Membrane currents elicited by prostaglandins, atrial natriuretic factor and oxytocin in follicle-enclosed Xenopus oocytes

Epithelium and/or theca are required for ATP-elicited K+ current in follicle-enclosed Xenopus oocytes

The Xenopus follicular cell membrane is endowed with ATP-sensitive K+ channels, which are operated by various transmitters. These generate the ionic response named IK,cAMP via a mechanism that involves intracellular cAMP synthesis. It is known that opening these K+ channels favors oocyte maturation. Follicle stimulation by adenosine (Ado) or ATP consistently generates a strong IK,cAMP via activation of P1 and P3 purinergic receptors; however, ATP can also inhibit IK,cAMP, apparently acting on a third receptor type. Here, we show that IK,cAMP might be elicited by ATP released within the follicle, and that current activation by ATP was entirely dependent on the presence of epithelial and/or theca layers. Morphological studies confirmed that removal of epithelium/theca in these follicles (e.t.r.) was complete, and activation of fast Cl- (Fin) currents by ATP in e.t.r. follicles confirmed that communication between oocyte and follicular cells remained unchanged. Thus, dependence on epithelium/theca was specific for ATP-elicited K+ current. Using UTP and betagamma-MeATP as specific purinergic agents for IK,cAMP inhibition and activation, respectively, it was found that inhibition of IK,cAMP elicited by ATP or UTP was robustly present in e.t.r. follicles but was absent or strongly decreased in whole follicles (w.f.). Accordingly, this indicated that in w.f., epithelium and/or theca downregulated the IK,cAMP inhibition evoked by ATP, and that this control mechanism was absent in e.t.r. follicles. We suggest that this notable action on follicular cells involves one or both of two mechanisms, a paracrine transmitter released from epithelial and/or theca layers and action of ecto-ATPases.

Electrophysiological responses of crayfish oocytes to biogenic amines

Intracellular recordings were made from immature, growing oocytes of the crayfish Pacifastacus leniusciulus. Oocytes had a relatively negative resting potential of -74.7+/-2.2 mV (n=26; range -53 to -90) and a mean input resistance of 0.86+/-0.19 MOmega (n=22; range 0.17-3.3). Octopamine induced a long-lasting response involving biphasic changes in input resistance, together with bi- or multiphasic changes in membrane potential. The resistance-decreasing phase involved (in different oocytes) membrane hyperpolarization, depolarization or both. The resistance-increasing phase was usually a depolarization. The hyperpolarizing form of the resistance-decreasing response, and the depolarizing resistance-increasing response reversed in polarity at membrane potentials of (respectively) -90 and -92 mV, suggesting increases and decreases in K(+) conductance underly the biphasic changes in input resistance. The threshold concentration for the response was remarkably low (>10(-12) M) and showed little or no dose-dependence over the concentration range 10(-12)-10(-6) M. Similar responses were evoked by dopamine and serotonin (at 10(-9) M), although a higher proportion of oocytes responded to octopamine and/or dopamine than to serotonin.

Identification of a novel complement-dependent serum-elicited inward current in the Xenopus oocyte provoking Ca2+ influx and subsequent activation of Cl- channels

The membrane spanning complement channel is assumed to be a nonselective ion 'pore', although little evidence is available to support this hypothesis. In this paper we provide evidence that Ca2+ entry and Cl- exit occur rapidly after complement activation and precede the development of a long-lasting complement-dependent inward current. Addition of rabbit serum (a source of heterologous complement) and mouse anti-human insulin receptor antibody to a single Xenopus oocyte expressing human insulin receptor was shown to stimulate an initial hyperpolarising current followed by a sustained depolarising current. On voltage clamping the oocyte, a novel long-lasting inward current generated by serum addition was detected. Complement classical pathway-stimulated calcium influx into the oocyte was directly demonstrated using 45Ca influx measurements. In addition, we found that Ca2+ influx was required for the stimulation of the complement alternative pathway-dependent inward current. The novel conductance elicited by the classical pathway was outwardly rectifying, had a reversal potential of -35 +/- 8 mV (or -52 +/- 7 mV in the presence of chloride channel inhibitors), was inhibited by nifedipine, and was observed in the presence but not in the absence of the pore-forming complement component C9. As overactivation of complement does play a role in many inflammatory or autoimmune diseases, inhibition of early complement-mediated ion flux might restrict tissue damage and aid recovery from such diseases.

Role of natriuretic peptides in ion transport mechanisms

Natriuretic peptides (NP) act as ligands on the guanylyl cyclase family of receptors. The NP binding site on these receptors is extracellular and the guanylyl cyclase and protein kinase domains are intracellular. The guanylyl cyclase receptor catalyzes the synthesis of the second messenger molecule, cGMP, which activates protein kinase. This in turn is involved in the phosphorylation of various ion transport proteins. Ion transport proteins, which are modulated by NP and are thought to underlie the natriuretic and diuretic actions of NP, include: (a) calcium-activated K+ channels; (b) ATP-sensitive K+ channels; (c) inwardly-rectifying K+ channels; (d) outwardly-rectifying K+ channels; (e) L-type Ca2+ channels; (f) Cl- channels including cystic fibrosis transmembrane conductance regulator Cl- channels; (g) Na+- K+ 2Cl- co-transporter; (h) Na+- K+ ATPase; (i) Na+ channels; (j) stretch-activated channels; and (k) water channels. It appears that NP modulate the kinetics, rather than the conductance, of ion channels. Some of these channels, like the Ca2+, ATP-sensitive K+ and stretch-activated channels, are also involved in NP secretion. In addition, the structural properties of the NP, e.g., ovCNP-22 and ovCNP-39, appear to confer on them the ability to form ion channels. These CNP-formed ion channels can modify the trans-membrane signal transduction and second messenger systems underlying NP-induced pathological effects.

Serotonergic modulation of muscle acetylcholine receptors of different subunit composition

Modulation of muscle acetylcholine (AcCho) receptors (AcChoRs) by serotonin [5-hydroxytryptamine (5HT)] and other serotonergic compounds was studied in Xenopus laevis oocytes. Various combinations of alpha, beta, gamma, and delta subunit RNAs were injected into oocytes, and membrane currents elicited by AcCho were recorded under voltage clamp. Judging by the amplitudes of AcCho currents generated, the levels of functional receptor expression were: alpha beta gamma delta > alpha beta delta > alpha beta gamma > alpha gamma delta. The alpha beta gamma delta and alpha beta delta AcChoR Subtypes were strongly blocked by 5HT, whereas the alpha beta gamma receptor was blocked only slightly. The order of blocking potency of AcChoRs by 5HT was: alpha beta delta > alpha beta gamma delta > alpha beta gamma. 5HT receptor antagonists, such as methysergide and spiperone, were even more potent blockers of AcChoRs than 5HT but did not show much subunit selectivity. Blockage of alpha beta gamma delta and alpha beta delta receptors by 5HT was voltage-dependent, and the voltage dependence was abolished when the delta subunit was omitted. These findings may need to be taken into consideration when trying to elucidate the mode of action of many clinically important serotonergic compounds.

Activation of ATP-sensitive potassium channels in follicle-enclosed xenopus oocytes by the epileptogenic agent pentylenetetrazol

For further investigation of the epileptogenic properties of pentylenetetrazol (PTZ), membrane currents elicited by PTZ were analysed in native Xenopus oocytes. PTZ elicited a sequence of membrane currents. Two inward currents have been described to be due to a decrease in potassium permeability and an increase in chloride permeability. Experiments performed up to 3 days after preparation of the oocytes showed that PTZ is also able to activate an outward current. This current is: (1) reversed near the potassium equilibrium potential, (2) associated with a decrease in membrane resistance, (3) reduced by tetraethylammonium and caesium, (4) abolished by defolliculation, and (5) blocked by glibenclamide. Thus, the current can be interpreted to be due to an activation of ATP-sensitive potassium (KATP) channels located in the follicle cells. An activation of KATP channels by PTZ may contribute to termination and re-initiation of seizure activity.

Incorporation of acetylcholine receptors and Cl- channels in Xenopus oocytes injected with Torpedo electroplaque membranes

A method was developed to transplant assembled nicotinic acetylcholine receptors (AcChoRs) and Cl- channels from the electric organ of Torpedo to the membrane of Xenopus oocytes. Membrane vesicles from Torpedo electroplaques were injected into the oocytes and, within a few hours, the oocyte membrane acquired AcChoRs and Cl- channels. The mechanism of expression of these receptors and channels is very different from that which follows the injection of mRNA, since the appearance of receptors after membrane injection does not require de novo protein synthesis or N-glycosylation. This, and other controls, indicate that the foreign receptor-bearing membranes fuse with the oocyte membrane and cause the appearance of functional receptors and channels. All this makes the Xenopus oocyte an even more powerful tool for studies of the structure and function of membrane proteins.

A monovalent cationic conductance that is blocked by extracellular divalent cations in Xenopus oocytes

1. Native Xenopus oocytes were voltage clamped and exposed to Ringer solutions containing low concentrations of divalent cations. Oocytes, held at -60 mV, developed a reversible non-inactivating smooth inward current (Ic) associated with an increase in membrane conductance. 2. Ic was selectively carried by cations (Na+, K+), indicating that the current was not the result of a non-specific membrane breakdown, but was due instead to removal of a blocking effect of divalent cations on a specific population of endogenous ionic channels located in the oocyte membrane. 3. The blocking effects of Ca2+ and Mg2+ were voltage dependent, implying action at a binding site within the pore of the cationic channel. For example, the half-maximal inhibition (IC50) of Ic by Ca2+ was 61 microM in oocytes held at -60 mV and 212 microM in oocytes held at 0 mV. 4. The Ic channels could be unblocked by depolarization of the membrane even in the presence of physiological concentrations of Ca2+ or Mg2+. The unblocking of the channels was observed as a slowly developing outward current. 5. The novel cationic current was substantially reduced following in vitro maturation of oocytes by treatment with progesterone (10 microM, 4-5 h). 6. The physiological role of Ic channels remains to be elucidated. Nonetheless, their characteristics explain the ionic basis of the sensitivity of oocytes to reductions in extracellular divalent cations and raise the possibility that the channels play a role in calcium homeostasis.

CGRP-induced activation of KATP channels in follicular Xenopus oocytes

The two-microelectrode voltage-clamp technique was used to monitor K+ channel activity in Xenopus oocyte follicular cells, which are electrically coupled to the oocyte itself by gap junctions. Endogenous vasodilators such as calcitonin gene-related peptide (CGRP), vasoactive intestinal peptide (VIP), prostaglandin E2 (PGE2) and adenosine activate glibenclamide-ATP-sensitive K+ (KATP) channels in Xenopus oocyte follicular cells. The mechanism of action of CGRP was studied in detail. CGRP effects undergo a rapid desensitization. CGRP acts via CGRPI receptors. Its effects are antagonised by the amino-truncated CGRP analog hCGRP(8-37). The second messenger for CGRP activation of KATP channels is cAMP. Phosphodiesterase inhibition by 3-isobutyl-1-methylxanthine enhances the CGRP response while adenyl cyclase inhibition by either 2',5'-dideoxyadenosine or progesterone nearly completely depresses the CGRP response. Vasoconstrictors such as ACh and angiotensin II also have receptors in follicular cells. ACh strongly inhibits the CGRP activation of K+ channels as it inhibits the activation of KATP channels by P1060, but angiotensin II does not. It is concluded that as in vascular smooth muscle cells, CGRP and probably other hyperpolarizing vasodilators open KATP channels in follicular cells by protein kinase A activation.

Atrial natriuretic factor potentiates glibenclamide-sensitive K+ currents via the activation of receptor guanylate cyclase in follicle-enclosed Xenopus oocytes

The effect of the atrial natriuretic factor (ANF) on K+ channel opener-induced glibenclamide-sensitive K+ currents was studied using follicle-enclosed Xenopus oocytes. K+ currents induced by the K+ channel opener Y-26763 were potentiated by ANF (0.5-50 nM) in a concentration-dependent manner. 50 nM ANF increased the peak amplitude of the current by 59.4 +/- 9.9% (mean +/- S.E., n = 8). ANF (1-1000 nM) increased the cGMP contents of follicle-enclosed oocytes; about 13-fold increase was achieved by 100 nM ANF, showing a peak at 5 min. The ANF-stimulated accumulation of cGMP was suppressed by HS-142-1 (a non-peptide antagonist of the ANF receptor), at concentrations of 3-300 micrograms/ml. The K+ current-potentiating effect of ANF was mimicked by membrane-permeable cGMP (1 mM 8-bromo cGMP). These results suggest that ANF potentiates glibenclamide-sensitive K+ currents via the activation of receptor guanylate cyclase and consequent accumulation of cGMP in follicle-enclosed Xenopus oocytes.

Activation by KRN2391 and nicorandil of glibenclamide-sensitive K+ channels in Xenopus oocytes

KRN2391 (N-cyano-N'-(2-nitroxyethyl)-3-pyridine-carboximidamide methanesulfonate) and nicorandil, a new class of K+ channel openers, each with an NO2 moiety, induced outward K+ currents in follicle-enclosed Xenopus oocytes. These K+ currents were suppressed concentration-dependently and reversibly by glibenclamide, phentolamine and trifluoperazine, all known to inhibit ATP-sensitive K+ channels. The nicorandil-induced K+ current was virtually abolished by defolliculation of oocytes, while the KRN2391 response was little affected by defolliculation. These results suggest that Xenopus oocyte has at least two types of glibenclamide-sensitive K+ channels, one is selectively sensitive to KRN2391 and is probably localized in the oocyte, and the other is sensitive to nicorandil and distributed in the follicle cells surrounding an oocyte.

Inactivation of glibenclamide-sensitive K+ channels in Xenopus oocytes by various calmodulin antagonists

In follicle-enclosed Xenopus oocytes, extracellular application of cromakalim (a K+ channel opener) or intracellular injection of cAMP induces the smooth outward K+ current which is inactivated by glibenclamide. We found that cromakalim- or cAMP-induced K+ currents in the oocytes were rapidly, reversibly and dose-dependently blocked by various drugs having a calmodulin antagonizing activity in common, namely, by a selective calmodulin antagonist (W-7), antipsychotics (trifluoperazine, chlorpromazine, haloperidol), an antidepressant (amitriptyline), a beta-adrenoceptor blocker (propranolol), a local anesthetic (lidocaine) and a calcium antagonist (prenylamine). W-7, trifluoperazine, chlorpromazine and prenylamine were relatively potent blockers. For example, IC50 values to block cromakalim (100 microM)-induced K+ currents were 12 microM for trifluoperazine and 16 microM for W-7, which were close to their IC50 values to inhibit Ca2+/calmodulin-dependent phosphodiesterase (an index of the potency of calmodulin antagonists). IC50 values to inhibit cAMP (20 pmol/oocyte)-induced K+ currents were 126 microM for prenylamine and 129 microM for chlorpromazine. The IC50 values of all drugs tested to block cromakalim or cAMP responses were significantly correlated with their calmodulin-antagonizing potencies. Isoproterenol-induced K+ currents in the oocytes were also dose-dependently inhibited by glibenclamide, W-7 and trifluoperazine. These results suggest the possibility that the activity of glibenclamide-sensitive K+ channels in follicle-enclosed oocytes are regulated by calmodulin or a calmodulin-dependent process.

Opening of glibenclamide-sensitive K+ channels in follicular cells promotes Xenopus oocyte maturation

The vasorelaxing K+ channel opener P1060 (a pinacidil analog), gonadotropins, and cAMP were shown to activate a glibenclamide-sensitive 86Rb+ efflux from fully grown follicle-enclosed Xenopus oocytes. Glibenclamide-sensitive K+ channels are located in follicular cells. Glibenclamide (i) depressed the gonadotropin- but not the progesterone-induced maturation and (ii) did not significantly modify progesterone production in oocytes exposed to Xenopus gonadotropin. In follicle-enclosed oocytes, the opener P1060 very significantly enhanced the oocyte sensitivity to progesterone. This increased sensitivity to the hormone induced by the K+ channel opener was reversed by glibenclamide. Thus these results suggest that the opening of glibenclamide-sensitive K+ channels in follicular cells by gonadotropins (and other activators of this channel) induces a hyperpolarization in the oocyte that greatly facilitates maturation by increasing the oocyte sensitivity to progesterone.

A potassium current evoked by growth hormone-releasing hormone in follicular oocytes of Xenopus laevis

1. Electrophysiological properties of the growth hormone-releasing hormone (GRH) receptor were studied in Xenopus oocytes with an intact follicle cell layer (i.e. follicular oocytes) by measuring whole-cell current using the two-electrode voltage-clamp method. 2. A slow transient outward current was elicited in oocytes, clamped at -60 mV, by the application of rat GRH but not bovine, porcine, or human GRH. 3. The response to GRH was not suppressed by blockers known to inhibit other endogenous receptors present in follicular Xenopus oocytes; blockers used were timolol (2 microM; beta-adrenergic blocker), theophylline (0.1 mM; purinergic blocker) and atropine (100 nM; muscarinic blocker). 4. The current response evoked by rat GRH occurred in a dose-dependent manner. The concentrations of GRH for threshold and maximum responses were 1 and 100 nM respectively and the estimated EC50 (half-maximal effective concentration) was approximately 7 nM. The amplitude and conductance of the response became larger and the latency, time-to-peak and half-decay time were shortened when the concentration of GRH was increased. 5. The GRH response was reversibly inhibited by a K+ channel blocker, tetraethylammonium+ (TEA+; 20 mM). The reversal potential for the GRH response was around -100 mV and was compatible with the reported value for a K+ current in Xenopus oocytes. Furthermore, a depolarizing shift of 40 mV in the reversal potential was observed when the external K+ concentration was increased from 2 to 10 mM, agreeing with the Nernst equation. In contrast, no significant shift in the reversal potential was observed by changing the external concentration of Na+ or Cl-. 6. The GRH response was not suppressed in oocytes treated with an acetoxy-methyl ester of bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA/AM; 10 microM) which penetrates the cell membrane and chelates internal Ca2+. 7. The GRH response was potentiated by pre-treatment with forskolin (0.4 microM; 5 min), which stimulates adenylate cyclase and increases the internal concentration of adenosine 3',5'-cyclic monophosphate (cyclic AMP). 8. The GRH response was not obtainable when follicle cells surrounding oocytes were removed mechanically with forceps or enzymically with collagenase (i.e. denuded oocytes). The response was also suppressed when gap junctions, which electrically couple follicle cells and the oocyte, were blocked by 1-octanol (1 mM). 9. The first amino acid is considered to be important for the binding of peptide ligands to their receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Hormone-regulated K+ channels in follicle-enclosed oocytes are activated by vasorelaxing K+ channel openers and blocked by antidiabetic sulfonylureas

Follicular oocytes from Xenopus laevis contain K+ channels activated by members of the recently recognized class of vasorelaxants that include cromakalim and pinacidil and blocked by antidiabetic sulfonylureas, such as glibenclamide. These channels are situated on the adherent follicular cells and are not present in denuded oocytes. Cromakalim-activated K+ channels are also activated by increases in intracellular cAMP, and cAMP-activated K+ channels are blocked by glibenclamide. Although cromakalim and cAMP effects are synergistic, cromakalim activation of K+ channels is drastically reduced or abolished by treatments that stimulate protein kinase C (e.g., muscarinic effectors, phorbol esters). Gonadotropins, known to play an essential role in ovarian physiology, also activate cromakalim and sulfonylurea-sensitive K+ channels. Follicular oocytes constitute an excellent system for studying regulation of cromakalim-sensitive K+ channels that are important in relation to a variety of disease processes, such as cardiovascular dysfunction and asthma, as well as brain function.

Angiotensin II receptors in Xenopus oocytes

Electrical recordings were used to study the sensitivity of native Xenopus oocytes to the octapeptide angiotensin II (AII). AII elicited oscillatory currents associated with an increase in membrane conductance to Cl-. Responsiveness to AII varied greatly between oocytes taken from different frogs, and to a lesser extent between oocytes from the same ovary. Oocytes from frogs showing high sensitivity had response thresholds between 0.5-1.0 nM AII, and at a holding potential of -60 mV, responded to 1 microM AII with currents greater than 3 microA. In contrast, oocytes from some frogs gave no response, even to 10 microM AII. A total of 618 oocytes from 79 frogs were tested for sensitivity to AII, and oocytes from 85% of frogs gave detectable electrical responses. Oscillatory Cl- currents elicited by AII were largely independent of extracellular Ca2+, were abolished by chelation of intracellular Ca2+ using EGTA and were mimicked by intraoocyte injection of inositol 1,4,5-trisphosphate (IP3). In addition to oscillatory Cl- currents, AII also evoked an influx of extracellular Ca2+, giving rise to a transient inward Cl- current on membrane hyperpolarizing steps. These experiments all suggested that AII responses were elicited through activation of an intracellular messenger pathway triggered by hydrolysis of inositolphospholipids, mobilization of intracellular Ca2+ by inositol polyphosphates, and activation of Ca(2+)-gated Cl- channels. The effect of manual or enzymic defolliculation on AII responses was studied in nine separate experiments recording from 70 defolliculated oocytes. Efficacy of defolliculation procedures was assayed using scanning electron microscopy, which confirmed removal of 90 to greater than 98% of follicular cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Receptors of the serotonin 1C subtype expressed from cloned DNA mediate the closing of K+ membrane channels encoded by brain mRNA

The modulation of K+ channels by serotonin (5-HT) receptors was studied by coinjecting Xenopus oocytes with mRNA transcribed in vitro from a cloned 5-HT 1C subtype (5-HT1C) receptor gene, together with size-fractionated mRNA isolated from rat cerebral cortex that expresses K+ channels. After intracellular loading with EGTA to block Ca2(+)-dependent chloride currents, these oocytes responded to 5-HT with an inward current associated with a decrease in membrane conductance. Membrane current responses were small or absent in oocytes injected with either mRNA alone. We conclude that 5-HT1C receptors are able to cause the closing of a class of K+ channels expressed by cortex mRNA in a Ca2(+)-independent manner. The coupling between the receptors and channels appears to be mediated by the inositol phospholipid second messenger pathway, since activation of this pathway by application of serum evoked a similar closing current.

Xenopus oocyte K+ current. II. Adenylyl cyclase-linked receptors on follicle cells

Xenopus ovarian follicles consist of single large oocytes surrounded by a layer of small follicle cells that are coupled to the oocyte by gap junctions. Hyperpolarizing K+ currents can be detected in the oocytes of follicles stimulated with adenosine, isoproterenol, follicle-stimulating hormone (FSH), or microinjected adenosine 3',5'-cyclic monophosphate (cAMP). We show that cAMP accumulation can be detected in follicles incubated with the adenosine agonist 5'-N-ethylcarboxamidoadenosine (NECA), isoproterenol, or FSH, but only if forskolin and a phosphodiesterase inhibitor are also added. Treatment of follicles with collagenase has been reported to reduce, but usually not to eliminate, cAMP-activated K+ currents. In this study we show that collagenase treatment alone does not completely remove follicle cells or receptor-mediated cAMP accumulation measured in follicles. cAMP accumulation and cAMP-dependent K+ currents are both eliminated when the follicle cells are completely removed by a technique involving treatment of follicles with collagenase and hypertonic saline. Oocytes completely stripped of follicle cells fail to accumulate cAMP in response to receptor agonists and forskolin. Isolated follicle cells derived from single follicles (but without the oocyte present) accumulate cAMP in response to these drugs to an extent equivalent to the response seen in single intact follicles. Adenylyl cyclase-linked receptors of Xenopus follicles thus appear to be located exclusively on follicle cells. The data suggest that cAMP-dependent K+ currents, although measured in oocytes, may be generated in follicle cells which communicate with oocytes. Another possibility is that a high resting K+ conductance in follicle cells is communicated to oocytes via cAMP-sensitive gap junctions.

Xenopus oocyte K+ current. I. FSH and adenosine stimulate follicle cell-dependent currents

Ovarian follicles of Xenopus laevis frogs consist of a single large oocyte surrounded by follicle cells attached to the oocyte by gap junctions. Adenosine has been found to activate an outward K+ current in follicles. This response is reduced by microinjection of protein kinase inhibitor (PKI), suggesting that adenosine 3',5'-cyclic monophosphate (cAMP) mediates the response. To investigate this further, we verified previous studies that indicate that several methods of elevating cAMP in follicles activate hyperpolarizing outward currents. The potency of two adenosine analogues to hyperpolarize follicles, 5'-N-ethylcarboxamidoadenosine (NECA) greater than cyclopentyladenosine, is indicative of A2 receptors that are characteristically coupled to adenylyl cyclase. We also report for the first time that another stimulator of adenylyl cyclase, follicle-stimulating hormone (FSH), also induces a hyperpolarizing current in follicles which is carried by K+ and attenuated by injection of PKI. We used a novel procedure to completely remove follicle cells from oocytes. Intact follicles, but not oocytes completely stripped of follicle cells, hyperpolarized in response to FSH, NECA, dibutyryl cAMP, microinjected cAMP, and forskolin, but not to dideoxyforskolin (which does not activate adenylyl cyclase). Injection of the catalytic subunit of cAMP-dependent protein kinase (which is too large to traverse gap junctions) into oocytes of intact follicles failed to activate a K+ current. These data suggest that FSH and adenosine hyperpolarize follicles by stimulating adenylyl cyclase and that cAMP-dependent protein kinase must be activated on both sides of follicle cell-oocyte gap junctions to elicit a hyperpolarizing K+ current.

A serum factor that activates the phosphatidylinositol phosphate signaling system in Xenopus oocytes

Blood sera from many vertebrate species elicit large oscillatory chloride currents in oocytes from the frog Xenopus laevis. Rabbit serum was active at dilutions as great as one part in 10 million. Intracellularly applied serum was ineffective, and externally applied serum failed to trigger oscillatory currents when the intracellular level of ionized calcium was prevented from rising by loading the oocyte with EGTA. The serum also caused an increase of inositol 1,4,5-trisphosphate in the oocyte. We conclude that serum contains a factor which activates a membrane receptor that is coupled to the phosphatidylinositol second messenger system. The active factor is a protein with an apparent molecular mass of 60-70 kDa in gel permeation chromatography. Although the normal function of the serum factor is still unknown, it may have far-reaching implications, because it acts on the multifunctional phosphatidylinositol phosphate signaling system. Also, because of its great potency the serum factor and Xenopus oocytes are very useful for probing the operation of the phosphatidylinositol system.

Membrane currents elicited by divalent cations in Xenopus oocytes

1. Membrane currents were recorded from voltage-clamped Xenopus oocytes in response to bath application of various divalent cations. 2. In oocytes from 93 of 160 frogs tested, Co2+ ions evoked slow, oscillatory membrane currents. Sensitivity to Co2+ varied greatly between oocytes from different frogs, but was relatively consistent for oocytes taken from the same ovary. Oocytes with high sensitivity had response thresholds of 5-10 microM, and gave currents greater than 1 microA to 1 mM-CoCl2. In contrast, oocytes from some frogs gave no oscillatory response even to 10 mM-CoCl2. With responsive oocytes, Cd2+, Ni2+, Zn2+, Mn2+ and Cr2+ ions (5 microM to 1 mM) also elicited oscillations, whereas Sr2+, Ba2+ and Ca2+ (0.1-10 mM) showed very little activity, and Mg2+ ions, none. 3. Responses to divalent cation were well preserved in defolliculated oocytes, indicating they were generated in the oocyte membrane itself, and were not dependent on the presence of enveloping follicular cells. 4. The oscillatory currents reversed around -20 mV (the chloride equilibrium potential) and rectified strongly at potentials more negative than about -60 mV. The oscillations were mimicked by intraoocyte injection of inositol 1,4,5-trisphosphate (IP3), were largely preserved after removal of external Ca2+, but were abolished following chelation of intracellular Ca2+ by EGTA. Intraoocyte injection of Co2+ ions failed to generate oscillatory currents. 5. Currents elicited by divalent cations resembled the oocyte's oscillatory responses to acetylcholine and a serum protein. However, the response to divalent cations was not blocked by atropine and furthermore, the relative sensitivities to these agonists varied independently between oocytes from different frogs. 6. We conclude that extracellular Cd2+, Ni2+, Zn2+, Co2+, Mn2+ and Cr2+ interact with the oocyte surface to raise cytosolic levels of inositol phosphates. This causes mobilization of intracellular Ca2+, in turn activating Ca2+-gated Cl- channels in the oocyte membrane. 7. In addition to the large oscillatory currents, divalent cations generated small (5-50 nA), smooth, maintained currents associated with decreases in membrane conductance. The size and ionic basis of these currents varied between oocytes from different frogs. 8. Zinc ions also elicited smooth currents, associated with an increase in membrane conductance, and carried predominantly by K+. This response was specific to Zn2+ and occurred independently of oscillatory Cl- currents. The K+ current was abolished by defolliculation, was potentiated by the cyclic AMP phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine,and showed facilitation with K+ currents generated by the adenylate cyclase activator forskolin.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of defolliculation on membrane current responses of Xenopus oocytes

1. Catecholamines, adenosine, gonadotrophins, vasoactive intestinal peptide (VIP) and E-series prostaglandins all elicit K+ currents in follicle-enclosed Xenopus oocytes. Evidence suggests that cyclic nucleotides act as intracellular messengers in the activation of this K+ conductance. Muscarinic agonists and some divalent cations (e.g. Co2+, Mn2+, Ni2+ and Cd2+) elicit slow oscillatory Cl- currents, which are activated through hydrolysis of inositol phospholipids and mobilization of intracellular calcium by inositol phosphates. 2. We investigated whether these membrane current responses were generated in the oocyte itself or in enveloping follicular cells which are coupled to the oocyte by gap junctions. Oocytes were defolliculated, either enzymatically using collagenase, or by manual dissection combined with rolling over poly-L-lysine-coated slides. Removal of follicular cells was checked using scanning electron microscopy. Membrane current responses of defolliculated oocytes were compared with responses seen in follicle-enclosed oocytes taken from the same ovary. 3. The K+ responses evoked by all the various hormones/neurotransmitters were either drastically reduced (greater than 90%) or abolished by defolliculation. K+ currents generated by the adenylate cyclase activator forskolin and by intraoocyte injection of adenosine 3',5'-cyclic monophosphate (cyclic AMP), or guanosine 3',5'-cyclic monophosphate were similarly reduced in defolliculated oocytes. In contrast, oscillatory Cl- currents to acetylcholine and divalent cations were selectively preserved through defolliculation. 4. Injection of cyclic AMP (1-20 pmol) into defolliculated oocytes had little or no effect on oscillatory Cl- currents elicited by ACh. However, the calcium-dependent transient Cl- current, activated by depolarization of the oocyte membrane, was consistently potentiated (100-900%) by injections of cyclic AMP (1-10 pmol). 5. These experiments suggest that cyclic nucleotide-activated K+ currents arise essentially in follicular cells and are monitored within the oocyte through electrical coupling by gap junctions. Oscillatory Cl- responses evoked by ACh and divalent cations are produced largely or wholly in the oocyte itself.