither produces a depolarization sufficient to block additional sperm entry, nor stimulates egg activation, and that simultaneous entries of multiple heterologous spermatozoa, as possibly reflected by the assembled consecutive depolarizations, induce cortical granule discharge and egg activation.

A calcium-activated sodium conductance produces a long-duration action potential in the egg of a nemertean worm

1. The egg of the nemertean worm Cerebratulus lacteus produced an action potential having a duration of about 9 min. We investigated the ionic conductances which accounted for this long-duration action potential. 2. The peak of the action potential was about +50 mV and depended on extracellular Ca2+, while the plateau potential was about +25 mV and depended on extracellular Na+. 3. Under voltage-clamp conditions, depolarization produced two temporally separate inward currents: a fast current which reached a peak at about 10 ms, and a slow current which took up to 1 min to reach its peak and lasted for several min. 4. The fast current was independent of extracellular Na+, but was blocked by removal of extracellular Ca2+. 5. The slow current was not seen when extracellular Na+ was replaced by choline+ or K+. 6. The slow current did not develop in Ca2+-free sea water, and was reduced to about half if Ca2+ was removed after the current had been initiated. 7. Microinjection of EGTA blocked the slow current, and reduced the action potential duration to about 1 min. 8. We concluded that a voltage-activated Ca2+ conductance produced the peak of the action potential, while a Ca2+-activated Na+ conductance produced its plateau.

Action potentials dependent on monovalent cations in developing mouse embryos

Action potentials were examined using intracellular recording techniques to study the ionic mechanisms of excitability in oocytes and embryos of the mouse from the 1-cell through to the 16-cell stages of development. At all stages examined, action potentials dependent on monovalent cations (Na+ or Li+) were observed under Ca2+-free conditions, and the maximum rate of rise (MRR) of the Na action potential was larger than that of the Li action potential at a given concentration of monovalent cations. Both the Na and Li action potentials were insensitive to tetrodotoxin, and they were blocked by inorganic (Co2+, Cd2+, Mn2+, La3+) and organic (diltiazem) Ca antagonists. These properties were exactly the same as those of the Ca channels present in the membranes of the mouse embryos. In addition, competition was observed between permeant monovalent and divalent cations: the overshoot and MRR of the Na or Li action potentials were reduced in the presence of Ca2+. These results suggest that Na+ or Li+ go through the Ca channels when the external Ca2+ concentration was very low, and that the Ca channels are more permeable to Na+ than to Li+. Separate Na channels could not be detected or induced at any stages of development.

The reduction of calcium current associated with early differentiation of the murine embryo

Membrane currents of intact oocytes and early embryos of the mouse and the hamster were analysed with voltage-clamp techniques. In both mouse and hamster the amplitude of Ca inward currents decreased with time during early development, and they were undetectable by the 8-cell stage, while the threshold potential, alkaline earth cation selectivity, and activation-inactivation kinetics remained unchanged. The reduction of Ca currents was further confirmed in the 2-cell embryo whose cleavage was arrested with use of cytochalasin D, but the process was slightly delayed by comparison with that of the intact embryo. Early differentiation of cytochalasin-D-treated embryos was comparable to that of the intact embryo in terms of intercellular couplings and intercellular fluid accumulation. But these processes were also delayed as in the case of Ca current reduction. The outward current in the hamster embryo which was reflected in the resting membrane conductance began to increase abruptly after the 2-cell stage and seemed to reach the maximum at the end of the 4-cell or 8-cell stage. The increase apparently occurred reciprocally with the decrease in Ca inward current. A similar but much smaller increase in resting membrane conductance also occurred in the cleavage-arrested mouse 2-cell embryo almost at the same development stage at which the abolition of Ca current was found. The possibility is discussed that Ca channels have a role in cell differentiation in early murine embryos.

Maturation involves suppression of voltage-gated currents in the frog oocyte

Voltage- and time-dependent currents having slow kinetics have been studied in plasma membranes of immature oocytes of the european frog, Rana esculenta. IK, corresponding to an outward flow of K+, is activated at potentials more positive than about -40 mV, and subserves outward rectification; Iir, corresponding to an outward flow of Cl-, is activated at potentials more negative than about -80 mV and subserves inward rectification. Such currents can act as negative feedback mechanisms in the control of membrane potential in the immature oocyte and limit to a somewhat restricted range its possible deviations from resting values. Besides IK, membrane depolarizations to potentials more positive than about +30 mV are capable of activating INa, corresponding to outflow of Na+. By contrast, the frog mature egg-cell has a single voltage- and time-dependent current, IM, activated at potentials more positive than +30 mV, with properties similar to INa. The disappearance of IK and Iir along with remarkable reduction in leakage lowers impedance in the egg membrane. It seems reasonable to suggest that the observed changes in membrane permeability reflect changes which have taken place along the maturation process and are of importance for successful fertilization.

Induction and disappearance of excitability in the oocyte of Xenopus laevis: a voltage-clamp study

Electrically excitable, sodium-selective channels are induced in the membrane of the oocytes of Xenopus laevis when it is submitted to prolonged positive potentials (Kado, Marcher & Ozon, 1979; Baud, Kado & Marcher, 1982). Under a long positive voltage-clamp step, the membrane current, initially outward, becomes inward with a sigmoidal time course. The mean time to half-maximal inward current (t 1/2) is about 18 s at 16 degrees C when stepping the membrane potential to +55 mV. The rate of channel induction was very temperature dependent (Q10 about 5). In an Arrhenius plot, the t 1/2 for induction at temperatures between 5 and 22 degrees C showed a single slope. The rate of induction was dependent on the membrane potential, increasing exponentially with positive membrane potential (e-fold for a 20 mV change). When the membrane was maintained at resting potential after induction, the ability to produce inward currents with short depolarizing steps slowly disappeared with a t 1/2 of 4 min at 16 degrees C. The temperature dependence for disappearance was larger than that found for induction (Q10 about 7). The rate of disappearance was not dependent on holding the membrane potential in the range -30 to -100 mV. Induction proceeded in calcium-free medium. Cycloheximide, a potent protein synthesis inhibitor had no effect (100 micrograms/ml) on the induction rate. Isobutylmethylxanthine (IBMX) or theophyllin (phosphodiesterase inhibitors) applied externally (10(-4) M) did not affect the induction or disappearance rates. From the present results, mechanisms such as protein synthesis or a second messenger (such as calcium or cyclic AMP) do not appear to be involved. During the depolarization necessary to obtain induction, another conductance was also activated. It was more slowly established, appeared to be non-saturable and had a reversal potential between zero and -10 mV. It was found to be very much reduced at temperatures below about 16 degrees C.

Excitability changes of presumptive ectoderm following mesodermal induction

Dissociated ectodermal cells of the early newt gastrula which have been treated with CMF (Ca-Mg-free saline) for 5 hr differentiate into muscle cells when cultured in HFCS (heated fetal calf serum) for up to 9-12 days. Similarly dissociated cells placed into FCS (fetal calf serum) culture differentiate into epidermis. Differences in cell-cluster formation have been found between HFCS and FCS in early cell cultures (6 hr), and membrane excitability phenomena associated with the differentiation of these clusters into the muscle cells or epidermal cells have been investigated, respectively. The HFCS cultures consist of cell clusters which have few of microvilli at their surfaces and which form loose contacts by means of lamellipodia. FCS cultures consist of cell clusters which have numerous microvilli at their surfaces and which make tight contacts between cells by means of ridge-structure precursors. The different reaggregation pattern of dissociated ectoderm cells in HFCS reflects changes in the cell membrane surface induced by HFCS. The sequential genesis of action potentials in cells destined to form muscle cells in HFCS is very similar to those produced by somitic muscle cells in vivo and their ionic dependence for generating action potentials is related to epidermal action potentials in vitro (FCS).

Determination of excitability types in blastomeres of the cleavage-arrested but differentiated embryos of an ascidian

Cleavage of the embryo of Halocynthia roretzi was arrested with cytochalasin B at 1- to 32-cell stages and the embryo was cultured in sea water containing cytochalsin B until a developmental time equivalent to the hatching of the control larva. Membrane properties of the blastomeres were studied with constant-current and voltage-clamp techniques. Four types of membrane response - neural, epidermal, muscular and non-excitable - were identified on the basis of the shapes and ionic dependence of action potentials in the blastomeres of 8- to 32-cell embryos. Only the epidermal type of response was found in the blastomeres of 1- to 4-cell embryos. The blastomeres with responses of neural type had Na, Ca, delayed K rectifier, anomalous K rectifier and Ca-induced K channels. Those of epidermal type had Ca, anomalous K rectifier and Ca-induced K channels. Those of muscular type had Ca, delayed K rectifier, anomalous K rectifier and possibly Ca-induced K channels. Those of non-excitable type had almost none or small amounts of outward- and inward-going rectifier channels. The characteristic responses of neural type were found in small blastomeres in the animal hemisphere, which included some presumptive neural regions. The responses of muscular type were found in large blastomeres of the vegetal hemisphere, which included some presumptive regions for muscle. Those of epidermal type were found in the blastomeres of the animal hemisphere which did not differentiate into the neural type. Those of non-excitable type were found in some blastomeres of the vegetal hemisphere. Blasomeres of 1- to 32-cell cleavage-arrested embryos, which were presumed to possess more than one possible developmental fate, did not develop mosaic membrane properties but differentiated into one of the four types, with a probability dependent upon a gradient of ooplasmic segregation at the time of arrest.

Comparison of properties of calcium channels between the differentiated 1-cell embryo and the egg cell of ascidians

In the ascidians Halocynthia roretzi and H. aurantium the Ca channels in the differentiated embryo whose cleavage was arrested with cytochalasin B at the 1-cell stage and in the unfertilized egg were studied using the voltage-clamp technique. In the cleavage-arrested 1-cell embryo, which differentiates into a cell of epidermal type after culturing until the time of hatching of the control larvae, Ca channel and Ca-induced K channel currents were observed upon depolarization of the membrane. Inward current through Ca channels in the embryo was analysed after suppressing Ca-induced K current by intracellular injection of EGTA. Sr or Ba ions could substitute for Ca ions as the charge carrier through Ca channels both in the cleavage-arrested embryo and in the egg. The selectivity ratios among these cations at their respective maximum inward currents were 1.0 (Ca):2.0 (Sr):4.5 (Ba) for the Ca channel in the embryo and 1.0 (Ca):1.9 (Sr):1.1 (Ba) for that in the egg. The time course of inactivation of Ca channels in Ca artificial sea water (ASW) was different from that in Sr or Ba ASW in the cleavage-arrested embryo. Fast inactivation was observed only in Ca ASW, and slight and slow inactivation was seen in Ba or Sr solution. In the egg, Ca, Sr and Ba currents through Ca channels all showed a similar time course of inactivation. The time course and voltage dependence of inactivation in Ca ASW were studied by measuring Ca tail current at a constant potential level of -28 mV. In the cleavage-arrested embryo the inactivation became slower and smaller in accordance with the decrease in inward Ca current when the potential level of the command pulse was increased in the positive direction from 10 to 80 mV. In the egg the time course of inactivation became faster when the potential level was similarly increased. The experimental results in (4) and (5) above suggest that the inactivation of the Ca channel in the cleavage-arrested embryo was dependent on Ca inward current while that in the egg was potential dependent. The developmental changes of Ca channels from egg type to epidermal type were studied in the cleavage-arrested 1-cell embryo. The epidermal-type Ca channels appeared at about 40 h after fertilization at 9 degrees C. The Ca channels in those blastomeres which differentiated to a cell of muscular type in the cleavage-arrested 8- or 16-cell embryo were studied after suppressing the outward current by tetraethylammonium and by intracellular injection of both Cs ions and EGTA.(ABSTRACT TRUNCATED AT 400 WORDS)

Voltage-operated channels induced by foreign messenger RNA in Xenopus oocytes

Poly(A)+ messenger RNA (mRNA) extracted from rat brains or from cat muscles was injected into Xenopus laevis oocytes. This led to the incorporation of voltage-operated Na+ and K+ channels into the oocyte membrane. These channels are not normally present in the oocyte and presumably result from the synthesis and processing of proteins coded by the injected mRNA. Tetrodotoxin blocked the Na+ channels induced by mRNA derived from either innervated or denervated muscle.

Spontaneous action potentials and resting potential shifts in fertilized eggs of the tunicate Clavelina

Electrical activity in the fertilized egg of the tunicate Clavelina was studied with microelectrode recording and voltage clamp techniques. The resting potential could assume either of two stable values (approximately -70 or -30 mV) and could be shifted between these values by direct current stimulation. Spontaneous shifts between two stable resting potentials were also seen. Egg cells produced action potentials spontaneously and in response to depolarizing stimuli. Inward currents were carried by both Na and Ca ions and a prominent outward potassium current was seen with depolarization to voltages above -15 mV. The steady-state current-voltage relationship (I-V curve) of the membrane showed two voltages where the net membrane current equaled zero: approximately -35 and -70 mV. Between these two voltages, membrane current was inward and carried by noninactivating Na and Ca currents. Inward rectification, which was blocked by external Rb, occurred at voltages below -70 mV. The voltage dependence of inward rectification is thought by the authors to be important for establishing the more negative resting potential; it is also thought the presence of inward current which does not inactivate completely at voltages more negative than about -20 mV is an important determinant of the more depolarized resting potential.

Spontaneous action potentials produced by Na and Cl channels in maturing Rana pipiens oocytes

The electrical excitability of maturing Rana pipiens oocytes was studied using intracellular recording and voltage-clamp techniques. Naturally ovulated oocytes, removed from the body cavity within a few hours after ovulation, possess voltage-sensitive Na and Cl channels that can produce action potentials (ap's). Young oocytes (sometime during metaphase I to first polar body stage) can generate trains of spontaneous action potentials: no chemical treatment or current injection is required. This is the first report of spontaneous repetitive firing in an egg cell membrane. As the oocyte matures, the duration of each ap increases because the outward Cl- current decreases. Middle-aged oocytes (about first polar body stage to metaphase II) have continuously positive membrane potentials (Vm's). Mature, activatable (metaphase II) oocytes have negative Vm's when impaled but can produce a long-lived ap when depolarizing current is injected. The ap's differ fundamentally from ap's in other excitable cells, including eggs: the Na+ current develops slowly and does not inactivate; most of the outward current is carried by Cl-, not by K+; the Cl channel is lost or is rendered insensitive to voltage as the oocyte matures.

Permeation of divalent and monovalent cations through the ovarian oocyte membrane of the mouse

Ovarian oocytes were isolated from adult mice and intracellular recording was performed using single glass micro-electrodes. The resting potential was - 7.0 +/- 1.8 mV in standard solution, and the oocyte showed a regenerative response at the cessation of hyperpolarizing current pulse. Ca spikes were observed under Na+-free conditions. The overshoot of the spike increased 28 mV for a 10-fold increase in [Ca2+]o and showed saturation as [Ca2+]o was elevated. The spike was insensitive to tetrodotoxin (TTX) and was blocked by polyvalent cations such as Co2+, Cd2+, Mn2+ and La3+. Sr2+ or Ba2+ substituted for Ca2+ in generating action potentials. Na spikes were observed under Ca2+-free conditions. The overshoot of the spike showed the slope of 39 mV for a 10-fold increase in [Na+]o and a saturation was detected when [Na+]o was raised. The spike was resistant to TTX and was blocked by Ca antagonists such as Co2+, Cd2+, Mn2+ or La3+. Li+ substituted for Na+ in producing spikes, while Rb+ did not. The overshoot and maximum rate of rise of the Na spike became smaller when Ca2+ was present in the bathing solution, indicating a competition between Na+ and Ca2+. Mn2+ acted not only as a Ca blocker but also as a charge carrier during excitation. Mn spikes were detected in Na+-, Ca2+-free solutions and were blocked by Ca antagonists. The resting membrane is permeable to not only Na+ but also to some extent to K+. It is suggested that the ovarian oocyte membrane of the mouse has voltage-dependent Ca channels, and both divalent (Ca2+, Sr2+, Ba2+, Mn2+) and monovalent (Na+, Li+) cations can pass through the Ca channels to generate action potentials.

Modulation of membrane conductance in rods of Bufo marinus by intracellular calcium ion

Double-barrel micropipettes were used to pressure-inject EGTA into the outer segments of rods in the isolated retina of Bufo marinus. We used these pipettes to point voltage clamp the cell to its resting membrane voltage during the injection of EGTA in order to prevent changes in membrane voltage from occurring. The input conductance of the rod was assessed by measuring the incremental membrane current required to hyperpolarize the membrane by less than or equal to 10 mV. When the retina was bathed in normal Ringer solution, the injection of EGTA during point voltage clamp evoked an inward membrane current and in increase in input conductance. This observation is consistent with an EGTA-evoked increase in conductance for an ion with an equilibrium potential more depolarized than the resting membrane potential. Injections of control solutions that did not contain EGTA had no effect. The effects of injected EGTA were not altered by variations in the pH or buffering capacity of the injection solution, or by the addition of equimolar Mg2+. Furthermore, injections of a solution containing equimolar Ca2+ and EGTA were without effect. Thus, the observed effects of injected EGTA were due to the lowering of the [Ca2+]i. Replacement of extracellular Na+ with choline+ abolished both the response to light and the EGTA-evoked increase in input conductance. A low [Na+]o solution containing 10(-8) M-Ca2+ reduced the response to injected EGTA by approximately the same amount as it reduced the response to light. Replacement of extracellular Cl- by methanesulphonate was without significant effect on either the response to light or to injected EGTA. These results are consistent with the interpretation that a lowered [Ca2+]i increases primarily the sodium conductance, gNa, of the plasma membrane of the rod outer segment. The conductance that is affected by a lowered [Ca2+]i appears to have the same specificity as the light-dependent conductance. This conclusion is consistent with a hypothesis for visual transduction involving modulation of gNa by light-evoked changes in the [Ca2+]i.

Differential action of tetrodotoxin on identified leech neurons

1. In leech segmental ganglia, the maximum rate of depolarization of action potentials was found to depend largely on Na in the Retzius (R) cell, the mechanosensory P, N and T cells and an identifiable neuron of unknown function, the X cell. 2. Tetrodotoxin (TTX) 15 100 mumol/l had little or no effect on R and X cells. In contrast, membrane excitation in N, P and T cells was depressed in dose- and use-dependent fashion. 3. The data imply the existence of two kinds of Na channels in normal, fully differentiated leech neurons. Correlation of such differences should lead to a better understanding of how particular neurons perform different functions.

Changes in holding and ion-channel currents during activation of an ascidian egg under voltage clamp

1. The unfertilized egg of an ascidian, Halocynthia roretzi, was activated by the divalent ionophore A23187 in natural or artificial sea water (nSW or ASW) or by an external solution containing a high concentration of Ca ions (high-Ca ASW) under voltage-clamp condition.2. Activation current began with an abrupt increase in the holding current and decayed relatively slowly with a common time course in various ASWs. Activation current was both Na- and Ca-dependent. The peak time, T(p), and the peak amplitude, D, of the activation current in nSW at 15 degrees C and -90 mV were 27 +/- 4 sec and -1.50 +/- 0.47 nA, respectively.3. The currents through Na, Ca and anomalous K channels were evoked by test pulses with constant intervals in nSW and high-Sr, high-Ca and high-K ASWs. Na-channel current was enhanced during activation. In contrast, Ca-channel current decreased. In high-Ca, Na-free ASW the Ca current through Na channels increased while the Ca current through Ca channels decreased. The time for the maximum of Na current, T(max), was (7.2 +/- 1.5) x 10 sec at 15 degrees C and index R, the ratio of the maximum amplitude to the amplitude before activation, was 2.31 +/- 0.18 in nSW. The time for the minimum of Ca-channel current, T(min), was about 70 sec, being almost the same as T(max) of Na current.4. The current through anomalous K channels increased initially and decreased later with a time lag behind the decrease in Ca-channel current.5. Both T(p) of activation current and T(max) of Na current were reduced by raising the temperature. Q(10) for T(p) and T(max) was 2.2 and 2.3, respectively.6. When the egg was activated in ASW containing scorpion toxin, the Na current through normal channels increased strongly while the Na current through toxinmodified channels increased less markedly.7. There was no significant change in the total membrane capacity before and during activation.

Membrane properties of solitary horizontal cells isolated from goldfish retina

1. Solitary horizontal cells were obtained by dissociating the adult goldfish retina using the enzyme papain. The cells were identified on morphological grounds and could be kept in culture for over a week. 2. Solitary horizontal cells, penetrated with micro-electrodes, had resting potentials of about -75 mV in normal solution. When external K+ concentration was changed, the membrane potential varied from EK calculated from the Nernst equation. 3. All solitary horizontal cells tested showed an action potential in response to superthreshold depolarizing current pulses. The action potential had an overshoot of about +20 mV and a plateau potential lasting for several seconds. 4. The action potential appeared to be Ca-dependent for the following reasons: (a) TTX or low [Na+] did not affect the action potential, (b) Sr2+, Ba2+ or high [Ca2+] enhanced the action potential, while (c) Co2+ or high [Mg2+] blocked it. No regenerative activity has been observed in horizontal cells in the retina but it is possible that the regenerative mechanism is suppressed normally. 5. A role for K+ was indicated by an increase in the duration and amplitude of the action potential on the application of tetraethylammonium. 6. The steady-state current--voltage (I--V) curve, measured by applying constant current pulses, was S-shaped (current on the abscissa) and composed of inward- and outward-going rectifying regions and a transitional region between them. A similar non-linear I--V relationship has been reported in vivo. 7. The transitional region was characterized by a sudden potential jump and hysteresis, suggesting the presence of a 'negative resistance'. This potential jump appeared not to be produced by the Ca-conductance mechanism mentioned above, since similar jumps were observed in the presence of Co2+.

Single K+ channel currents of anomalous rectification in cultured rat myotubes

The currents through single K+ channels of the anomalous (or inward) rectifier were recorded in tissue cultured rat myotubes by using the "gigohm seal" patch clamp technique developed by Sigworth and Neher. These unitary currents were detected as current fluctuations due to the blocking and unblocking of channels by Ba2+. The single-channel conductance was obtained from the slope of the linear relationship between unitary current amplitude and membrane potential. When the external solution contained 155 mM K+, the single-channel conductance was 10.4 +/- 2.6 pS (+/- SD; n = 6). This value was independent of the the concentration of blocking ions but increased with increasing external K+ concentration. The behavior of the unitary current agreed with that expected from the blocking kinetics of Ba2+ on the macroscopic K+ current of the anomalous rectifier. The density of the channel is likely to be small and may even be less than 1/micrometers 2.

Development of sodium, calcium and potassium channels in the cleavage-arrested embryo of an ascidian

1. The cleavage of the embryos of an ascidian, Halocynthia roretzi, was arrested with cytochalasin B (1 microgram/ml.) at the 16- or 64-cell stage. These cleavage-arrested embryos were still able to develop membrane excitability. 2. In the cleavage-arrested 64-cell embryos at the time when control embryos became hatched larvae, Ca spikes were evoked in the presumptive muscle blastomeres, and Na- and Ca-dependent action potentials were induced in some ectodermal blastomeres. 3. Membrane currents of the cleavage-arrested 16-cell embryos were recorded with the voltage-clamp technique and analysed as a function of developmental time at 15 degrees C. For this purpose, intact eggs, 4- and 8-cell embryos were also used. The cleavage-arrested embryos behaved electrically like single cells, due to tight electrical coupling between blastomeres. After the 25-hr stage decoupling occurred. 4. Both Na and Ca currents decreased during the initial 10 hr. Na current became less than one third and Ca current almost disappeared. At 17 hr both Na and Ca currents increased again. 5. The potential-dependence of the Na and Ca currents after 17 hr was similar to that in the egg, although substantial parallel shifts in the current-voltage relations were observed: a 5 mV positive shift for the Na current and a 15-20 mV negative shift for the Ca current. 6. Delayed (outward) K current developed gradually until 20 hr and then increased abruptly. The activation level for the delayed rectification was markedly negative (around -10 mV) in comparison with that of the egg (around + 100 mV). Anomalous (inward) K current, on the other hand, increased gradually without changes in the potential-dependence throughout development. 7. The results suggest that the differentiation of excitable membranes in the ascidian embryo does not involve changes in the properties of the individual channels, but rather changes in the numbers of various kinds of ion channels.

Neuromuscular transmission without sodium activation of the presynaptic nerve terminal in the lobster

1. We studed Na-independent synaptic transmission in the inhibitory synapse of the walking leg of the spiny lobster (Palinurus japonicus). 2. After loading the preparation with tetrodotoxin (TTX), brief depolarizing current injected in the inhibitory axon produced a small action potential, which propagated to the nerve terminal and gave rise to inhibitory post-synaptic potentials (i.p.s.p.) 3. The presynaptic action potential, in the presence of TTX, failed to propagate after removing Na+ in the solution. The TTX-resistant action potential was decreased, but not blocked by 30 mM-CoCl2. 4. When 4-aminopyridine (4-AP) was added to low Na+ or Na-free solution containing TTX synaptic transmission was restored. When the duration of the current pulse was increased, graded i.p.s.p. were evoked. 5. In high Ca2+ solutions containing K blockers, action potentials with prolonged duration were evoked. 6. The action potential of the presynaptic axon of the lobster neuromuscular junction depends on both Na+ and Ca2+.

Nonlinear current-voltage relationships in cultured macrophages

Intracellular recordings of cultured mouse thioglycolate-induced peritoneal exudate macrophages reveal that these cells can exhibit two different types of electrophysiological properties characterized by differences in their current-voltage relationships and their resting membrane potentials. The majority of cells had low resting membrane potentials (-20 to -40 mV) and displayed current-voltage relationships that were linear for inward-going current pulses and rectifying for outward-going pulses. Small depolarizing transients, occurring either spontaneously or induced by current pulses, were seen in some cells with low resting membrane potentials. A second smaller group of cells exhibited more hyperpolarized resting membrane potentials (-60 to -90 mV) and S-shaped current-voltage relationships associated with a high-resistance transitional region. Cells with S-shaped current-voltage relationships sometimes exhibited two stable states of membrane potential on either side of the high-resistance transitional region. These data indicate that macrophages exhibit complex electrophysiological properties often associated with excitable cells.

Tetrodotoxin sensitivity and Ca component of action potentials of mouse dorsal root ganglion cells cultured in vitro

In the mouse dorsal root ganglia cultured in vitro, neurons were classified into 3 groups according to the responses of their action potentials to tetrodotoxin (TTX) and removal of Na ions from bathing medium: (1) the neurons whose action potentials were not affected by TTX by TTX (10(-6) - 10 (-5)g/ml) and which generated Ca-dependent regenerative responses under Na-free condition, (2) the neurons whose spike potentials were resistant to TTX but failed to survive in Na-free saline and (3) the neurons whose action potentials were suppressed by TTX(10(-8)g/ml) as well as Na removal. The mean duration of spike and after-hyperpolarization was longest in the first group of the neurons and shortest in the third, probably reflecting the difference in the contribution of Ca currents to action potentials. The unresponsiveness of the neurons to TTX was shown to be due to the insensitivity of Na as well as Ca components of action potentials to the toxin. It was discussed that the occurrence of TTX-resistant action potentials to the toxin. It was discussed that the occurrence of TTX-resistant action might be related to the neuronal development.

Inactivation kinetics and steady-state current noise in the anomalous rectifier of tunicate egg cell membranes

1. Inward K current through the anomalous rectifier in the tunicate egg (Halocynthis roretzi, Drashe) was studied under voltage clamp. The transient inward current in response to a step change of membrane potential was measured. The steady-state current fluctuations were analysed using the power density spectrum (p.d.s.). 2. The inward current showed time-dependent changes, which were described by a pair of the first order kinetic parameters, n and s for activation and inactivation, respectively. The steady-state channel open probability due to the activation process (n infinity) was assumed to be 1.0 for V more negative than about--100 mV, but that of the inactivation process (s infinity) and the time constant of inactivation (taus) were membrane potential dependent in the same potential range; both decreased with increasing hyperpolarization. 3. The inward currents in Na-free choline medium did not inactivate, but were decreased in size. In Na-free Li medium, inactivation was very small; the steady-state conductance was not affected significantly. 4. After exposure to high Ca media, an increase of the conductance was observed. This effect is probably caused by an increase of intracellular Ca due to Ca ions entering through the Na channels. Mg ions slightly decreased the conductance. 5. In the hyperpolarized membrane (-160 less than or equal to V less than or equal to -80mV), steady-state current noise was recorded and analysed using p.d.s. A p.d.s. of the 1/[1 + (f/fc)2] type as well a p.d.s. of the 1/f type was observed; f, frequency, fc, cut-off frequency. 6. fc was translated into time constant tauN (= 1/2pIfC) and compared with the time constant of inactivation, taus. There was a significant correlation betwen these values with a regression coefficient of 0.82. 7. Changing from 400 mM-Li abloshied inactivation and changed the p.d.s. from the 1/[1 + (f/fc)2] into the 1/f type. These results (paragraphs 5--7)suggest that the fluctuations in the steady-state currents originatte in the inactivation gatin kinetics of the an ofthe anomalous rectifier. 8. The number of anomalous rectifier channels and the unit channel conductance were estimated from the 1/[1 + (f/fc)2] type current noise according to the formula : (see text), where I infinity = gamma Nninfinity s infinity (V--VK), gamma the unit channel conductance, N the maximum number of channels that can be opened by a hyperpolarizing pulse per egg. The unit conductance was 6 pmho in standard artificial sea water and the channel density was 0.028/micrometer2. 9. The unit channel conductance (gamma) was dependent upon external K concentration, but the number ofchannels (N) was not. 10. The increase in chord conductance evoked by higher Ca concentrations was due to the increase of the channel number. By contrast, Mg ions seem to decrease the unit channel conductance slightly.

Effects of internal free calcium upon the sodium and calcium channels in the tunicate egg analysed by the internal perfusion technique

1. The unfertilized egg of the tunicate, Halocynthia roretzi, was intracellularly perfused with various solutions. 2. The perfusion apparatus consisted of lower and upper compartments which were connected by a small glass funnel. A denuded egg cell without chorion was dropped into the funnel and brought into close contact with the glass wall of the funnel. The membrane of the egg faced to the lower compartment was ruptured by a slight difference of hydrostatic pressure and the inside of the egg was perfused with the internal solution flowing through the lower compartment. The current across the upper membrane was analysed by voltage-clamp technique. 3. The egg cell in contact with 400 mM-Na external solution and perfused intracellularly with 400 mM-Na for 30 min showed a relatively low Na reversal potential, +6 mV, in comparison with +60 mV in the intact egg in standard artificial sea water. The exchange efficiency was monitored by observing the shift of Na reversal potential during perfusion with high Na internal perfusate. 4. The internal perfusate containing F- ions stabilized the egg membrane and kept the excitability for 1--2 hr during the intracellular perfusion. With the internal F- perfusate the intracellular cationic content was changed to 400 mM-Na, K, Rb or Cs (external solution of 400 mM-Na) and permeability ratios of the egg Na channel were estimated as PNa:PK:PRb:PCs=1.0:0.14:0.05:0.04. The internal F- perfusate abolished Ca current which was consistently observed in the intact egg, while the internal Cl- perfusate kept both Na and Ca current as in the intact egg. However with the internal Cl- perfusate the egg cell could not be kept in good condition more than 20-30 min. 6. The effects of intracellular free Ca ions upon the egg Na and Ca channels were analysed by using Ca ion-buffered internal Cl- and high Na perfusate. The results showed that internal Ca ions above 10(-6) reduced the Ca current and enhanced the Na current at the same time. In the range between 10(-5) and 10(-4) M the Ca current became half of the control obtained with zero free Ca perfusate while the Na conductance at the zero current level doubled. The internal Ca ions above one mM seemed to abolish the Ca current and to reduce the Na current as well. The reciprocal effect of intracellular Ca ions upon the egg Na and Ca channels was demonstrated in the concentration range from 10(-6) to 10(-3) M.

Changes in excitability of the cell membrane during 'differentiation without cleavage' in the egg of the annelid, Chaetopterus pergamentaceus

1. The egg of the polychaete, Chaetopterus pergamentaceus, differentiates parthenogenetically without cleavage after 1 hr in a high K+ solution. The changes in the electrical properties of the membrane during differentiation have been investigated. 2. The treatment with the K solution for 40-60 min made unfertilized eggs become amoeboid cells in 4-5 hr and finally ciliated unicellular embryos in 14-16 hr. 3. In the untreated egg the action potential is Ca dependent and no Na component is found. The steady-state current-voltage relation has a marked inward rectification and shows a less marked outward rectification. 4. There are no significant changes in these properties of the cell membrane immediately after 40-60 min K treatment. 5. In the amoeboid cell stage (4-5 hr) the outward rectification increases significantly. 6. In the ciliated unicellular embryo the action potential is Ca and Na dependent. 7. The result suggests that Ca channels are present in the egg initially, K channels appear next and Na channels appear later.

Developmental changes in the inward current of the action potential of Rohon-Beard neurones

1. Rohon-Beard cells in the spinal cord of Xenopus tadpoles have been studied in animals from early neural tube to free-swimming larval stages. The onset and further development of electrical excitability of these neurones has been investigated in different ionic environments, to determine the ionic species carrying the inward current of the action potential.2. The cells appear inexcitable at early stages (Nieuwkoop & Faber stages 18-20) and do not give action potentials to depolarizing current pulses.3. The action potential is first recorded at stage 20. (A) The inward current is carried by Ca(2+) at stages 20-25, since it is blocked by mm quantitites of La(3+), Co(2+) or Mn(2+) and is unaffected by removal of Na(+) or the addition of tetrodotoxin (TTX). (B) The action potential is an elevated plateau of long duration (mean 190 msec at stages 20-22). The duration decreases exponentially with repetitive stimulation. (C) The specific Ca(2+) conductance (g(Ca)) at the onset of the plateau of the action potential is 2.6 x 10(-4) mho/cm(2). Calculations show that a single action potential raises [Ca(2+)](1) by more than 100-fold.4. At later times (stages 25-40), the inward current of the action potential is carried by both Na(+) and Ca(2+): the action potential has two components, an initial spike which is blocked by removal of Na(+) or addition of TTX, followed by a plateau which is blocked by La(3+), Co(2+) or Mn(2+).5. Finally (stages 40-51), the inward current is primarily carried by Na(+), since the action potential is blocked only by removal of Na(+) or addition of TTX, and the overshoot agrees with the prediction of the Nernst equation for a Na-selective membrane. When the outward current channel is blocked and cells exposed to Na-free solutions, 67% of cells at the latest stages studied were incapable of producing action potentials in which the inward current is carried by divalent cations.6. The duration of the action potential decreases from a maximum of about 1000 msec to about 1 msec during development. The maximum input resistance (R(in)) decreases from ca. 1000 to 100 MOmega.7. The calcium action potential may play a role in the development of excitability and the growth of the neurones.

Development of neuromuscular transmission in a larval tunicate

1. The time sequence of the development of acetylcholinesterase (AChE), acetylcholine (ACh) receptors and functional synapses on the embryonic muscle membrane in a tunicate larva (Halocynthia roretzi) was investigated in vivo.2. The fertilized tunicate egg was incubated in natural sea water at 9 degrees C. Sixty-eight hr after fertilization the free-swimming larva was hatched, which had six striated muscle fibres in the tail. The developmental stage of the embryo was indicated by the developmental hours after fertilization.3. The transmitter at the neuromuscular junction in the hatched larva is ACh. (i) Neuromuscular transmission was completely blocked by D-tubocurarine (1-5 x 10(-5)M). (ii) Eserine (5-10 x 10(-7)M) approximately doubled the time constant of the falling phase of miniature excitatory junctional currents (e.j.c.s). (iii) The reversal potential of the membrane response to iontophoretically applied ACh was -10 mV and similar to that of e.j.c.s. (iv) AChE was present on the muscle membrane surface.4. AChE activity became visible histochemically on the embryonic cell membrane in the presumptive muscle region as early as the late gastrula stage (27 hr after fertilization, 12 hr before the ACh response appeared).5. The response to iontophoretically applied ACh was present at 39 hr after fertilization but could not be evoked at 38 hr.6. Between 39 and 41 hr after fertilization, the ACh responses increased rapidly, then remained relatively unchanged until larval hatching.7. The stage of the initial appearance of the ACh response corresponded to the stage when the Ca current abruptly increased in the muscle membrane.8. The first sign of neuromuscular transmission was appearance of a giant excitatory junctional potential (e.j.p.) with uniform amplitude (about 15-20 mV) and slow time course (time constant of the falling phase of a giant e.j.c. was 23.4 +/- 6.9 msec, mean and S.D., at -60 mV and 11 degrees C).9. Within a few hours, these giant e.j.p.s disappeared and were successively replaced by medium-sized e.j.p.s and then e.j.p.s similar to those seen in hatched larvae (time constant of the falling phase of a miniature e.j.c. was 8.5 +/- 1.8 msec at -60 mV and 11 degrees C).

Ionic currents through the membrane of the mammalian oocyte and their comparison with those in the tunicate and sea urchin

1. The action potential and the membrane current of the mouse oocyte were analysed by current-clamp and voltage-clamp techniques and they were compared with those of other animal oocytes. 2. The matured and unfertilized oocyte of the mouse in standard medium with 6 mM-K showed the resting potential of -23-1+/-2-9 mV. The resting potential was relatively large in the medium with 20 mM-Ca or 10 mM-Mn, being -35-7+/-2-6 mV and further increased to -46-9+/-4-8 mV with replacement of Na in the medium by choline. 3. At the cessation of large hyperpolarization below -90 mV in standard medium, a regenerative potential was often elicited in the form of an off-response. The off-response depended upon the external concentration of Ca. In 20 mM-Ca medium it was constantly observed with hyperpolarization below -60 mV. Its critical level was -40 mV and its overshoot was +15 mV. 4. The time and potential-dependent inward current was observed both in standard and 20 mM-Ca media under voltage-clamp condition. In 20 mM-Ca medium the inward current was observed by depolarization beyond -40 mV and showed its maximum at -15 mV. It was greatly reduced by replacing the external Ca with Mn but retained by substituting Sr or Ba for Ca. The selectivity ratios among these alkali earth cations were Ca:Sr:Ba=1-0:1-4:0-7. 5. The current-voltage relation in Ca and Na-deficient and 10 mM-Mn medium was linear from -200 to +25 mV. The hyperpolarization below -200 mV revealed an inward-going rectification. The depolarization above +50 mV under voltage-clamp condition induced the outward surge current with activation and inactivation processes. 6. In contrast to the mouse oocyte, the matured and unfertilized oocyte of the sea urchin showed a large resting potential of -70 mV in 30 Ca ASW and the depolarization beyond -40 mV elicited an action potential with an overshoot of 20 mV. The action potential showed a notch in the rising phase and lasted about 1 to 2 sec. 7. Under the voltage-clamp condition both Ca inward current and the outward surge current were observed in the sea urchin oocyte membrane just as in the mouse oocyte membrane. 8. The selectivity ratios among alkali earth cations, Ca:Sr:Ba, for 'Ca channels' of the oocyte membranes were 1-0:1-4:0-7 in the mouse, 1-0:1-7:1-1 in the tunicate and 1-0:0-7:0-5 in the sea urchin. When the current density through Ca channels are revised in terms of the respective critical levels for Ca channels, the revised selectivity sequences become Ca greater than Sr greater than Ba, being common to all three species.

Surface potential reflected in both gating and permeation mechanisms of sodium and calcium channels of the tunicate egg cell membrane

1. Threshold changes of Na and Ca currents due to various polyvalent cations (stabilizing cations) or H(+) ions were studied in the egg cell membrane of a tunicate, Halocynthia roretzi, by using the voltage-clamp technique.2. With an increase in [Ca](o) or a decrease in pH in the external solution, the current-voltage (I-V) relations for the peak of the Na and Ca currents shifted along the voltage axis in the positive direction. These voltage shifts in the I-V relations, measured at a potential of V((1/2)) where inward current attains its half-maximum, were shown to be identical to shifts in voltage-dependence of the time courses of Na and Ca currents, and also identical to shifts in the inactivation curves of Na current along the voltage axis.3. The shifts in V((1/2)) produced by various polyvalent cations or H(+) ions were analysed by the Gouy-Chapman equation for the diffuse double layer, by assuming that a change in V((1/2)) directly corresponds to a change in the surface double layer potential.4. The V((1/2))-divalent cation concentration relations of Na current were exactly described by the predictions of the theory with a constant value of the surface charge density of 1e(-)/(9 A)(2). The weak stabilizing effects of Mg(2+), Sr(2+) and Ba(2+) were quite similar to each other and were explained in terms of a ;screening' effect. Other divalent cations, such as Ca(2+), Mn(2+) and Ni(2+), showed various different stabilizing effects which were explained in terms of a ;binding' effect. The binding constants (K(1)'s) for Ca(2+), Mn(2+) and Ni(2+) were 0.21, 0.45 and 0.94 M(-1), respectively.5. H(+) ions showed a powerful stabilizing effect upon the Na current with a K(H) of 6 x 10(4)M(-1). This value indicates that the acidic sites around Na channels have a pK(a) of 4.78. La(3+) ions also acted as a strong stabilizer upon the Na current with a K(La) of 15 M(-1). For both H(+) and La(3+), the V((1/2))-concentration relations were also exactly described by the Gouy-Chapman equation with the same charge density of 1e(-)/(9 A)(2) as estimated by varying divalent cations.6. The stabilizing effect of permeant cations such as Ca(2+), Sr(2+) and Ba(2+) on Ca channel currents was analysed. The effect of lowering pH was also studied. It was found that the surface charge density of 1e(-)/(9 A)(2) estimated by Na current is also applicable to the explanation for the V((1/2))-divalent cation concentration or - pH relationships. The estimated binding constants for H(+), Ca(2+) and Sr(2+) were 1.2x10(5), 0.58 and 0.035 M(-1), respectively. Ba(2+) does not bind to charged sites near to the Ca channels.7. It was noticed that a considerable reduction in the conductances of Na and Ca currents occurred in parallel with a stabilizing effect. This reduction was ascribed to a decrease in the concentration of permeant cations at the external surface of the cell membrane, as predicted by the theory of the diffuse double layer. The Goldman, Hodgkin-Katz equation for ionic currents was applied to explain the conductance suppression.8. The conductance suppressions of Na and Ca channel currents due to Ca(2+), Sr(2+) and Ba(2+) were found to be apparent ones, only reflecting decreases in the surface concentration of permeant cations without any changes in the permeability. After correction for the apparent suppression, the real permeability ratio among Ca(2+), Sr(2+) and Ba(2+) for Ca channels was determined as 1.00, 0.56 and 0.21 respectively.9. The conductance suppression of Na current by lowering pH was explained in terms of a real suppression or blocking which is superimposed on the apparent suppression. Considering the surface [Na](o), the plot of P(Na) against the surface pH yielded a blocking curve of Na channel by H(+) ions, which implies that two H(+) ions are necessary to block each Na channel. For Ca channels no real blockage was observed in acidic pH.10. It was concluded from the present experiment that there exists a surface potential capable of affecting both gating and permeation mechanisms of ionic channels in this tunicate egg cell membrane.

Electrical properties of developing oocytes of the migratory locust, Locusta migratoria

The electrical properties of developing nonfertilized oocytes of Locusta migratoria were studied, using intracellular microelectrodes. The inseries potential of the combined oomembrane and of the follicular cells was about 20 mV in the youngest oocytes. It increased as the oocytes developed and it reached a plateau of about 50 mV before full maturation, generally four to seven oocytes away from the fully-developed terminal oocyte. Current-voltage relations were always linear for hyperpolarizing currents. Most oocytes exhibited, however, rectification to outward current. Input resistance values varied with oocyte size from about 5 X 10(6) ohm for young oocytes to about 0.2 X 10(6) ohm for the more developed ones. Some oocytes displayed a transient depolarization on turning off a hyperpolarizing step of current. This depolarization was not correlated with the size of the oocyte or with any observed morphological feature. Any two adjacent oocytes were electrotonically coupled. A single ovariole thus represented a longitudinal chain of developing oocytes which were connected electrically. This was supported by electron microscope observations which revealed junctions partially impermeable to lanthanum and gap junctions between the follicular cells themselves and between follicular cells and oocytes. The coupling coefficient was dependent on the direction of current flow. The attenuation of voltage along an ovariole was always greater at the distal than at the proximal side.

Two components of the calcium current in the egg cell membrane of the tunicate

The Ca current of the egg cell membrane of a certain tunicate, Halocynthia roretzi Drasche, was studied by the voltage-clamp technique. 2. The Ca current in the standard artificial sea water (ASW) was produced at the critical membrane potential of -10 mV after inactivating the Na current by conditioning depolarization, -30 to -15 mV. The Ca current was abolished by replacing Ca in ASW with Mg2+ or Mn2+. The Ca current was not significantly influenced by replacing Na in ASW with choline or Cs. 3. The relation of Ca current to the external Ca concentration was a monotonously increasing function, but was not linear. The current tended to saturate above 50 mM-Ca. In 100 mM-Ca ASW, the maximum peak inward current of Ca ranged from 1 to 7 X 10(-9) A. 4. The kinetics of Ca current was accurately analysed because of the small contribution of K outward current and was found to be relatively slow in comparison with the Na current. The peak time and the half-decay time of the maximum Ca current at about 25 mV were about 25 and 100 msec respectively in 100 mM-Ca ASW at 15 degrees C. 5. Addition of 20 mM-Co2+ to 100 mM-Ca ASW reduced Ca current to one fourth and 1 mM-La3+ to 100 mM-Ca ASW abolished the current. 6. Sr and Ba could substitute for Ca in Ca channels. The selectivity ratios for the 'Ca channels's were Ca (1-00):Sr(1-17):Ba(0-71) at a potential level of +40 mV. The Ca current in the egg cell membrane appeared to be essentially the same as the Ca current in the common excitable membranes, such as the crustacean muscle fibre. 7. The polyvalent cations including Ca ion and monovalent H+ ion showed the stabilizing effect upon both Na and Ca currents, by shifting V-I relations along the voltage axis. From the prediction of a theory of the diffuse double layer, the shift in the V-I relation induced by those cations should be directly related to their binding powers to the membrane. Thus, the sequence of the binding powers was inferred as H+ greater than La3+ greater than Co2+ greater than Mn2+ greater than Ca2+ greater than Sr2+ larger than or equal to Ba2+ greater than Mg2+. 9. In Na-free ASW, such as isotonic Ca ASW, Ca current was composed of two components. The one component was the Ca current described in 1 to 6. The other was also dependent upon the external Ca concentration, but showed the more negative critical membrane potential and the faster kinetics. It was concluded that this component should be the Ca current through Na channels. 10. The selectivity among Ca, Sr and Ba for 'Ca' current through 'Na channels' was significantly different from that of 'Ca' current through 'Ca channels', being Ca greater than Sr larger than or equal to Ba = 0.

Membrane currents of the tunicate egg under the voltage-clamp condition

1. Ionic currents of the egg membrane of a certain tunicate. Halocynthia roretzi Drashe, were studied by the voltage-clamp technique. 2. The membrane depolarization beyond -55mV in standard artificial sea water induced mainly transient inward current and slight outward currents, when the holding potential was kept at -99 mV. 3. The transient inward current was composed of two components; the major one showed a faster time course, a more negative critical level of about -55 mV, and a reversal potential around +60 mV and the minor one showed a slower time course, a less negative critical level o -10 mV, and no definite reversal potential. 4. The major component became maximum at about -25 mV with the peak time of 6-9 msec at 15 degrees C, and the maximum currents ranged from 0-5 to 1-5 X 10(-5) A/cm2. 5. The major component of the inward current was abolished by the replacement of Na with choline or Tris or Cs ions, while it was almost unaltered by the replacement with Li. The minor component was independent of Na concentration in the external solution. 6. The major component showed the activation and inactivation identical with those of Na current of other excitable membranes. A conditioning depolarization over -90 mV inactivated the Na current and the half inactivation of the major inward current was obtained by a conditioning pulse to -56 mV, when the pulse duration was 400 msec and the temperature was at 15 degrees C. 7. The time course of the Na current was formulated with m and h parameters in the following equations: (see article). 8. The kinetic parameters taum and tauh of egg Na current were calculated and compared with those of the squid axon. The potential dependence of taum and tauh was almost identical with that of the axon, but the absolute values of both taum and tauh were ten- to twentyfold larger than those of the axon in any range of the membrane potential. 9. The temperature depdence of the kinetic parameters taum, tauh and of the chord conductance gNa was studied. The Q10's for taum and tauh were both 4-0, while the Q10 for gNa was 2-0 in the temperature range from 5 to 20 degrees C. 10. The outward and inward rectifying conductances of egg membrane were remarkably activated at the potential level above +100 mV and below -70 mV respectively in standard artificial sea water. Both increased currents were subsequently subject to inactivation. 11. It was suggested that Na, Ca, K inward rectifying and K outward rectifying conductances all exist separately in the egg cell membrane and the Na current was essentially identical with that through the Na channel in other excitable membranes.