A large conductance ion channel in the nuclear envelope of a higher plant cell

Studying Proton Gradients Across the Nuclear Envelope

The existence of nuclear pore complexes in the nuclear envelope has led to the assumption that ions move freely from the cytosol into the nucleus, and that the molecular mechanisms at the plasma membrane that regulate cytosolic pH also regulate nuclear pH. Furthermore, studies to measure pH in the nucleus have produced contradictory results, since it has been found that the nuclear pH is either similar to the cytosol or more alkaline than the cytosol. However, most studies of nuclear pH have lacked the rigor needed to understand pH regulation in the nucleus. A major problem has been the lack of in situ titrations in the nucleus and cytosol, since the intracellular environment is different in the cytosol and nucleus and the behavior of fluorescent pH probes is different in these environments. Here we present a method that uses the fluorescence of SNARF-1 that labels both cytosol and nucleus. Using ratio imaging microscopy, regions of interest corresponding to the nucleus and cytosol to perform steady-state pH measurements followed by in situ titrations, to correctly assign pH in those cellular domains.

A Pan-plant Protein Complex Map Reveals Deep Conservation and Novel Assemblies

Plants are foundational for global ecological and economic systems, but most plant proteins remain uncharacterized. Protein interaction networks often suggest protein functions and open new avenues to characterize genes and proteins. We therefore systematically determined protein complexes from 13 plant species of scientific and agricultural importance, greatly expanding the known repertoire of stable protein complexes in plants. By using co-fractionation mass spectrometry, we recovered known complexes, confirmed complexes predicted to occur in plants, and identified previously unknown interactions conserved over 1.1 billion years of green plant evolution. Several novel complexes are involved in vernalization and pathogen defense, traits critical for agriculture. We also observed plant analogs of animal complexes with distinct molecular assemblies, including a megadalton-scale tRNA multi-synthetase complex. The resulting map offers a cross-species view of conserved, stable protein assemblies shared across plant cells and provides a mechanistic, biochemical framework for interpreting plant genetics and mutant phenotypes.

Characterization of Single-Nucleus Electrical Properties by Microfluidic Constriction Channel

As key bioelectrical markers, equivalent capacitance (C_ne, i.e., capacitance per unit area) and resistance (R_ne, i.e., resistivity multiply thickness) of nuclear envelopes have emerged as promising electrical indicators, which cannot be effectively measured by conventional approaches. In this study, single nuclei were isolated from whole cells and trapped at the entrances of microfluidic constriction channels, and then corresponding impedance profiles were sampled and translated into single-nucleus C_ne and R_ne based on a home-developed equivalent electrical model. C_ne and R_ne of A549 nuclei were first quantified as 3.43 ± 1.81 μF/cm² and 2.03 ± 1.40 Ω·cm² (N_n = 35), which were shown not to be affected by variations of key parameters in nuclear isolation and measurement. The developed approach in this study was also used to measure a second type of nuclei, producing C_ne and R_ne of 3.75 ± 3.17 μF/cm² and 1.01 ± 0.70 Ω·cm² for SW620 (N_n = 17). This study may provide a new perspective in single-cell electrical characterization, enabling cell type classification and cell status evaluation based on bioelectrical markers of nuclei.

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.

Dynamic protoneural networks in plants: a new approach of spontaneous extracellular potential variations

Taking as a basis of discussion Kalanchoe's spontaneous and evoked extracellular activities recorded at the whole plant level, we put the challenging questions: do these low-voltage variations, together with endocellular events, reflect integrative properties and complex behavior in plants? Does it reflect common perceptive systems in animal and plant species? Is the ability of plants to treat short-term variations and information transfer without nervous system relevant? Is a protoneural construction of the world by lower organisms possible? More generally, the aim of this paper is to reevaluate the probably underestimated role of plant surface potentials in the plant relation life, carefully comparing the biogenesis of both animal and plant organisms in the era of plant neurobiology. Knowing that surface potentials participate at least to morphogenesis, cell to cell coupling, long distance transmission and transduction of stimuli, some hypothesis are given indicating that plants have to be studied as environmental biosensors and non linear dynamic systems able to detect transitional states between perception and response to stimuli. This study is conducted in the frame of the "plasticity paradigm," which gives a theoretical model of evolutionary processes and suggests some hypothesis about the nature of complexity, information and behavior.

Ion channels at the nucleus: electrophysiology meets the genome

The nuclear envelope is increasingly viewed from an electrophysiological perspective by researchers interested in signal transduction pathways that influence gene transcription and other processes in the nucleus. Here, we describe evidence for ion channels and transporters in the nuclear membranes and for possible ion gating by the nuclear pores. We argue that a systems-level understanding of cellular regulation is likely to require the assimilation of nuclear electrophysiology into molecular and biochemical signaling pathways.

Nuclear membrane ion channels mediate root nodule development

Previous work has implicated two predicted ion channels in mediating perinuclear calcium spiking, which is essential for rhizobia-induced root nodule formation in legumes. A new study demonstrates that these ion channels are preferentially permeable to cations, such as potassium, and are located in the nuclear envelope. Here, we consider ways in which the ion channels influence perinuclear calcium spiking and discuss a potentially broader role for nuclear membrane ion channels in signal transduction in plants.

Nucleocytoplasmic Trafficking in Plant Cells

In a eukaryotic cell, the nuclear envelope (NE) separates genetic information from the environment of biosynthesis and metabolism. Transfer of macromolecules across the NE involves the nuclear pores--large multisubunit protein complexes--and machinery that facilitates rapid, directional, and selective transport. While core elements of the transport process are conserved between kingdoms, different solutions to similar problems have also evolved. Although the structure and composition of the yeast and mammalian nuclear pore have been unraveled recently, the plant nuclear pore remains largely enigmatic. Like any other process, nucleocytoplasmic transport can be regulated. Several examples from plants are discussed that promise insights into the regulation of signaling pathways. While controlling the partitioning of cellular components, the nuclear envelope also presents an obstacle to viruses and transforming agents that need access to the genome, and different mechanisms have evolved to overcome this obstacle. Finally, the recent recognition of the importance of small RNAs for gene regulation emphasizes the need to understand small RNA nuclear export and the levels of its regulation. This review attempts to wed our molecular-mechanistic understanding of nucleocytoplasmic trafficking drawn from all model systems with the intriguing examples of regulated nucleocytoplasmic partitioning in plants.

Electrical dimension of the nuclear envelope

Eukaryotic chromosomes are confined to the nucleus, which is separated from the rest of the cell by two concentric membranes known as the nuclear envelope (NE). The NE is punctuated by holes known as nuclear pore complexes (NPCs), which provide the main pathway for transport of cellular material across the nuclear-cytoplasmic boundary. The single NPC is a complicated octameric structure containing more than 100 proteins called nucleoporins. NPCs function as transport machineries for inorganic ions and macromolecules. The most prominent feature of an individual NPC is a large central channel, ~7 nm in width and 50 nm in length. NPCs exhibit high morphological and functional plasticity, adjusting shape to function. Macromolecules ranging from 1 to >100 kDa travel through the central channel into (and out of) the nucleoplasm. Inorganic ions have additional pathways for communication between cytosol and nucleus. NE can turn from a simple sieve that separates two compartments by a given pore size to a smart barrier that adjusts its permeabiltiy to the metabolic demands of the cell. Early microelectrode work characterizes the NE as a membrane barrier of highly variable permeability, indicating that NPCs are under regulatory control. Electrical voltage across the NE is explained as the result of electrical charge separation due to selective barrier permeability and unequal distribution of charged macromolecules across the NE. Patch-clamp work discovers NE ion channel activity associated with NPC function. From comparison of early microelectrode work with patch-clamp data and late results obtained by the nuclear hourglass technique, it is concluded that NPCs are well-controlled supramolecular structures that mediate transport of macromolecules and small ions by separate physical pathways, the large central channel and the small peripheral channels, respectively. Electrical properties of the two pathways are still unclear but could have great impact on the understanding of signal transfer across NE and gene expression.

Patch-clamp study of liver nuclear ionic channels reconstituted into giant proteoliposomes

Nuclear ionic channels (NICs) represent ubiquitous structures of living cells, although little is known about their functional properties and encoding genes. To characterize NICs, liver nuclear membrane vesicles were reconstituted into either planar lipid bilayers or proteoliposomes. Reconstitution of nuclear envelope (NE) vesicles into planar lipid bilayer proceeded with low efficiency. NE vesicle reconstitution into proteoliposomes led to NIC observations by the patch-clamp technique. Large conductance, voltage-gated, K(+)-permeant and Cl(-)-permeant NICs were characterized. An 80-105-pS K(+)-permeant NIC with conducting sub-state was also recorded. Our data establish that NICs can be characterized upon reconstitution into giant proteoliposomes and retain biophysical properties consistent with those described for native NICs.

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.

A Ca2+- and voltage-dependent cation channel in the nuclear envelope of red beet

The patch-clamp technique was applied to study ion conductances in various configurations of the nuclear envelope of non-enzyme-treated red beet (Beta vulgaris L.) nuclei. With excised patches a non-selective cation channel was observed, that was activated by micromolar concentrations of Ca2+ on the nucleoplasmic side of the envelope. The channel activity was also voltage-dependent and the voltage threshold of channel activation changed with the nucleoplasmic Ca2+ concentration. The most prominent conductance level was 110+/-22 pS with 150 mM KCl in the bath and pipette. The channel was permeable to small cations: permeabilities relative to K+ were PK congruent with PNa=1, PCs=0.3, but PCl=0.09. Calcium ions also permeated the channel with PCa=0.43, estimated from reversal potential, or 0.14, estimated from conductance ratio. Zn2+ (1 mM) when applied to the cytoplasmic side of the envelope blocked the channel activity completely, while amiloride (2 mM) reduced the channel current by 86% from the nucleoplasmic side. The properties of the whole-nucleus current (voltage-, time- and Ca2+-dependence) paralleled those observed with excised patches. The channel may provide a Ca2+-regulated pathway for passive diffusion of cations across the nuclear envelope and thus may play an important role in Ca2+-dependent nuclear processes ranging from gene transcription to apoptosis.

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.

Single-channel kinetics, inactivation, and spatial distribution of inositol trisphosphate (IP3) receptors in Xenopus oocyte nucleus

Single-channel properties of the Xenopus inositol trisphosphate receptor (IP3R) ion channel were examined by patch clamp electrophysiology of the outer nuclear membrane of isolated oocyte nuclei. With 140 mM K+ as the charge carrier (cytoplasmic [IP3] = 10 microM, free [Ca2+] = 200 nM), the IP3R exhibited four and possibly five conductance states. The conductance of the most-frequently observed state M was 113 pS around 0 mV and approximately 300 pS at 60 mV. The channel was frequently observed with high open probability (mean P(o) = 0.4 at 20 mV). Dwell time distribution analysis revealed at least two kinetic states of M with time constants tau < 5 ms and approximately 20 ms; and at least three closed states with tau approximately 1 ms, approximately 10 ms, and >1 s. Higher cytoplasmic potential increased the relative frequency and tau of the longest closed state. A novel "flicker" kinetic mode was observed, in which the channel alternated rapidly between two new conductance states: F1 and F2. The relative occupation probability of the flicker states exhibited voltage dependence described by a Boltzmann distribution corresponding to 1.33 electron charges moving across the entire electric field during F1 to F2 transitions. Channel run-down or inactivation (tau approximately 30 s) was consistently observed in the continuous presence of IP3 and the absence of change in [Ca2+]. Some (approximately 10%) channel disappearances could be reversed by an increase in voltage before irreversible inactivation. A model for voltage-dependent channel gating is proposed in which one mechanism controls channel opening in both the normal and flicker modes, whereas a separate independent mechanism generates flicker activity and voltage-reversible inactivation. Mapping of functional channels indicates that the IP3R tends to aggregate into microscopic (<1 microm) as well as macroscopic (approximately 10 microm) clusters. Ca2+-independent inactivation of IP3R and channel clustering may contribute to complex [Ca2+] signals in cells.

Intranuclear distribution, function and fate of glutathione and glutathione-S-conjugate in living rat hepatocytes studied by fluorescence microscopy

The availability of fluorescent probes to detect soluble and protein-bound thiols has made it possible to investigate some aspects of reduced glutathione (GSH) metabolism and function in intact rat hepatocytes and in hepatocyte nuclei. Monochlorobimane (BmCl) has been employed to study the subcellular compartmentation of GSH and the formation and fate of the BmCl-GSH conjugate. The occurrence of relatively high concentrations of GSH within the nuclear matrix has been inferred from fluorescence quantitation using image analysis. Concomitant biochemical studies have revealed the presence of a GSH-stimulated ATP hydrolysis and of an ATP-stimulated GSH accumulation in isolated nuclei, providing the molecular basis for nuclear glutathione compartmentation. The contemporary use of fluorescent probes to label nuclear free sulfhydryl groups, proteins and chromatin status led to the demonstration that intranuclear accumulation of glutathione may modulate the thiol/disulfide redox status of nuclear proteins and control chromatin compacting and decondensation.

Nuclear versus perinuclear and cytoplasmic calcium in osteoclasts

We measured fluorescence from the calcium indicator Fluo-3 in multinucleated osteoclasts. In the initial state, each nucleus is surrounded by a ring of bright fluorescence. Following activation of purinergic receptors by 100 microM ATP there is a pulse of cellular fluorescence increase, and nuclear fluorescence intensity becomes greater than that of the cytoplasm. This is followed by a period during which the fluorescence of the cell decreases below that of the initial state. During the pulsed increase following purinergic receptor activation, the perinuclear fluorescence intensity does not increase as much as that in the nuclear centers and, following this pulse, the perinuclear fluorescence intensity decreases more than that in the nuclear centers, relative to the initial state. Measurements in which Mn2+ was introduced into the cell show that the number of Fluo-3 molecules per unit horizontal area in the nuclear centers is slightly greater than that in the perinuclear regions, and more than twice that in the surrounding cytoplasm. These results show that there is a much higher free calcium concentration in the perinuclear regions than in the nuclear centers in the initial state, with a release of free calcium from the perinuclear regions following activation of the purinergic receptors. These data also provide evidence that the free calcium concentration in the nuclear centers is lower than in the cytoplasm in the initial state.

Nuclear ion channel activity is regulated by actin filaments

Actin filaments are novel second messengers involved in ion channel regulation. Because cytoskeletal components interact with the nuclear envelope, the actin cytoskeleton may also control nuclear membrane function. In this report, the patch-clamp technique was applied to isolated nuclei from amphibian A6 epithelial cells to assess the role of actin filaments on nuclear ion channel activity under nucleus-attached or -excised conditions. The most prevalent spontaneous nuclear ion channel species, 76% (n = 46), was cation selective and had a maximal single-channel conductance of approximately 420 pS. Nuclear ion channels also displayed multiple subconductance states, including channel activity of 26 pS that was frequently observed. Nuclear ion channel activity on otherwise quiescent patches was induced by either addition of the actin cytoskeleton disrupter cytochalasin D (CD; 5 micrograms/ml, 60%, 3 of 5 patches) or actin (10-1,000 micrograms/ml) to the bathing solution of nucleus-attached patches (59%, 13 of 22 patches). Actin also induced ion channel activity in quiescent excised inside-out patches from the nuclear envelope (80%, 4 of 5 patches). In contrast, addition of bovine serum albumin (10-1,000 micrograms/ml) to the bathing solution of nucleus-attached patches was without effect on nuclear ion channel activity (5 of 5 patches). The monoclonal antibody MAb414, specific for nuclear pore complex proteins, completely prevented either spontaneous or cytosolic actin-induced nuclear ion channels under nucleus-attached conditions (4 of 4 patches) but not intranuclear actin-induced nuclear ion channels under excised inside-out conditions (3 of 3 patches). In nucleus-attached patches, channel activity was readily activated by addition of the G-actin-binding protein deoxyribonuclease I to nucleus-attached patches (56%, 5 of 9 patches) or further addition of the actin-cross-linker filamin in the presence of actin (57%, 4 of 7 patches). The data indicate that dynamic changes in actin filament organization may represent a novel mechanism to control nuclear function.

Nuclear pore complex ion channels (review)

It is currently thought that nuclear pore complexes (NPCs) primarily govern nucleocytoplasmic interactions via selective recognition and active transport of macromolecules. However, in various nuclear preparations, patch-clamp and fluorescence, luminiscence and ion microscopy support classical microelectrode measurements indicating that monoatomic ion flow across the nuclear envelope (NE) is strictly regulated. Gating of large conductance nuclear envelope ion channels (NICs) somewhat resembles that of gap junctional channels. In other respects, NICs are distinct in that they require cytosolic factors, are blocked by wheat germ agglutinin and are blocked and/or modified by antibodies to epitopes of NPC glycoproteins. Therefore, NIC activity, recorded as electrical current/conductance is likely to be intrinsic to NPCs. This observation suggests a potential use for the patch-clamp technique in establishing the mechanisms underlying nuclear pore gating in response to cytosolic and nucleosolic factors such as transcription and growth factors, oncogene and proto-oncogene products and receptors for retinoids, steroids and thyroid hormone. NIC activity may also be useful in evaluating the mechanisms of nuclear import of foreign nucleic acid material such as that contained in virons and viroids. Finally, in consideration to the electrophysiological data accumulated so far, the study of nuclear pore ion channel activity may help our understanding of other important issues such as cell suicide, programmed cell death or apoptosis.

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.

Restricted ion flow at the nuclear envelope of cardiac myocytes

Flow of small ions across the nuclear envelope (NE) is thought to occur without restriction through large diameter nuclear pore complexes (NPCs). However, investigations with electron and fluorescence microscopy, and with patch-clamp and microelectrode electrophysiology, suggest that in many animal and plant cell types small ions move through a barrier having the signature of large conductance nuclear ion channels (NICs). As nucleocytoplasmic transport and gene activity are regulated by cytoplasmic signals and as it has recently been shown by this investigator that cardiac NICs are sensitive to cAMP-dependent processes (1), it was considered relevant to further investigate the effects of various cytosolic signals on NIC activity. Ion species substitution demonstrated that K+ is the major species responsible for NIC currents. The Na-channel blocker tetrodotoxin (TTX, 100 microM) and the Ca-channel blocker diltiazem (100 microM) had no effect, indicating no relation of NICs to Na- or Ca-channels in transit to the cell surface membrane. Zn2+ (100 microM) blocked NIC activity, suggesting a dual role in nucleocytoplasmic transport and gene function. GTP did not produce measurable effect. However, its nonhydrolyzable analogue GTP-gamma-S (10 microM) suppressed NIC activity, suggesting a role for GTP hydrolysis in NIC function. Deoxynucleotides (dNTPs, 200 microM) produced a transient increase in NIC activity, pointing to a modulation of NIC function by nucleic acid substrates. These results indicate a role for NICs in mediating: (a) control of gene activity by transduction and other cytosolic signals, and (b) nuclear demands and response to such signals.