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Biomedical subjects

R B Silver

Publications and source records attributed to R B Silver.

At least 19 recordsLinked to original sources
PubMed2001

Coupling of histamine H3 receptors to neuronal Na+/H+ exchange: a novel protective mechanism in myocardial ischemia.

In myocardial ischemia, adrenergic nerves release excessive amounts of norepinephrine (NE), causing dysfunction and arrhythmias. With anoxia and the concomitant ATP depletion, vesicular storage of NE is impaired, resulting in accumulation of free NE in the axoplasm of sympathetic nerves. Intraneuronal acidosis activates the Na(+)/H(+) exchanger (NHE), leading to increased Na(+) entry in the nerve terminals. These conditions favor availability of the NE transporter to the axoplasmic side of the membrane, causing massive carrier-mediated efflux of free NE. Neuronal NHE activation is pivotal in this process; NHE inhibitors attenuate carrier-mediated NE release. We previously reported that activation of histamine H(3) receptors (H(3)R) on cardiac sympathetic nerves also reduces carrier-mediated NE release and alleviates arrhythmias. Thus, H(3)R activation may be negatively coupled to NHE. We tested this hypothesis in individual human SKNMC neuroblastoma cells stably transfected with H(3)R cDNA, loaded with the intracellular pH (pH(i)) indicator BCECF. These cells possess amiloride-sensitive NHE. NHE activity was measured as the rate of Na(+)-dependent pH(i) recovery in response to an acute acid pulse (NH(4)Cl). We found that the selective H(3)R-agonist imetit markedly diminished NHE activity, and so did the amiloride derivative EIPA. The selective H(3)R antagonist thioperamide abolished the imetit-induced NHE attenuation. Thus, our results provide a link between H(3)R and NHE, which may limit the excessive release of NE during protracted myocardial ischemia. Our previous and present findings uncover a novel mechanism of cardioprotection: NHE inhibition in cardiac adrenergic neurons as a means to prevent ischemic arrhythmias associated with carrier-mediated NE release.

Animals↗
PubMed2001

Porocytosis: a new approach to synaptic function.

We propose a new approach to address the question of how a single quantum of neurotransmitter is secreted from a presynaptic terminal whose clustered secretory vesicles are locally bathed in high levels of calcium ions [Proceedings of the Symposium on Bioelectrogenesis (1961) 297-309; The Physiology of Synapses (1964) Chapters 1, 4, 5, 6; How the Self Controls its Brain (1994) Chapters 1, 4, 5, 6; Science 256 (1992) 677-679]. This hypothesis, which we term 'porocytosis', posits that the post-synaptic quantal response results from transmitter secreted through an array of docked vesicle/secretory pore complexes. The transient increase in calcium ions, which results from the voltage activated calcium channels, stimulates the array of secretory pores to simultaneously flicker open to pulse transmitter. Porocytosis is consistent with the quantal nature of presynaptic secretion and transmission, and with available biochemical, morphological and physiological evidence. It explains the frequency dependency of quantal size as a function of the secretion process. It permits a signature amount of transmitter release for different frequencies allowing a given synapse to be employed in different behavioral responses. The porocytosis hypothesis permits fidelity of secretion and the seemingly apposed characteristic of synaptic plasticity. The dynamics inherent in an array insure a constant quantal size as a function of the number of units within the array. In this hypothesis, plasticity is a consequence of concurrent pre- and post-synaptic changes due to a change in array size. Changes in the number of docked vesicle-secretory pore complexes composing the array can explain facilitation, depletion, graded excitation-secretion and long term plasticity.

Animals↗
PubMed2000

Constitutive signaling by Kaposi's sarcoma-associated herpesvirus G-protein-coupled receptor desensitizes calcium mobilization by other receptors.

We coexpressed Kaposi's sarcoma-associated herpesvirus G protein-coupled receptors (KSHV-GPCRs) with thyrotropin-releasing hormone (TRH) receptors or m1-muscarinic-cholinergic receptors in Xenopus oocytes and in mammalian cells. In oocytes, KSHV-GPCR expression resulted in pronounced (81%) inhibition (heterologous desensitization) of Ca(2+)-activated chloride current responses to TRH and acetylcholine. Similar inhibitions of cytoplasmic free Ca(2+) responses to TRH were observed in human embryonic kidney HEK 293 EM cells and in mouse pituitary AtT20 cells. Further study of oocytes showed that this inhibition was partially reversed by interferon-gamma-inducible protein 10 (IP-10), an inverse agonist of KSHV-GPCR. The basal rate of (45)Ca(2+) efflux in oocytes expressing KSHV-GPCRs was 4.4 times greater than in control oocytes, and IP-10 rapidly inhibited increased (45)Ca(2+) efflux. In the absence of IP-10, growth-related oncogene alpha caused a further 2-fold increase in (45)Ca(2+) efflux. In KSHV-GPCR-expressing oocytes, responses to microinjected inositol 1,4,5-trisphosphate were inhibited by 74%, and this effect was partially reversed by interferon-gamma-inducible protein 10. Treatment with thapsigargin suggested that the pool of calcium available for mobilization by TRH was decreased in oocytes coexpressing KSHV-GPCRs. These results suggest that constitutive signaling by KSHV-GPCR causes heterologous desensitization of responses mediated by other receptors, which signal via the phosphoinositide/calcium pathway, which is caused by depletion of intracellular calcium pools.

Acetylcholine↗
PubMed2000

Potassium restriction downregulates ROMK expression in rat kidney.

The ROMK family of proteins has biophysical properties and distribution within the kidney similar to those of secretory potassium channels of the distal nephron. To study the regulation of ROMK during variations in dietary potassium, we measured the abundance of ROMK protein in rat kidney by immunoblotting. Neither 2 nor 5 days of a high-potassium diet had an effect on protein abundance in the cortex or medulla. Potassium deprivation (2 or 5 days) decreased ROMK protein content in both cortical and medullary fractions, to 51 and 40% of controls, respectively. To see whether the Na-K-2Cl cotransporter is similarly affected by potassium restriction, we analyzed immunoblots by using an antibody for the rat type 1 bumetanide-sensitive cotransporter (BSC-1). Like ROMK, BSC-1 protein content was found to decrease significantly in the renal medulla of potassium-deprived rats. In the thick ascending limb of Henle's loop, a decrease in ROMK and BSC-1 could result in decreased reabsorption of NaCl, a finding associated with hypokalemia.

Animals↗
PubMed2000

Potassium depletion increases proton pump (H(+)-ATPase) activity in intercalated cells of cortical collecting duct.

Intercalated cells (ICs) from kidney collecting ducts contain proton-transporting ATPases (H(+)-ATPases) whose plasma membrane expression is regulated under a variety of conditions. It has been shown that net proton secretion occurs in the distal nephron from chronically K(+)-depleted rats and that upregulation of tubular H(+)- ATPase is involved in this process. However, regulation of this protein at the level of individual cells has not so far been examined. In the present study, H(+)-ATPase activity was determined in individually identified ICs from control and chronically K(+)-depleted rats (9-14 days on a low-K(+) diet) by monitoring K(+)- and Na(+)-independent H(+) extrusion rates after an acute acid load. Split-open rat cortical collecting tubules were loaded with the intracellular pH (pH(i)) indicator 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, and pH(i) was determined by using ratiometric fluorescence imaging. The rate of pH(i) recovery in ICs in response to an acute acid load, a measure of plasma membrane H(+)-ATPase activity, was increased after K(+) depletion to almost three times that of controls. Furthermore, the lag time before the start of pH(i) recovery after the cells were maximally acidified fell from 93.5 +/- 13.7 s in controls to 24.5 +/- 2.1 s in K(+)-depleted rats. In all ICs tested, Na(+)- and K(+)-independent pH(i) recovery was abolished in the presence of bafilomycin (100 nM), an inhibitor of the H(+)-ATPase. Analysis of the cell-to-cell variability in the rate of pH(i) recovery reveals a change in the distribution of membrane-bound proton pumps in the IC population of cortical collecting duct from K(+)-depleted rats. Immunocytochemical analysis of collecting ducts from control and K(+)-depleted rats showed that K(+)-depletion increased the number of ICs with tight apical H(+)ATPase staining and decreased the number of cells with diffuse or basolateral H(+)-ATPase staining. Taken together, these data indicate that chronic K(+) depletion induces a marked increase in plasma membrane H(+)ATPase activity in individual ICs.

Acid-Base Equilibrium↗
PubMed2000

Chronic recording of regenerating VIIIth nerve axons with a sieve electrode.

A micromachined silicon substrate sieve electrode was implanted within transected toadfish (Opsanus tau) otolith nerves. High fidelity, single unit neural activity was recorded from seven alert and unrestrained fish 30 to 60 days after implantation. Fibrous coatings of genetically engineered bioactive protein polymers and nerve guide tubes increased the number of axons regenerating through the electrode pores when compared with controls. Sieve electrodes have potential as permanent interfaces to the nervous system and to bridge missing connections between severed or damaged nerves and muscles. Recorded impulses might also be amplified and used to control prosthetic devices.

Animals↗
PubMed1999

Permeability of rat heart myocytes to cytochrome c.

Rat heart myocytes undergoing progressive damage demonstrate morphological changes of shortening and swelling followed by the formation of intracellular vacuoles and plasma membrane blebbing. The damaged myocytes displayed impaired N,N'-tetramethyl-p-phenyldiamine (TMPD) ascorbate-stimulated respiratory activity which was restored by the addition of reduced cytochrome c to the cell culture medium. To clarify the role played by cytochrome c in the impairment of cell respiration, polarographic, spectrophotometric and fluorescence as well as electron microscopy imaging experiments were performed. TMPD/ascorbate-stimulated respiratory activity returned to control levels, at approximately 20 microM cytochrome c, establishing the threshold below which the turnover rate by cytochrome c oxidase in the cell depends on cytochrome concentration. Mildly damaged cardiac myocytes, as indicated by cell shortening, retention of visible striations and free-fluorescein exclusion, together with the absence of lactate dehydrogenase leakage and exclusion of trypan blue, were able to oxidize exogenous cytochrome c and were permeable to fluorescein-conjugated cytochrome c. The results, while consistent with an early cytochrome c release observed at the beginning of cell death, elucidate the role played by cytochrome c in the kinetic control of mitochondrial electron transfer under pathological conditions, particularly those involving the terminal part of the respiratory chain. These data are the first to demonstrate that the sarcolemma of cardiac myocytes, damaged but still viable, is permeable to cytochrome c.

Animals↗
PubMed1999

H+-K+-ATPases: regulation and role in pathophysiological states.

Molecular cloning experiments have identified the existence of two H+-K+-ATPases (HKAs), colonic and gastric. Recent functional and molecular studies indicate the presence of both transporters in the kidney, which are presumed to mediate the exchange of intracellular H+ for extracellular K+. On the basis of these studies, a picture is evolving that indicates differential regulation of HKAs at the molecular level in acid-base and electrolyte disorders. Of the two transporters, gastric HKA is expressed constitutively along the length of the collecting duct and is responsible for H+ secretion and K+ reabsorption under normal conditions and may be stimulated with acid-base perturbations and/or K+ depletion. This regulation may be species specific. To date there are no data to indicate that the colonic HKA (HKAc) plays a role in H+ secretion or K+ reabsorption under normal conditions. However, HKAc shows adaptive regulation in pathophysiological conditions such as K+ depletion, NaCl deficiency, and proximal renal tubular acidosis, suggesting an important role for this exchanger in potassium, HCO-3, and sodium (or chloride) reabsorption in disease states. The purpose of this review is to summarize recent functional and molecular studies on the regulation of HKAs in physiological and pathophysiological states. Possible signals responsible for regulation of HKAs in these conditions will be discussed. Furthermore, the role of these transporters in acid-base and electrolyte homeostasis will be evaluated in the context of genetically altered animals deficient in HKAc.

Acidosis↗
PubMed1998

Examination of the cupula and stereocilia of the horizontal semicircular canal in the toadfish Opsanus tau.

We imaged the horizontal semicircular canal (HSCC) crista and cupula of toadfish, Opsanus tau, by using a) confocal light microscopy of isolated vital HSCC; b) serial sections of fixed, trichrome-stained HSCC; and c) scanning electron microscopy of fixed HSCCs. HSCC were dissections which included an ampulla and an attached canal tube (long and slender canal portion), and, in some cases, a small portion of the utricular wall. Cupulae were seen as multipartite mucus connective tissue shells rising from the crista and extending toward the ampullary roof. They were composed of several refractile bands traversing the cupulae perpendicular to longitudinal fibers extending from the cupular base to its apex. Alcian green-stained cupulae showed an asymmetric alcianphilic, dark, X-shaped structure, indicating that the pillar is rich in mucin and carbohydrate, an interpretation supported by images of trichrome-stained sections. The cupular antrum is devoid of prominent refractile fibers. No tubes or channels were observed in the cupula or antrum of vital preparations. Cupular shell fibers cover the surface of the crista, are roughly parallel, and are associated with a translucent material having a refractive index greater than the surrounding endolymph. Stereocilia were thin, 100-microm-long structures, with little longitudinal curvature, which end with no end bulb. No strands extend from stereocilia to the roof or other portions of the cupular antrum. Gross movements of stereocilia were not seen in mechanically quiescent preparations. Within the cupular antrum, stereocilia were parallel to connective tissue fibers, all embedded in an isotropic gel. This fiber-reinforced gel and cupular matrix are sensitive to N-acetlyneuraminidase and beta-N-acetyl glucosaminidase, and minimally sensitive to beta-N-acetyl hexosaminidase. Connective tissue fibers may serve to stiffen the gel, whose matrix would restrict lateral motion of embedded fibers and stereocilia thereby providing mechanical support for stereocilia.

Acetylglucosaminidase↗
PubMed1998

Low-NaCl diet increases H-K-ATPase in intercalated cells from rat cortical collecting duct.

Extracellular K+-dependent H+ extrusion after an acute acid load, an index of H/K exchange, was monitored in intercalated cells (ICs) from rat cortical collecting tubule (CCT) using ratiometric fluorescence imaging of the intracellular pH (pHi) indicator, 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). The hypothesis tested was that 12- to 14-day NaCl deprivation increases H-K-ATPase in rat ICs. The rate of H/K exchange in the low-NaCl ICs was double that of controls. In control ICs, H/K exchange was inhibited by Sch-28080 (10 microM). In the low-NaCl ICs, it was partially blocked by Sch-28080 or ouabain (1 mM). Simultaneous addition of both inhibitors abolished K-dependent pHi recovery. The induced H/K exchange observed with NaCl restriction was not due to elevated plasma aldosterone as evidenced by experiments on ICs from rats implanted with osmotic minipumps administering aldosterone such that plasma levels were comparable to those of NaCl-deficient rats. The results suggest that NaCl deficiency induces two isoforms of H-K-ATPase in ICs and that this effect is not mediated by elevated plasma aldosterone.

Aldosterone↗
PubMed1997

Characterization and regulation of H-K-ATPase in intercalated cells of rabbit cortical collecting duct.

K-dependent H+ extrusion was investigated using fluorescence techniques in rabbit cortical collecting tubules (CCTs). Experiments were performed in split-open tubules from normal animals exposed to the intracellular pH indicator 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF). This preparation permitted the study of individual intercalated cells (ICs). In the ICs, partial recovery of pH(i) was observed in response to an acute acid load upon readdition of 5 mM K to the superfusate. This recovery was SCH 28080-inhibitable (10(-5) M) and ouabain-insensitive suggesting the process is mediated by a gastric-type H-K ATPase. To see if H-K ATPase plays a role in acid secretion its function was evaluated under chronic metabolic acidosis (CMA) conditions. CMA was induced by replacing drinking water with 75 mM NH4Cl in 5% sucrose for 10-14 days. The SCH 28080-inhibitable K-dependent pH(i) recovery rate was three-fold higher in CMA ICs compared to controls. To determine the location of the H-K ATPase, CCTs were microperfused and individual peanut lectin binding (PNA) ICs studied. K-dependent pH(i) recovery was measured in response to an NH4Cl pulse. An apical SCH 28080-inhibited K-dependent pH(i) recovery process was observed in control and CMA ICs. Taken together these data confirm the existence of a gastric-type H-K ATPase in ICs of rabbit CCT. Based on our findings the H-K ATPase is found on the apical side of the cell and is stimulated under conditions of CMA.

Acid-Base Equilibrium↗
PubMed1997

H-K-ATPase activity in PNA-binding intercalated cells of newborn rabbit cortical collecting duct.

Functional and immunocytochemical studies indicate that intercalated cells in the adult rabbit cortical collecting duct (CCD) possess an H-K-adenosinetriphosphatase (H-K-ATPase). Because growing subjects must retain K+ and excrete H+, we sought to determine whether H-K-ATPase is present in the CCD early in life and, if so, to assess its activity and polarity. H-K-ATPase activity was defined as the initial rate of Sch-28080-inhibitable K+-dependent cell pH (pHi) recovery observed, in the absence of Na+, in response to an in vitro acid load. Transporter activity was assayed in intercalated cells labeled with the pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein and apical cell surface marker rhodamine peanut lectin (PNA) in split-open CCDs isolated from neonatal and adult New Zealand White rabbits. In Na+-free N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-buffered solutions (nominal absence of CO2/HCO3-), the rate of K+-dependent pH(i) recovery from a NH4Cl-induced acid load was similar in newborn (0.056 +/- 0.015 pH U/min, n = 9) and adult (0.060 +/- 0.019 pH U/min; n = 9, P = not significant) cells. This rate of K+-dependent pH(i) recovery was significantly reduced by 10-20 pM Sch-28080, an inhibitor of gastric H-K-ATPase, in both newborns (0.009 +/- 0.003 pH U/min, n = 7) and adults (0.013 +/- 0.007 pH U/min, n = 9) (P < 0.05 compared with rates in absence of inhibitor). To determine whether the location of the transporter is consistent with a role in K+ absorption and H+ secretion, pH(i) recovery of acutely acid-loaded intercalated cells in neonatal CCDs (n = 7) microperfused and bathed in the absence of Na+ and K+ was monitored after selective addition of K+ to either the luminal or basolateral membrane. Addition of 5 mM K+ led to a significantly greater rate of pH(i) recovery when it was added to the luminal rather than the peritubular solution (0.049 +/- 0.005 vs. 0.018 +/- 0.005 pH U/min, P < 0.05). We conclude that PNA-binding intercalated cells of the neonatal CCD possess H-K-ATPase activity, predominantly located in the apical membrane. This provides a mechanism for H secretion and K+ retention, processes required for growth.

Animals↗
PubMed1996

Calcium, BOBs, QEDs, microdomains and a cellular decision: control of mitotic cell division in sand dollar blastomeres.

The role of Ca2+ in controlling cell processes (e.g. mitosis) presents an enigma in its ubiquity and selectivity. Intracellular free Ca2+ (Ca2+i) is an essential regulator of specific biochemical and physiological aspects of mitosis (e.g. nuclear envelope breakdown (NEB)). Changes in Ca2+i concentrations during mitosis in second cell-cycle sand dollar (Echinaracnius parma) blastomeres were imaged as Ca(2+)-dependent luminescence of the photoprotein aequorin with multi-spectral analytical video microscopy. Photons of this luminescence were seen as bright observable blobs (BOBs). Spatiotemporal patterns of BOBs were followed through one or more cell cycles to detect directly changes in Ca2+i, and were seen to change in a characteristic fashion prior to NEB, the onset of anaphase chromosome movement, and during cytokinesis. These patterns were observed from one cell cycle to the next in a single cell, from cell to cell, and from egg batch to egg batch. In both mitosis and synaptic transmission increases in Ca2+i concentration occurs in discrete, short-lived, highly localized pulses we name quantum emission domains (QEDs) within regions we named microdomains. Signal and statistical optical analyses of spatiotemporal BOB patterns show that many BOBs are linked by constant displacements in space-time (velocity). Linked BOBs are thus nonrandom and are classified as QEDS. Analyses of QED patterns demonstrated that the calcium signals required for NEB are nonrandom, and are evoked by an agent(s) generated proximal to a Ca2+i-QED; models of waves, diffusible agonists and Ca(2+)-activated Ca2+ release do not fit pre-NEB cell data. Spatial and temporal resolution of this multispectral approach significantly exceeds that reported for other methods, and avoids the perturbations associated with many fluorescent Ca2+ reporters that interfere with cells being studied (Ca(2+)-buffering, UV toxicity, etc.). Spatiotemporal patterns of Ca2+i-QED can control so many different processes, i.e. specific frequencies used to control particular processes. Predictive and structured patterns of calcium signals (e.g. a language expressed in Ca2+) may selectively regulate specific Ca(2+)-dependent cellular processes.

Animals↗