Nuclear electrophysiology

Vacuolar H+-ATPase in the nuclear membranes regulates nucleo-cytosolic proton gradients

The regulation of the luminal pH of each organelle is crucial for its function and must be controlled tightly. Nevertheless, it has been assumed that the nuclear pH is regulated by the cytoplasmic proton transporters via the diffusion of H⁺ across the nuclear pores because of their large diameter. However, it has been demonstrated that ion gradients exist between cytosol and nucleus, suggesting that the permeability of ions across the nuclear pores is restricted. Vacuolar H⁺-ATPase (V-H⁺-ATPase) is responsible for the creation and maintenance of trans-membrane electrochemical gradient. We hypothesize that V-H⁺-ATPase located in the nuclear membranes functions as the primary mechanism to regulate nuclear pH and generate H⁺ gradients across the nuclear envelope. We studied the subcellular heterogeneity of H⁺ concentration in the nucleus and cytosol using ratio imaging microscopy and SNARF-1, a pH indicator, in prostate cells. Our results indicate that there are proton gradients across the nuclear membranes that are generated by V-H⁺-ATPase located in the outer and inner nuclear membranes. We demonstrated that these gradients are mostly dissipated by inhibiting V-H⁺-ATPase. Immunoblots and V-H⁺-ATPase activity corroborated the existence of V-H⁺-ATPase in the nuclear membranes. This study demonstrates that V-H⁺-ATPase is functionally expressed in nuclear membranes and is responsible for nuclear H⁺ gradients that may promote not only the coupled transport of substrates, but also most electrochemically driven events across the nuclear membranes. This study represents a paradigm shift that the nucleus can regulate its own pH microenvironment, providing new insights into nuclear ion homeostasis and signaling.

Endogenous voltage gradients as mediators of cell-cell communication: strategies for investigating bioelectrical signals during pattern formation

Alongside the well-known chemical modes of cell-cell communication, we find an important and powerful system of bioelectrical signaling: changes in the resting voltage potential (Vmem) of the plasma membrane driven by ion channels, pumps and gap junctions. Slow Vmem changes in all cells serve as a highly conserved, information-bearing pathway that regulates cell proliferation, migration and differentiation. In embryonic and regenerative pattern formation and in the disorganization of neoplasia, bioelectrical cues serve as mediators of large-scale anatomical polarity, organ identity and positional information. Recent developments have resulted in tools that enable a high-resolution analysis of these biophysical signals and their linkage with upstream and downstream canonical genetic pathways. Here, we provide an overview for the study of bioelectric signaling, focusing on state-of-the-art approaches that use molecular physiology and developmental genetics to probe the roles of bioelectric events functionally. We highlight the logic, strategies and well-developed technologies that any group of researchers can employ to identify and dissect ionic signaling components in their own work and thus to help crack the bioelectric code. The dissection of bioelectric events as instructive signals enabling the orchestration of cell behaviors into large-scale coherent patterning programs will enrich on-going work in diverse areas of biology, as biophysical factors become incorporated into our systems-level understanding of cell interactions.

Calcium, ATP and nuclear pore channel gating

Nuclear envelope (NE) cisternal Ca2+ and cytosolic ATP are required for nuclear-pore-complex-(NPC-) mediated transport of DNAs, RNAs, transcription factors and other large molecules. Isolated cardiomyocyte nuclei, capable of macromolecular transport (MMT), have intrinsic NPC ion channel behavior. The large ion conductance (gamma) activity of the NPC channel (NPCC) is blocked by the NPC monoclonal antibody mAb414, known to block MMT, and is also silenced during periods of MMT. In cardiomyocytes, neither cytosolic Ca2+ nor ATP alone directly affects NPCC gating. To test the role of Ca2+ and ATP in NPCC activity, we carried out the present patch-clamp study with the pipette attached to the outer NE membrane of nuclei isolated from cultured Dunning G prostate cancer cells. Our investigations demonstrate that in these isolated nuclei neither cytosolic Ca2+ nor ATP alone directly affects NPCC gating. However, when simultaneously applied to the bath and pipette, they transiently silence NPCC activity through stimulation of MMT by raising the Ca2+ concentration in the NE cisterna ([Ca2+]NE). Our fluorescence microscopy observations with nuclear-targeted macromolecular fluorochromes (B-phycoerythrin and plasmid for the enhanced green fluorescence protein EGFP, pEGFP-C1) and with FITC-labeled RNA support the view that channel silence accompanies MMT. Repeated Ca2+ loading of the NE with Ca2+ and ATP, after unloading with 1-5 microM inositol 1,4,5-trisphosphate (IP3), thapsigargin (TSG) or 5 mM BAPTA or EGTA, failed to affect channel gating. This result indicates that other factors are involved in this phenomenon and that they are exhausted during the first cycle of NE Ca2+ loading/unloading--in agreement with current theories of NPC-mediated MMT. The results explain how Ca2+ and IP3 waves may convert the NE into an effective Ca2+ barrier and, consequently, affect the regulation of gene activity and expression through their feedback on MMT and NPCC gating. Thus, [Ca2+]NE regulation by intracellular messengers is an effective mechanism for synchronizing gene activity and expression to the cellular rhythm.

Nuclear hourglass technique: an approach that detects electrically open nuclear pores in Xenopus laevis oocyte

Nuclear pore complexes (NPCs) mediate both active transport and passive diffusion across the nuclear envelope (NE). Determination of NE electrical conductance, however, has been confounded by the lack of an appropriate technical approach. The nuclear patch clamp technique is restricted to preparations with electrically closed NPCs, and microelectrode techniques fail to resolve the extremely low input resistance of large oocyte nuclei. To address the problem, we have developed an approach for measuring the NE electrical conductance of Xenopus laevis oocyte nuclei. The method uses a tapered glass tube, which narrows in its middle part to 2/3 of the diameter of the nucleus. The isolated nucleus is sucked into the narrow part of the capillary by gentle fluid movement, while the resulting change in electrical resistance is monitored. NE electrical conductance was unexpectedly large (7.9 +/- 0.34 S/cm(2)). Evaluation of NPC density by atomic force microscopy showed that this conductance corresponded to 3.7 x 10(6) NPCs. In contrast to earlier conclusions drawn from nuclear patch clamp experiments, NPCs were in an electrically "open" state with a mean single NPC electrical conductance of 1.7 +/- 0.07 nS. Enabling or blocking of active NPC transport (accomplished by the addition of cytosolic extracts or gp62-directed antibodies) revealed this large NPC conductance to be independent of the activation state of the transport machinery located in the center of NPCs. We conclude that peripheral channels, which are presumed to reside in the NPC subunits, establish a high ionic permeability that is virtually independent of the active protein transport mechanism.

Gating mechanism of the nuclear pore complex channel in isolated neonatal and adult mouse liver nuclei

Several types of ionic channels on the outer membrane of the nuclear envelope communicate with the nuclear cisternae. These are distinct from nucleocytoplasmic pathways, the nuclear pores that span the double membrane of the envelope and are the route for RNA and protein traffic in the nucleus. Recent data indicate that the nuclear pores may also function as ion channels. The most probable candidate for nucleocytoplasmic ion flux is a 300-400 pS pathway observed in many nuclear preparations. Morphological and functional studies of nuclear envelope suggest a tight relationship between the large conductance channel and the pore complex. However, there is no direct evidence for gating of the nuclear pore or its ability to open and close as a conventional channel. This study shows that in liver nuclei isolated from newborn mouse, there is a substantial correspondence between the number of pores and the number of channels recorded during patch-clamp. This is not the case for adult nuclei. Although pore density is comparable, some nuclear cytoskeletal components, such as actin and nonmuscle myosin, show a significant increase in the adult preparation. Previous studies demonstrate the presence of these two proteins in association with the pore complex. Here we show that by using actin filament disrupter, we were able to increase the number of active channels in adult isolated nuclei. We suggest that a functional interaction between actin filaments and the nuclear pore complex could regulate nucleocytoplasmic permeability.

Regulation of cytosol-nucleus pH gradients by K+/H+ exchange mechanism in the nuclear envelope of neonatal rat astrocytes

In order to study the subcellular heterogeneity of intracellular H+ concentration in reactive astrocytes, the pH in the nucleus and cytosol of cultured astrocytes was measured using a confocal laser scanning microscope (CLSM) and pH indicator dye, 5'(and 6')-carboxyseminaphthofluorescein (carboxy SNAFL-1). The change in intracellular pH was indexed by the fluorescence ratio (F535/F610) at an excitation wavelength of 514.5 nm. The in vitro fluorescence ratio increased as pH decreased. This ratio in the nucleus was significantly lower than that in the cytosol of astrocytes when perfused by HEPES-buffered Hanks' balanced salt solution (HHBSS) at pH 7.4. Acid stimulations of cells (pH 5.0) raised the fluorescence ratio in both nucleus and cytosol. However, the increase in the fluorescence ratio of the nucleus was less than that of cytosol. Treatment with a K+/H+ ionophore, nigericin (20 microM), reversibly nullified this cytosol-nucleus pH gradient. These findings suggest that a buffering mechanism(s) for maintaining of intracellular pH exists between the nucleus and cytosol, and a K+/H+ exchanger may act on the nuclear envelope to eventuate intranuclear pH maintenance in the living cells.

Patch-clamp detection of macromolecular translocation along nuclear pores

The present paper reviews the application of patch-clamp principles to the detection and measurement of macromolecular translocation along the nuclear pores. We demonstrate that the tight-seal 'gigaseal' between the pipette tip and the nuclear membrane is possible in the presence of fully operational nuclear pores. We show that the ability to form a gigaseal in nucleus-attached configurations does not mean that only the activity of channels from the outer membrane of the nuclear envelope can be detected. Instead, we show that, in the presence of fully operational nuclear pores, it is likely that the large-conductance ion channel activity recorded derives from the nuclear pores. We conclude the technical section with the suggestion that the best way to demonstrate that the nuclear pores are responsible for ion channel activity is by showing with fluorescence microscopy the nuclear translocation of ions and small molecules and the exclusion of the same from the cisterna enclosed by the two membranes of the envelope. Since transcription factors and mRNAs, two major groups of nuclear macromolecules, use nuclear pores to enter and exit the nucleus and play essential roles in the control of gene activity and expression, this review should be useful to cell and molecular biologists interested in understanding how patch-clamp can be used to quantitate the translocation of such macromolecules into and out of the nucleus.

Ion Permeability of the Nuclear Envelope

The nuclear envelope mediates nucleocytoplasmic communication. Nuclear pores transport proteins and RNA into and out of the nucleus. The pore is believed to allow free ion diffusion. Using an electrophysiological approach, we show the possible semipermeable properties of the envelope. To accomplish these functions we hypothesize a mechanism in which the pore complex acts as a molecular diaphragm.

Atomic force microscopy visualizes ATP-dependent dissociation of multimeric TATA-binding protein before translocation into the cell nucleus

The TATA-binding protein (TBP) is a universal transcription factor which plays an essential role in eukaryotic gene expression. As a karyophilic molecule, this cytosolic protein reaches its DNA-binding site through the transport channel of the nuclear pore complex. As occurs with other major cellular proteins, TBP forms multimers in solution, which is a limiting factor for nuclear translocation. While studying the nuclear translocation of TBP, we detected ATP-dependent multimerization of TBP with atomic force microscopy. In physiological solutions containing ATP, 14-molecule multimers dissociated into four-molecule multimers with a half-maximum dissociation constant of 10 microM. Electrophysiological experiments using isolated cell nuclei of cultured kidney cells revealed that TBP translocates into the cell nucleus only in the presence of ATP. When ATP was replaced with its slowly hydrolysing analogue, ATP[gamma-S] [i.e. adenosine 5'-o-(3-thiotriphosphate)], the aggregates remained intact and nuclear translocation was not possible. Taken together, our investigations suggest that TBP exhibits ATPase activity similar to that observed in relation to molecular chaperons. This activity secures physiological translocation of the transcription factor into the nucleus.

Aldosterone activates the nuclear pore transporter in cultured kidney cells imaged with atomic force microscopy

Nuclear pore complexes (NPC), located in the nuclear envelope, functionally connect the cell nucleus with the cytoplasm and serve as a crucial pathway for macromolecule exchange. A Madin-Darby canine kidney (MDCK) clone that resembles principal cells of the collecting duct was shown recently to respond to sustained aldosterone exposure with a significant increase in the NPC number per nucleus. The present study elucidates the molecular nature of the NPC pathway and its regulation by aldosterone applying atomic force microscopy. We imaged individual NPC in situ and searched for a putative so-called transporter in the NPC centre. In aldosterone-depleted cells we found numerous macromolecules docked to individual NPC waiting for translocation into the nucleoplasm (standby mode=inactive pore). In contrast, in aldosterone-treated cells NPC were frequently found free of macromolecules, indicating that the translocation process kept pace with docking under hormone-stimulated conditions (transport mode=active pore). In the NPC centre we detected a ring-like structure with a central invagination. We assume that the ring is the putative transporter and that the invagination is the channel entrance used for translocation of macromolecules. Transporters were found in open and closed configurations. In conclusion, the results provide evidence for the existence of a nuclear transporter as part of the translocation machinery of an individual NPC. Aldosterone increases the activity of the nuclear transporter and thus facilitates steroid-mediated gene expression.

Reconstitution of ionic channels from inner and outer membranes of mammalian cardiac nuclei

Recent reports suggest that the nuclear envelope possesses specific ion transport mechanisms that regulate the electrolyte concentrations within the nucleoplasm and perinuclear space. In this work, intact nuclei were isolated from sheep cardiac cells. After chromatin digestion, the nuclear envelopes were sonicated and four nuclear vesicle populations were separated by sucrose step gradients (SF1-SF4). These fractions were compared by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and their protein content was analyzed by Western blot, using lamin and SEC 61 antibodies. The lamins, which are associated with the inner nuclear membrane, were present in three fractions, SF2, SF3, and SF4, with a lower amount in SF2. The SEC 61 protein, a marker of the rough endoplasmic reticulum, was detected in small amounts in SF1 and SF2. Upon fusion of vesicles into bilayers, the activities of nuclear ionic channels were recorded in 50 mM trans/250 mM cis KCl or CsCl, pH 7.2. Two types of Cl- selective channels were recorded: a large conducting 150-180-pS channel displaying substates, and a low conducting channel of 30 pS. They were both spontaneously active into bilayers, and their open probability was poorly voltage dependent at negative voltages. Retinoic acid (10(-8) M) increases the po of the large Cl- conducting channel, whereas ATP modifies the kinetics of the low conductance anion selective channel. Our data also suggest that this anionic channel is mainly present in the SF3 and SF4 population. The presence of a 181 +/- 10 pS cation-selective channel was consistently observed in the SF2 population. The behavior of this channel was voltage dependent in the voltage range -80 to +60 mV. Furthermore, we report for the first time the activity of a channel exclusively present in the SF3 and SF4 fractions, shown to contain mainly inner membrane vesicles. This cation selective channel displays a 75-pS conductance in 50 mM trans/250 mM cis K-gluconate. It is concluded that the bilayer reconstitution technique is an attractive approach to studying the electrophysiological properties of the inner and outer membranes of the nuclear envelope.

Nuclear pH gradient in mammalian cells revealed by laser microspectrofluorimetry

Intracellular pH has been measured by laser microspectrofluorimetry, using the pH-sensitive dyes SNARF-1, SNARF-calcein and SNARF-1-dextran. By this technique it was possible to accurately determine pH in volumes as small as 0.5 × 0.5 × 1 microns 3. The probes were loaded into the cells either by diffusion of their acetoxymethylester derivatives (SNARF-1-AM, SNARF-calcein-AM) or by microinjection (SNARF-1-dextran). When the five types of cells were studied in RPMI medium, the nuclear pH was consistently found to be 0.3 to 0.5 units above that of the cytosol. Although the presence of pores in the nuclear membrane has been taken as evidence that free diffusion of ions and small molecules can occur in and out the nucleus, we conclude that the nuclear membrane of these cells presents a permeability barrier to H+. The pH gradient was not observed in cells suspended in PBS.

Visualizing life on biomembranes by atomic force microscopy

Since its invention in 1986, the atomic force microscope (AFM) has become one of the most widely used near-field microscopes. Surfaces of hard samples are imaged routinely with atomic resolution. Soft biological samples, however, are still challenging. In this brief review, the AFM technique is introduced to the experimental biologist. We discuss recent data on imaging molecular structures of biomembranes, and give detailed information on the application of the AFM with two representative examples. One is imaging plasma membrane turnover of transformed renal epithelial cells during migration in vivo, and the other is visualizing macromolecular pore complexes of the nuclear envelope of aldosterone-sensitive kidney cells.

The ion channel behavior of the nuclear pore complex

Macromolecule-conducting pores have been recently recognized as a distinct class of ion channels. The poor role of macromolecules as electrical charge carriers can be used to detect their movement along electrolyte-filled pores. Because of their negligible contribution to electrical ion currents, translocating macromolecules reduce the net conductivity of the medium inside the pore, thus decreasing the measured pore ion conductance. In the extreme case, a large translocating macromolecule can interrupt ion flow along the pore lumen, reflected as a negligible pore conductance. Therefore, ion conductance serves as a measurement of macromolecular transport, with lesser values indicating greater macromolecular translocation (in size and/or number). Such is the principle of operation of the widely used Coulter counter, an instrument for counting and sizing particles. It has long been known that macromolecules translocate across the central channel of nuclear pore complexes (NPCs). Recently, large conductance ion channel activity (100-1000 pS) was recorded from the nuclear envelope (NE) of various preparations and it was suggested that NPCs may be the source of this activity. Despite its significance to understanding the regulation of transcription, replication, mRNA export, and thus gene expression of normal and pathological states, no report has appeared demonstrating that this channel activity corresponds to ion flow along the central channel of the NPC. Here we present such a demonstration in adult mouse cardiac myocyte nuclei. In agreement with concepts introduced for macromolecule-conducting channels, our patch clamp experiments showed that ion conductance is reduced, and thus that ion flow is restricted during translocation of macromolecules containing nuclear targeting signals. Ion flow was blocked by mAb414, a monoclonal antibody raised against a major NPC glycoprotein and known to localize on the NPC channel where it blocks macromolecular transport. These results also establish patch clamp as a useful technique for the measurement of macromolecular translocation along the large central channel of the NPC and provide a basis for the design of future investigations of nuclear signaling for control of gene activity, mRNA export for gene expression, as well as other processes subservient to NPC-mediated nucleocytoplasmic exchange.

Patch clamp and atomic force microscopy demonstrate TATA-binding protein (TBP) interactions with the nuclear pore complex

The universal TATA-binding protein, TBP, is an essential component of the multiprotein complex known as transcription factor IID (TFIID). This complex, which consists of TBP and TBP-associated factors (TAFs), is essential for RNA polymerase II-mediated transcription. The molecular size of human TBP (37.7 kD) is close to the passive diffusion limit along the transport channel of the nuclear pore complex (NPC). Therefore, the possibility exists that NPCs restrict TBP translocation to the nuclear interior. Here we show for the first time, with patch-clamp and atomic force microscopy (AFM), that NPCs regulate TBP movement into the nucleus and that TBP (10(-15)-10(-10)M) is capable of modifying NPC structure and function. The translocation of TBP was ATP-dependent and could be detected as a transient plugging of the NPC channels, with a concomitant transient reduction in single NPC channel conductance, gamma, to a negligible value. NPC unplugging was accompanied by permanent channel opening at concentrations greater than 250 pM. AFM images demonstrated that the TBP molecules attached to and accumulated on the NPC cytosolic side. NPC channel activity could be recorded for more than 48 hr. These observations suggest that three novel functions of TBP are: to stabilize NPC, to force the NPC channels into an open state, and to increase the number of functional channels. Since TBP is a major component of transcription, our observations are relevant to the understanding of the gene expression mechanisms underlying normal and pathological cell structure and function.

Patch clamp detection of transcription factor translocation along the nuclear pore complex channel

Transcription factors (TFs) are cytoplasmic proteins that play an essential role in gene expression. These proteins form multimers and this phenomenon is thought to be one of the mechanisms that regulate transcription. TF molecules reach their DNA binding sites through the large central channel of the nuclear pore complex (NPC). However, the NPC channel is known to restrict the translocation of molecules > or = 20-70 kD. Therefore, during their translocation, TF molecules and/or their multimers may plug the NPC channel and thus, interrupt ion flow through the channel, with a concomitant reduction in the ion conductance of the channel (gamma). Here we show with patch clamp that gamma is reduced during translocation of three major TFs: c-Jun (40 kD), NF-kappa B (approximately equal to 50 kD), and SP1 (approximately equal to 100 kD). Within a minute, femtomolar concentrations of these proteins reduced gamma suggesting a purely mechanical interaction between single TF molecules and the inner wall of the NPC channel. NPCs remained plugged for 0.5-3 hr in the absence of ATP but when ATP was added, channel plugging was shortened to < 5 min. After unplugging, channel closures were rarely observed and the number of functional channels increased. The transcription factors also stabilized the NPCs as shown by the extended duration of the preparations which allowed recordings for up to 72 hr. These observations are the first direct demonstration of the important role of NPCs in mediating nuclear translocation of TFs and, therefore, in forming part of the mechanisms regulating gene expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium release from the nucleus by InsP3 receptor channels

The nucleus is surrounded by a double membrane separating it from the cytoplasm. The perinuclear space is continuous with endoplasmic reticulum, and the nuclear outer membrane shares many features with the reticular membrane. We now show that inositol 1,4,5-trisphosphate (InsP3) receptors associated with the nucleus release Ca2+ from isolated Xenopus laevis oocyte nuclei. Electrophysiological measurements of the intracellular InsP3 receptor in its native membrane have not been possible on the fine filamentous endoplasmic reticulum. In this paper, we directly measure InsP3-dependent receptor channels in isolated nuclei. The nuclear InsP3 receptor is activated by InsP3 and modulated by Ca2+. The channel is weakly regulated by ATP, is mildly voltage dependent, and has a greater conductance with monovalent cations than with divalent cations.

Imaging nuclear pores of aldosterone-sensitive kidney cells by atomic force microscopy

In nuclei of renal target cells, aldosterone enhances transcriptional activity followed by the translocation of specific RNA molecules across the nuclear envelope. Trafficking between cell nucleus and cytoplasm occurs via nuclear pore complexes (NPCs) located in the double-layered nuclear envelope. We investigated the nucleocytoplasmic transport route by structure-function analysis at subcellular level in quiescent and aldosterone-stimulated cells. With atomic-force microscopy (AFM) we imaged individual pores of the nuclear surface of cultured kidney cells and related the number of pores per micron2 to nuclear envelope conductance (Gn, per micron2) evaluated electrically by current injection into the isolated nucleus. NPCs were equally distributed resembling "donut-like" structures with outer diameters of 134 +/- 12 nm (n = 50), each equipped with a central channel. Six hours of aldosterone exposure (0.1 microM) increased the number of NPCs per micron 2 of nuclear surface from 7.4 +/- 0.4 to 9.8 +/- 0.4 (n = 12; P < 0.01). At the same time Gn rose from 6900 +/- 520 to 9600 +/- 610 pS/micron2 paralleled by an increase of the intranuclear electrical potential from -2.8 +/- 0.2 to -6.2 +/- 0.4 mV (n = 18; P < 0.01). Assuming that NPCs represent the sole conductive pathway in the nuclear envelope, we calculate a mean single NPC conductance of 932 and 980 pS, in the absence and presence of aldosterone, respectively. We conclude that aldosterone facilitates nucleocytoplasmic transport by increasing the number of NPCs but not by modifying their biophysical properties. Possibly, aldosterone controls similar transport mechanisms in both plasma membrane and nuclear envelope.

Relationship between [Ca2+] changes in nucleus and cytosol

The free calcium concentration in nucleus ([Ca2+]n) and in cytoplasm ([Ca2+]c) of single cells were estimated by confocal laser microscopy using the Ca(2+)-indicator Indo-1. It is shown that in various cell types a nucleo-cytosolic Ca(2+)-gradient is present at rest and during stimulation. The direction and the extent of the nucleo-cytosolic Ca(2+)-gradient may vary with the cell type, differentiation status, phosphorylation conditions and also with the type of agonist. Evidence is given for the role of extra- and intranuclear storage sites as well as for Ca(2+)-influx. Finally potential artefactual interference with the measurements is discussed.

Open states of nuclear envelope ion channels in cardiac myocytes

Prevalent nucleocytoplasmic transport theory views flow of monoatomic ions as completely unrestricted, resulting from the presence of large diameter pore complexes (NPCs) that perforate, but hold together, the two separate membranes of the nuclear envelope (NE). However, three lines of investigations indicate that, at least in some cell types, monoatomic ion flow is restricted. (i) Patch clamp reveals quantized, ion channel-like activity in several NE preparations; activity thought to result from nuclear ion channels (NICs) connected to NPCs. (ii) Ratiometric fluorescence microscopy demonstrates that ions, as well as small molecules relevant to signal transduction, do distribute as if there is a NE barrier. (iii) Electron microscopy shows that NPCs contain material that behaves like a plug. NICs' large conductance (up to 1,000 pS) makes them a major determinant of nuclear ion concentrations which, in turn, influence nuclear processes. Therefore, NICs are an important modulating force of gene and transcriptional activities--two major determinants of gene expression. As nuclear processes may take from seconds (e.g., signaling) to minutes (e.g., transcription), the time the channels dwell in the ion-conducting open state is relevant to understanding NICs' role in nuclear function. Consequently, dwell-times and lifetimes of open NIC states were studied in 61 patch-clamped adult mouse cardiac myocyte nuclei. Upon voltage stimulation, NICs opened to main states of large conductance (281 +/- 198 pS, range = 120-490 pS, n = 55) and wide-range mean dwell-times (approximately 100 msec, 1-10 sec, and min). Closed states (0 pS) also had widely distributed mean dwell-times (approximately 100 msec, 1-10 sec, and min). Putative open substates (37 +/- 11 pS, range = 25-50, pS, n = 6) of high bursting frequency (< 1 msec) were observed without intervening main states (approximately 5% of patches). Fast (approximately 0.1 msec) and slow (approximately 10 msec) state-transitions were also detected. These observations suggest a role of NICs in mediating cytoplasmic signal control of cardiomyocyte gene expression.