Permeability of the non-selective channel in brown adipocytes to small cations

The bovine TRPV3 as a pathway for the uptake of Na+, Ca2+, and NH4+

Absorption of ammonia from the gastrointestinal tract results in problems that range from hepatic encephalopathy in humans to poor nitrogen efficiency of cattle with consequences for the global climate. Previous studies on epithelia and cells from the native ruminal epithelium suggest functional involvement of the bovine homologue of TRPV3 (bTRPV3) in ruminal NH₄⁺ transport. Since the conductance of TRP channels to NH₄⁺ has never been studied, bTRPV3 was overexpressed in HEK-293 cells and investigated using the patch-clamp technique and intracellular calcium imaging. Control cells contained the empty construct. Divalent cations blocked the conductance for monovalent cations in both cell types, with effects higher in cells expressing bTRPV3. In bTRPV3 cells, but not in controls, menthol, thymol, carvacrol, or 2-APB stimulated whole cell currents mediated by Na⁺, Cs⁺, NH₄⁺_, and K⁺, with a rise in intracellular Ca²⁺ observed in response to menthol. While only 25% of control patches showed single-channel events (with a conductance of 40.8 ± 11.9 pS for NH₄⁺ and 25.0 ± 5.8 pS for Na⁺), 90% of bTRPV3 patches showed much larger conductances of 127.8 ± 4.2 pS for Na⁺, 240.1 ± 3.6 pS for NH₄⁺, 34.0 ± 1.7 pS for Ca²⁺, and ~ 36 pS for NMDG⁺. Open probability, but not conductance, rose with time after patch excision. In conjunction with previous research, we suggest that bTRPV3 channels may play a role in the transport of Na⁺, K⁺, Ca²⁺ and NH₄⁺ across the rumen with possible repercussions for understanding the function of TRPV3 in other epithelia.

Norepinephrine-induced sustained inward current in brown fat cells: alpha(1)-mediated by nonselective cation channels

The nature of the sustained norepinephrine-induced depolarization in brown fat cells was examined by patch-clamp techniques. Norepinephrine (NE) stimulation led to a whole cell current response consisting of two phases: a first inward current, lasting for only 1 min, and a sustained inward current, lasting as long as the adrenergic stimulation was maintained. The nature of the sustained current was here investigated. It could be induced by the alpha(1)-agonist cirazoline but not by the beta(3)-agonist CGP-12177A. Reduction of extracellular Cl(-) concentration had no effect, but omission of extracellular Ca(2+) or Na(+) totally eliminated it. When unstimulated cells were studied in the cell-attached mode, some activity of approximately 30 pS nonselective cation channels was observed. NE perfusion led to a 10-fold increase in their open probability (from approximately 0.002 to approximately 0.017), which persisted as long as the perfusion was maintained. The activation was much stronger with the alpha(1)-agonist phenylephrine than with the beta(3)-agonist CGP-12177A, and with the Ca(2+) ionophore A-23187 than with the adenylyl cyclase activator forskolin. We conclude that the sustained inward current was due to activation of approximately 30 pS nonselective cation channels via alpha(1)-adrenergic receptors and that the effect may be mediated via an increase in intracellular free Ca(2+) concentration.

Beta(3)-adrenergic stimulation and insulin inhibition of non-selective cation channels in white adipocytes of the rat

Single-channel currents were recorded from the plasma membrane of white adipocytes of 6-8-week-old male Sprague-Dawley rats. In outside-out patches (high K(+), no Ca(2+) in pipette), a voltage-dependent K-channel (delayed rectifier) with a single-channel conductance (gamma) of 16 pS (24 degrees C) in modified Ringer's was active at a density of 0.5/microm(2). It was blocked by TEA (IC(50)=1.5 mM). A Ca(2+)-activated non-selective cation channel (NSC-channel) appeared at a mean density of 1/microm(2) in inside-out patches ([Ca(2+)](i)=1.2 mM). gamma was 28 pS (24 degrees C). The NSC showed weak voltage dependence and was blocked by mefenamic acid and by internal ATP. In the cell-attached mode spontaneous activity could be blocked reversibly by 100 nM insulin. Noradrenaline (NA, 100 nM) induced a flickering activity of the NSC-channels. Isoproterenol (100 nM) caused activity of the NSC-channel as well. After 1 microM propranolol even 1 microM NA did not induce any activity. The alpha-antagonist phentolamine had no effect on isoproterenol- or on NA-induced currents. The beta(3)-agonists BRL 37344 and BRL 35135A induced activity of the NSC-channel at 100 nM as well. We conclude that white adipocytes express ion channels which are comparable to those in brown adipocytes and that beta-receptor activation opens NSC-channels thus allowing for Na(+) entry into white adipocytes.

A calcium-activated nonselective cationic channel in the basolateral membrane of outer hair cells of the guinea-pig cochlea

The patch-clamp technique was used to investigate ion channels in the basolateral perilymph-facing membrane of freshly isolated outer hair cells (OHCs) from the guinea-pig cochlea. These sensory cells probably determine, via their motile activity, the fine tuning of sound frequencies and the high sensitivity of the inner ear. A Ca(2+)-activated nonselective cationic channel was found in excised inside-out membrane patches. The current/voltage relationship was linear with a unit conductance of 26.3 +/- 0.3 pS (n = 15) under symmetrical inger conditions. The channel excluded anions (PNa/PCl = 18 where PNa/PCl denotes the relative permeability of Na to Cl); it was equally permeant to the Na+ and K+ ions and exhibited a low permeability to N-methyl-D-glucamine and Ba2+ or Ca2+. Channel opening required a free Ca2+ concentration of about 10(-6) mol/l on the internal side of the membrane and the open probability (Po) was maximal at 10(-3) mol/l (Po = 0.72 +/- 0.06, n = 12). Adenosine 5'mono-, tri- and di-phosphate reduced Po to 29 +/- 14 (n = 5), 42 +/- 10 (n = 8) and 51 +/- 12 (n = 5) % of control Po, respectively, when they were added at a concentration of 10(-3) mol/l to the internal side. The channel was partially blocked by flufenamic acid (10(-4) mol/l) and 3',5'-dichlorodiphenylamine-2-carboxylic acid (DCDPC, 10(-5) mol/l). This type of channel, together with Ca(2+)-activated K+ channels, might participate in the control of membrane potential and modulate the motility of OHCs.

A voltage-dependent and a voltage-independent potassium channel in brown adipocytes of the rat

Single-channel recordings of a voltage-dependent potassium channel in brown adipocytes of the rat confirm recordings of macroscopic currents. Single-channel conductance (gamma) is 8 pS at 20 degrees C in KF solution inside vs. a modified Ringer's solution outside. With KCl solution outside, gamma is 17 pS for outward currents and 21 pS for inward currents. The majority of the channels inactivate with a time constant around 200 ms; deactivation occurs within milliseconds. The channel is blocked by tetraethylammonium (TEA) with an inhibiting constant of 1.8 mM. The type of block is fast. Selectivity sequence for monovalent cations is K+ > Rb+ >> NH4+ >> Li+ > or = Na+ approximately Cs+. Cs+ at the outside causes a voltage-dependent block of inward currents. This channel is remarkably similar to the delayed rectifier of the F-type in the node of Ranvier. Occasionally, an additional K+ channel was found. This channel is voltage-insensitive, not blocked by 10 mM TEA, and has not been recorded in brown adipocytes before. Physiological relevance of this channel could be the steady-state membrane potential.

Nonselective ion pathways in human endothelial cells

Four probably different transmembrane pathways are described in human endothelial (EN) cells that are all nonselective for cations. i) A nonselective cation channel that is more permeable for Na+ and K+ than for Ca2+ can be gated by agonists such as histamine. This channel provides an agonist-gated entry route for Ca2+ into EN cells with a single-channel conductance of 25 pS for Na+, K+, and approximately 4 pS for Ca2+ (110 mM). ii) Another Ca(2+)-permeable pathway can be activated by shear stress. This supposedly mechanically activated channel is more permeable for divalent than for monovalent cations and provides mechano-sensing properties to EN cells. iii) A third ionic current, activated by the selective Ca(2+)-ATPase blocker thapsigargin, seems to be related to Ca(2+)-release from Ca(2+)-stores in the endoplasmic reticulum. In EN cells, this Ca(2+)-entry route is cation selective, but cannot differentiate between Na+ and K+. Activation of this nonselective current is associated with an increase in intracellular Ca2+. We therefore assume a Ca(2+)-entry through this thapsigargin-activated pathway. iv) A nickel-blockable, Ca(2+)-permeable, nonselective leak is described that is present in nonstimulated EN cells. It will be discussed whether agonist-gated channels and leak channels might be related to the Ca(2+)-release activated Ca(2+)-entry mechanism.

A calcium-permeable channel in the apical membrane of primary cultures of the rabbit distal bright convoluted tubule

Calcium is actively reabsorbed in the distal nephron segments and recent studies have demonstrated the presence of Ca2+ channels in these epithelial cells, which could be involved in transepithelial transport. To test this possibility, single-channel currents were recorded by the patch-clamp technique in the apical membrane of primary cultures of the rabbit distal bright convoluted tubule cells (DCTb). In the cell-attached mode with 100 mmol/l BaCl2 in the pipette and 145 mmol/l NaCl in the bath, inward negative currents, consistent with Ba2+ currents, were recorded. In these conditions, the single-channel conductance was 15 pS. In excised inside-out patches, the single-channel conductance was 13 pS and the current reversal potential of +60 mV was close to the Nernst equilibrium potential for Ba2+ (> +58 mV). Similar experiments conducted with Ca2+ as the main charge carrier showed that this ion was less permeant through the channel than Ba2+ (PBa/PCa approximately 1.4). We also showed that the Ca(2+)-channel blocker, lanthanum (1 mumol/l La3+), added on the cytosolic side of the membrane, reversibly blocked the channel activity. On the other hand, verapamil (0.1 mmol/l) and nifedipine (10 mumol/l), perfused on the cytosolic side of the membrane, abolished the channel activity but this effect was not reversible. Another type of channel was also identified in the apical membrane of cultured DCTb cells. Ion-substitution experiments showed that this 21-pS conductance channel did not discriminate between Na+ and K+ and did not conduct Ba2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca(2+)-activated nonselective cation, maxi K+ and Cl- channels in apical membrane of marginal cells of stria vascularis

Patch clamp recordings on the apical membrane of marginal cells of the stria vascularis of the gerbil were made in the cell-attached and excised configuration. Marginal cells are thought to secrete K+ into and absorb Na+ from endolymph. Four types of channel were identified; the most frequently observed channel was a small, nonselective cation channel which was highly similar to that found in the apical membrane of vestibular dark cells (Marcus et al., (1992) Am. J. Physiol. 262, C1423-C1429). The small nonselective cation channel was equally conductive (26.7 +/- 0.3 pS; N = 49) for K+, Na+, Rb+, Li+ and Cs+, 1.6 times more permeable to NH4+, but not permeable to Cl-, Ca2+, Ba2+ or N-methyl-D-glucamine. This channel yielded linear current-voltage relations which passed nearly through the origin (intercept: -2.2 +/- 0.4 mV, N = 49) when conductive monovalent cations were present on both sides of the membrane in equal concentrations. Channel activity required the presence of Ca2+ at the cytosolic face but not the extracellular (endolymphatic) face; there was essentially no activity for cytosolic Ca2+ less than or equal to 10(-7) M Ca2+ and full activity for greater than or equal to 10(-5) M. Cell-attached recordings had a conductance of 28.6 +/- 2.2 pS (N = 6) and a reversal voltage of -2.2 +/- 5.2 mV (N = 3) which was interpreted to reflect the intracellular potential of marginal cells under the present conditions. The three other types of channel were a Cl- channel (approximately 50 pS; N = 2), a maxi-K+ channel (approximately 230 pS; N = 1), and another large channel, probably cation nonselective (approximately 170 pS; N = 1). The 27 pS nonselective cation channel may be involved in K+ secretion and Na+ absorption under stimulated conditions which produce an elevated intracellular Ca2+; however, consideration of the apparent channel density in relation to the total transepithelial K+ flux suggests that these channels are not sufficient to account for K+ secretion.

Ca(2+)-activated nonselective cation channel in apical membrane of vestibular dark cells

Patch-clamp recordings were made on cell-attached and excised apical membrane from dark cells of the semicircular canal of the gerbil. These cells are thought to secrete K+ and absorb Na+ from the luminal fluid (endolymph). Single-channel events were identified as being equally conductive (27.6 +/- 0.4 pS; n = 48) for K+, Na+, Rb+, Li+, and Cs+ and 1.4 times more permeable to NH4+ but not permeable to Cl-, Ca2+, Ba2+, nor to N-methyl-D-glucamine. The channels displayed linear current-voltage relations that passed nearly through the origin (intercept: -2.6 +/- 0.5 mV; n = 48) when conductive monovalent cations were present on both sides of the membrane in equal concentrations. Channel activity required the presence of Ca2+ at the cytosolic face; there was no activity at less than or equal to 10(-7) M Ca2+ and full activity at greater than or equal to 10(-5) M Ca2+. Cell-attached recordings had a mean reversal voltage of -36.4 +/- 7.9 mV (n = 7), which was interpreted to reflect the intracellular potential of dark cells under the present conditions. We have identified a nonselective cation channel in the apical membrane of vestibular dark cells that might participate in K+ secretion or Na+ absorption under stimulated conditions, but the density appears to be insufficient to fully account for the transepithelial K+ flux.

Activation of nonselective cation channels in the basolateral membrane of rat distal colon crypt cells by prostaglandin E2

Ion channels in the basolateral membrane of colonic crypts were investigated with the patch-clamp technique during stimulation of secretion. Intact crypts were isolated from rat distal colon and the cell potential was recorded by addition of nystatin to the pipette solution. The cell resting potential in the base of the crypt was -74 +/- 1 mV (n = 90). Addition of 100 microM carbachol to the bath resulted in a transient hyperpolarization by 9 mV, which was probably due to the opening of basolateral K+ channels. In contrast, application of prostaglandin E2 (PGE2, 1 nM-1 microM) caused a dose-dependent depolarization in the base of the crypt. With 1 microM PGE2 cells depolarized from -74 +/- 1 to -27 +/- 2 mV (n = 26). Cell potential recordings in the midcrypt showed only a slight and transient depolarization after application of PGE2, whereas cells close to the surface of the crypt had no response. In the base of the crypt the PGE2-induced depolarization could be completely inhibited by addition of 50 microM flufenamic acid, a known blocker of nonselective cation channels. After substitution of all monovalent cations by N-methyl-D-glucamine in the bath, PGE2 had no significant effect on the cell potential. Cell-attached experiments with no nystatin in the patch pipette revealed the activation of ion channels in the basolateral membrane after application of PGE2. After excision of the membrane patch, these channels could be identified as nonselective cation channels. Experiments involving substitution of the bath solution showed that the channel is impermeable for Cl- and scarcely permeable for Ca2+ ions. The permeability sequence for monovalent cations, as calculated from reversal potentials, is NH4+ greater than Na+ = K+ greater than Rb+ = Li+ much greater than TRIS+ = NMDG+. Single channels are completely inhibited by flufenamic acid (50 microM), mefenamic acid (200 microM), as well as by 3',5-dichlorodiphenylamine-2-carboxylate. In conclusion, PGE2 activates nonselective cation channels in the basolateral membrane of cells in the base of colonic crypts. It is suggested that this mechanism initiates the secretion of K+ ions. Na+ influx through the nonselective cation channel will stimulate the Na+/K+ pump and active uptake of K+ at the basolateral side. K+ can leave the cell at the luminal side through K(+)-selective channels.

Cation selective channel in fetal alveolar type II epithelium

A cation selective channel was identified in the apical membrane of fetal rat (Wistar) alveolar type II epithelium using the patch clamp technique. The single channel conductance was 23 +/- 1.2 pS (n = 16) with symmetrical NaCl (140 mM) solution in the bath and pipette. The channel was highly permeable to Na+ and K+ (PNa/PK = 0.9) but essentially impermeant to chloride and gluconate. Membrane potential did not influence open state probability when measured in a high Ca2+ (1.5 mM) bath. The channel reversibly inactivated when the bath was exchanged with a Ca(2+)-free (less than 10(-9) M) solution. The Na+ channel blocker amiloride (10(-6) M) applied to the extracellular side of the membrane reduced P(open) relative to control patches; P(control) = 0.57 +/- 0.11 (n = 5), P(amiloride) = 0.09 +/- 0.07 (n = 4, p less than 0.01), however, amiloride did not significantly influence channel conductance (g); g(control) 19 +/- 0.9 pS (n = 5), 18 +/- 3.0 pS (n = 4). More than one current level was observed in 42% (16/38) of active patches; multiple current levels (ranging from 2 to 6) were of equal amplitude suggesting the presence of multiple channels or subconductance states. Channel activity was also evident in cell attached patches. Since monolayers of these cells absorb Na+ via an amiloride sensitive transport mechanism we speculate that this amiloride sensitive cation selective channel is a potential apical pathway for electrogenic Na+ transport in the alveolar region of the lung.

Inner mitochondrial membrane anion channel is present in brown adipocytes but is not identical with the uncoupling protein

Vesicles of inner mitochondrial membrane, mitoplasts, from rat brown adipose tissue were prepared by osmotic swelling and studied using the patch-clamp technique. Current events of a 107.8 +/- 8.7 pS (n = 16, 21 degrees C) channel were recorded in the mitoplast-attached mode. This channel was selective for anions and its kinetics resembled those of channels previously found in liver and heart mitochondria of mouse and ox. In whole-mitoplast mode each of five purine nucleotides (20 microM) blocked the channel. This is the first demonstration of pharmacological blockade of this type of channel. Although a similar anion channel in mouse and ox mitochondria was suggested to be the uncoupling protein (UCP) associated with nonshivering thermogenesis, we present several arguments against this possibility. Thus we describe a high-conductance, purine-nucleotide-binding, anion selective mitochondrial channel, that is not the UCP.

Permeation properties of a non-selective cation channel in human vascular endothelial cells

Endothelial cells obtained from human umbilical chord have been studied by the patch clamp method. An ion channel is described that is activated by microM concentrations of histamine and shows a slow run-down in cell-attached patches. After excision, channel activity quickly runs down to zero open probability. In symmetrical potassium concentrations (140 mM K in the bath and the pipette), the single channel conductance is 28 +/- 2 pS and the reversal potential is 0.3 +/- 0.8 mV (mean +/- SEM, n = 4). With 140 mM Na in the pipette, the conductance is 26 +/- 2 pS. A reversal potential of -1.5 +/- 0.9 mV (n = 7) was measured. With 60 mM Ca and 70 mM Na in the pipette, 140 mM K in the bath, the reversal potential was -11 +/- 3 mV, the single channel conductance in 16 +/- 3 pS (n = 5). The single channel conductance in 110 mM Ca (pipette) and 140 mM K (bath) is 8 +/- 2 pS and the reversal potential is -18 +/- 6 mV (n = 3). From analysis of the reversal potentials, a permeation ratio of K:Na:Ca = 1:0.9:0.2 was calculated. This ligand-gated non-selective cation channel in human endothelial cells is Ca permeable and could induce a sustained agonist mediated Ca influx.