An electrophysiological study of the membrane properties of the immature and mature oocyte of the batstar, Patiria miniata

The arm of the starfish: The far-reaching applications of Patiria miniata as a model system in evolutionary, developmental, and regenerative biology

Many species of echinoderms have long been considered model research organisms in biology. Historically, much of this research has focused on the embryology of sea urchins and the use of their extensive gene regulatory networks as a tool to understand how the genome controls cell state specification and patterning. The establishment of Patiria miniata, the bat sea star, as a research organism has allowed us to expand on the concepts explored with sea urchins, viewing these genetic networks through a comparative lens, gaining great insight into the evolutionary mechanisms that shape developmental diversity. Extensive molecular tools have been developed in P. miniata, designed to explore gene expression dynamics and build gene regulatory networks. Echinoderms also have a robust set of bioinformatic and computational resources, centered around echinobase.org, an extensive database containing multiomic, developmental, and experimental resources for researchers. In addition to comparative evolutionary development, P. miniata is a promising system in its own right for studying whole body regeneration, metamorphosis and body plan development, as well as marine disease.

Rapid increase in plasma membrane chloride permeability during wound resealing in starfish oocytes

Plasma membrane wound repair is an important but poorly understood process. We used femtosecond pulses from a Ti-Sapphire laser to make multiphoton excitation–induced disruptions of the plasma membrane while monitoring the membrane potential and resistance. We observed two types of wounds that depolarized the plasma membrane. At threshold light levels, the membrane potential and resistance returned to prewound values within seconds; these wounds were not easily observed by light microscopy and resealed in the absence of extracellular Ca²⁺. Higher light intensities create wounds that are easily visible by light microscopy and require extracellular Ca²⁺ to reseal. Within a few seconds the membrane resistance is ∼100-fold lower, while the membrane potential has depolarized from −80 to −30 mV and is now sensitive to the Cl⁻ concentration but not to that of Na⁺, K⁺, or H⁺. We suggest that the chloride sensitivity of the membrane potential, after wound resealing, is due to the fusion of chloride-permeable intracellular membranes with the plasma membrane.

Cortical granule translocation during maturation of starfish oocytes requires cytoskeletal rearrangement triggered by InsP3-mediated Ca2+ release

Cortical granules (secretory vesicles located under the cortex of mature oocytes) release their contents to the medium at fertilization. Their exocytosis modifies the extracellular environment, blocking the penetration of additional sperm. The granules translocate to the surface during the maturation process, and it has been suggested that they move to the cortex via cytoskeletal elements. In this paper we show that the increase in intracellular Ca2+, which the maturing hormone 1-methyladenine (1-MA) induces in starfish through the activation of inositol 1,4, 5-trisphosphate (InsP3) receptors, triggers changes in filamentous actin, which then direct the correct movement and reorientation of the cortical granules and the elevation of the fertilization envelope.

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.

Extracellular Mg2+ induces a loss of microvilli, membrane retrieval, and the subsequent cortical reaction, in the oocyte of the prawn Palaemon serratus

Surface changes induced by sea water were analyzed in the ovulated oocyte of the prawn Palaemon serratus. They depended on the presence of external Mg2+ but not on external Ca2+ alone. Increasing external Mg2+ from 0 mM to 30 mM stimulated first a progressive disappearance of preexisting microvilli, which was over within 30 min of incubation. This is correlated with membrane removal via internalization of coated vesicles, ascertained by observations of endocytosis of an extracellular fluid-phase marker and by measurement of a diminution in membrane capacitance (Cm). Thirty-five minutes after sea water contact, the prawn oocyte underwent a cortical reaction independent of fertilization. It consists in a heavy exocytosis of ring-shaped elements, leading to the deposition of a thick capsule, and requiring a threshold Mg2+ concentration of greater than or equal to 10 mM and at least a 3-min incubation with Mg2+. Concurrently, the values of the membrane capacitance (Cm) and conductance (Gm) increased about 2 and 10 times their initial values, respectively. The calcium ionophore ionomycin, added to Mg(2+)-free artificial sea water, stimulated the cortical reaction with requirement of external Ca2+. Other divalent cations (Mn2+, Zn2+, Co2+, Ni2+, Cd2+) instead of Mg2+, induced the cortical reaction, but Ba2+, Sr2+, and La3+ did not. When eggs are fertilized, the cortical reaction takes place in two steps, the first being a discrete exocytosis of a foamy material and the second always involving ring-shaped elements.

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.

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.

Rabbit oocyte maturation: changes of membrane resistance, capacitance, and the frequency of spontaneous transient depolarizations

Immature oocytes from rabbits were examined with electrophysiological techniques to determine if their membrane properties change during maturation. The input resistance increased and input capacitance decreased during maturation, although no significant change in membrane potential was observed. The changes observed were consistent with a decrease of corona radiata-oocyte electrical coupling accompanying maturation. Spontaneous transient depolarizations were recorded from immature oocytes surrounded by corona radiata, but not from mature ova. Each event consisted of a rapid depolarization, sustained for 1-100 sec, and a slow repolarization to the resting potential. Spontaneous inward currents with a time course similar to the spontaneous transient depolarizations occurred when the oocyte's membrane potential was held constant by voltage clamp. The frequency with which spontaneous transient depolarizations occurred decreased during maturation. These findings are consistent with a model in which spontaneous depolarizations originate in corona radiata cells and are detected in the oocyte via electrical coupling.

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.

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)

Calcium antagonists and calmodulin inhibitors block cytokinin-induced bud formation in Funaria

The plant hormone cytokinin stimulates nuclear migration followed by an asymmetric cell division in target cells of the protonema of the moss Funaria hygrometrica, leading to bud formation. The role of calcium in this developmental event was investigated by examining the effects of various calcium antagonists on the cytokinin-induced division. Calcium-free medium (buffered with EGTA), the extracellular Ca2+ antagonist La3+ (lanthanum), and the Ca2+ channel inhibitors D 600 and verapamil all block bud formation. These inhibitions are partially reversed by washing the cells or by raising the extracellular [Ca2+]. The Ca2+ ionophore A23187 partially reversed the effects of D 600 and verapamil. Bud formation is also inhibited by the intracellular Ca2+ antagonist TMB-8 (8-diethylamino)octyl 3,4,5-trimethoxybenzoate HCl), and this inhibition is partially reversed by washing or raising the extracellular [Ca2+]. The cross walls of both the filaments and bud initial cells formed during TMB-8 exposure exhibit a distorted morphology. High concentrations of TMB-8 block nuclear migration. The calmodulin inhibitor trifluoperazine stops cytokinin-induced budding more effectively than the related compound chlorpromazine. Low concentrations of these two compounds do not affect nuclear migration; however, the target cell does not enter mitosis. These results support the hypothesis that a rise in intracellular calcium mediates cytokinin-induced bud formation in Funaria. It is concluded that the proposed cytokinin-induced rise in intracellular calcium may be effected in part by the activation of calmodulin. The essential source of Ca2+ appears to be extracellular, because blocking Ca2+ uptake with Ca2+ transport inhibitors can block both nuclear migration and subsequent division.

Hydrostatic pressure inhibition of hormone-induced resumption of meiotic maturation in isolated oocytes

Prior to ovulation, fully grown oocytes of both the amphibian (Rana pipiens) and the starfish (Piaster ochraceus), like those of many other organisms, are arrested in late prophase I of meiosis. Reinitiation of meiotic maturation in oocytes from either of these organisms is hormonally induced. Although the meiosis-inducing substance (MIS) for each organism is chemically dissimilar (a steroid in the frog and a purine in the starfish) induction of oocyte maturation in both biological systems appears to be initiated by the interaction of the MIS with the plasma membrane of the oocyte. The objective of this investigation was to determine if elevated hydrostatic pressure affected hormonal induction of oocyte maturation and to compare the effect of pressure on oocytes stimulated with different meiosis-inducing substances. In isolated oocytes from either the frog or the starfish, increasing ambient pressure reduced the percentage of oocytes which matured, and a stepwise increase in pressure resulted in a corresponding shift in the dose response curves of hormone-induced, oocyte maturation to the right. In experiments using only starfish oocytes, this inhibitory effect of pressure was found to be reversible and limited to the initial period of the maturation event. Taken together, these data suggest that elevated ambient pressure inhibits an early cellular event in the hormonal induction of meiotic maturation which is common to both amphibian and starfish oocytes.

Is calcium the second messenger of 1-methyladenine in meiosis reinitiation of starfish oocytes?

Microinjection of EGTA into prophase-blocked oocytes does not inhibit hormone-induced meiosis reinitiation, although it prevents oocyte activation by fertilization, by ionophore A23187, or by subsequent microinjection of otherwise efficient Ca2+ buffers. In contrast microinjection of Ca2+ buffers inhibits 1-methyladenine-induced meiosis reinitiation. Oocytes can be released from Ca2+ inhibition by raising hormone concentration or by the subsequent transfer of cytoplasm taken from maturing oocytes. Ca2+-microinjected oocytes remain inhibited up to 1 h after microinjection, although free Ca2+ concentration comes back to its resting value less than 30 sec after microinjection. Cyanide, which decreases ATP content and depresses Ca2+-pumping activity, reversibly inhibits 1-methyladenine-induced meiosis reinitiation. These results do not support the hypothesis that Ca2+ is the second messenger of the hormone in meiosis reinitiation of starfish oocytes, although they support the view that elimination of Ca2+ from some component of the oocyte cortex (perhaps the plasma membrane) might be a compulsory event for transduction of the hormonal message.

Developmental regulation of Ca2+ and K+ currents during hormone-induced maturation of starfish oocytes

Changes in the electrical properties of starfish oocytes during hormone-induced maturation (the reinitiation of meiosis prior to fertilization) were studied by using the voltage-clamp technique. Three voltage-dependent ionic currents dominate the current-voltage relation of the immature oocyte: an inward Ca2+ current, a fast transient K+ current similar to the "A current" of molluscan neurons, and an inwardly rectifying K+ current. During in vitro maturation stimulated by the natural maturing hormone 1-methyladenine, gradual changes in the amplitudes of all three currents were seen: the Ca2+ currents became larger, and both K+ currents became smaller. The kinetics of the currents were not significantly altered during maturation. As a result of these changes, action potentials in the mature egg had lower thresholds, faster rates of rise, and larger overshoots than those of the immature oocyte. We also found that the total membrane capacitance decreased substantially during maturation, perhaps indicating a decrease in membrane surface area triggered by the hormone. The significance of these results is discussed in terms of the preparation of the immature oocyte for fertilization and the mechanisms of modification of ion channel properties during development.

Ionic regulation of egg activation

Developing cells have constantly to make decisions: when to proliferate and divide, when and how to differentiate. It is an increasingly attractive idea that these decisions involve changes in intracellular cation concentrations. Our ideas about the mechanisms of changes in intracellular cations come largely from the application of biophysical techniques in the study of excitable tissues. These ideas are proving very valuable to the investigation of the control of proliferation and cell development and it is evident that the ionic mechanisms which pertain in nerve and muscle have their counterparts in other cells. Just as alterations in intracellular ion concentrations serve a signalling function in excitable tissue, so too they act as signals during development. Since almost all the quantitative data on the ionic mechanisms of fertilization come from work on sea urchins we have confined our review to sea urchin eggs.

Possible role of "prenervous" neurotransmitters in cellular interactions of early embryogenesis: a hypothesis

Evidence is presented in support of the working hypothesis that "prenervous" neurotransmitters directly participate in cell-cell interactions occurring during the first several cleavage divisions of sea urchin embryos, a function which may occur during the early development of higher animals as well. This intercellular signaling could be a link in the evolutionary progression from the use of these substances as intracellular regulators to their participation in cell-cell interactions occurring during synaptic transmission.

A repeating unit of higher order chromatin structure in chick red blood cell nuclei

The organization of nucleosomes in higher order chromatin structures has been studied by electron microscopy of chick red blood cell nuclei. Chromatin appears as a thick fiber with an average diameter of approximately 300 A when prepared for electron microscopy in buffers which approximate physiological ionic strength. Progressive steps of disassembly of the thick fiber into individual nucleosomes could be induced either by ionic strength reduction or by tRNA treatment (which removes histone H1 and some non-histone chromosomal proteins). When disassembly was induced by ionic strength reduction in the presence of Mg++ (or Ca++), the lengths of the intermediate disassembly products were found to be multiples of 330 A. The diameter of these structures was estimated to be 275 A. This intermediate in the disassembly process is not observed if thick fiber disassembly is induced by ionic strength reduction in the absence of divalent cations. To investigate whether the higher order structural unit is present in the thick fiber at physiological ionic strengths, tRNA treatment was used to induce thick fiber disassembly under physiological monovalent ionic conditions. In this case, either with or without divalent cations, a supranucleosomal unit was found with dimensions similar to those given above. This data provides evidence for a slightly oblong supranucleosomal structure (330 x 275 A) whick forms a repeating unit in the chromatin thick fiber.

Calcium content and distribution in egg vesicles of Galleria mellonella (Lepidoptera) as determined by X-ray microanalysis

In egg vesicles of Galleria mellonella (Lepidoptera) electron microprobe analysis reveals calcium in concentrations of 9 and 3 mmoles per 1,000 g tissue wet weight in oocyters and accompanying trophic cells, respectively. This high average level of calcium characterizes both pre- and postvitellogenic oocytes, but the distribution of calcium is not uniform. In postvitellogenic vesicles the central area of the ooplasm shows a higher content of Ca than peripheral one, what may be correlated with the distribution of mature yolk platelets within the ooplasm.

Membrane response to 1-methyladenine requires the presence of the nucleus

Meiosis is re-initiated in starfish oocytes by the action of the hormone 1-methyladenine (1-MA). It is thought that the primary trigger is localised at the plasma membrane, and early changes reported are the activation of a Na+ pump and variation in membrane potential and conductance. We report here that external application of 1-MA results in an irreversible switching of the starfish oocyte membrane from one stable electrical state to another. This induced change requires the presence of the germinal vesicle.

Blocking effects of barium and hydrogen ions on the potassium current during anomalous rectification in the starfish egg

1. The blocking effects of Ba+ and H+ on the inward K current during anomalous rectification of the giant egg membrane of the starfish, Mediaster aequalis, were studied using voltage clamp techniques. 2. External Ba2+ at a low concentration (10--100 micron) suppresses the inward K current; the extent of suppression, expressed as the ratio of currents with and without Ba2+, can be described by a conventional bimolecular adsorption isotherm, K/(K + [Ba2+]o), K being an apparent dissociation constant. 3. The dissociation constant, K, decreases as the membrane potential V becomes more negative and can be expressed by K(V) = K(0) exp (zmuFV/RT), where K(0) is the K at V = 0, z is the charge of the blocking ion, and mu is a parameter for the membrane potential dependence of Ba2+ blockage. The value of mu ranges between 0.64 and 0.68. 4. Upon a sudden change in membrane potential the change in the blocking effect of Ba2+ follows first order kinetics; the forward rate constant is membrane-potential-dependent whereas the backward constant is potential-independent. 5. The blocking effect of Ba2+ appears to be independent of the activation of K channels during anomalous rectification. 6. The blocking effect of Ba2+ depends on V alone, in contrast to the activation of the K channel during anomalous rectification which depends on V--VK. 7. In these respects, the effect of Ba2+ is equivalent to the introduction of inactivation into the anomalous rectification. 8. SI2+ and Ca2+ show small but observable blocking effects only at much higher concentrations (about 10--20 mM). 9. The inward K current is suppressed when the external pH is reduced below 6.0. The blocking effect of H+ shows no significant potential dependence. The concentration dependence suggests that three H+ ions simultaneously titrate the acidic groups of each channel (pK = 5.3--5.4). 10. The implications of these results are discussed in terms of molecular models of the potassium channel of anomalous rectification and possible mechanisms of K channel inactivation.

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.