Amiloride selectively blocks the low threshold (T) calcium channel

Multiple actions of Bay K 8644 on high-threshold Ca channels in adult rat sensory neurons

omega-Conotoxin (omega-CgTx, 6.4 microM) failed to fully block the high-threshold Ba currents (HVA; L,N) of adult rat sensory neurons, and showed only a minor inhibitory action on the low-voltage activated current (LVA,T). In most of the CgTx-treated neurons the residual high-threshold Ba current was strongly agonized by the 1,4-dihydropyridine Bay K 8644. 1 microM Bay K 8644 enhanced 3- to 4-fold the size of this current at low membrane potentials and prolonged its deactivation kinetics by one order of magnitude. As in cardiac cells, in some neurons Bay K 8644 sped up about 3-fold the inactivation time course of the omega-CgTx-resistant Ba current, suggesting multiple actions of the dihydropyridine on neuronal high-threshold Ca channels.

Effects of amiloride on metabolism and contractility during reoxygenation in perfused rat hearts

Myocardial recovery after hypoxia may be determined not only by the extent of metabolic depression during the hypoxic period but also by changes in cation contents as well. Calcium overload during reoxygenation, mediated in part by Na-Ca exchange and supported by the rise in cell sodium during hypoxia, may be one factor. The effects of amiloride (0.1 mM), a diuretic that inhibits Na(+)-H+ and Na-Ca exchanges in cardiac sarcolemma and mitochondria preparations, were studied during hypoxia-reoxygenation in the isovolumic, isolated rat heart. During hypoxia, cell sodium, measured using potassium ethylenediamine tetraacetate cobaltate as an extracellular marker, increased in amiloride and amiloride-free hearts, but there was no increase in cell calcium (3.3 +/- 0.3 vs. 3.6 +/- 0.9 mumol/g dry wt; p = NS). Amiloride did not alter developed pressure (DP), end-diastolic pressure (EDP), pH, or integrated areas of adenosine triphosphate (ATP) and phosphocreatine (PCr) (determined by phosphorus-31-nuclear magnetic resonance spectroscopy) during hypoxia or normal perfusion conditions. Forty minutes after reoxygenation, however, cell calcium was significantly lower in the amiloride (5.1 +/- 1.3 mumol/g dry wt) than in the amiloride-free group (10.4 +/- 1.8 mumol/g dry wt; p less than 0.001), and there was improved recovery of DP (percent of initial) (72 +/- 12% vs. 41 +/- 12%; p less than 0.001), PCr (99 +/- 9% vs. 70 +/- 14%; p less than 0.001), and pH (7.17 +/- 0.17 vs. 6.88 +/- 0.16; p less than 0.001) in the amiloride group. To determine whether this dose of amiloride inhibits the manifestations of sodium-mediated calcium gain in the same model during normoxia, the metabolic and functional sequelae of lithium-substituted low sodium (50 mM) perfusion were studied. Amiloride significantly limited the manifestations of sodium-mediated calcium gain as indexed (all expressed as percent of control) by a lower peak DP (221 +/- 25% vs. 284 +/- 20%) at 3 minutes, improved preservation of PCr (85 +/- 10% vs. 68 +/- 9%) and ATP (104 +/- 12% vs. 84 +/- 9%), lower rise in inorganic phosphate (201 +/- 74% vs. 332 +/- 106%), and a smaller fall in intracellular pH (7.01 +/- 0.04 vs. 6.70 +/- 0.15, p less than 0.05) for all metabolic parameters during a 20-minute period.(ABSTRACT TRUNCATED AT 400 WORDS)

Ca currents in human neuroblastoma IMR32 cells: kinetics, permeability and pharmacology

We have investigated the kinetics, permeability and pharmacological properties of Ca channels in in vitro differentiated IMR32 human neuroblastoma cells. The low-threshold (LVA, T) Ca current activated positive to -50 mV and inactivated fully within 100 ms in a voltage-dependent manner. This current persisted in the presence of 3.2 microM omega-conotoxin (omega-CgTx) or 40 microM Cd and showed a weaker sensitivity to Ni and amiloride than in other neurons. The high-threshold Ca currents (HVA,L and N) turned on positive to -30 mV, and inactivated slowly and incompletely during pulses of 200 ms duration. The amplitude of the HVA currents and the number of 125I-omega-CgTx binding sites increased markedly during cell differentiation. In agreement with recent reports, 6.4 microM omega-CgTx blocked only about 85% of the Ba currents through HVA channels in 50% of the cells. Residual omega-CgTx-resistant currents proved to be more sensitive to dihydropyridines (DHP) than total HVA currents. Bay K 8644 (1 microM) had a clear agonistic action on omega-CgTx-resistant currents and was preferred to other Ca antagonists for identifying HVA DHP-sensitive channels. Compared to the omega-CgTx-sensitive, the DHP-sensitive currents turned on at slightly more negative potentials and showed a weaker sensitivity to voltage. The two HVA currents were otherwise hardly distinguishable in terms of activation/inactivation kinetics, Ca/Ba permeability and sensitivity to holding potentials. This suggests that currently used criteria for identifying multiple types of neuronal Ca channels (T;L,N) may be widely misleading if not supported by pharmacological assays.

Amiloride inhibits contraction and serotonin modulation of Aplysia muscle

1. Serotonin (5-HT) potentiates acetylcholine (ACh)-elicited contractions of Aplysia buccal muscles. Serotonin potentiation was significantly reduced by 0.03 mM, 0.1 mM, and 0.3 mM amiloride. 2. Unpotentiated ACh-elicited contractions were significantly reduced by 0.1 mM and 0.3 mM amiloride. 3. Amiloride reduced ACh-elicited depolarization. The reduction in contraction caused by 0.3 mM amiloride (to 16% of control) was larger than could be explained by the reduction in depolarization (86% of control). 4. Amiloride had no effect on tension in skinned muscle fibers, indicating that amiloride probably did not have a direct effect on contractile mechanisms. 5. Potentiation of contraction produced by zero sodium (Tris substituted, 0 Na-Tris) medium could be abolished by 0.3 mM amiloride. 6. Zero Na-Tris increased 45Ca influx 2.7-fold. In the presence of 0.3 mM amiloride, 0 Na-Tris increased 45Ca influx only 1.4-fold. 7. Amiloride (0.3 mM) reduced the elevation of muscle cAMP caused by 10(-6) M 5-HT by 60%. Zero Na-Tris did not cause a change in muscle cAMP.

Modulation of neuronal T-type calcium channel currents by photoactivation of intracellular guanosine 5'-O(3-thio) triphosphate

Low voltage-activated T-type Ca2+ channel currents were recorded from cultured rat dorsal root ganglion neurons using the whole-cell clamp technique with Ba2+ as the charge carrier. The T-type Ca2+ channel current was identified by its low threshold of activation (Vc -50 to -20 mV from VH - 90 mV), its kinetics of inactivation and its sensitivity to NiCl2 (100 microM). It was also sensitive to 1-octanol (1 microM). omega-Conotoxin (1 microM) markedly reduced the high threshold voltage-activated Ca2+ channel currents but did not inhibit the T-type Ca2+ channel current. Photorelease of intracellular guanosine 5'-O(3-thio) triphosphate from a photolabile "caged" precursor had dose-dependent effects on the T-type Ca2+ channel current. At a concentration of 6 microM, guanosine 5'-O(3-thio) triphosphate enhanced the current, but further photorelease of guanosine 5'-O(3-thio) triphosphate (up to 20 microM) inhibited the current. Only the inhibitory response was sensitive to pertussis toxin. These data suggest that more than one G-protein is involved in T-type Ca2+ channel current modulation. Inclusion of guanosine 5'-O(2-thio) diphosphate (1 mM) in the patch solution prevented guanosine 5'-O(3-thio) triphosphate from potentiating the current, and greatly attenuated the inhibitory effects observed when larger amounts of guanosine 5'-O(3-thio) triphosphate were photoreleased. Photorelease of guanosine 5'-O(2-thio) diphosphate had no effect on T-type current but did significantly increase the high voltage-activated current. A low concentration of (-)-baclofen (2 microM), potentiated T-type current, while 100 microM(-)-baclofen inhibited T-type current.

Cation transport probes: the amiloride series

The use of amiloride and its analogs in the study of ion transport requires a knowledge of the pharmacology of inhibition of transport proteins, and of effects on enzymes, receptors, and other cellular processes, such as DNA, RNA, and protein synthesis, and cellular metabolism. We have reviewed the pharmacology of inhibition of these processes by amiloride an its analogs, as well as the use of amiloride analogs as potential probes for the characterization of ion transport systems.

Studies on the mechanism of action of the bipyridine milrinone on the heart

Milrinone is a positive inotropic and vasodilator agent when tested in experimental animals and in human heart-failure patients. It is generally believed that milrinone acts by inhibiting phosphodiesterase IV, thus increasing cyclic AMP, [Ca++]i and cardiac contractile force and relaxation. Maximal force produced by milrinone is greater when single-dose response curves are compared to cumulative dose-response curves. In vitro, milrinone produces a tachyphylaxis, the extent of which is both dose- and time-dependent. Recovery of tachyphylaxis is both dose- and time-dependent and is not influenced by inhibitors of protein or RNA synthesis. There is a specific cross-tachyphylaxis between milrinone and amrinone, theophylline, papaverine, and Bay K8644. This tachyphylaxis may explain the low maximal contractile response of the cumulative dose-response observed in isolated tissues. Milrinone increased cyclic AMP in dog and guinea pig cardiac muscle. As previously shown by Endoh et al., milrinone in low doses produced a biphasic effect on cyclic AMP. The early increase (first 60-70 s) in cyclic AMP shows a good correlation with contractile force changes. If cyclic AMP is determined at maximal contractile force this correlation was poor. Here we also present instances where the increase in cyclic AMP after milrinone (determined at maximal effect) does not correlate with the contractile response. The cross-tachyphylaxis of milrinone with Bay K8644 suggests that milrinone has an action on the sarcolemmal Ca++ channels. Bay K8644 suppresses the positive inotropic effect of catecholamines by 50%, but not the cyclic AMP response. The inotropic effect of milrinone, in contrast to norepinephrine is highly sensitive to [Ca++]0, stimulation rate, and [K+]0. In this respect milrinone behaves more like Bay K8644. We postulate that the main inotropic action of milrinone is due to a sarcolemmal effect. The early cyclic AMP production described could be in the sarcolemmal compartment and this may explain some of the similarities of milrinone's actions with those of Bay K8644. The tachyphylaxis observed with the inotropic effect of milrinone does not extend to the decreases in relaxation time. This and other findings to be discussed suggest that the positive inotropic and reduction in relaxation time by milrinone depend on different mechanisms, possibly through differential compartmentalization of cyclic AMP.

Effects of amiloride analogues on the production of prostacyclin by aortic endothelial cells

1. The release of prostacyclin (PGI2) from bovine aortic endothelial cells stimulated by adenosine 5'-triphosphate (ATP) was decreased by amiloride analogues bearing alkyl groups on the 5-amino nitrogen atom, like 5-(N-ethyl-N-isopropyl)amiloride (EIPA), which are inhibitors of the Na+/H+ exchanger. Analogues substituted on a terminal guanidino nitrogen atom were not inhibitory. 2. The release of PGI2 induced by ATP was not significantly depressed in a Na+-poor medium or in a medium acidified to pH 6.9, two conditions known to inhibit the Na+/H+ exchanger. 3. Cytoplasmic alkalinization by ammonium chloride did not suppress the inhibitory action of EIPA. By itself, ammonium chloride decreased the response of endothelial cells to ionophore A23187 and ATP, whereas sodium acetate had no effect. 4. EIPA did not decrease the mobilization of free arachidonic acid induced by ATP. It inhibited the conversion of exogenous arachidonate into PGI2 and prostaglandin E2 (PGE2). 5. Although the intracellular pH was not measured in this study, it seems unlikely that cytoplasmic alkalinization via the activation of the Na+/H+ exchanger plays a significant role in the stimulatory action of ATP on the release of PGI2 from endothelial cells. The inhibition of that release by EIPA and other amiloride analogues might involve a direct effect on cyclo-oxygenase, although an action on the reacylation of free arachidonic acid cannot be excluded.

Ca2+ and calmodulin-sensitive inositol trisphosphate kinase from bovine parathyroid

A Ca2+ and calmodulin-activated inositol 1,4,5 trisphosphate kinase activity was detected in both soluble and membrane fractions from bovine parathyroid glands. Ca2+ activated the soluble enzyme in the concentration range 100 nM to 1 microM, which corresponds to the Ca2+ concentration range observed in the intact cell following maximal variation in extracellular Ca2+, the principal regulator of parathyroid hormone release. The Ca2+ sensitivity of the enzyme was absolutely dependent upon calmodulin. A similar activity was detected in the membranes but could be progressively removed by repeated washing at low ionic strength. This, together with data demonstrating binding of the enzyme to the hydrophobic matrix, Phenyl-Sepharose, suggests that the association of the enzyme with the membrane is likely to involve a significant hydrophobic component. The organic base, amiloride was identified as an inhibitor of the activity, the degree of inhibition being most marked in the presence of Ca2+ and calmodulin (K0.5 approx. 0.1 mM). The Ca2+ concentration dependence of the IP3 kinase suggests that inositol 1,3,4,5 tetrakisphosphate may be a messenger in the signal transduction pathway for the feedback inhibition of PTH secretion by extracellular Ca2+.

Role of intracellular Na+ in Ca2+ overload and depressed recovery of ventricular function of reperfused ischemic rat hearts. Possible involvement of H+-Na+ and Na+-Ca2+ exchange

The roles of H+-Na+ and Na+-Ca2+ exchange in the depression of ventricular function were studied in the reperfused isolated ischemic rat heart. Zero-flow global ischemia was induced for either 15 or 30 minutes and was followed by 30 minutes of aerobic reperfusion. Intracellular Na+ (Na+i) and 45Ca2+ uptake were measured during ischemia and reperfusion. Accumulation of Na+i was modified by prior glycogen depletion and by treatment with amiloride, a H+-Na+ exchange inhibitor, or monensin, a Na+ ionophore. Na+i rose continuously during ischemia and rapidly during the first two minutes of reperfusion. The larger inhibitory effect of amiloride and preischemic glycogen depletion was on Na+i accumulation during reperfusion; this finding suggests that the uptake occurs by H+-Na+ exchange. Reduction of Na+i accumulation by glycogen depletion was associated with less lactate and, presumably, H+ production and accumulation during ischemia. The rapid increase in Na+i during early reperfusion may reflect the readjustment of the low intracellular pH resulting from ischemia. The level of Na+i at the end of ischemia and especially after two minutes of reperfusion were linearly correlated with 45Ca2+ uptake and depression of ventricular function during subsequent reperfusion. This highly significant correlation between Na+i and 45Ca2+ uptake when Na+i was varied by several independent procedures, including monensin, strongly suggests that reperfusion 45Ca2+ uptake occurs at least in part by Na+-Ca2+ exchange. The rate of 45Ca2+ uptake during reperfusion was linearly and highly significantly correlated with elevation of diastolic pressure, reduced developed pressure, and decreased recovery of ventricular function. The data strongly support a mechanism of ischemic cell damage that involves excessive production and accumulation of H+ during ischemia that exchanges for extracellular Na+ during ischemia and rapidly during the first few minutes of reperfusion. Increased Na+i then causes excessive 45Ca2+ uptake and depressed recovery of cellular functions with continued reperfusion. Increased levels of Na+i may be a major event that couples a decreased intracellular pH during ischemia to excessive 45Ca2+ uptake and depressed recovery of cellular function with reperfusion.

Hippocampal CA1 pyramidal cells of rats have four voltage-dependent calcium conductances

In isolated rat hippocampal neurons, we observed 4 voltage- and extracellular Ca2+-dependent conductances; i.e. the T-, N- and L-type Ca2+ currents and tetrodotoxin-sensitive transient Ca2+ current. Intracellular perfusion with F- suppressed irreversibly the L-type Ca2+ current and partially the N-type one. omega-Conotoxin inhibited selectively the L-type Ca2+ current. Amiloride reduced strongly the T-type Ca2+ current without affecting the L-type one. Gd3+, nicardipine, phenytoin and octanol had no specific inhibition on the T-, N- and L-type Ca2+ currents. Thereby, the pharmacological property of mammalian CNS neurons for Ca2+ channel blockers considerably differs from that in the peripheral and cultured cells.

Expression of depolarizing voltage- and transmitter-activated currents in neuronal precursor cells from the rat brain is preceded by a proton-activated sodium current

The early expression of amiloride-sensitive proton-activated sodium currents (INa(H] was demonstrated using the giga-seal whole-cell voltage clamp technique in cells from the primordial tectum of E12 rat embryos. Less than 10% of these cells stained for tetanus toxin receptors after 2 h in vitro. However, after 10 h in vitro all cells with neuronal geometry were tetanus toxin-positive and capable of generating voltage-activated Na currents (INa(V] and high-voltage activated Ca2+-currents (ICa(HV]. INa(H) was expressed roughly in parallel with INa(V) and ICa(HV), but exceeded the former currents in amplitude by 50-100 times, reaching 600 pA and more. In 25% of the cells tested within the first 5 h in vitro INa(H) was, in fact, the only cationic inward current resolved. Responses to quisqualate and kainate appeared only after 3 days in vitro, and responses to N-methyl-D-aspartate/glycine were seen only after 4 days in vitro. These results suggest that the channels carrying INa(H) are present at the earliest stages of neuronal development.

T-type calcium channels mediate the transition between tonic and phasic firing in thalamic neurons

Thalamic neurons undergo a shift from tonic to phasic (burst) firing upon hyperpolarization. This state transition results from deinactivation of a regenerative depolarizing event referred to as the low-threshold spike. Isolated adult guinea pig thalamic (dorsal lateral geniculate) neurons exhibited low-threshold spikes that could be blocked by low concentrations of nickel but were unaffected by the dihydropyridine nimodipine. Whole-cell voltage-clamp recordings from these cells demonstrated a low-threshold, rapidly inactivating (T) Ca2+ current that manifested similar voltage dependency and time course as the low-threshold spike. Like low-threshold spikes, the T-type Ca2+ current was eliminated by nickel but was unaffected by nimodipine. In thalamic neurons, T-type Ca2+ channels underlie the low-threshold spike and, therefore, play a critical role in regulating the firing pattern of these cells.

Low-voltage-activated calcium current in rat aorta smooth muscle cells in primary culture

1. Electrical and pharmacological properties of the low-voltage-activated Ca2+ current (ICa, LVA) in rat aorta smooth muscle cells (SMC) in primary culture were examined, particularly in comparison with the high-voltage-activated Ca2+ current (ICa, HVA). Both types of Ca2+ currents were recorded in external solution containing 20 mM-Ca2+, using the whole-cell voltage-clamp technique. 2. ICa, LVA was evoked by step depolarizations to potentials more positive than -60 mV from a holding potential of -100 mV, and reached a peak in the current-voltage (I-V) relationship around -30 mV. ICa, HVA was activated at -20 mV, and reached a peak at +20 mV. 3. The intracellular dialysis of 5 mM-F- irreversibly suppressed ICa, HVA, with time, while it has little effect on the ICa, LVA. The ICa, LVA could be separated from the ICa, HVA by either selecting the holding and test potential levels or by perfusing intracellularly with F-. 4. The ratio of peak amplitude of Ba2+, Sr2+ and Ca2+ currents in the respective I-V relationship was 1.6:1.2:1.0 for high-voltage-activated Ca2+ channels and was 1.0:1.4:1.0 for low-voltage-activated ones. 5. The inactivation phase of ICa, HVA was fitted by a sum of double-exponential functions, the time constants of which were larger when the current was carried by Ba2+ than by Ca2+. The inactivation time course of ICa, LVA was fitted by a single-exponential function, and the time constant was practically the same when the current was carried by Ba2+ or by Ca2+. Activation and inactivation processes of ICa, LVA were potential-dependent. 6. The steady-state inactivation curve of ICa, LVA was fitted by the Boltzmann equation, having a mid-potential of -80 mV and a slope factor of 5.0. The recovery time course from steady-state inactivation was fitted by a sum of two exponential functions. The time constants of the faster phase were 230 and 380 ms, and those of slower phase were 2.8 and 1.8 s at the repolarization potentials of -120 and -100 mV, respectively. 7. The amplitude of ICa, LVA depended on the external Ca2+ concentration ([Ca2+]o), approaching saturation at 95 mM [Ca2+]o. 8. Various polyvalent cations blocked both types of Ca2+ current reversibly in the order (IC50 in M): La3+ (8 x 10(-8)) greater than Cd2+ (6 x 10(-6)) greater than Ni2+ (1 x 10(-5)) greater than Zn2+ (2 x 10(-5)) for ICa, HVA, and La3+ (6 x 10(-7)) greater than Zn2+ (3 x 10(-5)) greater than Cd2+ (4 x 10(-4)) greater than Ni2+ (6 x 10(-4)) for ICa, LVA.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of neurally-evoked transmitter release by calcium channel antagonists in rat parasympathetic ganglia

1. Excitatory postsynaptic potentials (e.p.s.ps) were recorded from the submandibular parasympathetic ganglia of newborn rats (10-20 days old), by intracellular microelectrode recording and a suction electrode to deliver stimulus trains to the lingual nerve (15 stimuli at 0.1, 0.3, 1, 3, and 10 Hz, 22 degrees C). Only evoked responses without voltage-dependent action potentials were analyzed (observed at membrane potentials negative to -70 mV), and e.p.s.p. amplitudes were determined for the plateau responses during each train (5-15th response). 2. Cadmium, an inorganic calcium channel antagonist, reduced e.p.s.p. amplitudes in a dose-dependent manner (Kd 74 microM, P less than 0.01). Nickel (1-300 microM) did not attenuate the amplitude of evoked responses. 3. Verapamil (0.1-30 microM), a phenylamine, had no significant effects upon e.p.s.p. amplitudes at any frequency examined. Higher concentrations of verapamil (100 microM) blocked neurally evoked responses in a manner consistent with the antagonism of voltage-sensitive sodium currents. 4. Diltiazem, a benzothiazepine, reduced e.p.s.p. amplitudes in a dose-dependent manner, the depression being accentuated at high stimulation frequencies (80% block at 30 microM and 10 Hz). The pure (-)-cis enantiomer of diltiazem (10-30 microM) was without effect. 5. Amlodipine, a 1,4-dihydropyridine, did not antagonize synaptic transmission at any stimulus frequency examined (10-30 microM, 0.1-10 Hz, n = 3). 6. Amiloride, a potassium-sparing diuretic, depressed the amplitudes of evoked responses in a dose-dependent manner (one-site Kd 31 microM, P less than 0.005), although the extent of the block was alleviated with high stimulus frequencies. The effects of 30 microM amiloride were unlikely to be of post-synaptic origin as both the amplitudes of miniature e.p.s.ps, and the iontophoretic potentials induced by exogenous acetylcholine, were not attenuated by treatment with this compound. The amiloride derivative, 3',4'-dichlorobenzamil was ineffective in reducing the amplitude of e.p.s.ps (30-100 microM). 7. omega-Conotoxin GVIA, a marine neurotoxin, which depressed whole cell calcium currents recorded from cultured rat parasympathetic cardiac neurones (up to 90% block at 10 nM), was ineffective at blocking synaptic transmission in submandibular ganglia (0.1-1 microM). 8. The differential effects of these calcium channel antagonists upon synaptic transmission in rat parasympathetic ganglia, suggest that either more than one type of calcium channel may be involved in transmitter release, or that the presynaptic calcium channels possess pharmacological sensitivities different from those of channel types described in ne

Calcium currents in rat thalamocortical relay neurones: kinetic properties of the transient, low-threshold current

1. Calcium currents were recorded with whole-cell voltage-clamp procedures in relay neurones of the rat thalamus which had been acutely isolated by an enzymatic dissociation procedure. 2. Low-threshold and high-threshold Ca2+ currents were elicited by depolarizing voltage steps from holding potentials more negative than -60 mV. A transient current, analogous to the T-current in sensory neurones, was activated at low threshold near -65 mV and was completely inactivating at command steps up to -35 mV. Voltage steps to more depolarized levels activated a high-threshold current that inactivated slowly and incompletely during a 200 ms step depolarization. 3. The high-threshold current contained both non-inactivating and slowly inactivating components which were insensitive and sensitive to holding potential, respectively. 4. A 'T-type' current was prominent in relay neurones, in both absolute terms (350 pA peak current average) and in relation to high-threshold currents. The average ratio of maximum transient to maximum sustained current was greater than 2. 5. T-current could be modelled in a manner analogous to that employed for the fast Na+ current underlying action potential generation, using the m3h format. The rate of activation of T-current was voltage dependent, with a time constant (tau m) varying between 8 and 2 ms at command potentials of -60 to -10 mV at 23 degrees C. The rate of inactivation was also voltage dependent, and the time constant tau h varied between 50 and 20 ms over the same voltage range. With command potentials more positive than -35 mV, the inactivation of Ca2+ current could no longer be fitted by a single exponential. 6. Steady-state inactivation of T-current could be well fitted by a Boltzman equation with slope factor of 6.3 and half-inactivated voltage of -83.5 mV. 7. Recovery from inactivation of T-current was not exponential. The major component of recovery (70-80% of total) was not very voltage sensitive at potentials more negative than -90 mV, with tau r of 251 ms at -92 mV and 23 degrees C, compared to 225 ms at -112 mV. A smaller, voltage-sensitive component accounted for the remainder of recovery. 8. All kinetic properties, including rates of activation, inactivation, and recovery from inactivation, as well as the amplitude of T-current, were temperature sensitive with Q10 (temperature coefficient) values of greater than 2.5.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuraminidase selectively enhances transient Ca2+ current in cardiac myocytes

Sialic acid, an anionic sugar moiety found peripherally on membrane glycoconjugates, is specifically hydrolyzed from the cell surface by neuraminidase. Because neuraminidase has previously been demonstrated to augment myocardial cell calcium content, the effects of neuraminidase on Ca channel function were studied on voltage-clamped guinea pig ventricular myocytes. In 25-50% of cells, neuraminidase treatment (0.12 U/ml for 20 min) enhanced current through the transient (T) Ca channel by 304 +/- 35% without significantly altering the magnitude of the long-lasting (L) Ca channel current. Exposure to neuraminidase did not affect the voltage dependence of activation or inactivation, nor did it affect the selective inhibition of the T-channel current by amiloride or the L-channel current by nifedipine. After neuraminidase treatment, the T-channel current inactivated more rapidly (time constant decreasing from 8.9 +/- 0.9 to 7.7 +/- 0.6 ms), whereas there was no change in the rate of inactivation of the L-channel current. Neuraminidase treatment removed approximately 20% of the total cellular sialic acid. These results indicate that neuraminidase treatment selectively modulates the function of the T Ca channel in ventricular myocytes, possibly through removal of sarcolemmal sialic acid, suggesting that glycosylation of membrane macromolecules may influence membrane function.

Calcium channels reconstituted from the skeletal muscle dihydropyridine receptor protein complex and its alpha 1 peptide subunit in lipid bilayers

In the first part of this study, we show that sDHPR and pDHPR preparations reconstituted into lipid bilayers formed on the tips of patch pipettes exhibit two divalent cation-selective conductance levels of 9 and 20 pS, similar in single-channel conductance to VSCC reported in a variety of intact preparations (see Pelzer et al. and Tsien et al. for review). The larger conductance level is similar to the VSCC identified in intact rat t-tubule membranes and described in sDHPR and pDHPR preparations, and shares many properties in common with activity from L-type VSCC. It is sensitive to augmentation by the DHP agonist (+/-)-BAY K 8644 and cAMP-dependent phosphorylation, and to block by the phenylalkylamine (+/-)-D600 and the inorganic blocker CoCl2. Its open-state probability and open times are increased upon depolarization as expected for a voltage-dependent activation process. Upon depolarization beyond the reversal potential, however, open-state probability and open times decline again. A reasonable way to explain the bell-shaped dependence of open times and open-state probability on membrane potential is to assume voltage-dependent ion-pore interactions that produce closing of the channel at strong negative and positive membrane potentials. By contrast, the smaller conductance level may be similar to the 10.6-pS t-tubule VSCC described by Rosenberg et al. and may best be compared with T-type VSCC. It is largely resistant to augmentation by (+/-)-BAY K 8644 and cAMP-dependent phosphorylation or block by (+/-)-D600, but is sensitive to block by CoCl2. Its open times and open-state probability show a sole dependence on membrane potential where depolarization increases both parameters sigmoidally from close to zero up to a saturating level. Both elementary conductance levels do not exhibit significant inactivation over a wide potential range, which may suggest that skeletal muscle VSCC inactivation is either poorly or not voltage-dependent at all. This possibility seems in agreement with bilayer recordings on reconstituted intact t-tubule membranes and voltage-clamp recordings on intact fibers. It supports the idea that the decline of Ca2+ current in intact skeletal muscle fibers may be due to Ca2+ depletion from the t-tubule system and/or to inactivation induced by Ca2+ release from the sarcoplasmic reticulum. We consistently observe two conductance levels of 9 and 20 pS, either singly, or together in the same bilayer from solubilized DHPR samples and even highly purified DHPR preparations.(ABSTRACT TRUNCATED AT 400 WORDS)

Development alters the expression of calcium currents in chick limb motoneurons

Calcium current types expressed in vertebrate spinal motoneurons have not been previously resolved. We have resolved three types in chick limb motoneurons identified by retrograde labeling and report that dramatic changes in their expression take place during development in vivo. T-, N-, and L-type calcium currents were distinguished on the basis of kinetics, voltage dependence, and unique pharmacological sensitivities. Developmental changes were characterized by studying motoneurons isolated from embryos at three stages spanning neuromuscular system development. T currents were dominant at the earliest stage. Motoneurons from embryos 2 days older showed much reduced T currents and much increased N and L currents. We suggest that mature motoneurons will be dominated by N- and L-type calcium currents and that T current may serve developmental roles.

Effects of amiloride in guinea-pig and rat left atrial contraction as affected by frequency of stimulation and [Ca2+]0-[Na+]0 ratio: role of Na+/Ca2+ exchange

1. The effect of amiloride (0.5 mM) on guinea-pig and rat left atria driven at various rates of stimulation and different [Ca2+]0-[Na+]0 ratios has been studied. 2. Amiloride elicited a positive inotropic response in guinea-pig left atria driven at 0.1 Hz, 0.5 Hz and 1 Hz when [Ca2+]0 was 3.6 mM, 1.8 mM and 0.9 mM respectively but not when [Ca2+]0 was 2.7 mM at 0.1 Hz, 0.9 mM at 0.5 Hz and 0.45 mM at 1 Hz. 3. A positive inotropic response was obtained in guinea-pig left atria driven at 0.1 Hz and 1 Hz when [Ca2+]0-[Na+]0(2) was increased respectively from 8 x 10(-5) to 16 x 10(-5) and from 2 x 10(-5) to 8 x 10(-5). The positive inotropic effect was evident only when the ratio was increased by increasing [Ca2+]0 and not by decreasing [Na+]0. 4. In the presence of amiloride, the force of contraction of guinea-pig left atria decreased instead of increasing, when the rate of stimulation was lowered from 1 Hz to 0.01 Hz. Amiloride inhibited the post-rest potentiation. 5. In rat left atria amiloride was devoid of any effect in all the above-mentioned experimental conditions. 6. It is suggested that the pattern of cardiac actions of amiloride can be explained by the inhibition of the Na+/Ca2+ exchange system.

Serotonin modulates calcium-dependent plateau of action potentials recorded from bull frog A-type sensory neurons which is omega-conotoxin GVIA-sensitive, but dihydropyridine-insensitive

Tetraethylammonium-treated bull frog sensory neurons exhibit a calcium-dependent plateau on the falling limb of the action potential, which is reduced in duration by 5-HT. The portion of the calcium-dependent plateau which is reduced by 5-HT is blocked by omega-conotoxin GVIA but is not reduced by nifedipine or enhanced by Bay K 8644. The lack of effect of 5-HT, when retested after exposure of a previously 5-HT sensitive neuron to conotoxin favors the hypothesis that 5-HT reduces the duration of the calcium-dependent plateau by inactivating voltage-dependent calcium channels, rather than by increasing a voltage-dependent potassium current.

The plasma membrane potential of human neutrophils. Role of ion channels and the sodium/potassium pump

Calcium-depleted human neutrophils are depolarised when suspended in calcium-free media containing sodium ions, and are repolarised by extracellular replenishment of Ca2+. The depolarisation is due to a high inward sodium current, which is blocked by calcium and by several other divalent cations, but not by barium. Addition of calcium results in a rise in the cytosolic concentration from approx. 20 nM to the resting level of approx. 130 nM. Calcium influx is strongly accelerated by a voltage-gated calcium channel. This channel might be responsible for the depolarising Na+ current in the absence of divalent cations. In the polarised state the neutrophil membrane has a high intrinsic permeability to K+, which may be low or absent in the depolarised state. Generation of membrane potential from the depolarised state is mainly due to the electrogenic sodium/potassium pump. However, the resting potential of about -75 mV is maintained primarily by the K+ conductance, and only to a small extent by the sodium/potassium pump.

Accelerated ion fluxes during differentiation in zoospores of Phytophthora palmivora

Zoospores of Phytophthora palmivora show increased fluxes of Na+ and Ca2+ 3-5 min after they have been stimulated to differentiate with pectin. Both spontaneous and pectin-induced encystment are reduced below pH 6 and accelerated above pH 7. The ionophores monensin and A23187 induce slow differentiation when added together, but not when added separately. Ethanol (0.5%) also induces slow differentiation. Amiloride and verapamil inhibit pectin-induced differentiation and also reduce the onset of the Na+ and Ca2+ flux. A requirement for Ca2+ for differentiation is confirmed, but a requirement for Na+ could not be demonstrated.

Effect of amiloride on cell volume regulation in renal straight proximal tubules

Amiloride has been shown to impair cell volume regulatory decrease in amphiuma red cells. The present study has been performed to test for the influence of amiloride on volume regulatory decrease and electrical properties in isolated perfused mouse straight proximal tubules. Replacement of 40 mmol/l NaCl with 80 mmol/l mannitol in bath perfusate does not appreciably affect the cell volume or the potential difference across the basolateral cell membrane. Reduction of osmolarity by omission of mannitol leads to cell swelling by 16.7 +/- 0.7% (n = 7), followed by volume regulatory decrease to 107.2 +/- 1.2% (n = 7) of original cell volume within 2 min. 1 mmol/l amiloride (but not 0.1 mmol/l amiloride) in the bath depolarizes the basolateral cell membrane from -63 +/- 1 mV (n = 24) by +16 +/- 1 mV (n = 16), decreases the apparent potassium transference number from 0.69 +/- 0.02 (n = 5) to 0.36 +/- 0.05 (n = 5), and significantly impairs volume regulatory decrease without appreciably modifying cell volume in isotonic solutions. 1 mmol/l amiloride in the luminal perfusate leads to a slight hyperpolarization of the basolateral cell membrane but does not interfere with volume regulatory decrease. Reduction of bath osmolarity depolarizes the basolateral cell membrane within 30 s by +7.8 +/- 0.8 mV (n = 18) in the absence and by +18 +/- 2 mV (n = 8) in the presence of amiloride. In the presence of reduced bath osmolarity and amiloride the potassium transference number amounts to 0.36 +/- 0.04 (n = 8). The hyperpolarization following luminal application of amiloride is most likely due to inhibition of luminal sodium channels, whereas bath amiloride depolarizes the basolateral cell membrane by reduction of basolateral potassium selectivity. As in amphiuma red cells amiloride impairs volume regulatory decrease in proximal straight renal tubules.

Low- and high-voltage-activated calcium currents: their relationship to the site of neurotransmitter release in an identified neuron of Helisoma

In this study we have characterized two types of Ca2+ currents in identified neuron B5 of Helisoma and have examined the relationship between these currents and neurotransmitter release. Neuron B5 contains low-voltage-activated (LVA) and high-voltage-activated (HVA) Ca2+ currents. These currents have distinct electrophysiological and pharmacological properties. To gain access to the site of neurotransmitter release, we used a model system in which somata that do not extend neurites assume the role of neurotransmitter release. Before somata gain the ability to release neurotransmitter, they contain LVA and HVA Ca2+ currents. After 3 days of culture, when spherical somata have gained the secretory capacity, only the HVA Ca2+ current is present. Experiments were also performed when neurite extension was permitted. These data indicate that neurons with processes have a differential distribution of Ca2+ currents. The soma, which does not release neurotransmitter, contains both LVA and HVA Ca2+ currents, while distal secretory processes contain only HVA current.

Amiloride-blockable sodium currents in isolated taste receptor cells

Isolated taste receptor cells from the frog tongue were investigated under whole-cell patch-clamp conditions. With the cytosolic potential held at -80 mV, more than 50% of the cells had a stationary inward Na current of 10 to 700 pA in Ringer's solution. This current was in some cells partially, in others completely, blockable by low concentrations of amiloride. With 110 mM Na in the external and 10 mM Na in the internal solution, the inhibition constant of amiloride was (at -80 mV) near 0.3 microM. In some cells the amiloride-sensitive conductance was Na specific; in others it passed both Na and K. The Na/K selectivity (estimated from reversal potentials) varied between 1 and 100. The blockability by small concentrations of amiloride resembled that of channels found in some Na-absorbing epithelia, but the channels of taste cells showed a surprisingly large range of ionic specificities. Receptor cells, which in situ express these channels in their apical membrane, may be competent to detect the taste quality "salty." The same cells also express TTX-blockable voltage-gated Na channels.