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O Alvarez

Publications and source records attributed to O Alvarez.

At least 37 records · Page 2Linked to original sources
PubMed1992

Modeling ion permeation through batrachotoxin-modified Na+ channels from rat skeletal muscle with a multi-ion pore.

The mechanism of ion permeation through Na+ channels that have been modified by batrachotoxin (BTX) and inserted into planar bilayers has been generally described by models based on single-ion occupancy, with or without an influence of negative surface charge, depending on the tissue source. For native Na+ channels there is evidence suggestive of a multi-ion conduction mechanism. To explore the question of ion occupancy, we have reexamined permeation of Na+, Li+, and K+ through BTX-modified Na+ channels from rat skeletal muscle. Single-channel current-voltage (I-V) behavior was studied in neutral lipid bilayers in the presence of symmetrical Na+ concentrations ranging from 0.5 to 3,000 mM. The dependence of unitary current on the mole fraction of Na+ was also examined in symmetrical mixtures of Na(+)-Li+ and Na(+)-K+ at a constant total ionic strength of 206 and 2,006 mM. The dependence of unitary conductance on symmetrical Na+ concentration does not exhibit Michaelis-Menten behavior characteristic of single-ion occupancy but can be simulated by an Eyring-type model with three barriers and two sites (3B2S) that includes double occupancy and ion-ion repulsion. Best-fit energy barrier profiles for Na+, Li+, and K+ were obtained by nonlinear curve fitting of I-V data using the 3B2S model. The Na(+)-Li+ and Na(+)-K+ mole-fraction experiments do not exhibit an anomalous mole-fraction effect. However, the 3B2S model is able to account for the biphasic dependence of unitary conductance on symmetrical [Na+] that is suggestive of multiple occupancy and the monotonic dependence of unitary current on the mole fraction of Na+ that is compatible with single or multiple occupancy. The best-fit 3B2S barrier profiles also successfully predict bi-ionic reversal potentials for Na(+)-Li+ and Na(+)-K+ in both orientations across the channel. Our experimental and modeling results reconcile the dual personality of ion permeation through Na+ channels, which can display features of single or multiple occupancy under various conditions. To a first approximation, the 3B2S model developed for this channel does not require corrections for vestibule surface charge. However, if negative surface charges of the protein do influence conduction, the conductance behavior in the limit of low [Na+] does not correspond to a Gouy-Chapman model of planar surface charge.

Animals↗
PubMed1992

Thrombocytosis and hyposplenism in an infant with fetal hydantoin syndrome.

An infant boy with fetal hydantoin syndrome was evaluated for the finding of thrombocytosis. Delayed splenic maturation was identified by a decreased splenic uptake of 99mTc sulfur colloid and by an increased number of pitted erythrocytes at 6 months of age. We speculate that the patient's thrombocytosis may be related to delayed maturation of splenic function and conclude that hyposplenism and secondary thrombocytosis may be associated with fetal hydantoin syndrome.

Abnormalities, Drug-Induced↗
PubMed1992

Equine protozoal myelitis in Panamanian horses and isolation of Sarcocystis neurona.

Schizonts of Sarcocystis neurona were identified microscopically in hematoxylin-eosin-stained spinal cord sections from 2 native Panamanian horses that exhibited clinical signs of equine protozoal myelitis (EPM). Spinal cord homogenate from a third Panamanian horse with EPM was inoculated onto monolayers of cultured bovine monocytes (M617). Intracytoplasmic schizonts containing merozoites arranged in rosette forms surrounding a central residual body first were observed 13 wk postinoculation. Parasites divided by endopolygeny and lacked rhoptries. Schizonts from each horse reacted with Sarcocystis cruzi antiserum in an immunohistochemical test.

Animals↗
PubMed1991

Intracarotid chemotherapy with etoposide and cisplatin for malignant brain tumors.

Chemotherapy for tumors of the central nervous system has a limited efficacy presumably because of restricted blood-brain barrier permeability. The advantage of regional intra-arterial administration of anticancer drugs is an increased uptake during the first passage of the drugs through tumor capillaries. Twenty patients with high-grade astrocytomas (HGA) and 28 patients with metastatic brain tumors (MBT) received intracarotid/intravertebral infusion of etoposide and cisplatin. Eight patients with HGA who underwent incomplete resection responded to chemotherapy alone. Four additional patients had complete resection of the tumor. Median survival time of the group (responders and nonresponders) has been 14 months. Twelve patients with MBT responded to chemotherapy alone (six had complete response [CR], and six had partial response [PR]) with a median survival time of 7 months. Intra-arterial chemotherapy (IAC) appears to be effective with acceptable toxicities. Accrual of additional patients is required before a final conclusion can be reached.

Adult↗
PubMed1991

Kinetics of iron passage through subcellular compartments of rabbit reticulocytes.

The kinetics of the separate processes of Fe2(III)-transferrin binding to the transferrin receptor, transferrin-receptor internalization, iron dissociation from transferrin, iron passage through the membrane, and iron mobilization into the cytoplasm were studied by pulse-chase experiments using rabbit reticulocytes and 59Fe, 125I-labeled rabbit transferrin. The binding of 59Fe-transferrin to transferrin receptors was rapid with an apparent rate constant of 2 x 10(5) M-1 sec-1. The rate of internalization of 59Fe-transferrin was directly measured at 520 +/- 100 molecules of Fe2(III)-transferrin internalized/sec/cell with 250 +/- 43 sec needed to internalize the entire complement of reticulocyte transferrin receptors. Subsequent to Fe2(III)-transferrin internalization the flux of 59Fe was followed through three compartments: internalized transferrin, membrane, and cytosol. A process preceding iron dissociation from transferrin and a reaction involving membrane-associated iron required 17 +/- 2 sec and 34 +/- 5 sec, respectively. Apparent rate constants of 0.0075 +/- 0.002 sec-1 and 0.0343 +/- 0.0118 sec-1 were obtained for iron dissociation from transferrin and iron mobilization into the cytosol, respectively. Iron dissociation from transferrin is the rate-limiting step. An apparent rate constant of 0.0112 +/- 0.0025 sec-1 was obtained for processes involving iron transport through the membrane although at least two reactions are likely to be involved. Based on mechanistic considerations, iron transport through the membrane may be attributed to an iron reduction step followed by a translocation step. These data indicate that the uptake of iron in reticulocytes is a sequential process, with steps after the internalization of Fe2(III)-transferrin that are distinct from the handling of transferrin.

Animals↗
PubMed1990

Rapid degradation of extracellular matrix proteins by normal human uroepithelial cells.

The degradation of subendothelial and smooth muscle matrices by normal and neoplastic uroepithelial cells grown under serum-free conditions was examined. Normal urothelial cells were compared with neoplastic cells derived from a low grade papillary tumor (RT4) and a more invasive carcinoma (EJ). Low levels of degradation were observed with all cell types in serum-free medium alone. Supplementing the medium with plasminogen increased the degradative activity of each cell type. Logarithmically growing normal urothelial cells degraded extracellular matrix proteins 6 to 14 times faster on a per cell basis than their transformed counterparts. Analysis of the residual matrix constituents revealed that, while the levels of glycoprotein breakdown by the normal and neoplastic cells were similar, the normal cells degraded more of the collagen components than the neoplastic cells. Epidermal growth factor and cell density were examined as possible regulators of degradative activity. The neoplastic cells were not responsive to cell density as a regulatory factor and were only slightly responsive to epidermal growth factor. However, epidermal growth factor increased the degradative activity of logarithmically growing normal urothelial cells in the presence of plasminogen and the activity of confluent cells was increased to an even greater extent. Gelatin substrate gel analysis confirmed that the normal urothelial cells elaborated a more diverse set of gelatinases than the tumorigenic cells. Although normal urothelial cells had higher degradative abilities than their malignant counterparts, it is significant that the neoplastic cells were less responsive to regulatory signals in our model system. Thus, loss of regulatory mechanisms for protease secretion and matrix degradation may be a more important determinant of invasive ability in vivo than protease secretion or matrix degradation in vitro.

Animals↗
PubMed1990

Primary structure of peptides and ion channels. Role of amino acid side chains in voltage gating of melittin channels.

Melittin produces a voltage-dependent increase in the conductance of planar lipid bilayers. The conductance increases when the side of the membrane to which melittin has been added (cis-side) is made positive. This paper reports observations on the effect of modifying two positively charged amino acid residues within the NH2-terminal region of the molecule: lysine at position 7 (K7), and the NH2-terminal glycine (G1). We have synthesized melittin analogues in which K7 is replaced by asparagine (K7-N), G1 is blocked by a formyl group (G1-f), and in which both modifications of the parent compound were introduced (G1-f, K7-N). The time required to reach peak conductance during a constant voltage pulse was shorter in membranes exposed to the analogues than in membranes modified by melittin. The apparent number of monomers producing a conducting unit for [K7-N]-melittin and [G1-f]-melittin, eight, was found to be greater than the one for [G1-f], K7-N]-melittin and for melittin itself, four. The apparent gating charge per monomer was less for the analogues, 0.5-0.3 than for melittin, one. Essentially similar results were obtained with melittin analogues in which the charge on K7 or G1 or both was blocked by an uncharged N-linked spin label. These results show that the positive charges in the NH2-terminal region of melittin play a major but not exclusive role in the voltage gating of melittin channels in bilayers.

Amino Acid Sequence↗
PubMed1989

Streaming potential measurements in Ca2+-activated K+ channels from skeletal and smooth muscle. Coupling of ion and water fluxes.

Streaming potentials arising across large-conductance Ca2+-activated K+ channels incorporated into planar lipid bilayers were measured. Ca2+-activated channels obtained either from skeletal muscle or from smooth muscle membranes were used. Streaming potentials were extracted from the current-voltage relationship for the open channel obtained in the presence of an osmotic gradient. The osmotic gradient was established by adding glucose to one side of the membrane. At 300 mM KCl, the average streaming potential was 0.72 mV/osmol per kg for t-tubule channels and 0.83 mV/osmol per kg for smooth muscle channels. Streaming potential values depend on KCl concentration, they decrease as KCl concentration increases, and the value obtained by extrapolation to zero KCl concentration is 0.85 mV/osmol per kg. Assuming that water and ions cannot pass each other, at least in a region of the channel, the streaming potential values obtained indicate that this region contains a minimum of two and a maximum of four water molecules. It is concluded that the channel has a narrow region with a length of 0.6-1.2 nm.

Animals↗
PubMed1989

Lyme neuroborreliosis: central nervous system manifestations.

We evaluated 85 patients with serologic evidence of Borrelia burgdorferi infection. Manifestations included encephalopathy (41), neuropathy (27), meningitis (2), multiple sclerosis (MS) (6), and psychiatric disorders (3). We performed lumbar punctures in 53, brain MRI in 33, and evoked potentials (EPs) in 33. Only patients with an MS-like illness had abnormal EPs, elevated IgG index, and oligoclonal bands in the cerebrospinal fluid. Twelve of 18 patients with encephalopathy, meningitis, or focal CNS disease had evidence of intrathecal synthesis of anti-B burgdorferi antibody, compared with no patients with either MS-like or psychiatric illnesses, and only 2/24 patients with neuropathy. MRIs were abnormal in 7/17 patients with encephalopathy, 5/6 patients with an MS-like illness, and no others. We conclude that (1) intrathecal concentration of specific antibody is a useful marker of CNS B burgdorferi infection; (2) Lyme disease causes an encephalopathy, probably due to infection of the CNS; (3) MS patients with serum immunoreactivity against B burgdorferi lack evidence of CNS infection with this organism.

Acute Disease↗
PubMed1988

Probing a Ca2+-activated K+ channel with quaternary ammonium ions.

A series of quaternary ammonium (QA) ions were used to probe the gross architecture of the ion conduction pathway in a Ca2+-activated K+ channel from rat muscle membrane. The channels were inserted into planar phospholipid membranes and the single channel currents were measured in the presence of the different QA ions. Internally applied monovalent QA ions (e.g. tetramethylammonium and analogues) induced a voltage-dependent blockade with a unique effective valence of the block equal to 0.30, and blocking potency increases as the compound is made more hydrophobic. Blockade is relieved by increasing the K+ concentration of the internal or external side of the channel. The effective valence of block is independent of K+ concentration. These results suggest that, from the internal side, all monovalent QA ions interact with a site located in the channel conduction system. Divalent QA ions of the type n-alkyl-bis-alpha,beta-trimethylammonium (bisQn) applied internally also block the channel in a voltage dependent fashion. For short chains (bisQ2-bisQ5), the effective valence decreases with chain length from 0.41 to 0.27, it remains constant for bisQ5 to bisQ6 and increases up to 0.54 for bisQ10. This dependence of block with chain length implies that 27% of the voltage drop within the channel occurs over a distance of approximately 1 nm. Externally applied monovalent QA ions also block the channel. The site is specific for tetraethylammonium; increasing or decreasing the side chains in one methylene group decrease potency by about 400-fold. It is concluded that the Ca2+-activated K+ channel has wide mouths located at each end and that they are different in molecular nature.

Animals↗
PubMed1988

Activation by divalent cations of a Ca2+-activated K+ channel from skeletal muscle membrane.

Several divalent cations were studied as agonists of a Ca2+-activated K+ channel obtained from rat muscle membranes and incorporated into planar lipid bilayers. The effect of these agonists on single-channel currents was tested in the absence and in the presence of Ca2+. Among the divalent cations that activate the channel, Ca2+ is the most effective, followed by Cd2+, Sr2+, Mn2+, Fe2+, and Co2+. Mg2+, Ni2+, Ba2+, Cu2+, Zn2+, Hg2+, and Sn2+ are ineffective. The voltage dependence of channel activation is the same for all the divalent cations. The time-averaged probability of the open state is a sigmoidal function of the divalent cation concentration. The sigmoidal curves are described by a dissociation constant K and a Hill coefficient N. The values of these parameters, measured at 80 mV are: N = 2.1, K = 4 X 10(-7) mMN for Ca2+; N = 3.0, K = 0.02 mMN for Cd2+; N = 1.45, K = 0.63 mMN for Sr2+; N = 1.7, K = 0.94 mMN for Mn2+; N = 1.1, K = 3.0 mMN for Fe2+; and N = 1.1 K = 4.35 mMN for Co2+. In the presence of Ca2+, the divalent cations Cd2+, Co2+, Mn2+, Ni2+, and Mg2+ are able to increase the apparent affinity of the channel for Ca2+ and they increase the Hill coefficient in a concentration-dependent fashion. These divalent cations are only effective when added to the cytoplasmic side of the channel. We suggest that these divalent cations can bind to the channel, unmasking new Ca2+ sites.

Animals↗