Action potential and non-linear current-voltage relation in starfish oocytes

Dithiothreitol Affects the Fertilization Response in Immature and Maturing Starfish Oocytes

Immature starfish oocytes isolated from the ovary are susceptible to polyspermy due to the structural organization of the vitelline layer covering the oocyte plasma membrane, as well as the distribution and biochemical properties of the actin cytoskeleton of the oocyte cortex. After the resumption of the meiotic cycle of the oocyte triggered by the hormone 1-methyladenine, the maturing oocyte reaches fertilizable conditions to be stimulated by only one sperm with a normal Ca2+ response and cortical reaction. This cytoplasmic ripening of the oocyte, resulting in normal fertilization and development, is due to the remodeling of the cortical actin cytoskeleton and germinal vesicle breakdown (GVBD). Since disulfide-reducing agents such as dithiothreitol (DTT) are known to induce the maturation and GVBD of oocytes in many species of starfish, we analyzed the pattern of the fertilization response displayed by Astropecten aranciacus oocytes pre-exposed to DTT with or without 1-MA stimulation. Short treatment of A. aranciacus immature oocytes with DTT reduced the rate of polyspermic fertilization and altered the sperm-induced Ca2+ response by changing the morphology of microvilli, cortical granules, and biochemical properties of the cortical F-actin. At variance with 1-MA, the DTT treatment of immature starfish oocytes for 70 min did not induce GVBD. On the other hand, the DTT treatment caused an alteration in microvilli morphology and a drastic depolymerization of the cortical F-actin, which impaired the sperm-induced Ca2+ response at fertilization and the subsequent embryonic development.

The Therapeutic and Diagnostic Potential of Phospholipase C Zeta, Oocyte Activation, and Calcium in Treating Human Infertility

Oocyte activation, a fundamental event during mammalian fertilisation, is initiated by concerted intracellular patterns of calcium (Ca2+) release, termed Ca2+ oscillations, predominantly driven by testis-specific phospholipase C zeta (PLCζ). Ca2+ exerts a pivotal role in not just regulating oocyte activation and driving fertilisation, but also in influencing the quality of embryogenesis. In humans, a failure of Ca2+ release, or defects in related mechanisms, have been reported to result in infertility. Furthermore, mutations in the PLCζ gene and abnormalities in sperm PLCζ protein and RNA, have been strongly associated with forms of male infertility where oocyte activation is deficient. Concurrently, specific patterns and profiles of PLCζ in human sperm have been linked to parameters of semen quality, suggesting the potential for PLCζ as a powerful target for both therapeutics and diagnostics of human fertility. However, further to PLCζ and given the strong role played by Ca2+ in fertilisation, targets down- and up-stream of this process may also present a significantly similar level of promise. Herein, we systematically summarise recent advancements and controversies in the field to update expanding clinical associations between Ca2+-release, PLCζ, oocyte activation and human fertility. We discuss how such associations may potentially underlie defective embryogenesis and recurrent implantation failure following fertility treatments, alongside potential diagnostic and therapeutic avenues presented by oocyte activation for the diagnosis and treatment of human infertility.

Starfish oocytes of A. pectinifera reveal marked differences in sperm-induced electrical and intracellular calcium changes during oocyte maturation and at fertilization

Although changes in membrane potential and intracellular Ca²⁺ (Ca_i ) during fertilization in starfish oocytes have been known for long time, little is known precisely about how and what kind of channels are involved during oocyte maturation and in fertilization, and how the mechanisms of changes in Ca_i in oocytes develop during oocyte maturation. Since in starfish, oocyte maturation-inducing hormone, 1-methyladenine (1MA) is well known, we took advantage of it to investigate the developmental process of channel-function and changes in Ca_i in three different developmental stages using 1MA. Sperm-induced membrane current at voltage clamp and changes in Ca_i in starfish oocytes, Asterina pectinifera, were examined in stages of immature, partly mature (a state in 15-20 min after sufficient concentration, 1 µM of 1MA addition, or 30-40 min exposure to subthreshold concentration of 1MA), and mature oocytes (MO). We found some immature and many partly MOs showed fluctuating responses in membrane current, membrane potential, and corresponding changes in Ca_i , which are distinct from those in MOs. The responses in immature and partly MOs indicate physiologically characteristic responses of insufficient changes in Ca_i and its corresponding electrical responses at the partial developmental stage during maturation. Our data should shed light on the mechanism of egg activation and oocyte maturation in terms of examining membrane current and corresponding changes in Ca_i .

Modulators of calcium signalling at fertilization

Calcium (Ca2+) signals initiate egg activation across the animal kingdom and in at least some plants. These signals are crucial for the success of development and, in the case of mammals, health of the offspring. The mechanisms associated with fertilization that trigger these signals and the molecules that regulate their characteristic patterns vary widely. With few exceptions, a major contributor to fertilization-induced elevation in cytoplasmic Ca2+ is release from endoplasmic reticulum stores through the IP3 receptor. In some cases, Ca2+ influx from the extracellular space and/or release from alternative intracellular stores contribute to the rise in cytoplasmic Ca2+. Following the Ca2+ rise, the reuptake of Ca2+ into intracellular stores or efflux of Ca2+ out of the egg drive the return of cytoplasmic Ca2+ back to baseline levels. The molecular mediators of these Ca2+ fluxes in different organisms include Ca2+ release channels, uptake channels, exchangers and pumps. The functions of these mediators are regulated by their particular activating mechanisms but also by alterations in their expression and spatial organization. We discuss here the molecular basis for modulation of Ca2+ signalling at fertilization, highlighting differences across several animal phyla, and we mention key areas where questions remain.

Early and later studies on action potential and fertilization potential of echinoderm oocytes and Ca2+ response of mammalian oocytes

This is a personal essay starting from the early study on fertilization signals in echinoderm and mammalian oocytes. It presents actual examples showing that a unexpected discovery leads to unimaginable development of the research in diverse directions in later years and yields a common concept after long years' effort and accumulation. Those outcomes are the happiest gift for researchers. We also learn many precepts in our own research life.

Fertilization in Starfish and Sea Urchin: Roles of Actin

Marine animals relying on "external fertilization" provide advantageous opportunities to study the mechanisms of gamete activation and fusion, as well as the subsequent embryonic development. Owing to the large number of eggs that are easily available and handled, starfish and sea urchins have been chosen as favorable animal models in this line of research for over 150 years. Indeed, much of our knowledge on fertilization came from studies in the echinoderms. Fertilization involves mutual stimulation between eggs and sperm, which leads to morphological, biochemical, and physiological changes on both sides to ensure successful gamete fusion. In this chapter, we review the roles of actin in the fertilization of starfish and sea urchin eggs. As fertilization is essentially an event that takes place on the egg surface, it has been predicted that suboolemmal actin filaments would make significant contributions to sperm entry. A growing body of evidence from starfish and sea urchin eggs suggests that the prompt reorganization of the actin pools around the time of fertilization plays crucial regulatory roles not only in guiding sperm entry but also in modulating intracellular Ca²⁺ signaling and egg activation.

De novo assembly of a transcriptome from the eggs and early embryos of Astropecten aranciacus

Starfish have been instrumental in many fields of biological and ecological research. Oocytes of Astropecten aranciacus, a common species native to the Mediterranean Sea and the East Atlantic, have long been used as an experimental model to study meiotic maturation, fertilization, intracellular Ca²⁺ signaling, and cell cycle controls. However, investigation of the underlying molecular mechanisms has often been hampered by the overall lack of DNA or protein sequences for the species. In this study, we have assembled a transcriptome for this species from the oocytes, eggs, zygotes, and early embryos, which are known to have the highest RNA sequence complexity. Annotation of the transcriptome identified over 32,000 transcripts including the ones that encode 13 distinct cyclins and as many cyclin-dependent kinases (CDK), as well as the expected components of intracellular Ca²⁺ signaling toolkit. Although the mRNAs of cyclin and CDK families did not undergo significant abundance changes through the stages from oocyte to early embryo, as judged by real-time PCR, the transcript encoding Mos, a negative regulator of mitotic cell cycle, was drastically reduced during the period of rapid cleavages. Molecular phylogenetic analysis using the homologous amino acid sequences of cytochrome oxidase subunit I from A. aranciacus and 30 other starfish species indicated that Paxillosida, to which A. aranciacus belongs, is not likely to be the most basal order in Asteroidea. Taken together, the first transcriptome we assembled in this species is expected to enable us to perform comparative studies and to design gene-specific molecular tools with which to tackle long-standing biological questions.

Kir2.1 and K2P1 channels reconstitute two levels of resting membrane potential in cardiomyocytes

KEY POINTS

Outward and inward background currents across the cell membrane balance, determining resting membrane potential. Inward rectifier K⁺ channel subfamily 2 (Kir2) channels primarily maintain the resting membrane potential of cardiomyocytes. Human cardiomyocytes exhibit two levels of resting membrane potential at subphysiological extracellular K⁺ concentrations or pathological hypokalaemia, however, the underlying mechanism is unclear. In the present study, we show that human cardiomyocytes derived from induced pluripotent stem cells with enhanced expression of isoform 1 of Kir2 (Kir2.1) channels and mouse HL-1 cardiomyocytes with ectopic expression of two pore-domain K⁺ channel isoform 1 (K2P1) recapitulate two levels of resting membrane potential, indicating the contributions of Kir2.1 and K2P1 channels to the phenomenon. In Chinese hamster ovary cells that express the channels, Kir2.1 currents non-linearly counterbalance hypokalaemia-induced K2P1 leak cation currents, reconstituting two levels of resting membrane potential. These findings support the hypothesis that Kir2 currents non-linearly counterbalance inward background cation currents, such as K2P1 currents, accounting for two levels of resting membrane potential in human cardiomyocytes and demonstrating a novel mechanism that regulates excitability.

ABSTRACT

Inward rectifier K⁺ channel subfamily 2 (Kir2) channels primarily maintain the normal resting membrane potential of cardiomyocytes. At subphysiological extracellular K⁺ concentrations or pathological hypokalaemia, human cardiomyocytes show both hyperpolarized and depolarized resting membrane potentials; these depolarized potentials cause cardiac arrhythmia; however, the underlying mechanism is unknown. In the present study, we show that inward rectifier K⁺ channel subfamily 2 isoform 1 (Kir2.1) currents non-linearly counterbalance hypokalaemia-induced two pore-domain K⁺ channel isoform 1 (K2P1) leak cation currents, reconstituting two levels of resting membrane potential in cardiomyocytes. Under hypokalaemic conditions, both human cardiomyocytes derived from induced pluripotent stem cells with enhanced Kir2.1 expression and mouse HL-1 cardiomyocytes with ectopic expression of K2P1 channels recapitulate two levels of resting membrane potential. These cardiomyocytes display N-shaped current-voltage relationships that cross the voltage axis three times and the first and third zero-current potentials match the two levels of resting membrane potential. Inhibition of K2P1 expression eliminates the phenomenon, indicating contributions of Kir2.1 and K2P1 channels to two levels of resting membrane potential. Second, in Chinese hamster ovary cells that heterologously express the channels, Kir2.1 currents non-linearly counterbalance hypokalaemia-induced K2P1 leak cation currents, yielding the N-shaped current-voltage relationships, causing the resting membrane potential to spontaneously jump from hyperpolarization at the first zero-current potential to depolarization at the third zero-current potential, again recapitulating two levels of resting membrane potential. These findings reveal ionic mechanisms of the two levels of resting membrane potential, demonstrating a previously unknown mechanism for the regulation of excitability, and support the hypothesis that Kir2 currents non-linearly balance inward background cation currents, accounting for two levels of resting membrane potential of human cardiomyocytes.

Ion currents modulating oocyte maturation in animals

Growing oocytes are arrested at the first prophase of meiosis which is morphologically identified by the presence of a large and vesicular nucleus, called the germinal vesicle. The dissolution of the germinal vesicle marks the resumption of meiosis during which the oocyte undergoes massive modifications up to the second meiotic block, which is removed at fertilization. The interval between the first and the second meiotic block is defined as maturation and the events occurring during this period are crucial for ovulation, fertilization, and embryo development. Oocytes are excitable cells that react to stimuli by modifying their electrical properties as a consequence of ion currents flowing through ion channels on the plasma membrane. These electrical changes have been largely described at fertilization whereas little information is available during oocyte maturation. The aim of this review is to give an overview on the involvement of ion channels and ion currents during oocyte maturation in species from invertebrates to mammals. The results summarized here point to the possible functional role of ion channels underlying oocyte growth and maturation.

Pharmacological characterization of NAADP-induced Ca2+ signals in starfish oocytes

The recently discovered second messenger nicotinic acid adenine dinucleotide phosphate (NAADP) is central to the onset of intracellular Ca2+ signals induced by several stimuli, including fertilization. The nature of the Ca2+ pool mobilized by NAADP is still controversial. Depending on the cell type, NAADP may target either an acidic compartment with lysosomal properties or ryanodine receptors (RyRs) on endoplasmic reticulum. In addition, NAADP elicits a robust Ca2+ influx into starfish oocytes by activating a Ca2+-mediated current across the plasma membrane. In the present study, we employed the single-electrode intracellular recording technique to assess the involvement of either acidic organelles or RyRs in NAADP-elicited Ca2+ entry. We found that neither drugs which interfere with acidic compartments nor inhibitors of RyRs affected NAADP-induced depolarization. These data further support the hypothesis that a yet unidentified plasma membrane Ca2+ channel is the target of NAADP in starfish oocytes.

NAADP triggers the fertilization potential in starfish oocytes

In invertebrates oocytes or eggs, the fertilization or activation potential establishes the fast electrical block to polyspermy and, in some species, provides the Ca2+ influx which contributes to the following intracellular Ca2+ wave. In echinoderms, the molecule triggering the activation potential is still unknown. The aim of this study was to assess whether nicotinic acid-adenine dinucleotide phosphate (NAADP) elicited the fertilization potential in starfish oocytes. The changes in membrane potential induced by the sperm were measured in oocytes held at a low resting potential, so that the Ca2+-action potential was inactivated and only the initial slower depolarization caused by the sperm could be studied. Decreasing extracellular Na+ concentration did not prevent the onset of the fertilization potential, while removal of external Ca2+ abolished it. The pre-incubation with SK&F 96365 and verapamil and the pre-injection of BAPTA inhibited the fertilization potential, while the injection of heparin only reduced its duration. The biophysical and pharmacological properties of the sperm-elicited depolarization were similar to those displayed by the NAADP-activated Ca2+-mediated current recently described in starfish oocytes. Indeed, the desensitization of NAADP-receptors prevented the onset of the fertilization potential. Taken together, these data suggest that NAADP could trigger the fertilization potential in starfish oocytes.

A Mueller matrix model of Haidinger's brushes

Stokes vectors and Mueller matrices are used to model the polarisation properties (birefringence, dichroism and depolarisation) of any optical system, in particular the human eye. An explanation of the form and behaviour of the entoptic phenomenon of Haidinger's brushes is derived that complements and expands upon a previous study. The relationship between the appearance of Haidinger's brushes and intrinsic ocular retardation is quantified and the model allows prediction of the effect of any retarder of any orientation placed between a source of polarised light and the eye. The simple relationship of minimum contrast of Haidinger's brushes to the cosine of total retardation is derived.

Ca2+ signalling and membrane current activated by cADPr in starfish oocytes

Cyclic ADP-ribose (cADPr) is a second messenger that regulates intracellular free [Ca2+] ([Ca2+](i)) in a variety of cell types, including immature oocytes from the starfish Astropecten auranciacus. In this study, we employed confocal laser scanning microscopy and voltage clamp techniques to investigate the source of the cADPr-elicited Ca2+ wave originating from the cortical Ca2+ patches we have described previously. The Ca2+ swing was accompanied by a membrane current with a reversal potential of approximately +20 mV. Decreasing external Na+ almost abolished the current without affecting the Ca2+ response. Removal of extracellular Ca2+ altered neither the Ca2+ transient nor the ionic current, nor did the holding potential exert any effect on the Ca2+ wave. Both the Ca2+ response and the membrane current were abolished when BAPTA, ruthenium red or 8-NH(2)-cADPr were preinjected into the oocytes, while perfusion with ADPr did not elicit any [Ca2+](i) increase or ionic current. However, elevating [Ca2+](i) by uncaging Ca2+ from nitrophenyl- (NP-EGTA) or by photoliberating inositol 1,4,5-trisphosphate (InsP(3)) induced an ionic current with biophysical properties similar to that elicited by cADPr. These results suggest that cADPr activates a Ca2+ wave by releasing Ca2+ from intracellular ryanodine receptors and that the rise in [Ca2+](i) triggers a non-selective monovalent cation current that does not seem to contribute to the global Ca2+ elevation.

Evidence that fertilization activates starfish eggs by sequential activation of a Src-like kinase and phospholipase cgamma

Recent evidence has indicated a requirement for a Src family kinase in initiating Ca(2+) release at fertilization in starfish eggs (Giusti, A. F., Carroll, D. J., Abassi, Y. A., Terasaki, M., Foltz, K. R., and Jaffe, L. A. (1999) J. Biol. Chem. 274, 29318-29322). We now show that injection of Src protein into starfish eggs initiates Ca(2+) release and DNA synthesis, as occur at fertilization. These responses depend on the phosphorylation state of the Src protein; only the kinase active form is effective. Like Ca(2+) release at fertilization, the Ca(2+) release in response to Src protein injection is inhibited by prior injection of the SH2 domains of phospholipase Cgamma. These findings support the conclusion that in starfish, sperm-egg interaction causes egg activation by sequential activation of a Src-like kinase and phospholipase Cgamma. Injection of the SH2 domain of Src, which inhibits Ca(2+) release at fertilization, does not inhibit Ca(2+) release caused by Src protein injection. This indicates that the requirement for a Src SH2 domain interaction is upstream of Src activation in the pathway leading to Ca(2+) release at fertilization.

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.

Calcium release at fertilization in starfish eggs is mediated by phospholipase Cgamma

Although inositol trisphosphate (IP3) functions in releasing Ca2+ in eggs at fertilization, it is not known how fertilization activates the phospholipase C that produces IP3. To distinguish between a role for PLCgamma, which is activated when its two src homology-2 (SH2) domains bind to an activated tyrosine kinase, and PLCbeta, which is activated by a G protein, we injected starfish eggs with a PLCgamma SH2 domain fusion protein that inhibits activation of PLCgamma. In these eggs, Ca2+ release at fertilization was delayed, or with a high concentration of protein and a low concentration of sperm, completely inhibited. The PLCgammaSH2 protein is a specific inhibitor of PLCgamma in the egg, since it did not inhibit PLCbeta activation of Ca2+ release initiated by the serotonin 2c receptor, or activation of Ca2+ release by IP3 injection. Furthermore, injection of a PLCgamma SH2 domain protein mutated at its phosphotyrosine binding site, or the SH2 domains of another protein (the phosphatase SHP2), did not inhibit Ca2+ release at fertilization. These results indicate that during fertilization of starfish eggs, activation of phospholipase Cgamma by an SH2 domain-mediated process stimulates the production of IP3 that causes intracellular Ca2+ release.

Cortical changes in starfish (Asterina pectinifera) oocytes during 1-methyladenine-induced maturation and fertilisation/activation

Maturation of the starfish oocyte cortex to produce an effective cortical granule reaction and fertilisation envelope is believed to develop in three phases: (1) pre-methyladenine (1-MA) stimulation; (2) post-1-MA stimulation, pregerminal vesicle breakdown; and (3) post-germinal vesicle breakdown. The present study was initiated to identify what each of these phases may encompass, specifically with respect to structures associated with the oocyte cortex, including cortical granules, microvilli and vitelline layer. 1-MA treatment brought about an orientation of cortical granules such that they became positioned perpendicular to the oocyte surface, and an approximately 4-fold decrease in microvillar length. A-23187 activation of immature oocytes treated with (10 min; pregerminal vesicle breakdown) or without 1-MA resulted in a reduction in cortical granule number of 21% and 41%, respectively (mature oocytes underwent a 96% reduction in cortical granules). Elevation of the fertilisation envelope in both cases was significantly retarded compared with activated mature oocytes. In activated mature oocytes, the vitelline layer elevated 20.0 +/- 5.4 mu m from the egg's surface, whereas in immature oocytes treated with just A-23187 or with 1-MA (10 min) and A-23187, it lifted 0.35 +/- 0.1 and 0.17 +/- 0.04 mu m, respectively. The fertilisation envelopes of activated (or fertilised) immature oocytes also differed morphologically from those of mature oocytes. In activated, immature oocytes, the fertilisation envelope was not uniform in its thickness and possessed thick and thin regions as well as fenestrations. Additionally, it lacked a complete electron-dense stratum that characterised the fertilisation envelopes of mature oocytes. The nascent perivitelline space of immature oocytes was also distinguished by the presence of numerous vesicles which appeared to be derived from microvilli. Differences in the morphology of cortices from activated (fertilised) and non-activated, immature and mature oocytes substantiate previous investigations demonstrating three phases of cortical maturation, and are consistent with physiological changes that occur during oocyte maturation, involving ionic conductance of the plasma membrane, establishment of slow and fast blocks to polyspermy and elevation of a fertilisation envelope.

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.

Changes in voltage-dependent currents and membrane area during maturation of starfish oocytes: species differences and similarities

Full grown starfish oocytes are arrested at meiotic prophase I in the ovary. The natural hormone 1-methyladenine triggers oocyte maturation which involves meiosis reinitiation along with a variety of morphological, biochemical, and electrical changes. In studying oocytes of two species, Henricia leviuscula and Asterina miniata, using the voltage-clamp technique, we found interesting differences and similarities in the electrophysiological changes which occurred during maturation. Oocytes of both species have three major voltage-dependent currents: an inward Ca2+ current, an inwardly rectifying K+ current, and a transient outward K+ current (A-current). The Ca2+ current and the A-current were similar in the two species but the inward rectifier in Henricia had activation kinetics that were more than 10-fold slower than in Asterina. Nonetheless, all three currents were affected similarly during maturation: the inward Ca2+ currents remained constant in both species, while the two K+ currents decreased in amplitude. In Henricia the membrane surface area decreased substantially during maturation, while in Asterina it remained constant. This may be explained by the more highly infolded state of the membrane in the immature Henricia oocyte. The selective loss of K+ current followed the time course of the area decrease in Henricia, but the same percentage decrease in current occurred in Asterina without a net membrane loss.

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.

Calcium current and calcium-activated inward current in the oocyte of the starfish Leptasterias hexactis

1. Inward currents in the immature oocyte of the starfish Leptasterias hexactis were studied with a two-micro-electrode voltage clamp. Experiments investigated the role of Ca2+ in the Na+-dependent plateau of the action potential. 2. Voltage steps more positive than -55 mV produced inward currents in normal sea water that activated and then decayed to a non-zero level with a double-exponential time course. Returning the voltage to the resting potential produced an inward tail current that relaxed slowly to zero with a time course of seconds. 3. Replacing Na+ with choline abolished the slowly decaying component as well as the slow tail current which followed the end of the voltage pulse. This suggested that inward current in Na+-containing sea water consisted of a rapidly decaying component that flowed through Ca2+ channels and a more slowly decaying component carried by Na+. 4. Ca2+ current was isolated in Na+-free sea water. Activation followed a sigmoidal time course that could be described with m2 kinetics. Inactivation during a maintained depolarization followed first-order kinetics and was voltage dependent. 5. When Ba2+ was substituted for Ca2+ as the divalent ion charge carrier, inward currents in Na+-containing sea water decayed along a single-exponential time course. The absence of a slowly decaying Na+ current in Ba2+-containing sea water suggested that Na+ current depended on Ca2+ influx. 6. The effects of altering Ca2+ influx on the time course of Na+ current were investigated. Na+ current decayed more rapidly as the test pulse potential was made more positive, while raising [Ca2+]o slowed the decaying phase without altering its dependence on membrane potential. 7. Tail currents measured after rapidly stepping the membrane potential back to the resting level consisted of a fast component associated with the closing of Ca2+ channels and a slow component that was abolished by removing Na+. 8. The variation of the amplitude of the slow component of tail current with the duration of the voltage-clamp pulse indicated that Na+ current is associated with a time-dependent component of membrane conductance. 9. Possible mechanisms for the slowly decaying Na+ current are considered. The results are discussed in relation to the idea that the conductance change to Na+ follows the time course of Ca2+ accumulation and removal from the cytoplasm.

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.

Hormone-induced loss of surface membrane during maturation of starfish oocytes: differential effects on potassium and calcium channels

Prior to fertilization, starfish oocytes undergo meiotic maturation, triggered by the hormone 1-methyladenine (1-MA). Maturation involves a variety of complex biochemical, morphological, and electrical changes, many of which are similar to those caused by progesterone in vertebrates. Using voltage-clamp and ultrastructural techniques to study maturation in starfish, we have discovered a novel process by which 1-MA alters the electrical properties of the oocyte. The surface area of the oocyte decreases by more than 50% during the first hour of maturation, due to the elimination of microvilli, but the calcium and potassium currents present are affected differently by the loss of membrane. The amplitudes of both the transient K current ("A-current") and the inwardly rectifying K current decrease, following the time course of the decrease in surface area, while the Ca current amplitude remains virtually unaffected, and may even increase in some oocytes. The kinetics of the currents do not change. This selective removal of K channels results in a larger and more rapidly rising action potential in the mature egg, which may aid in the fast block to polyspermy. The differential accessibility of various ion channels to mechanisms of membrane removal and insertion may play an important role in the development of excitable cells.

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.

A comparative study of the membrane potential from before fertilization through early cleavage in two frogs, Rana pipiens and Xenopus laevis

The membrane potential of Rana pipiens eggs (-55.0 mV +/- 11.2(16)) was more likely to recover from impalement and was always more negative than that of eggs of Xenopus laevis (-19.3 mV +/- 4.2(68)). It was also much more negative than previously reported. Essentially similar membrane resistance changes were measured in the two frog species through fertilization and cleavage. Small transient depolarizations only associated with the onset of the fertilization potential in Xenopus could be prevented by hyperpolarizing the egg membrane prior to fertilization. Repolarization was variable and longer in Rana and often accompanied by large transient spontaneous depolarizations. Insemination time, the time between fertilization and cleavage and the first cleavage division cycle, were all about twice as long in Rana. Xenopus egg cleavage was invariably accompanied by pronounced transient hyperpolarizations that were essentially absent in Rana.

Voltage-clamp study of the conductance activated at fertilization in the starfish egg

Ionic currents underlying the fertilization potential of the egg of the starfish Mediaster aequalis were studied using a two-micro-electrode voltage clamp. Mature eggs were fertilized in vitro under voltage-clamp conditions. The fertilization current, here termed IF, was induced by adding sperm to sea water bathing the egg. At a holding potential of -70 mV, IF was inward. It reached a peak within 2-4 min and then decayed over the next approximately 20 min with a rate which depended on the holding membrane potential. Instantaneous current-voltage relations measured at different times during IF were approximately linear and reversed at a potential of +6.0 +/- 5.8 mV (mean +/- S.D., n = 11). Membrane chord conductance was highest at the peak of inward current and the declining phase of IF was due to a decrease in conductance towards the pre-fertilization level. When the membrane potential was rapidly stepped to levels more positive than about -45 mV, the conductance underlying IF decreased in a manner which depended on both membrane potential and time. The fertilization-specific conductance showed a sigmoidal activation curve between -50 and +10 mV with a half-activation level of -25 mV. Analysis of the steady-state voltage dependence indicated that at the peak of the fertilization potential (+10 to +15 mV) only 4-5% of the total available channels would be open. Current relaxations followed first-order kinetics and the relaxation time constant depended upon the membrane potential during the voltage pulse. The relation between the time constant and voltage was bell-shaped, decreasing at potentials more negative than -40 and more positive than 0 mV. Both the steady-state conductance-voltage relation and the kinetics of the current relaxations were consistent with a simple two-state gating model in which the probability of a channel being open is determined by a single gating particle with an effective valency of -1.7 moving through the entire membrane field. The shifts in reversal potential with changes in external Na (at 10 mM-external K) were analysed using the constant field expression, which gave a relative permeability of Na to K of approximately 0.6.(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.

Ionic dependence and transmission of epidermal action potentials in a newt embryo

The ionic dependency and transmission of epidermal action potentials have been examined from tailbud to hatching stages of newt embryos. Previously we have reported that the epidermal action potential is composed of a fast- and slow-action component; only the slow-action component, however, is transmitted to other cells. We address in this report the mechanism by which these responses are mediated. The slow-action potential is not produced in Na+-free saline, tricaine saline, or following the application of TTX, and thus appears to be Na+ dependent. The fast-action potential on the other hand is blocked by application of Co2+ and verapamil saline and thus appears to be Ca2+ dependent. The slow-action potentials appear to be chemically transmitted since they are transmitted even to those cells which are electrically uncoupled at low intracellular pH (NaHCO3 + HCl, pH 6.2). Furthermore 1 microM curare and atropine are inhibitory to transmission of the slow potential. Epidermal cells of the newt embryo are sensitive to acetylcholine (ACh) applied by hydrostatic ejection through a micropipet. The latter observation further suggests that propagation of the slow-action potential is, in part, a chemical event.

Enhancement of ionic currents through voltage-gated channels in the mouse oocyte after fertilization

1. The changes of voltage-gated ion channels in the mouse oocyte after fertilization were investigated under voltage clamp.2. About 60 min after introduction of sperm suspension into the fertilization medium, the amplitude of inward current through Ca(2+)-channels increased, which occurred at anaphase during the second meiotic division. The peak amplitude of the maximum inward current per unit membrane capacity of the oocytes at metaphase was 20+/-3 muA/muF in 50 mM-Sr medium. It was 28+/-8 muA/muF at anaphase, and 32+/-5 muA/muF at telophase. The kinetic properties as well as selectivity among Ca, Sr and Mn ions were not altered after fertilization.3. The outward surge current which was found at the higher membrane potential over +50 mV also increased in amplitude after fertilization, simultaneously with the increase in amplitude of inward current through Ca(2+)-channels. The means and the standard deviations of the surge current per unit membrane capacity at 120 mV were 31+/-8 muA/muF at metaphase, and 48+/-7 muA/muF at telophase. The kinetic properties of the outward surge current were not altered after fertilization.4. Application of colcemid (10(-7) mole/l.) or cytochalasin B (2 x 10(-5) mole/l.) did not prevent the increase in amplitude of both inward current through Ca channels and the outward surge current.5. The membrane currents in N-18 mouse neuroblastoma cells in logarithmic growth phase were examined under voltage clamp. The N-18 neuroblastoma cells possessed the Ca inward current and the delayed outward current. The kinetic properties and the steady-state inactivation of Ca(2+)-channels in N-18 neuroblastoma cells were compared with those in mouse oocytes. It was concluded that they could be regarded as identical between the mouse oocyte and the N-18 neuroblastoma cell.

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+.

Transmembrane potential of rat hepatocytes in primary monolayer culture

Transmembrane potentials of rat hepatocytes in primary monolayer culture on collagen gels were measured with glass microelectrodes. Potentials for cells in culture for 23-30 h comprised two populations. The mean +/- SD for a population of stable low potentials was -9.7 +/- 2.0 mV (n = 93). This was compared with -23.6 +/- 9.4 mV (n = 42), the mean value for stable potentials that followed spontaneous increases in the low potentials, 0.5-2.0 min after the impalement. The estimated input resistance increased during these spontaneous hyperpolarizations. In some cells, after 48 h in culture, the transmembrane potential oscillated rhythmically, with an amplitude of 25 mV and a period of 7 min. Suffusing the cells with 120 mM potassium chloride decreased the potential and eliminated the oscillations. The stable high potentials were considered more accurate estimates of the hepatocyte transmembrane potential, based on comparison with values for intact liver. Low potentials may have resulted from current leaking through an electrode shunt resistance, followed by an increase in potential as the membrane "sealed" the shunt pathway. However, these events may also reflect cells capable of two stable transmembrane potentials.

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.

Zinc-dependent action potentials in giant neurons of the snail, Euhadra quaestia

In giant neurons of subesophageal ganglion of the Japanese land snail, Euhadra quaestia Deshayes, permeation of Zn ions through Ca channels were investigated with a conventional current clamp method. All-or-none action potentials of long duration (90 to 120 sec) were evoked in 24 mM Zn containing salines. The overshoots were about +10 mV and the maximum rate of rises (MRRs) was about 2.9 V/sec. The amplitudes and the MRRs of the action potentials depended on external Zn ion concentrations. The action potentials were suppressed by specific Ca-channel inhibitors such as Co2+, La3+ and Verapamil, but they were resistant to Na-channel inhibitor, tetrodotoxin, even at 30 microM. It is concluded that these action potentials are generated by Zn ions permeating Ca channels in snail neuronal membrane. On the basis of Hagiwara and Takahashi's (S. Hagiwara & K. Takahashi, 1967, J. Gen. Physiol. 50:583) model of Ca channels, it is inferred that Zn ions are 5 to 10 times stronger in affinity to Ca channels than Ca ions, but 10 to 20 times less permeable.

A potassium conductance activated by hyperpolarization in paramecium

Voltage clamp studies show that the wild-type membrane of Paramecium tetraurelia contains a conductance component which is sensitive to hyperpolarization. This component manifests itself as "anomalous", or "inward going", rectification of membrane voltage in response to applied constant current pulses and as a "hyperpolarizing spike" when no K is added to the external solution (Y. Satow, C. Kung, 1977. J. Comp. Physiol. 119:99). Like the conductances which underlie anomalous rectification in other cells, the hyperpolarization-sensitive conductance in Paramecium is specific for K, and the magnitude of the voltage-dependent conductance change depends not only on voltage but also on external potassium concentration. The internal potassium ion concentration of Paramecium is calculated to be between 17 and 18mM.

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.

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.

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.