Activation of K+ current in macrophages by platelet activating factor.

Puff application of platelet activating factor (10(-8) M) onto peritoneal macrophages from thioglycollate-stimulated mice induced an outward current at a holding potential of -63 mV. The current was suppressed by an antagonist Y-24180 but not by CV-3988. Charybdotoxin (10(-6) M) suppressed the current. Reversal potentials were dependent on external K+ concentrations. The current was not suppressed in Ca(2+)-free EGTA-containing solution but was completely abolished in BAPTA-AM containing solution. These results suggest that platelet activating factor activates a Ca(2+)-dependent K+ channel.     

Membrane currents in canine bronchial artery and their regulation by excitatory agonists

The bronchial vasculature plays an important role in airway physiology and pathophysiology. We investigated the ion currents in canine bronchial smooth muscle cells using patch-clamp techniques. Sustained outward K(+) current evoked by step depolarizations was significantly inhibited by tetraethylamonium (1 and 10 mM) or by charybdotoxin (10(-6) M) but was not significantly affected by 4-aminopyridine (1 or 5 mM), suggesting that it was primarily a Ca(2+)-activated K(+) current. Consistent with this, the K(+) current was markedly increased by raising external Ca(2+) to 4 mM but was decreased by nifedipine (10(-6) M) or by removing external Ca(2+). When K(+) currents were blocked (by Cs(+) in the pipette), step depolarizations evoked transient inward currents with characteristics of L-type Ca(2+) current as follows: 1) activation that was voltage dependent (threshold and maximal at -50 and -10 mV, respectively); 2) inactivation that was time dependent and voltage dependent (voltage causing 50% maximal inactivation of -26 +/- 22 mV); and 3) blockade by nifedipine (10(-6) M). The thromboxane mimetic U-46619 (10(-6) M) caused a marked augmentation of outward K(+) current (as did 10 mM caffeine) lasting only 10-20 s; this was followed by significant suppression of the K(+) current lasting several minutes. Phenylephrine (10(-4) M) also suppressed the K(+) current to a similar degree but did not cause the initial transient augmentation. None of these three agonists elicited inward current of any kind. We conclude that bronchial arterial smooth muscle expresses Ca(2+)-dependent K(+) channels and voltage-dependent Ca(2+) channels and that its excitation does not involve activation of Cl(-) channels.     

The neuropeptide, neuromedin C, activates a potassium current in mouse macrophages.

Neuromedin C (NmC) induced an outward current (Io(NmC)) in macrophages. Reversal potentials were dependent on external K+ concentrations ([K+]o) and independent of [Cl-]o. Tetraethylammonium (TEA) and quinidine effectively suppressed Io(NmC). Charybdotoxin (ChTX) and apamin had little effect. Io(NmC) was abolished in Ca(2+)-free EGTA-containing solution. These results suggest that MnC activates a Ca(2+)-dependent K+ current (IK,Ca) and can modulate activities in macrophages.     

Transient outward current carried by inwardly rectifying K+ channels in guinea pig ventricular myocytes dialyzed with low-K+ solution

There have been periodic reports of nonclassic (4-aminopyridine insensitive) transient outward K+ current in guinea pig ventricular myocytes, with the most recent one describing a novel voltage-gated inwardly rectifying type. In the present study, we have investigated a transient outward current that overlaps inward Ca2+ current (I(Ca,L)) in myocytes dialyzed with 10 mM K+ solution and superfused with Tyrode's solution. Although depolarizations from holding potential (Vhp) -40 to 0 mV elicited relatively small inward I(Ca,L) in these myocytes, removal of external K+ or addition of 0.2 mM Ba2+ more than doubled the amplitude of the current. The basis of the enhancement of I(Ca,L) was the suppression of a large transient outward K+ current. Similar enhancement was observed when Vhp was moved to -80 mV and test depolarizations were preceded by short prepulses to -40 mV. Investigation of the time and voltage properties of the outward K+ transient indicated that it was inwardly rectifying and unlikely to be carried by voltage-gated channels. The outward transient was attenuated in myocytes dialyzed with high-Mg2+ solution, accelerated in myocytes dialyzed with 100 microM spermine solution, and abolished with time in myocytes dialyzed with ATP-free solution. These and other findings suggest that the outward transient is a component of classic "time-independent" inwardly rectifying K+ current.     

Muscarinic activation of transient inward current and contraction in canine colon circular smooth muscle cells

Muscarinic receptor mediated membrane currents and contractions were studied in isolated canine colon circular smooth muscle cells. Carbachol (10(-5) M) evoked a slow transient inward current that was superimposed by a transient outward current at holding potentials greater than -50 mV. Carbachol contracted the cells by 70 +/- 2%. The effects of carbachol were blocked by atropine (10(-6) M), tetraethyl ammonium (20 mM), and BAPTA-AM (25 mM applied for 20 min). The inward current and contraction were not sensitive to diltiazem (10(-5) M), nitrendipine (3 x 10(-7) M), niflumic acid (10(-5) M), or N-phenylanthranilic acid (10(-4) M), but were gradually inhibited after repetitive stimulations in Ca2+ free solution. Ni2+ (2 mM) inhibited the inward current by 67 +/- 4%. The inward current reversed at +15 mV. The outward component could be selectively inhibited by iberiotoxin (20 nM) or by intracellular Cs+. Repeated stimulation in the presence of cyclopiazonic acid (CPA, 3 microM) inhibited the carbachol-induced outward current and partially inhibited contraction. CPA did not inhibit the inward current. In conclusion, muscarinic receptor stimulation evoked a CPA-sensitive calcium release that caused contraction and a CPA-insensitive transient inward current was activated that is primarily carried by Ca2+ ions and is sensitive to Ni2+.     

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

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

Rapid charge translocation by the cardiac Na(+)-Ca2+ exchanger after a Ca2+ concentration jump.

The kinetics of Na(+)-Ca2+ exchange current after a cytoplasmic Ca2+ concentration jump (achieved by photolysis of DM-nitrophen) was measured in excised giant membrane patches from guinea pig or rat heart. Increasing the cytoplasmic Ca2+ concentration from 0.5 microM in the presence of 100 mM extracellular Na+ elicits an inward current that rises with a time constant tau 1 < 50 microseconds and decays to a plateau with a time constant tau 2 = 0.65 +/- 0.18 ms (n = 101) at 21 degrees C. These current signals are suppressed by Ni2+ and dichlorobenzamil. No stationary current, but a transient inward current that rises with tau 1 < 50 microseconds and decays with tau 2 = 0.28 +/- 0.06 ms (n = 53, T = 21 degrees C) is observed if the Ca2+ concentration jump is performed under conditions that promote Ca(2+)-Ca2+ exchange (i.e., no extracellular Na+, 5 mM extracellular Ca2+). The transient and stationary inward current is not observed in the absence of extracellular Ca2+ and Na+. The application of alpha-chymotrypsin reveals the influence of the cytoplasmic regulatory Ca2+ binding site on Ca(2+)-Ca2+ and forward Na(+)-Ca2+ exchange and shows that this site regulates both the transient and stationary current. The temperature dependence of the stationary current exhibits an activation energy of 70 kj/mol for temperatures between 21 degrees C and 38 degrees C, and 138 kj/mol between 10 degrees C and 21 degrees C. For the decay time constant an activation energy of 70 kj/mol is observed in the Na(+)-Ca2+ and the Ca(2+)-Ca2+ exchange mode between 13 degrees C and 35 degrees C. The data indicate that partial reactions of the Na(+)-Ca2+ exchanger associated with Ca2+ binding and translocation are very fast at 35 degrees C, with relaxation time constants of about 6700 s-1 in the forward Na(+)-Ca2+ exchange and about 12,500 s-1 in the Ca(2+)-Ca2+ exchange mode and that net negative charge is moved during Ca2+ translocation. According to model calculations, the turnover number, however, has to be at least 2-4 times smaller than the decay rate of the transient current, and Na+ inward translocation appears to be slower than Ca2+ outward movement.     

Inhibition of Ca2+-activated K+ channels in pig pancreatic acinar cells by Ba2+, Ca2+, quinine and quinidine

Patch-clamp whole-cell and single-channel current recordings were made from pig pancreatic acinar cells to test the effects of quinine, quinidine, Ba2+ and Ca2+. Voltage-clamp current recordings from single isolated cells showed that high external concentrations of Ba2+ or Ca2+ (88 mM) abolished the outward K+ currents normally associated with depolarizing voltage steps. Lower concentrations of Ca2+ only had small inhibitory effects whereas 11 mM Ba2+ almost blocked the K+ current. 5.5 mM Ba2+ reduced the outward K+ current to less than 30% of the control value. Both external quinine and quinidine (200-500 microM) markedly reduced whole-cell outward K+ currents. In single-channel current studies it was shown that external Ba2+ (1-5 mM) markedly reduced the probability of opening of high-conductance Ca2+ and voltage-activated K+ channels whereas internal Ba2+ (6 X 10(-6) to 3 X 10(-5) M) caused activation at negative membrane potentials and inhibition at positive potentials. Quinidine (200-400 microM) evoked rapid chopping of single K+ channel openings acting both from the outside and inside of the membrane and in this way markedly reduced the total current passing through the channels.     

Ryanodine Inhibits Caffeine-Evoked Ca2+ Mobilization and Catecholamine Secretion from Cultured Bovine Adrenal Chromaffin Cells

The effects of ryanodine, a selective inhibitor of the Ca(2+)-induced Ca2+ release mechanism, on caffeine-evoked changes in cytosolic Ca2+ concentration ([Ca2+]i) and catecholamine secretion were investigated using cultured bovine adrenal chromaffin cells. Caffeine (5-40 mM) caused a concentration-dependent transient rise in [Ca2+]i and catecholamine secretion in Ca2+/Mg(2+)-free medium containing 0.2 mM EGTA. Ryanodine (5 x 10(-5) M) alone had no effect on either [Ca2+]i or catecholamine secretion. Although the application of ryanodine plus caffeine caused the same increase in both [Ca2+]i and catecholamine secretion as those induced by caffeine alone, ryanodine (4 x 10(-7) - 5 x 10(-5) M) irreversibly prevented the increase in both [Ca2+]i and catecholamine secretion resulting from subsequent caffeine application over a range of concentrations. The secretory response to caffeine was markedly enhanced by replacement of Na+ with sucrose in Ca2+/Mg(2+)-free medium, and this enhanced response was also blocked by ryanodine. Caffeine was found to decrease the susceptibility of the secretory apparatus to Ca2+ in digitonin-permeabilized cells. These results indicate that caffeine mobilizes Ca2+ from intracellular stores, the function of which is irreversibly blocked by ryanodine, resulting in the increase in catecholamine secretion in the bovine adrenal chromaffin cell.     

Ca2+- and voltage-dependent K+ conductance in dispersed parathyroid cells

The membrane ionic conductances of dispersed parathyroid cells kept in primary culture were studied using the "whole-cell" and "inside-out excised patch" variants of the patch-clamp technique. The major component of the total current was a voltage-dependent outward K+ current without an appreciable inward current. The amplitude of the K+ current was markedly reduced when free internal Ca2+ was buffered by addition of 10 mM EGTA. Recordings of single-channel current in excised membrane patches revealed the presence of K+ channels with large unitary conductance (200 pS in symmetrical 130 mM K+ solutions) which were also activated by depolarization when internal Ca2+ concentration was about 10(-5)-10(-6) M. At any membrane voltage these channels were closed most of the time at internal Ca2+ concentrations lower than 10(-10) M. These results demonstrate the existence of a Ca2+- and voltage-dependent K+ permeability in parathyroid cells which may participate in the unusual membrane potential changes induced by alterations of external Ca2+ and, possibly, in the regulation of parathormone secretion.     

Reverse Na(+)/Ca(2+)-exchange mediated Ca(2+)-entry and noradrenaline release in Na(+)-loaded peripheral sympathetic nerves

[(3)H]noradrenaline ([(3)H]NA) released from sympathetic nerves in the isolated main pulmonary artery of the rabbit was measured in response to field stimulation (2Hz, 1ms, 60V for 3min) in the presence of uptake blockers (cocaine, 3 x10(-5)M and corticosterone, 5 x10(-5)M). The [(3)H]NA-release was fully blocked by the combined application of the selective and irreversible 'N-type' voltage-sensitive Ca(2+)-channel (VSCC)-blocker omega-conotoxin (omega-CgTx) GVIA (10(-8)M) and the 'non-selective' VSCC-blocker aminoglycoside antibiotic neomycin (3x10(-3)M). Na(+)-loading (Na(+)-pump inhibition by K(+)-free perfusion) was required to elicit further NA-release after blockade of VSCCs (omega-CgTx GVIA+neomycin). In K(+)-free solution, in the absence of functioning VSCCs (omega-CgTx GVIA+neomycin), the fast Na(+)-channel activator veratridine (10(-5)M) further potentiated the nerve-evoked release of [(3)H]NA. This NA-release was significantly inhibited by KB-R7943, and fully blocked by Ca(o)(2+)-removal. However, Li(+)-substitution was surprisingly ineffective. The non-selective K(+)-channel blocker 4-aminopyridine (4-AP, 10(-4)M) also further potentiated the nerve-evoked release of NA in K(+)-free solution. This potentiated release was concentration-dependently inhibited by KB-R7943, significantly inhibited by Li(+)-substitution and abolished by Ca(o)(2+)-removal. It is concluded that in Na(+)-loaded sympathetic nerves, in which the VSCCs are blocked, the reverse Na(+)/Ca(2+)-exchange-mediated Ca(2+)-entry is responsible for transmitter release on nerve-stimulation. Theoretically we suppose that the fast Na(+)-channel and the exchanger proteins are close to the vesicle docking sites.     

Phosphoinositides and synaptic function in NG108-15 neuroblastoma x glioma hybrid cells.

1. The second-messengers system of bradykinin (BK) receptors was examined in NG108-15 neuroblastoma x glioma hybrid cells. 2. An application of BK induced an immediate outward (K+) current and acetylcholine (ACh) release, which are generated through inositol 1,4,5-trisphosphate (InsP3)-dependent Ca2+ ions. 3. Application of phorbol dibutyrate (a protein kinase C activator) produced a voltage-dependent inward current and inhibited another K+ (M)-current. 4. A similar current response has been produced by ACh in NG108-15 cells transfected with rodent muscarinic ACh receptor I and III subtype genes. 5. These results suggest a dual and time-dependent role for these two intracellular messengers in the control of neuronal signalling by BK and ACh.     

Nonselective cationic currents activated by acetylcholine in swine tracheal smooth muscle cells

Membrane currents in isolated swine tracheal smooth muscle cells were investigated using a pipette solution containing BAPTA-Ca2+ buffer and Cs+ as the major cation. With a pipette solution containing 100 nM free Ca2+, acetylcholine (ACh; 1-100 microM), in a concentration-dependent manner, activated a current without inducing shortening of cells, although neither 1 mM histamine nor 1 microM leukotriene D4 activated the current (n = 7, n is the number of cells). The effect of 100 microM ACh was suppressed by pretreatment with 100 microM atropine (n = 6) or intracellular application of preactivated pertussis toxin at a concentration of 0.1 microg x mL(-1) (n = 8). Genistein (0.1-100 microM), in a concentration-dependent manner, suppressed the activation of the inward current by 100 microM ACh, whereas it did not significantly suppress that of the outward current (n = 6-8). With a pipette solution containing 50 nM free Ca2+, outward current, but not inward current, was activated by 100 microM ACh (n = 10). When the pipette solution had free Ca2+ concentrations greater than 50 nM, the inward current together with the outward current was activated. The ratio between the amplitude of the inward and outward currents was significantly increased as the free Ca2+ concentration in the pipette solution increased. The steady-state activation curve of the ACh-activated current with the 50 nM free Ca2+ pipette solution was fitted by a single Boltzmann distribution (Vh = +69.8 mV, k = -11.9 mV, n = 10). The activation time constant became smaller as the membrane potential was more depolarized (164.3+/-5.9 ms at +40 mV to 92.4+/-6.3 ms at +120 mV, n = 10). The reversal potential was not significantly changed by reducing extracellular Cl- concentration to one-tenth of the control (n = 8), suggesting that the current is a nonselective cationic current. These results suggest that ACh activates an outward nonselective cationic current via pertussis toxin-sensitive G-protein(s) coupled with muscarinic receptors. Involvement of genistein-sensitive tyrosine kinase in the activation process of the current is unlikely.     

Characterization and Ca2+ Requirement of Histamine-Induced Catecholamine Secretion in Cultured Bovine Chromaffin Cells

The stimulation of cultured bovine chromaffin cells with histamine induced a continuous catecholamine secretion (EC50 = 3 x 10(-7) M) via the H1 receptor, in addition to an initial catecholamine burst due to a nonspecific stimulatory effect at higher doses (greater than or equal to 10(-4) M). The continuous secretion showed little desensitization and lasted for more than 1 h. In fura-2-loaded cells, the stimulation with histamine evoked a transient rise of intracellular free Ca2+ concentration ([Ca2+]i) which lasted only for a few minutes and was followed by a sustained [Ca2+]i rise which continued for more than 20 min. The addition of an activator for the L-type voltage-sensitive Ca2+ channel, i.e., Bay K 8644 (1 microM), facilitated the sustained [Ca2+]i rise, as well as the secretion, whereas the addition of relatively high concentrations of Ca(2+)-channel blockers (10 microM) suppressed the sustained [Ca2+]i rise and part of the secretion. Removal of extracellular Ca2+ completely abolished continuous secretion and sustained [Ca2+]i rise. When the external Ca2+ level was elevated, both sustained [Ca2+]i rise and continuous secretion were enhanced in a similar Ca(2+)-dependent manner, showing saturation with around 1-3 mM Ca2+. This Ca2+ dependence was clearly different from that observed with high K+ and nicotine, which is mediated by the L-type Ca2+ channel, in which the responses showed little or no saturation when the Ca2+ level was increased. The results indicate that stimulation with histamine induces a continuous secretion via the H1 receptor, in addition to a transient and nonspecific secretion at higher doses.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interaction of DNA with divalent metal ions

The interaction between the native DNA macromolecules and Ca2+, Mn2+, Cu2+ ions in solutions of low ionic strength (10(-3) M Na+) is studied using the methods of differential UV spectroscopy and CD spectroscopy. It is shown that the transition metal ions Mn2+ exercise binding to the nitrogen bases of DNA at concentrations approximately 5 x 10(-6) M and form chelates with guanine of N7-Me(2+)-O6 type. Only at high concentrations in solution (5 x 10(-3) M) do Ca2+ ions interact with the nitrogen bases of native DNA. In the process of binding to Ca2+ and Mn2+ the DNA conformation experiences some changes under which the secondary structure of the biopolymer is within the B-form family. The DNA transition to the new conformation is revealed by its binding to Cu2+ ions.     

Ionic currents in the soma of an identified cockroach motoneurone recorded under voltage-clamp

1. Membrane currents have been recorded from the soma of a bifunctional basalar/coxal depressor motoneurone in the metathoracic ganglion of the cockroach (Periplaneta americana) using a two-electrode voltage-clamp technique. 2. This motoneurone cell body is normally inexcitable when studied under current-clamp. Appropriate depolarizing command steps evoke rapid transient outward currents and late outward currents. 3. Late outward currents are dominated by a Ca-dependent component that confers an N-shaped I-V relationship on the neurone. 4. The Ca-dependent outward current is suppressed by Cd2+ (1 mM), Mn2+ (5 mM) or verapamil (50 microM). 5. Externally applied tetraethylammonium ions (TEA+) (25 mM) block the Ca-dependent current, but also appear to suppress a component of the late outward current that is independent of Ca2+. 6. Aminopyridines cause only minor suppression of late outward currents, but shift the peak in the N-shaped I-V relationship to more negative potentials. 7. The reversal potential of tail currents recorded following pre-pulses to +50 mV were dependent upon the pre-pulse duration; increasing the duration from 10 to 50 msec caused a +17 mV shift in tail current reversal potential. 8. A five-fold increase in the K+ concentration of the solution bathing the preparation only produced small and inconsistent changes in the reversal potential of tail currents. 9. Five-fold reduction in external Cl- caused no change. 10. The dependence of tail current reversal potential upon pre-pulse duration and the limited effect of alterations in the composition of the bathing solution are discussed in the context of restricted ion movements near the external surface of the cell membrane.     

Role of the Outward Delayed Rectifier K+ Current in Ceramide-Induced Caspase Activation and Apoptosis in Cultured Cortical Neurons

We studied the novel hypothesis that an up-modulation of channels for outward delayed rectifier K+ current (I(K)) plays a key role in ceramide-induced neuronal apoptosis. Exposure for 6-10 h to the membrane-permeable C2-ceramide (25 microM) or to sphingomyelinase (0.2 unit/ml), but not to the inactive ceramide analogue C2-dihydroceramide (25 microM), enhanced the whole-cell I(K) current without affecting the transient A-type K+ current and increased caspase activity, followed by neuronal apoptosis 24 h after exposure onset. Tetraethylammonium (TEA) or 4-chloro-N,N-diethyl-N-heptylbenzenebutanaminium tosylate (clofilium), at concentrations inhibiting I(K), attenuated the C2-ceramide-induced caspase-3-like activation as well as neuronal apoptosis. Raising extracellular K+ to 25 mM similarly blocked the C2-ceramide-induced cell death; the neuroprotection by 25 mM K+ or TEA was not eliminated by blocking voltage-gated Ca2+ channels. An inhibitor of tyrosine kinases, herbimycin A (10 nM) or lavendustin A (0.1-1 microM), suppressed I(K) enhancement and/or apoptosis induced by C2-ceramide. It is suggested that ceramide-induced I(K) current enhancement is mediated by tyrosine phosphorylation and plays a critical role in neuronal apoptosis.     

Effect of Bay K 8644 on tetraethylammonium-induced excitability of the rabbit pulmonary artery

The effect of Bay K 8644 on the electrical activity of the smooth muscle cells in the main pulmonary artery of the rabbit was examined. In normal physiological solution, the resting membrane potential was -56 +/- 0.6 mV, and the cells were electrically quiescent. Tetraethylammonium (5 mM) depolarized the membrane to about -45 mV, and electrical stimulation elicited action potentials. To suppress contractile responses and thereby facilitate sustained impalements, the muscle strips were bathed with a hypertonic solution containing sucrose. The mean amplitude of the tetraethylammonium-induced action potentials in the hypertonic solution was 35 +/- 0.9 mV. The action potentials were dependent upon the extracellular Ca2+ concentration and were abolished by diltiazem (10(-6) M). Spontaneous action potentials were occasionally generated in the presence of tetraethylammonium alone and could be induced by the further addition of Ba2+ (0.5 mM). The Ca2+ agonist Bay K 8644 (10(-8) to 10(-6) M) had no effect on the resting membrane potential or excitability in normal solution. However, in the hypertonic solution containing tetraethylammonium, Bay K 8644 caused a further depolarization and oscillatory potential changes, which were not prevented by tetrodotoxin. The oscillations were suppressed or abolished by diltiazem or nilvadipine. Thus, active responses can occur in the normally quiescent smooth muscle cells of the rabbit pulmonary artery when the outward K+ current(s) are suppressed.     

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

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

Effective diameter of pores in rat liver mitochondria inner membrane and selectivity of transport of monovalent cations with a submicromolar concentration of Ca2+ ions or A23187 and EDTA]

Studies of swelling of rat liver mitochondria in isoosmotic solutions of nonelectrolytes in the presence of respiration inhibitors revealed that submicromolar concentrations of Ca2+ increase the diameter of pores in the inner mitochondrial membrane--from 5.5-6.0 A (10(-8) M Ca2+) up to 7.5 A (3 x 10(-7) M Ca2+) and 8.0-8.5 A (6 x 10(-7) M Ca2+); these increases are prevented by cyclosporin A. The inner mitochondrial membrane with an effective pore diameter of 7.5 A is readily permeable for potassium but not for sodium ions, although with an increase in the effective pore diameter up to 8.0-8.5 A the selectivity of the K(+)-Na+ channel decreases. A conclusion is drawn that in the presence of submicromolar concentrations of Ca2+ the conductivity of the cyclosporin-sensitive pore for monovalent cations increases in the following order: K > Na > Li.