Spatial Organization of Calcium Signaling Involved in Cell Volume Control in the Fucus Rhizoid

Subprotoplasts prepared from different regions of rhizoid and thallus cells of Fucus zygotes displayed mechanosensitive plasma membrane channels in cell-attached patch-clamp experiments by using laser microsurgery. In excised patches, this channel was found to be voltage gated, carrying K+ outward and Ca2+ inward, with a relative permeability of Ca2+/K+ of 0.35 to 0.5, and an increased open probability at membrane potentials more positive than -80 mV. No significant difference was found in the density of this channel type from different regions of rhizoid or thallus cells. Hypoosmotic treatment of intact zygotes induced dramatic transient elevations of cytoplasmic Ca2+, initiating at the rhizoid apex and propagating in a wavelike manner to subapical regions. Localized initiation of the Ca2+ transient correlated with greater osmotic swelling at the rhizoid apex compared with other regions of the zygote. Ca2+ transients exhibited a refractory period between successive hypoosmotic shocks, during which additional transients could not be elicited and the ability to osmoregulate was impaired. Buffering the Ca2+ transients with microinjected Br2BAPTA similarly reduced the ability of rhizoid cells to osmoregulate. Ca2+ influx was associated with the initiation of the Ca2+ transient in apical regions, whereas intracellular sources contributed to its propagation. Thus, localized signal transduction is patterned by interactions of the cell wall, plasma membrane, and intracellular Ca2+ stores.     

A calcium-dependent transient outward current in Xenopus laevis oocytes

Membrane currents were investigated in Xenopus laevis oocytes under voltage clamp. Depolarizing pulses, given from a holding potential of about-100 mV, elicited a transient outward current when the membrane potential was made more positive than about-20 mV. As the potential was made increasingly positive the transient outward current first increased and then decreased. The amplitude of the transient current increased when the external Ca2+ concentration was raised; and the current was abolished by Mn2+. It appears that when the membrane is depolarized Ca2+ ions enter the oocyte and trigger an outward current, possibly by opening C1- channels.     

A study of the "outward potassium channel" (OPC) in the frog oocyte. I. A study of the "cell-attached" configuration

A single channel current was studied in the membrane of the immature oocyte of the european frog (Rana esculenta) by using the "patch clamp" technique in the "cell attached" configuration. Single channel activity appeared as short outward currents when membrane potential was made positive inside; full activation required seconds to be complete, no inactivation being appreciable. Deactivation (or current block) upon membrane repolarization was so fast that no inward current could be detected in any case. The reversal potential, estimated by interpolating the I/V diagrams, was -30 mV using standard Ringer as electrode filling solution, and the elementary conductance was 95 pS. Neither reversal potential nor elementary conductance were affected by removal of external Ca2+ (Mg2+ or Ba2+ substitution) or external Cl- (methanesulphonate substitution). The reversal potential moved towards positive potentials by substituting external Na+ with K+, the magnitude of the shifts being consistent with a ratio PK/PNa = 6.4. A distinctive property of the current/voltage relation for this K-current is its anomalous bell-shape, the outward current displaying a maximum at membrane potentials around 75 mV with standard Ringer as electrode filling solution and tending to zero with more positive potentials.     

A Fast Transient Outward Monovalent Current in Rat Saphenous Myocytes Passing Through Ca2+ Channels

Transient outward currents in rat saphenous arterial myocytes were studied using the perforated configuration of the patch-clamp method. When myocytes were bathed in a Na-gluconate solution containing TEA to block large-conductance Ca2+-activated K+ (BK) currents, depolarizing pulses positive to +20 mV from a holding potential of -100 mV induced fast transient outward currents. The activation and inactivation time constants of the current were voltage dependent, and at +40 mV were 3.6 +/- 0.8 ms and 23.9 +/- 6.4 ms (n = 4), respectively. The steady-state inactivation of the transient outward current was steeply voltage dependent (z = 1.7), with 50% of the current inactivated at -55 mV. The current was insensitive to the A-type K+ channel blocker 4-AP (1-5 mM), and was modulated by external Ca, decreasing to approximately 0.85 of control values upon raising Ca2+ from 1 to 10 mM, and increasing approximately 3-fold upon lowering it to 0.1 mM. Transient outward currents were also recorded following replacement of internal K+ with either Na+ or Cs+, raising the possibility that the current was carried by monovalent ions passing through voltage-gated Ca2+ channels. This hypothesis was supported by the finding that the transient outward current had the same inactivation rate as the inward Ba2+ current, and that both currents were effectively blocked by the L-type Ca2+ channel blocker, nifedipine and enhanced by the agonist BAYK8644.     

Effects of tetraethylammonium on potassium currents in a molluscan neurons

The effects of tetraethylammonium (TEA) on the delayed K+ current and on the Ca2+-activated K+ current of the Aplysia pacemaker neurons R-15 and L-6 were studied. The delayed outward K+ current was measured in Ca2+-free ASW containing tetrodotoxin (TTX), using brief depolarizing clamp pulses. External TEA blocks the delayed K+ current reversibly in a dose-dependent manner. The experimental results are well fitted with a Michaelis-Menten expression, assuming a one-to-one reaction between TEA and a receptor site, with an apparent dissociation constant of 6.0 mM. The block depends on membrane voltage and is reduced at positive membrane potentials. The Ca2+-activated K+ current was measured in Ca2+-free artificial seawater (ASW) containing TTX, using internal Ca2+ ion injection to directly activate the K+ conductance. External TEA and a number of other quaternary ammonium ions block the Ca2+-activated K+ current reversibly in a dose-dependent manner. TEA is the most effective blocker, with an apparent dissociation constant, for a one-to-one reaction with a receptor site, of 0.4 mM. The block decreases with depolarization. The Ca2+-activated K+ current was also measured after intracellular iontophoretic TEA injection. Internal TEA blocks the Ca2+-activated K+ current (but the block is only apparent at positive membrane potentials), is increased by depolarization, and is irreversible. The effects of external and internal TEA can be seen in measurements of the total outward K+ current at different membrane potentials in normal ASW.     

Electrophysiology of cardiac myocytes of Aplysia brasiliana

The electrical properties of Aplysia brasiliana myogenic heart were evaluated. Two distinct types of action potentials (APs) were recorded from intact hearts, an AP with a slow rising phase followed by a slow repolarizing phase and an AP with a 'fast' depolarizing phase followed by a plateau. Although these two APs differ in their rates of depolarization (2.2 x 0.3 V/s), both APs were abolished by the addition of Co2+, Mn2+ and nifedipine or by omitting Ca2+ from the external solution. These data suggest that a Ca2+ inward current is responsible for the generation of both types of APs. Two outward currents activated at -40 mV membrane potential were prominent in isolated cardiac myocytes: a fast activating, fast inactivating outward current similar to the A-type K+ current and a slow activating outward current with kinetics similar to the delayed rectifier K+ current were recorded under voltage clamp conditions. Based on the effects of 4-AP and TEA on the electrical properties of ventricular myocytes, we suggest that the fast kinetic outward current substantially attenuates the peak values of the APs and that the slow activating outward current is involved on membrane repolarization.     

Activation of three types of membrane currents by various divalent cations in identified molluscan pacemaker neurons

We investigated membrane currents activated by intracellular divalent cations in two types of molluscan pacemaker neurons. A fast and quantitative pressure injection technique was used to apply Ca2+ and other divalent cations. Ca2+ was most effective in activating a nonspecific cation current and two types of K+ currents found in these cells. One type of outward current was quickly activated following injections with increasing effectiveness for divalent cations of ionic radii that were closer to the radius of Ca2+ (Ca2+ greater than Cd2+ greater than Hg2+ greater than Mn2+ greater than Zn2+ greater than Co2+ greater than Ni2+ greater than Pb2+ greater than Sr2+ greater than Mg2+ greater than Ba2+). The other type of outward current was activated with a delay by Ca2+ greater than Sr2+ greater than Hg2+ greater than Pb2+. Mg2+, Ba2+, Zn2+, Cd2+, Mn2+, Co2+, and Ni2+ were ineffective in concentrations up to 5 mM. Comparison with properties of Ca2(+)-sensitive proteins related to the binding of divalent cations suggests that a Ca2(+)-binding protein of the calmodulin/troponin C type is involved in Ca2(+)-dependent activation of the fast-activated type of K+ current. Th sequence obtained for the slowly activated type is compatible with the effectiveness of different divalent cations in activating protein kinase C. The nonspecific cation current was activated by Ca2+ greater than Hg2+ greater than Ba2+ greater than Pb2+ greater than Sr2+, a sequence unlike sequences for known Ca2(+)-binding proteins.     

Intracellular calcium and the control of neuronal pacemaker activity

Pacemaker activity of the Aplysia bursting pacemaker neuron R-15 was analyzed. It was shown that the free intracellular Ca2+ concentration, as measured by arsenazo III, increases during the depolarizing phase of the pacemaker cycle and declines throughout the hyperpolarizing phase that follows. This increase in Ca2+ results from the activation of voltage-dependent Ca2+ channels that open during the depolarizing phase of the cycle. The extracellular K+ concentration also increases during the depolarizing phase of the cycle and is correlated with an outward K+ current that opposes the inward current carried by Ca2+ ions. The increase in internal Ca2+ is sufficient to activate a K+ conductance that depends on the magnitude of the change in internal Ca2+ and on membrane potential, which is responsible for the hyperpolarizing phase of the cycle. It is proposed that the membrane oscillation depends on three separate but linked systems, which include a voltage-dependent Ca2+ channel, the internal Ca2+ concentration, and a Ca2+-activated K+ channel.     

Depolarization of macrophage polykaryons in the absence of external sodium induces a cyclic stimulation of a calcium-activated potassium conductance

Macrophage polykaryons associated with the foreign body granuloma display several electrophysiological properties when studied with intracellular microelectrodes. One of the most evident properties is the slow hyperpolarization (2-5 s long, 10-60 mV amplitude), due to transient openings of Ca2+-dependent K+ channels, that is similar to those observed in macrophages. How this oscillation of membrane potential is triggered is not well known and the only way to repeatedly activate it under experimental control is through the intracellular injection of Ca2+. Although this technique is important for understanding the properties of the K+ channels, no information has been obtained about the way Ca2+ levels are raised and controlled in the cytosol. Slow hyperpolarizations can also be triggered by electrical stimulation, but reproducibility is low with cells bathed in physiological solutions. We then decided to investigate the effect of depolarization on the electrophysiological properties of macrophage polykaryons exposed to bathing solutions of several ionic compositions. We show in this paper that cell membrane depolarization induced by a long current pulse can trigger several patterns of membrane potential changes and that, in the absence of extracellular Na+, repetitive oscillations of decaying amplitudes are observed in almost all the cells. They are very similar to the slow hyperpolarizations, are dependent on the presence of extracellular Ca2+, and are blocked by quinine and D-600. Whole-cell patch clamp recording under voltage clamp conditions showed an outward current that oscillates and that also exhibits decaying amplitudes. The data presented here indicate that these oscillations are a consequence of the cyclic opening of the Ca2+-activated K+ channels and support the hypothesis that favors the participation of Ca2+ channels and of the Ca2+/Na+ exchange system in their triggering. These two mechanisms are not enough to explain either why the K+ channels close or why the membrane potential returns to the original level at the end of each cycle. The possibility of using these oscillations as a model to study the slow hyperpolarization is discussed.     

Elevation of a potassium current in differentiating human leukemic (HL-60) cells

Human promyelocytic leukemia (HL-60) cells display a novel voltage-dependent outward current under voltage clamp. This current is present at low levels in the proliferative state and in granulocytes derived from HL-60 cells which were induced to differentiate with retinoic acid. It is elevated in macrophages derived from HL-60 cells after exposure to phorbol-12-myristate-13-acetate (PMA). The current is carried primarily by K+, is blocked by Cs+ and by increased intracellular concentrations of Cl-. From a holding potential of -80 mV, significant activation required depolarization to +20 mV membrane potential. Activation was not influenced by intracellular Ca2+ (1-2 X 10(-6) M). These properties appear to differ significantly from the Ca2+-activated K+ channel and the delayed rectifier. The increase of this voltage-activated current in differentiation toward the macrophage, but not the granulocyte, suggests that this current is correlated specifically with macrophage differentiation.     

Calcium- and voltage-dependent ion channels in Saccharomyces cerevisiae.

Ion channels in both the tonoplast and the plasma membrane of Saccharomyces cerevisiae have been characterized at the single channel level by patch-clamp techniques. The predominant tonoplast channel is cation selective, has an open-channel conductance of 120 pS in 100 mM KCl, and conducts Na+ or K+ equally well, and Ca2+ to a lesser extent. Its open probability (Po) is voltage-dependent, peaking at about -80 mV (cytoplasm negative), and falling to near zero at +80 mV. Elevated cytoplasmic Ca2+, alkaline cytoplasmic pH, and reducing agents activate the channel. The predominant plasma membrane channel is highly selective for K+ over anions and other cations, and shows strong outward rectification of the time-averaged current-voltage curves in cell-attached experiments. In isolated inside-out patches with micromolar cytoplasmic Ca2+, this channel is activated by positive going membrane voltages: mean Po is zero at negative membrane voltages and near unity at 100 mV. At moderate positive membrane voltages (20-40 mV), elevating cytoplasmic Ca2+ activates the channel to open in bursts of several hundred milliseconds duration. At higher positive membrane voltages, however, elevating cytoplasmic Ca2+ blocks the channel in a voltage-dependent fashion for periods of 2-3 ms. The frequency of these blocking events depends on cytoplasmic Ca2+ and membrane voltage according to second-order kinetics. Alternative cations, such as Mg2+ or Na+, block the yeast plasma-membrane K+ channel in a similar but less pronounced manner.     

Voltage-dependent ionic currents in taste receptor cells of the larval tiger salamander

Voltage-dependent membrane currents of cells dissociated from tongues of larval tiger salamanders (Ambystoma tigrinum) were studied using whole-cell and single-channel patch-clamp techniques. Nongustatory epithelial cells displayed only passive membrane properties. Cells dissociated from taste buds, presumed to be gustatory receptor cells, generated both inward and outward currents in response to depolarizing voltage steps from a holding potential of -60 or -80 mV. Almost all taste cells displayed a transient inward current that activated at -30 mV, reached a peak between 0 and +10 mV and rapidly inactivated. This inward current was blocked by tetrodotoxin (TTX) or by substitution of choline for Na+ in the bath solution, indicating that it was a Na+ current. Approximately 60% of the taste cells also displayed a sustained inward current which activated slowly at about -30 mV and reached a peak at 0 to +10 mV. The amplitude of the slow inward current was larger when Ca2+ was replaced by Ba2+ and it was blocked by bath applied CO2+, indicating it was a Ca2+ current. Delayed outward K+ currents were observed in all taste cells although in about 10% of the cells, they were small and activated only at voltages more depolarized than +10 mV. Normally, K+ currents activated at -40 mV and usually showed some inactivation during a 25-ms voltage step. The inactivating component of outward current was not observed at holding potentials more depolarized -40 mV. The outward currents were blocked by tetraethylammonium chloride (TEA) and BaCl2 in the bath or by substitution of Cs+ for K+ in the pipette solution. Both transient and noninactivating components of outward current were partially suppressed by CO2+, suggesting the presence of a Ca2(+)-activated K+ current component. Single-channel currents were recorded in cell-attached and outside-out patches of taste cell membranes. Two types of K+ channels were partially characterized, one having a mean unitary conductance of 21 pS, and the other, a conductance of 148 pS. These experiments demonstrate that tiger salamander taste cells have a variety of voltage- and ion-dependent currents including Na+ currents, Ca2+ currents and three types of K+ currents. One or more of these conductances may be modulated either directly by taste stimuli or indirectly by stimulus-regulated second messenger systems to give rise to stimulus-activated receptor potentials. Others may play a role in modulation of neurotransmitter release at synapses with taste nerve fibers.(ABSTRACT TRUNCATED AT 400 WORDS)     

Transmembrane outward hydrogen current in intracellularly perfused neurones of the snail Helix pomatia

The ionic nature and pharmacological properties of the outward current activated by membrane depolarization were studied on isolated neurones of the snail Helix pomatia, placed in Na+- and Ca2+-free extracellular solutions and intracellularly perfused with K+-free solution ("nonspecific outward current"). It was shown that the amplitude and reversal potential of this current (estimated from instantaneous current-voltage characteristics) are determined mainly by the transmembrane gradient for H+ ions. Lowering of pHi induced an increase in the current amplitude and a shift of the reversal potential to more negative values; the shift magnitude was comparable with that predicted for the hydrogen electrode. Raising pHi, as well as lowering pHo, induced a decrease in the current amplitude and a displacement of the current activation curve to more positive potentials. Addition of EGTA (8 mmol/l) to the intracellular perfusate did not affect the current amplitude. Extracellular 4-aminopyridine (10 mmol/l), verapamil (0.25 mmol/l) or Cd2+ (0.5 mmol/l) blocked the current. It is concluded that the current studied is carried mainly by H+ ions. In the same neurones the nature of the fast decay of the calcium inward current was also studied (in the presence of extracellular Ca2+ ions). This decay considerably slowed when pHi was raised or pHo was lowered, and it became less pronounced upon extracellular application of 4-aminopyridine or upon intracellular introduction of phenobarbital (4 mmol/l) and tolbutamide (3 mmol/l). It is suggested that the fast decay of the calcium inward current is due to activation of a Ca-sensitive component of the hydrogen current which depends on accumulation of Ca2+ ions. The possible physiological role of the transmembrane hydrogen currents is discussed.     

High extracellular Ca2+ hyperpolarizes human parathyroid cells via Ca(2+)-activated K+ channels

Membrane potential has a major influence on stimulus-secretion coupling in various excitable cells. The role of membrane potential in the regulation of parathyroid hormone secretion is not known. High K+-induced depolarization increases secretion from parathyroid cells. The paradox is that increased extracellular Ca2+, which inhibits secretion, has also been postulated to have a depolarizing effect. In this study, human parathyroid cells from parathyroid adenomas were used in patch clamp studies of K+ channels and membrane potential. Detailed characterization revealed two K+ channels that were strictly dependent of intracellular Ca2+ concentration. At high extracellular Ca2+, a large K+ current was seen, and the cells were hyperpolarized (-50.4 +/- 13.4 mV), whereas lowering of extracellular Ca2+ resulted in a dramatic decrease in K+ current and depolarization of the cells (-0.1 +/- 8.8 mV, p < 0.001). Changes in extracellular Ca2+ did not alter K+ currents when intracellular Ca2+ was clamped, indicating that K+ channels are activated by intracellular Ca2+. The results were concordant in cell-attached, perforated patch, whole-cell and excised membrane patch configurations. These results suggest that [Ca2+]o regulates membrane potential of human parathyroid cells via Ca2+-activated K+ channels and that the membrane potential may be of greater importance for the stimulus-secretion coupling than recognized previously.     

Voltage-dependent calcium and calcium-activated potassium currents of a molluscan photoreceptor

Two-microelectrode voltage clamp studies were performed on the somata of Hermissenda Type B photoreceptors that had been isolated by axotomy from all synaptic interaction as well as any impulse-generating (i.e., active) membrane. In the presence of 2-10 mM 4-aminopyridine (4-AP) and 100 mM tetraethylammonium ion (TEA), which eliminated two previously described voltage-dependent potassium currents (IA and the delayed rectifier), a voltage-dependent outward current was apparent in the steady state responses to command voltage steps more positive than -40 mV (absolute). This current increased with increasing external Ca++. The magnitude of the outward current decreased and an inward current became apparent following EGTA injection. Substitution of external Ba++ for Ca++ also made the inward current more apparent. This inward current, which was almost eliminated after being exposed for approximately 5 min to a solution in which external Ca++ was replaced with Cd++, was maximally activated at approximately 0 mV. Elevation of external potassium allowed the calcium (ICa++) and calcium-dependent K+ (IC) currents to be substantially separated. Command pulses to 0 mV elicited maximal ICa++ but no IC because no K+ currents flowed at their new reversal potential (0 mV) in 300 mM K+. At a holding potential of -60 mV, which was now more negative than the potassium equilibrium potential, EK+, in 300 mM K+, IC appeared as an inward tail current after positive command steps. The voltage dependence of ICa++ was demonstrated with positive steps in 100 mM Ba++, 4-AP, and TEA. Other data indicated that in 10 mM Ca++, IC underwent pronounced and prolonged inactivation whereas ICa++ did not. When the photoreceptor was stimulated with a light step (with the membrane potential held at -60 mV), there was also a prolonged inactivation of IC. In elevated external Ca++, ICa++ also showed similar inactivation. These data suggest that IC may undergo prolonged inactivation due to a direct effect of elevated intracellular Ca++, as was previously shown for a voltage-dependent potassium current, IA. These results are discussed in relation to the production of training-induced changes of membrane currents on retention days of associative learning.     

Complexation and ionophoric properties of taxol and colchicine: complex formation and transport of sodium, potassium, magnesium and calcium ions across a liquid membrane

We report the activities of taxol (an anticancer drug) and colchicine, which are inhibitors of microtubule organization, on the complexation and transport of Na+, K+, Mg2+ and Ca2+ ions across a liquid membrane, using a spectrophotometric procedure. Taxol, a diterpenoid compound, that has been demonstrated to possess a potent antitumour activity, is shown to extract Na+, K+, Mg2+ and Ca2+ ions from the aqueous solution to the organic phase with preference for Ca2+ ions. A kinetic study of the transport and complexation of Na+, K+, Mg2+ and Ca2+ ions through a liquid membrane revealed that the K+ ion is more rapidly transported and the Ca2+ ion is more rapidly complexed than other ions. However, colchicine, another alkaloid compound, extracted and transported only the divalent ions tested, Mg2+ and Ca2+. In both complexation and transport, the flux of the ions increases with the concentration of taxol or colchicine. Complexation and ionophoric properties of taxol and colchicine sheds new lights on therapeutic properties of these drugs. The treatment of disease states by the administration of these drugs to alter membrane permeability will prove to be a valuable therapeutic concept.     

Ionic currents in cardiac muscle: a framework for glycoside action.

This paper briefly reviews the current state of understanding of cardiac excitation--contraction coupling and its relation to glycoside action. Evidence that inotropic action of glycosides might result from increased influx of Ca2+ during action potential is reviewed. Recent voltage clamp studies that show little if any direct effect on Ca2+ influx during the action potential are cited. It is suggested that the primary inotropic effects derive from altered ionic exchange mechanisms secondary to inhibition of Na+,K+-ATPase. The role of ionic currents in glycoside toxicity is considered, with discussion of a dynamic, depolarizing current that appears shortly after action potential. This current is apparently an inward movement of positive ions that is strongly mediated by extracellular Ca2+ levels. It is noted that such spontaneous depolarizations of the membrane have been observed in several other circumstances where strong positive inotropism has been induced. The conclusion is reached that membrane ionic currents probably play only a secondary role in glycoside inotropism and in many of the toxic effects.     

Physiological basis of a steady endogenous current in rat lumbrical muscle

In an attempt to determine the mechanism by which rat skeletal muscle endplates generate a steady outward current, we measured the effects of several drugs (furosemide, bumetanide, 9-anthracene carboxylic acid [9-AC]) and changes in external ion concentration (Na+, K+, Cl-, Ba++) on resting membrane potential (Vm) and on the steady outward current. Each of the following treatments caused a 10-15-mV hyperpolarization of the membrane: replacement of extracellular Cl- with isethionate, addition of furosemide or bumetanide, and addition of 9-AC. These results suggest that Cl- is actively accumulated by the muscle fibers and that the equilibrium potential of Cl- is more positive than the membrane potential. Removal of external Na+ also caused a large hyperpolarization and is consistent with evidence in other tissues that active Cl- accumulation requires external Na+. The same treatments greatly reduced or abolished the steady outward current, with a time course that paralleled the changes in Vm. These results cannot be explained by a model in which the steady outward current is assumed to arise as a result of a nonuniform distribution of Na+ conductance, but they are consistent with models in which the steady current is produced by a nonuniform distribution of GCl or GK. Other treatments (Na+-free and K+-free solutions, and 50 microM BaCl2) caused a temporary reversal of the steady current. Parallel measurements of Vm suggested that in none of these cases did the electrochemical driving force for K+ change sign, which makes it unlikely that the steady current arises as a result of a nonuniform distribution of GK. All of the results, however, are consistent with a model in which the steady outward current arises as a result of a nonuniform distribution of Cl- conductance, with GCl lower near the endplate than in extrajunctional regions.     

Ion currents and membrane domains in the cleaving Xenopus egg

We used an extracellular vibrating probe to measure ion currents through the cleaving Xenopus laevis egg. Measurements indicate sharp membrane heterogeneities. Current leaves the first cleavage furrow after new, unpigmented membrane is inserted. This outward current may be carried by K+ efflux. No direct involvement of the Na+,K+-ATPase in the generation of this outward current is detected at first cleavage. Inward current enters the old, pigmented membrane; however, it does not enter uniformly. The inward current is largest at the old membrane bordering the new membrane. This suggests a heterogeneous ion channel distribution within the old membrane. Experiments suggest that the inward current may be carried by Na+ influx, Ca2+ influx, and Cl- efflux. No steady currents were detected during grey crescent formation, the surface contraction waves preceding cleavage, or with groove formation at the beginning of cleavage.     

Thyrotropin-releasing hormone-induced rise in cytosolic calcium and activation of outward K+ current monitored simultaneously in individual GH3B6 pituitary cells

Thyrotropin-releasing hormone (TRH) acts on pituitary cells to raise the cytosolic free Ca2+ concentration ([Ca2+]i) and causes simultaneously a transient hyperpolarization of the plasma membrane. The combination of the microfluorimetric monitoring of [Ca2+]i with electrophysiological recordings obtained using the patch clamp technique in its whole cell configuration, allows the analysis of the correlation between changes in [Ca2+]i and the alterations in ionic currents at the plasma membrane. It was shown that in the absence of hormone stimulation, a depolarization-induced change in steady state [Ca2+]i, as well as the internal perfusion with Ca2+ at microM levels at constant membrane potential led to the activation of outward K+ current. TRH stimulation resulted in a marked but transient rise in [Ca2+]i; concomitantly, there was an increase in membrane conductance and an enhancement of outward current. During the time course of an individual response, an excellent correlation between the changes in [Ca2+]i and those in conductance or current was observed. The relative changes of current and conductance during the TRH response were consistent with the activation of a single type of ionic current, the apparent reversal potential of which coincided with the equilibrium potential for K+. A strong correlation between the TRH-induced changes in [Ca2+]i and K+, conductance was demonstrated in a large number of cells with varied kinetic features: significant correlation coefficients were found both for the transition time from basal to maximal values (r = 0.85, p less than 0.001) as well as for the total duration of the responses (r = 0.68, p less than 0.002). It is concluded that during the early phase of TRH action, the hormone-induced rise in [Ca2+]i is the principal cause of enhanced K+ channel activation.