Zn2+ interaction with Alzheimer amyloid beta protein calcium channels

The Alzheimer disease 40-residue amyloid beta protein (AbetaP[1-40]) forms cation-selective channels across acidic phospholipid bilayer membranes with spontaneous transitions over a wide range of conductances ranging from 40 to 4000 pS. Zn2+ has been reported to bind to AbetaP[1-40] with high affinity, and it has been implicated in the formation of amyloid plaques. We now report the functional consequences of such Zn2+ binding for the AbetaP[1-40] channel. Provided the AbetaP[1-40] channel is expressed in the low conductance (<400 pS) mode, Zn2+ blocks the open channel in a dose- dependent manner. For AbetaP[1-40] channels in the giant conductance mode (>400 pS), Zn2+ doses in the millimolar range were required to exert substantial blockade. The Zn2+ chelator o-phenanthroline reverses the blockade. We also found that Zn2+ modulates AbetaP[1-40] channel gating and conductance only from one side of the channel. These data are consistent with predictions of our recent molecular modeling studies on AbetaP[1-40] channels indicating asymmetric Zn(2+)-AbetaP[1-40] interactions at the entrance to the pore.

Induced mitotic death of HeLa cells by abnormal expression of c-H-ras

When HeLa cells were selected for stable expression of a neo gene, linked either to mutated or wt c-H-ras genes, morphological examination of selected clones from several experiments revealed formation of giant multinucleated cells. These morphological alterations culminate in cell death, as a consequence of mitotic catastrophe (or mitotic death). Although clones expressing the mutated gene produced significantly larger numbers of these giant cells, those transfected with the normal allele were also found to produce significantly more giant multinucleated cells than non-transfected HeLa cells. Northern blot analysis of mRNA revealed overexpression of the normal H-ras gene in these clones. Chromatin structure analysis of these clones showed gross alterations, including the presence of micronuclei and heteroploid nuclei. Interestingly, odd numbers of nuclei were found in colonies of these giant cells. In addition, alterations in cell cycle parameters were observed, including the appearance of a subpopulation of cells with an abnormal content of DNA, probably representing dying cells. Our data support the notion that abnormal expression of H-ras contributes to mitotic catastrophe and death of a subpopulation of HeLa cells.

Immortalized GnRH neurons express large-conductance calcium-activated potassium channels

The expression and function of large-conductance Ca2+ -activated K+ (BK) channels in the GT1-7 line of immortalized gonadotropin-releasing hormone (GnRH) neurons was investigated. Ionic currents were recorded using the patch-clamp technique, cytoplasmic free Ca2+ concentration ([Ca2+]i) was monitored using the fluorescent indicator, fura-2, and GnRH secretion was measured by radioimmunoassay. In cell-attached and inside-out patch-clamp recordings, K+ channels with a single-channel conductance of approximately 200 pS were detected. Depolarizing the patch increased the unitary current size and the open probability. In perforated-patch recordings, depolarizing pulses (50 ms) to potentials of -10 to +60 mV from a holding potential of -90 mV elicited outward current with early transient and sustained components. The transient current peaked 2-10 ms after the beginning of each pulse and increased in a voltage-dependent manner. This current was: (1) unaffected by the small-conductance Ca2+ -activated K+ channel blocker, apamin (100 nM); (2) reduced by the BK channel blocker, charybdotoxin (5 nM); (3) abolished by the Ca2+ channel blocker, CdCl2 (25 mu M), and (4) prolonged by the endoplasmic reticulum Ca2+ -ATPase inhibitor, thapsigargin (1 mu M). Based on the single-channel and whole-cell conductances, the number of channels per patch, the patch area, and the surface area of the cell, each GT1-7 cell contains 30-60 BK channels. The functional role of BK channels in GT1-7 cells was evaluated by measuring the effect of charybdotoxin (100 nM) on basal [Ca2+]i and GnRH secretion, as well as on the [Ca2+]i and GnRH secretory responses to gamma-aminobutyric acid (GABA, 100 mu M), an excitatory neurotransmitter in this system. Charybdotoxin had no effect on basal [Ca2+]i or GnRH secretion, or on the GABA-evoked [Ca2+]i and GnRH secretory responses. These results indicate that GT1-7 cells express BK channels; however, the physiological role of BK channels in GT1-7 cells remains elusive.

Reprotoxic and genotoxic studies of vanadium pentoxide in male mice

Effects of vanadium pentoxide (V2O5) treatment on reproductive function and testicular DNA in male mice were investigated. These functions were evaluated with fertility rate, implants, resorptions, sperm counts, motility, and morphology. The DNA damage in individual testis cells was analyzed by single-cell gel electrophoresis technique (COMET assay). V2O5 treatment resulted in a decrease in fertility rate, implantations, live fetuses, and fetal weight, and an increase in the number of resorptions/dam. Sperm count, motility, and morphology were impaired with the advancement of treatment. Vanadium treatment induced DNA damage depending on the dose in the testis cells that was expressed and detected as DNA migration in the COMET assay. The distribution of DNA migration among cells, a function of dose, revealed that the majority of cells of treated animals expressed more DNA damage than cells from control animals. It is concluded that vanadium pentoxide was a reprotoxic and genotoxic agent in mice.

Muscarinic receptor activated Ca2+ channels in non-excitable cells

We have provided preliminary characterization of a single channel Ca2+ conductance in CHO cells. We have demonstrated that the channel conducts Ca2+, is regulated by m5 receptors, is voltage-independent, has an extremely low conductance, and is second messenger-independent. This channel may be the receptor-operated channel required for downstream activation of several signaling events. It is not known what other cell types express the channel or if it is one of a larger group of related channels. It seems likely that Ca2+ influx-dependent signaling pathways, activated by the muscarinic m5 receptor, would utilize a plasma membrane resident Ca2+ channel to provide a steady source of Ca2+ from outside the cell. The transient nature of IP3-activated increases in intracellular Ca2+ make it an unlikely source of the sustained Ca2+ rise required for phospholipase regulation. This is especially surprising, since levels of intracellular Ca2+ achieved from the release of intracellular Ca2+ stores can be at least one order of magnitude higher than those achieved from extracellular influx (Berridge, 1993). The phospholipase A2 and phospholipase D involved in muscarinic receptor-mediated signaling have not been purified or cloned. It is possible that receptor-activated and Ca2+ influx-dependent phospholipases are integral membrane proteins located adjacent to both receptors and channels. The phospholipases may also translocate to the membrane following activation where they would gain access to the continuous Ca2+ flow. Purification and cloning of this and other related channels should provide better insight into their role in cell signaling.

[Excretion of promastigotes of Leishmania pifanoi by experimentally infected Lutzomyia youngi]

The increase in the promastigotes population of Leishmania pifanoi in Lutzomyia youngi experimentally infected and kept on 50% sacarose under constant conditions of temperature and humidity is described. Two stages in the differentiation and growth of the parasites are recognised between two and twenty-four hours after meals. The pleomorphic differentiation of the amastigotes in short promastigotes which multiply by binary division for 60 hours, when the rupture of the peritrophic membrane occurs, takes place within 48 hours. The second stage occurs between 72 and 96 hours when some of the parasites migrate to the esophagic valve and the rest of the free parasites are excreted in fecal drops as large, active promastigotes. The first drops excreted react positively to glucose or contain crystals of urate. The excess promastigotes of the second stage of development are eliminated in the last excretions and react positively to the Hemoscreen and Biuret tests for total proteins as also for glucose and account for 82% of the of drops excreted. The excretion of parasites by Lu. youngi is a normal stage in the growth of L. pifanoi into a vector.

GnRH-induced cytosolic calcium oscillations in pituitary gonadotrophs: phase resetting by membrane depolarization

Cultured rat pituitary gonadotrophs under whole-cell voltage clamp conditions respond to the hypothalamic hormone GnRH with synchronized oscillatory changes in both cytosolic Ca2+ concentration ([Ca2+]i) and [Ca2+]i-activated, apamin-sensitive K+ current (IK(Ca)). We found, and report here for the first time, that in GnRH-stimulated cells a brief depolarizing pulse can elicit a transient [Ca2+]i rise similar to the endogenous cycle. Furthermore, Ca2+ entry during a single depolarizing pulse was found to shift the phase of subsequent endogenous [Ca2+]i oscillations, which thereafter continue to occur at their previous frequency before the pulse. Application of two consecutive depolarizing pulses showed that the size of the [Ca2+]i rise evoked by the second pulse depended on the time lapsed between two consecutive pulses, indicating that each endogenous or evoked [Ca2+]i rise cycle leaves the Ca2+ release mechanism of the gonadotroph in a refractory state. Recovery from this condition can be described by an exponential function of the time lapsed between the pulses (time constant of ca. 1 s). We propose that the underlying mechanism in both refractoriness after endogenous cycles and phase resetting by a brief pulse of Ca2+ entry involves the InsP3 receptor-channel molecule presumed to be located on the cytosolic aspect of the endoplasmic reticulum membrane.

Ion channel hypothesis for Alzheimer amyloid peptide neurotoxicity

1. Alzheimer's disease (AD) is a chronic dementia and neurodegenerative disorder affecting the oldest portions of the population. Brains of AD patients accumulate large amount of the A beta P peptide in amyloid plaques. 2. The A beta P[1-40] peptide is derived by proteolytic processing from a much larger amyloid precursor protein (APP), and has been circumstantially identified as the toxic principle causing cell damage in the disease. 4. The A beta P[1-40] peptide is able to form quite characteristic calcium channels in planar lipid bilayers. These channels have conductances in the nS range, and can dissipate ion gradients quickly. The peptide can also cause equivalent cation conductances in cells. 5. We suggest that amyloid channel blocking agents might be therapeutically useful in Alzheimer's Disease, and have constructed molecular models of the channels to aid in the design of such compounds.

Identification of mammalian noggin and its expression in the adult nervous system

The multiple roles of noggin during dorsal fate specification in Xenopus embryos, together with noggin's ability to directly induce neural tissue, inspired an effort to determine whether a similar molecule exists in mammals. Here we describe the identification of human and rat noggin and explore their expression patterns; we also localize the human NOGGIN gene to chromosome 17q22, and the mouse gene to a syntenic region of chromosome 11. Mammalian noggin is remarkably similar in its sequence to Xenopus noggin, and is similarly active in induction assays performed on Xenopus embryo tissues. In the adult mammal, noggin is most notably expressed in particular regions of the nervous system, such as the tufted cells of the olfactory bulb, the piriform cortex of the brain, and the Purkinje cells of the cerebellum, suggesting that one of the earliest acting neural inducers also has important roles in the adult nervous system.

Direct control of a large conductance K(+)-selective channel by G-proteins in adrenal chromaffin granule membranes

We report here the presence of a Ca(2+)-independent K(+)-channel of large conductance in adrenal chromaffin cell secretory vesicle membranes which is controlled by inhibitory as well as stimulatory heterotrimeric GTP-binding proteins. Using antibodies against specific alpha subunits for immunoblot analysis, we were able to identify the presence of the inhibitory G(i)2 and G(i)3 subtypes, as well as the stimulatory G(o) and Gs subtypes, but not G(i)1 in adrenal chromaffin granules. Furthermore, functional analysis of the K(+)-channel incorporated into planar lipid bilayers showed that GDP beta S and GTP gamma S have opposite effects on channel activity inducing interconversions between a low and a high open-probability state. Consistent with these findings, the same antibodies antagonized the effects of the nonhydrolyzable analogues on the open probability of the K(+)-channel.

Receptor tyrosine kinase specific for the skeletal muscle lineage: expression in embryonic muscle, at the neuromuscular junction, and after injury

While a number of growth factors have been described that are highly specific for particular cell lineages, neither a factor nor a receptor uniquely specific to the skeletal muscle lineage has previously been described. Here we identify a receptor tyrosine kinase (RTK) specific to skeletal muscle, which we term "MuSK" for muscle-specific kinase. MuSK is expressed at low levels in proliferating myoblasts and is induced upon differentiation and fusion. In the embryo, it is specifically expressed in early myotomes and developing muscle. MuSK is then dramatically down-regulated in mature muscle, where it remains prominent only at the neuromuscular junction; MuSK is thus the only known RTK that localizes to the neuromuscular junction. Strikingly, MuSK expression is dramatically induced throughout the adult myofiber after denervation, block of electrical activity, or physical immobilization. In humans, MuSK maps to chromosome 9q31.3-32, which overlaps with the region reported to contain the Fukuyama muscular dystrophy mutation. Identification of MuSK introduces a novel receptor-factor system that seems sure to play an important and selective role in many aspects of skeletal muscle development and function.

Magnitude and modulation of pancreatic beta-cell gap junction electrical conductance in situ

The parallel gap junction electrical conductance between a beta-cell and its nearest neighbors was measured by using an intracellular microelectrode to clamp the voltage of a beta-cell within a bursting islet of Langerhans. The holding current records consisted of bursts of inward current due to the synchronized oscillations in membrane potential of the surrounding cells. The membrane potential record of the impaled cell, obtained in current clamp mode, was used to estimate the behavior of the surrounding cells during voltage clamp, and the coupling conductance was calculated by dividing the magnitude of the current bursts by that of the voltage bursts. The histogram of coupling conductance magnitude from 26 cells was bimodal with peaks at 2.5 and 3.5 nS, indicating heterogeneity in extent of electrical communication within the islet of Langerhans. Gap junction conductance reversibly decreased when the temperature was lowered from 37 to 30 degrees C and when the extracellular calcium concentration was raised from 2.56 to 7.56 mM. The coupling conductance decreased slightly during the active phase of the burst. Activation of adenylate cyclase with forskolin (10 microM) resulted in an increase in cell-to-cell electrical coupling. We conclude that beta-cell gap junction conductance can be measured in situ under near physiological conditions. Furthermore, the magnitude and physiological regulation of beta-cell gap junction conductance suggest that intercellular electrical communication plays an important role in the function of the endocrine pancreas.

Identification of full-length and truncated forms of Ehk-3, a novel member of the Eph receptor tyrosine kinase family

Factors that bind and activate receptor tyrosine kinases are known to play key roles during development and in the adult. The Eph-related receptors constitute the largest known family of receptor tyrosine kinases. Members of the Eph family exhibit intriguing patterns of expression in the embryo, implicating them in a variety of developmental processes, and their expression is often restricted to particular subpopulations of postmitotic neurons in the adult. We describe the identification and characterization of a novel member of the Eph receptor family, which we have termed Ehk-3 for Eph Homologous Kinase 3. Ehk-3 displays all the major structural features shared by other members of the Eph family, including a cysteine-rich region and tandem fibronectin type-III domains in its extracellular portion. Ehk-3 is expressed in two forms in a developmentally-regulated fashion: a conventional full-length version containing the intracellular tyrosine kinase domain, as well as a truncated form that lacks this domain. Both forms of Ehk-3 are quite restricted to the nervous system in the adult, but Ehk-3 is more widely expressed in the embryo, suggesting that Ehk-3 mediates different functions during development and in the adult.

Modulatory mechanism of ACTH on insulin secretion: effect on cytosolic Ca2+, membrane potential and Ca(2+-ATPase activity

The aim of this work was to get some insight into the mechanism by which ACTH produces its enhancing effect on glucose-induced insulin secretion. For this purpose we have determined: a) the release of insulin by isolated rat islets incubated with 3.3 or 16.6 mM glucose with or without the addition of 500 pg/ml ACTH, together with the changes induced by ACTH on b) cytosolic [Ca2+] of isolated B cells, c) islet plasma membrane Ca(2+)-ATPase activity and d) changes in membrane potential of single mouse islets. ACTH significantly enhanced the release of insulin elicited by either 3.3 or 16.6 mM glucose. This hormone concentration also induced a significant increase in the cytosolic [Ca2+] in isolated B cells. ACTH did not produce B cell membrane depolarization. Conversely, ACTH produced a significant decrease in islet plasma membrane Ca(2+)-ATPase activity. These results suggest that ACTH in concentrations similar to those attained by the endogenous peptide at the islet interstitium exerts its positive modulation on glucose-induced secretion of insulin, at least partly through its increasing effect on cytosolic [Ca2+] of B cells. The latter might be the consequence of the decreasing effect of ACTH on Ca(2+)-ATPase activity rather than to stimulation of voltage-dependent Ca(2+)-channels.

[Leishmania braziliensis: isolation from lesions by inoculation of hamsters with and without addition of salivary gland lysate from Lutzomyia youngi]

Homogenized biopsy tissue from the cutaneous leishmaniasis lesions of 50 patients from Trujillo, Venezuela, were inoculated subcutaneously into the tarsi of male hamsters. Homogenized tissue either alone or mixed with salivary gland lysates of Lutzomyia youngi were used for inoculation. Homogenized tissue alone yielded 58.5% of infections with a mean of twelve weeks for prepatency, while those mixed with sandfly lysate resulted in 92% of infections with a mean prepatency of three weeks.

Oxotremorine-m potentiation of glucose-induced insulin release from rat islets involves M3 muscarinic receptors

cDNAs encoding for M1 and M3 muscarinic acetylcholine (ACh) receptors were detected in rat pancreatic islet cells by polymerase chain reaction (PCR) amplification techniques. A new cholinergic agonist, oxotremorine-m (oxo-m), in the presence of glucose (5.6 mM), produced a dose-dependent potentiation of insulin secretion saturating at approximately 5 microM. This effect was suppressed by the L-type Ca2+ channel blocker nifedipine. Higher doses of oxo-m (50 microM) induced a biphasic insulin response both at low (5.6 mM) or high (16.7 mM) glucose concentrations. In a Ca(2+)-deficient medium containing glucose (5.6 mM), oxo-m evoked only a reduced first phase of insulin secretion. The potentiating effects of oxo-m were inhibited by the muscarinic receptor antagonists 4-diphenylacetoxy-N-methylpiperidine methiodide (M3), hexahydro-sila-difenidol hydrochloride, p-fluoro analogue (M3 > M1 > M2), and pirenzepine (M1) in a dose-dependent manner; half-maximal inhibitory concentration values were approximately 5, 20, and 340 nM, respectively. The PCR results demonstrate the presence of M1 and M3 muscarinic ACh receptors in the islet tissue, and the secretion data strongly suggest that the potentiation of glucose-induced insulin release evoked by oxo-m depends on the activation of a muscarinic M3-subtype receptor present in the beta-cell membrane.

Genomic organization and chromosomal localization of the human and mouse genes encoding the alpha receptor component for ciliary neurotrophic factor

Ciliary neurotrophic factor (CNTF) has recently been found to share receptor components with, and to be structurally related to, a family of broadly acting cytokines, including interleukin-6, leukemia inhibitory factor, and oncostatin M. However, the CNTF receptor complex also includes a CNTF-specific component known as CNTF receptor alpha (CNTFR alpha). Here we describe the molecular cloning of the human and mouse genes encoding CNTFR. We report that the human and mouse genes have an identical intron-exon structure that correlates well with the domain structure of CNTFR alpha. That is, the signal peptide and the immunoglobulin-like domain are each encoded by single exons, the cytokine receptor-like domain is distributed among 4 exons, and the C-terminal glycosyl phosphatidylinositol recognition domain is encoded by the final coding exon. The position of the introns within the cytokine receptor-like domain corresponds to those found in other members of the cytokine receptor superfamily. Confirming a recent study using radiation hybrids, we have also mapped the human CNTFR gene to chromosome band 9p13 and the mouse gene to a syntenic region of chromosome 4.

Effects of selected synthetic compounds on growth of Mucor rouxii

The in vitro activity of four synthetic compounds was tested on fungal cells from Mucor rouxii. The compounds included phenylenediamine, two phenolamines, and quinone. At the concentrations tested (10(-2)-10(-4)M) the compounds exhibited antifungal activity, with the exception of quinone. On the basis of their effects on spore germination, and development of hyphae, phenylenediamine was the more active. The mechanism of action of the compounds is still unknown, but hyphae show morphological alterations and disturb the distribution of calcofluor in the cell wall. This suggests variations in the genesis of the cell wall.

Single-microelectrode voltage clamp measurements of pancreatic beta-cell membrane ionic currents in situ

A conventional patch clamp amplifier was used to test the feasibility of measuring whole-cell ionic currents under voltage clamp conditions from beta-cells in intact mouse islets of Langerhans perifused with bicarbonate Krebs buffer at 37 degrees C. Cells impaled with a high resistance microelectrode (ca. 0.150 G omega) were identified as beta-cells by the characteristic burst pattern of electrical activity induced by 11 mM glucose. Voltage-dependent outward K+ currents were enhanced by glucose both in the presence and absence of physiological bicarbonate buffer and also by bicarbonate regardless of the presence or absence of glucose. For comparison with the usual patch clamp protocol, similar measurements were made from single rat beta-cells at room temperature; glucose did not enhance the outward currents in these cells. Voltage-dependent inward currents were recorded in the presence of tetraethylammonium (TEA), an effective blocker of the K+ channels known to be present in the beta-cell membrane. Inward currents exhibited a fast component with activation-inactivation kinetics and a delayed component with a rather slow inactivation; inward currents were dependent on Ca2+ in the extracellular solution. These results suggest the presence of either two types of voltage-gated Ca2+ channels or a single type with fast and slow inactivation. We conclude that it is feasible to use a single intracellular microelectrode to measure voltage-gated membrane currents in the beta-cell within the intact islet at 37 degrees C, under conditions that support normal glucose-induced insulin secretion and that glucose enhances an as yet unidentified voltage-dependent outward K+ current.

Theoretical models of the ion channel structure of amyloid beta-protein

Theoretical methods are used to develop models for the ion channel structure of the membrane-bound amyloid beta-protein. This follows recent observations that the beta-protein forms cation-selective channels in lipid bilayers in vitro. Amyloid beta-protein is the main component of the extracellular plaques in the brain that are characteristic of Alzheimer's disease. Based on the amino acid sequence and the unique environment of the membrane, the secondary structure of the 40-residue beta-protein is predicted to form a beta-hairpin followed by a helix-turn-helix motif. The channel structures were-designed as aggregates of peptide subunits in identical conformations. Three types of models were developed that are distinguished by whether the pore is formed by the beta-hairpins, the middle helices, or by the more hydrophobic C-terminal helices. The latter two types can be converted back and forth by a simple conformational change, which would explain the variable conduction states observed for a single channel. It is also demonstrated how lipid headgroups could be incorporated into the pore lining, and thus affect the ion selectivity. The atomic-scale detail of the models make them useful for designing experiments to determine the real structure of the channel, and thus further the understanding of peptide channels in general. In addition, if beta-protein-induced channel activity is found to be the cause of cell death in Alzheimer's disease, then the models may be helpful in designing counteracting drugs.

Rise in cytoplasmic Ca2+ induced by monensin in bovine medullary chromaffin cells

Monensin, a Na+/H+ exchanger, induces catecholamine secretion from adrenal chromaffin cells by an unknown mechanism. We found and report here that in bovine chromaffin cells, monensin evokes profound changes in [Ca2+]i which were measured by means of the fluorescent Ca2+ indicator Indo-1. Application of monensin (10 microM) generated a marked [Ca2+]i rise. Removal of external Ca2+ did not prevent the elevation of [Ca2+]i, though it was significantly decreased. In the presence of nifedipine (10 microM) or tetrodotoxin (3 microM) the monensin-induced [Ca2+]i rise remained unchanged. In contrast, in the absence of extracellular Na+ the [Ca2+]i rise was abolished. Addition of caffeine (40 mM) at the peak response generated by monensin produced a further increase in [Ca2+]i, which was independent of external [Ca2+] or [Na+]. After depletion of the IP3-sensitive compartment by thapsigargin (1 microM), caffeine still induced a rise in [Ca2+]i while the monensin response was absent. We concluded that the origin of the Ca2+ for the [Ca2+]i increase elicited by the Na+/H+ exchanger in chromaffin cells is not the extracellular space. Clearly there seems to be at least two intracellular Ca2+ stores, one of which is affected by monensin. This Ca2+ pool, which is different than the pool stimulated by caffeine, is sensitive to the extracellular [Ca2+] and to thapsigargin. Our data are compatible with the idea that the monensin mediated Na+ entry could activate the production of inositol trisphosphate and this in turn could trigger Ca2+ release from the endoplasmic reticulum.

The diabetogenic agent alloxan increases K+ permeability by a mechanism involving activation of ATP-sensitive K(+)-channels in mouse pancreatic beta-cells

The effects of the diabetogenic agent, alloxan, on membrane potential, input resistance and electrical activity of normal mouse pancreatic beta-cells were studied. Tetraethylammonium (TEA), quinine and Glyburide were used to block K(+)-channels and to elucidate the mechanisms underlying alloxan's effects on beta-cell membrane potential. Exposure of the islet to alloxan (75-100 microM) in the presence of glucose (11 mM), produced a rapid (15 sec), transient inhibition of electrical activity, often accompanied by hyperpolarization of the membrane, and this was followed by recovery of the burst pattern. This early effect of alloxan was followed after approximately 15 min by a complete inhibition of electrical activity and hyperpolarization. The inhibition accompanied by hyperpolarization was associated with a decrease in input resistance, indicating increased K(+)-conductance. Both the transient and delayed effects of alloxan were blocked by glucose (33 mM), quinine and glyburide but not by other conditions which induce continuous electrical activity such as elevated external [K+] (10 mM), ouabain, K+ removal, or TEA (20 mM). The transient inhibition induced by alloxan may be due to a direct competition with glucose transport/metabolism since it did not occur when alpha-keto isocaproic acid (KIC) was used to induce electrical activity. The delayed inhibition may reflect indirect effects of accumulation of this agent or its metabolites within the cell. Since both effects of alloxan are blocked by glyburide they appear to involve activation of the ATP-sensitive K(+)-channel (K-ATP).

beta-Amyloid Ca(2+)-channel hypothesis for neuronal death in Alzheimer disease

The Alzheimer's Disease (AD) amyloid protein (A beta P[1-40]) forms cation selective channels when incorporated into planar lipid bilayers by fusion with liposomes containing the peptide. Since the peptide has been proposed to occur in vivo in both membrane-bound and soluble forms, we also tested the possibility of direct incorporation of the soluble A beta P[1-40] into the membrane. We found the peptide can also form similar channels in acidic phospholipid bilayers formed at the tip of a patch pipet, as well as in the planar lipid bilayer system. As in the case of liposome mediated incorporation, the A beta P[1-40]-channel in the solvent-free membrane patch exhibits multiple cation selectivity (Cs+ > Li+ > Ca2+ > or = K+), and sensitivity to tromethamine. The fact that equivalent A beta P[1-40] amyloid channels can be detected by two different methods thus provides additional validation of our original observation. Further studies with a beta P-channels incorporated into planar lipid bilayers from the liposome complex have also revealed that the channel activity can express spontaneous transitions to a much higher range of conductances between 400 and 4000 pS. Under these conditions, the amyloid channel continues to be cation selective but loses its tromethamine sensitivity. By contrast, amyloid channels were insensitive to nitrendipine at either conductance range. We calculate that if such channels were expressed in cells, the ensuing ion fluxes down their electrochemical potential gradients would disrupt cellular homeostasis. We therefore interpret these data as providing further support for our beta-amyloid Ca(2+)-channel hypothesis for neuronal death in Alzheimer's Disease.