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T Hoshi

Publications and source records attributed to T Hoshi.

At least 145 records · Page 8Linked to original sources
PubMed1990

On the optimality principle in a bio-flow absorption system: an analysis of the Jmax-Kt relationship in the proximal tubule of the kidney.

The absorption process of sugars and amino acids in the proximal tubule of the kidney is one of the typical examples of operation of bio-flow absorption systems in the living body. The relationship between two kinetic parameters of the Michaelis-Menten equation, Jmax and Kt, which are the main determinants of the reaction velocity in the absorption process, was investigated by using experimentally obtained data for several actively transported substances and a mathematical model of the proximal tubule. When examined the data of the parameters for sugars and amino acids obtained from both amphibian (Triturus) and mammalian (rat) kidneys, a distinct and common tendency was found such that a high affinity system has a low capacity and vice versa. For all substances tested in this study, the calculated concentration profile along the tubule monotonically reduced to zero at around the middle portion of the tubule, although each substance has considerably different values of Jmax and Kt. From the digital simulation, the observed Jmax-Kt relationship among the tested substances could be explained in terms of the optimality principle, by which all the systems could accomplish absorption with a sufficient reserve capacity within a limited length of the tubule. The importance of such an elaborated arrangement of the transport systems in the renal proximal tubule is discussed from the viewpoints of evolution and functional speciality of the organ.

Absorption↗
PubMed1989

Simulated weightlessness and bone metabolism: decreases of protein and DNA syntheses in the femoral diaphysis of rats.

The effect of simulated weightlessness on bone protein and DNA syntheses was investigated in the skeletal unloading for up to 4 days. The skeletal unloading was designed by using the model of hindlimb hang in rats. The femoral-diaphyseal fragments obtained from rats which bred with skeletal unloading were cultured for 3 h at 37 degrees C in 5% CO2/95% air in Dulbecco's Modified Eagle Medium (high glucose). When the bone tissues were pulsed with [3H]proline, the incorporation of [3H]proline into the bone protein was significantly decreased by skeletal unloading. In the pulse with [14C]uridine, the incorporation of [14C]uridine into the bone RNA was significantly reduced by skeletal unloading. Moreover, the incorporation of [3H]thymidine into the acid-insoluble residues of bone tissue was significantly decreased by skeletal unloading. Also, DNA content in the femoral diaphysis was significantly reduced by skeletal unloading. These findings suggest that skeletal unloading causes the decreases of protein and DNA syntheses in the femoral diaphysis of rats.

Animals↗
PubMed1989

Simulated weightlessness and bone metabolism: decrease of alkaline phosphatase activity in the femoral diaphysis of rats.

The effect of simulated weightlessness on bone metabolism was investigated in the skeletal unloading for up to 9 days. The skeletal unloading was designed by using the model of hindlimb hang in rats. The food ingestion of rats with the skeletal unloading was not altered in comparison to that of normal rats. Calcium concentration in the serum was not significantly altered by the skeletal unloading for 9 days, while the serum inorganic phosphorus concentration was significantly decreased at 6 and 9 days. Calcium content in the femoral diaphysis was not altered by the skeletal unloading for 9 days. However, the activities of alkaline and acid phosphatases in the femoral diaphysis were markedly decreased by the skeletal unloading. The decrease in bone alkaline phosphatase activity was seen at 2 days with the skeletal unloading. The present results demonstrate that the skeletal unloading with hindlimb hang can induce the disorder of bone metabolism. This model is useful for studying the effects of simulated weightlessness on bone metabolism.

Alkaline Phosphatase↗
PubMed1989

Properties of ShB A-type potassium channels expressed in Shaker mutant Drosophila by germline transformation.

We have used P element-mediated germline transformation to express ShB channels in Shaker mutant Drosophila and have examined their properties by patch-clamp of embryonic myotubes. The transformed ShB cDNA was placed under the transcriptional control of a heat shock promoter (hsp70). Northern blots revealed that transformed DNA is efficiently transcribed in response to heat shock. Cultured myotubes from the transformants produced large A-type potassium currents in response to heat shock. Although qualitatively similar to native Shaker A-currents in wild-type myotubes, transformant A-current inactivates more rapidly and recovers from inactivation more rapidly, similar to ShB channels expressed in Xenopus oocytes. Unlike the channels in oocytes, however, the transformant A-current is insensitive to 50 nM charybdotoxin.

Animals↗
PubMed1989

Gating of single Shaker potassium channels in Drosophila muscle and in Xenopus oocytes injected with Shaker mRNA.

The voltage-dependent gating mechanism of single A-type potassium channels coded for by the Shaker locus of Drosophila was studied by single-channel recording. A-type channels expressed in Xenopus oocytes injected with Shaker B and Shaker D mRNA exhibited gating and voltage dependence that were qualitatively similar to those of the native Shaker A-types channels from embryonic myotubes. In all three channel types the molecular transition rates leading to the first opening were voltage-dependent, whereas all transitions after the first opening, including inactivation, were independent of voltage. While these channels exhibit some quantitative differences in their transition rates that account for the observed differences in macroscopic currents, in all three cases the voltage dependence of the macroscopic currents is determined by a voltage dependence in the time to first opening. This gating mechanism is similar to that of the vertebrate voltage-gated sodium channel and, together with the sequence similarities in the S4 region of the proteins, suggests a conserved mechanism for activation and inactivation.

Animals↗
PubMed1989

Regulation of ionic currents in pheochromocytoma cells by nerve growth factor and dexamethasone.

Growth factors and hormones induce differentiation of clonal pheochromocytoma (PC12) cells, which are derived from rat adrenal medulla chromaffin cells. On application of nerve growth factor (NGF), PC12 cells extend neurites and express properties characteristic of autonomic ganglion cells. In contrast, incubation of PC12 cells with a corticosteroid, dexamethasone (DEX), does not induce neurite formation but causes an increase in tyrosine hydroxylase activity, suggesting that the cells become chromaffin cell-like. The ability of NGF and DEX to regulate ionic currents has been less well studied. Therefore, we examined how long-term NGF and DEX treatments affected voltage-dependent Na, Ca, and K currents in PC12 cells. Voltage-dependent Na currents were observed only in a small fraction of the PC12 cells in the absence of NGF or DEX. Virtually all NGF-treated cells expressed Na currents within 7 d. DEX increased the number of cells expressing voltage-dependent Na current slowly over 3 weeks, but, unlike NGF, DEX did not change Na current density. Both NGF and DEX also affected the expression of voltage-dependent Ca currents. Most of the untreated cells had only sustained, high-threshold voltage-dependent Ca currents. Chronic application of NGF or DEX increased the fractions of the cells that showed transient, low-threshold T-type Ca currents in addition to the high-threshold currents. The T-type Ca current density, however, increased significantly only in NGF-treated cells. Neither DEX nor NGF affected the voltage-dependent K currents. These results suggest that the expression of voltage-dependent Na and Ca currents are differentially regulated by NGF and DEX. The distinction between treated and untreated cells decreased after 3 weeks in culture as older untreated cells showed increases in the fraction of cells expressing both Na and low-threshold Ca currents. A PC12 subline selected for adherence to uncoated plastic also showed increased fraction of cells expressing these currents, suggesting that interactions with substrate may also influence ionic current expression.

Adrenal Gland Neoplasms↗
PubMed1988

Effect of forskolin on voltage-gated K+ channels is independent of adenylate cyclase activation.

Forskolin is commonly used to stimulate adenylate cyclase in the study of modulation of ion channels and other proteins by adenosine 3',5'-monophosphate (cAMP)-dependent second messenger systems. In addition to its action on adenylate cyclase, forskolin directly alters the gating of a single class of voltage-dependent potassium channels from a clonal pheochromocytoma (PC12) cell line. This alteration occurred in isolated cell-free patches independent of soluble cytoplasmic enzymes. The effect of forskolin was distinct from those of other agents that raise intracellular cAMP levels. The 1,9-dideoxy derivative of forskolin, which is unable to activate the cyclase, was also effective in altering the potassium channel activity. This direct action of forskolin can lead to misinterpretation of results in experiments in which forskolin is assumed to selectively activate adenylate cyclase.

1-Methyl-3-isobutylxanthine↗
PubMed1988

Na+-dependent elevation of the acidic cell surface pH (microclimate pH) of rat jejunal villus cells induced by cyclic nucleotides and phorbol ester: possible mediators of the regulation of the Na+/H+ antiporter.

The effects of cyclic nucleotides and phorbol ester on the acidic cell surface pH of rat jejunal villi were studied by using single-barrelled pH-sensitive microelectrodes. Addition of dibutyryl cAMP (1 mM) to the mucosal bathing solution caused an elevation of the cell surface pH from 6.19 +/- 0.04 (n = 12 measurements from three animals) to 6.53 +/- 0.03 (12) in the presence of Na+ in the medium. However, dibutyryl cAMP had no significant effect in the absence of Na+ and presence of 1 mM amiloride. Dibutyryl cGMP (1 mM) also had an Na+-dependent inhibitory effect on the cell surface pH. A phorbol ester, phorbol 12-myristate 13-acetate, caused an elevation of the cell surface pH only in the presence of Na+ from 6.14 +/- 0.07 (12) to 6.46 +/- 0.08 (12). Phorbol and phorbol 13-acetate, which do not stimulate protein kinase C, were without significant effects. These results suggest that increased levels of the intracellular cyclic nucleotides and activation of protein kinase C raise the acidic cell surface pH by inhibiting the activity of the brush-border Na+/H+ antiporter in the rat jejunal villus cells.

Amiloride↗
PubMed1988

Gating kinetics of four classes of voltage-dependent K+ channels in pheochromocytoma cells.

Clonal pheochromocytoma (PC-12) cells have four different types of voltage-dependent K+ channels whose activation does not require high concentrations of Ca++ on the cytoplasmic side of the membrane (Hoshi, T., and R. W. Aldrich, 1988, Journal of General Physiology, 91:73-106). The durations of open and closed events of these four different types of voltage-dependent K+ channels were measured using the excised configuration of the patch-clamp method. The open durations of a class of K+ channels termed the Kz channel, which activates rapidly and inactivates slowly in response to depolarizing pulses, had two exponential components. The closed durations of the Kz channel had at least four exponential components. The time constants of the fastest of the two exponential components in the closed durations were very similar to those of the two exponential components present in the first-latency distribution. The first latencies of the Kz channel decreased steeply with depolarization, contributing to the increased probability of the channel being open with depolarization. The Kz channel also had a very slow gating process that resulted in a clustering of blank sweeps. A gating scheme containing two open states and five closed states is consistent with the observations. The Ky channel had one exponential component in the open durations and three exponential components in the closed durations. The first latencies varied greatly depending on the prepulse voltage and duration. The results were consistent with a sequential model with a large number of closed states and one open state. The Kx channel, which required large hyperpolarizing prepulses to remove steady state inactivation and did not show inactivation with maintained depolarization, had two exponential components in the open durations and three exponential components in the closed durations. The burst behavior of the Kx channel involved many more than two states. The transient Kw channel had one exponential component in the open durations and the mean open time increased with depolarization. The first latencies of the Kw channel were steeply dependent on the voltage, decreasing with depolarization.

Animals↗
PubMed1988

Voltage-dependent K+ currents and underlying single K+ channels in pheochromocytoma cells.

Properties of the whole-cell K+ currents and voltage-dependent activation and inactivation properties of single K+ channels in clonal pheochromocytoma (PC-12) cells were studied using the patch-clamp recording technique. Depolarizing pulses elicited slowly inactivating whole-cell K+ currents, which were blocked by external application of tetraethylammonium+, 4-aminopyridine, and quinidine. The amplitudes and time courses of these K+ currents were largely independent of the prepulse voltage. Although pharmacological agents and manipulation of the voltage-clamp pulse protocol failed to reveal any additional separable whole-cell currents in a majority of the cells examined, single-channel recordings showed that, in addition to the large Ca++-dependent K+ channels described previously in many other preparations, PC-12 cells had at least four distinct types of K+ channels activated by depolarization. These four types of K+ channels differed in the open-channel current-voltage relation, time course of activation and inactivation, and voltage dependence of activation and inactivation. These K+ channels were designated the Kw, Kz, Ky, and Kx channels. The typical chord conductances of these channels were 18, 12, 7, and 7 pS in the excised configuration using Na+-free saline solutions. These four types of K+ channels opened in the presence of low concentrations of internal Ca++ (1 nM). Their voltage-dependent gating properties can account for the properties of the whole-cell K+ currents in PC-12 cells.

4-Aminopyridine↗
PubMed1987

Role of Na+/H+ antiport in intracellular pH regulation by rabbit enterocytes.

The steady-state intracellular pH (pHi) of isolated rabbit enterocytes was determined using 9-aminoacridine, a fluorescent weak base, and the null-point method with digitonin. When cells are incubated in a Na+-containing solution, the estimated value of pHi was in the range of 7.10-7.20, whereas it was 6.60-6.70 when cells were incubated in a Na+-free solution, indicating an important role of external Na+ in maintaining pHi at a slightly alkaline level. Pulse injection of Na+ into a Na+-free cell suspension induced a slowly developing alkalinization of pHi. The time course of the alkalinization was found to be dependent on the Na+ concentration. Li+ had the same effect as Na+, while K+ had a slight effect. Amiloride inhibited the effects of Na+ dose-dependently. These results indicate that the Na+/H+ antiport plays an important role in maintaining the pHi at a neutral or slightly alkaline level in the intact enterocytes.

Amiloride↗
PubMed1987

Transient Ca2+-channel current characterized by a low-threshold voltage in zona glomerulosa cells of rat adrenal cortex.

Voltage-gated Ca2+-current was identified in single isolated cells of the zona glomerulosa of adrenal cortex and its properties were studied by the "tight-seal" whole cell recording technique. The Ca2+ channel current was dissected from the net current by dialyzing the cells with ScCL. The identified Ca2+-current was found to be activated by a relatively small depolarization only when the cells were held at a large negative holding potential, but it was inactivated within 10-30 ms. The time course of activation and inactivation was voltage-dependent and become faster when the amplitude of depolarization was increased. The transmembrane potential of the glomerulosa cells was highly sensitive to [K+]e, the slope of the potential change per tenfold change in [K+]e being 48 mV. An increase in [K+]e from 4.7 to 10 mM induce a membrane depolarization by 15 mV, which was sufficient to cause the membrane to reach the threshold potential (-60 mV) for activation of the Ca2+-current at physiological concentration of [Ca2+]e (2.5 mM-CaCl2). The observed properties of the Ca2+-current and K+-dependence of the membrane potential may give reasonable explanation for the mechanism of Ca2+-uptake and consequent aldosterone secretion induced by a small increase in [K+]e, which is known to be one of the major stimulations for aldosterone secretion.

Adrenal Cortex↗
PubMed1987

Characteristics of transmural potential changes associated with the proton-peptide co-transport in toad small intestine.

1. Ionic dependence and kinetic properties of the peptide-evoked potentials across everted toad intestine were investigated with eighteen dipeptides and four tripeptides. All peptides evoked saturable increases in the mucosal negativity regardless of the presence of Na+. 2. The peptide-evoked potentials recorded in the absence of Na+ were sensitive to external pH (pHo); lowering pHo from 7.4 to 6.5 and 5.5 caused stepwise increases in their amplitude. 3. Loading epithelial cells with 9-aminoacridine or acetate caused a significant increase or decrease in amplitude of the Gly-Gly-evoked potential, suggesting intracellular alkalinization or acidification also has a great influence on the peptide-evoked potential. 4. Kinetically, Na+-independent peptide-evoked potentials conformed to simple Michaelis-Menten kinetics, and lowering pHo caused a decrease of the half-saturation concentration (Kt) for Gly-Gly without changing the maximum potential difference increase. Similar affinity-type kinetic effect was also seen for Gly-Gly influx. 5. Simultaneous measurements of Gly-Gly-induced increase in short-circuit current and Gly-Gly influx revealed that the coupling ratio of H+ and Gly-Gly flows was 1.78 +/- 0.12, suggesting the stoichiometry of the H+-peptide co-transport being 2:1. 6. Kinetic analyses of the peptide-evoked potentials indicated that all glycyl-dipeptides tested (Gly-Gly, Gly-Pro, Gly-Sar, Gly-Leu, Gly-Phe) and other dipeptides (Ala-Ala, Ala-Phe, Phe-Ala) shared a common carrier. Gly-Gly-Gly and Ala-Ala-Ala were also found to share the same carrier, while Phe-Phe, Leu-Leu and Phe-Leu appeared to be transported by a different carrier. 7. Kt values for di- and tripeptides, which apparently shared a common carrier, fell in a narrow range (0.5-2.2 mM). There was no clear correlation between 1/Kt value and molecular weight.

Animals↗
PubMed1987

Large depolarization induces long openings of voltage-dependent calcium channels in adrenal chromaffin cells.

Single Ca2+-channel currents in bovine adrenal chromaffin cells were studied with the patch-clamp technique using Ba2+ as the charge carrier. Depolarizing pulses to voltages less than +10 mV from holding voltage of -60 mV elicited short openings with a mean life time of less than 1 msec. Depolarization to more positive voltages elicited longer openings with a mean life time of about 3 msec in addition to the short openings similar to those observed at less positive voltages. Following large depolarizing prepulses, 2 types of "tail" openings, one with a mean duration of less than 1 msec and the other with a mean duration of 4 msec, were observed. In the presence of a dihydropyridine BAY K 8644, openings with a mean duration of more than 12 msec were present. Depolarization-induced long openings and BAY K 8644-produced long openings differed in the first latency and open-time properties. The results could be explained in terms of multiple open states of one type of Ca2+ channel. A kinetic model with at least 2 open states is required to explain activation of Ca2+ channels in chromaffin cells.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1986

Hydrogen ion-coupled transport of D-glucose by phlorizin-sensitive sugar carrier in intestinal brush-border membranes.

In rabbit intestinal brush-border membrane vesicles, Na+-independent D-glucose uptake in the presence of an inside-negative transmembrane potential was found to be stimulated by an imposed pH gradient. Na+-independent, pH-dependent and phlorizin-sensitive D-glucose-evoked potentials could be recorded from isolated toad intestine. The obtained data suggest that phlorizin-sensitive D-glucose carriers of intestinal brush-border membrane can interact with H+ when Na+ is absent.

Action Potentials↗