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Biomedical subjects

M M White

Publications and source records attributed to M M White.

At least 73 records · Page 4Linked to original sources
PubMed1988

Control of Torpedo acetylcholine receptor biosynthesis in Xenopus oocytes.

The nicotinic acetylcholine receptor (AcChoR) of Torpedo electroplax is a multisubunit transmembrane glycoprotein complex with a subunit stoichiometry of alpha 2 beta gamma delta. The RNAs for the separate subunits were transcribed in vitro from cDNAs inserted in pSP64T vectors and microinjected in Xenopus oocytes. Microinjected in vitro-transcribed RNAs were stable, with a half-life of 72 hr. Xenopus oocytes assembled functional AcChoRs from the subunit-specific RNAs. These receptors were inserted in the cell membrane and could be detected as early as 6 hr after RNA microinjection. The oocyte-expressed AcChoR subunits could be immunoprecipitated with anti-Torpedo AcChoR subunit antibodies. Expression of the AcChoR in oocytes proceeded linearly for 72 hr after microinjection. While the amount of RNA injected did not alter the linearity of the expression time course, the rate of receptor expression in oocytes showed a saturable dependence on RNA concentration. Varying the relative amount of alpha-subunit RNA microinjected into oocytes had a striking effect on receptor expression. This effect was specific for the alpha-subunit. These results suggest that transcript availability may control receptor expression in Xenopus oocytes. In addition, the availability of the alpha-subunit may be a limiting factor for receptor expression.

Animals↗
PubMed1988

Studies of esterase 6 in Drosophila melanogaster. XVIII. Biochemical differences between the slow and fast allozymes.

Most natural populations of Drosophila melanogaster are polymorphic for two major electrophoretic variants at the esterase-6 locus. The frequency of the EST 6F allozyme is greatest in populations in warmer latitudes, whereas the EST 6S allozyme is predominant in colder latitudes. Latitudinal clines in electromorph frequencies are found on three continents. Purified preparations of the allozymes have been characterized for their pH optimum, substrate specificity, organophosphate inhibition, alcohol activation, thermal stability, and kinetic parameters. These and previous analyses of the EST 6 allozymes reveal that the two variants have differences in their physical and kinetic properties that may provide a basis for the selective maintenance of the polymorphisms and an explanation of the clinal variation observed in natural populations.

Alcohols↗
PubMed1988

Forskolin alters acetylcholine receptor gating by a mechanism independent of adenylate cyclase activation.

The diterpene forskolin activates adenylate cyclase in a receptor-independent fashion and is commonly used to obtain a rapid elevation of intracellular cAMP levels. Application of 10-20 microM forskolin to Xenopus oocytes that express Torpedo nicotinic acetylcholine (ACh) receptors leads to an acceleration in the decay of ACh-elicited currents, which could be taken as evidence for modulation of ACh receptor gating by cAMP-dependent protein kinase. However, the effect is not mimicked by phosphodiesterase inhibitors or intracellular injection of a cAMP analog. In addition, 1,9-dideoxyforskolin, which is unable to activate adenylate cyclase, has a similar effect. Finally, the action of forskolin is rapidly reversible, with full onset and recovery occurring within the exchange time of the recording chamber. These results suggest that forskolin is a potent local anesthetic and that this property of this widely used compound must be taken into account when using it to study ion channel modulation.

Adenylyl Cyclases↗
PubMed1987

Technique and evaluation of the double contrast ileostomy enema.

A simple method for double contrast examination of the distal ileum in patients with a terminal ileostomy is described, using stomal intubation with a Foley catheter, syringe injection of barium, and air insufflation with smooth muscle relaxation. In 40 examinations the median length of ileum filled was 55 cm (range 20 to 140) with 38% of this length visualised in double contrast in a single view. The prestomal ileum was demonstrated clearly in 85%. Nine out of 18 patients with an abnormal ileostomy enema also had a barium follow through examination. The ileostomy enema made a significant contribution to the diagnosis in five of the nine patients.

Adolescent↗
PubMed1985

Mouse-Torpedo hybrid acetylcholine receptors: functional homology does not equal sequence homology.

The nicotinic acetylcholine (AcCho) receptor (AcChoR) is a multisubunit protein complex of stoichiometry alpha 2 beta gamma delta. The several subunits show homology with each other within a given species; in addition, homology is found between analogous subunits between species. We have used the phage SP6 RNA polymerase transcription system to produce single-species RNA in vitro for various AcChoR subunits from cDNAs. Injection of an equimolar mixture of RNA for the alpha, beta, gamma, and delta subunits of Torpedo californica AcChoR into Xenopus oocytes results in the appearance of functional receptors in the oocyte membrane. No response to AcCho is detected when the beta or gamma subunit RNA is omitted, and a small response is seen when the delta subunit RNA is omitted. Replacement of Torpedo delta subunit RNA by the mouse BC3H-1 cell line AcChoR delta subunit RNA leads to the formation of functional receptors that show a 3-4-fold greater response to AcCho than does the full Torpedo complex. No response is seen when the mouse delta RNA replaces Torpedo gamma RNA. By amino acid homology profile comparisons, the mouse delta subunit appears to be moderately but not highly similar to the Torpedo delta subunit; the apparent similarity to the Torpedo gamma subunit is only slightly less. Therefore, the features of the primary sequence that determine the functional delta character of the mouse polypeptide are not revealed by simple homology comparisons.

Amino Acid Sequence↗
PubMed1985

Activation of squid axon K+ channels. Ionic and gating current studies.

We have used data obtained from measurements of ionic and gating currents to study the process of K+ channel activation in squid giant axons. A marked improvement in the recording of K+ channel gating currents (IKg) was obtained by total replacement of Cl- in the external solution by NO-3, which eliminates approximately 50% of the Na+ channel gating current with no effect on IKg. The midpoint of the steady state charge-voltage (Qrel - V) relationship is approximately 40 mV hyperpolarized to that of the steady state activation (fo - V) curve, which is an indication that the channel has many nonconducting states. Ionic and gating currents have similar time constants for both ON and OFF pulses. This eliminates any Hodgkin-Huxley nx scheme for K+ channel activation. An interrupted pulse paradigm shows that the last step in the activation process is not rate limiting. IKg shows a nonartifactual rising phase, which indicates that the first step is either the slowest step in the activation sequence or is voltage independent. These data are consistent with the following general scheme for K+ channel activation: (formula; see text)

Animals↗
PubMed1984

Dimeric structure of single chloride channels from Torpedo electroplax.

The inhibition by 4,4'-diisothiocyano-2,2'-stilbenedisulfonate (DIDS) of Cl- channels from Torpedo electroplax incorporated in planar phospholipid bilayer membranes is studied. DIDS irreversibly and rapidly inhibits the macroscopic conductance of membranes containing many channels. At the single-channel level, the effect of DIDS is more complicated. The uninhibited single channel displays three "substates" of conductances 20, 10, and 0 pS. Short exposure (5-30 s) to 10 microM DIDS converts this three-level active channel into a "conventional" channel of 10-pS conductance. Longer exposure eliminates all channel fluctuations. The results are taken as strong evidence that the Cl- channel is constructed as a functional dimer of identical protein subunits.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1984

Dose response of topical carbamylcholine chloride (carbachol) in normotensive and early glaucomatous beagles.

Single-dose loading with a placebo (0.5% methylcellulose) and 0.75%, 1.5%, 2.25%, or 3% carbamylcholine chloride (carbachol) was evaluated in normotensive and glaucomatous Beagles to determine dose response. The 4 carbachol concentrations significantly (P less than 0.05) lowered intraocular pressure and reduced pupil size in the normotensive and early glaucomatous Beagles at most time intervals as compared with base-line values, nontreated fellow eyes, and placebo-treated eyes. There were no significant (P less than 0.24) effects on intraocular pressure or pupil size by unilateral carbachol instillations as compared with base-line values or placebo-treated eyes.

Administration, Topical↗
PubMed1981

Chloride permeability of membrane vesicles isolated from Torpedo californica electroplax.

The Cl- permeability of membrane vesicles prepared from the electric organ of the marine ray Torpedo californica was studied by means of radioactive tracer exchange and by measuring the changes in the scattered-light intensity caused by osmotically induced volume changes. Both types of experiments indicate that a substantial fraction of the vesicles is extremely permeable of Cl-. Furthermore, this permeability pathway is inhibited by 4,4'-diisothiocyano-2,2'-disulfonic acid stilbene, a well-known inhibitory of anion transport in a variety of systems. The properties of this permeability pathway are consistent with its identification as the voltage-aged Cl- channel studied in planar bilayers.

Animals↗
PubMed1981

Probes of the conduction process of a voltage-gated Cl- channel from Torpedo electroplax.

The open-channel conductance properties of a voltage-gated Cl- channel derived from Torpedo californica electroplax and incorporated into planar bilayers were studied by several approaches. In neutral bilayers the channel conductance saturates with Cl- activity according to a rectangular hyperbolic relation with a half-saturation activity of 75 mM and a maximum conductance of 32 pmho. The observation of identical behavior in charged membranes implies that ions permeating the channel do not sense the surface potential of the bulk membrane. The Cl-:Br- permeability ratio, measured under biionic conditions, is independent of salt concentration. SCN- ion reversibly blocks the channel. The voltage dependence of the block implies the existence of two separate blocking sites within the channel: one accessible from the cis side only (the side to which vesicles are added) and the other accessible from the trans side only. The block at each site is competitive with Cl-. The results are consistent with a single-ion Eyring model of the conduction process in which the ion must traverse three kinetic barriers as it permeates the channel and in which the channel can accommodate at most one ion at a time.

Animals↗
PubMed1979

A voltage-gated anion channel from the electric organ of Torpedo californica.

Addition of membrane vesicles prepared from the electric organ of Torpedo californica to the aqueous phase of a planar phospholipid bilayer system results in a large (up to 3 orders of magnitude) stepwise increase in membrane conductance. This increased conductance consists of two components: an ohmic background "leak" and a voltage-dependent, ideally anion-selective conductance. The anion conductance is low at voltages greater than +10 mV, rises sharply as the voltage becomes negative, and then saturates as the voltage becomes highly negative. (The trans side of the bilayer, to which vesicles are not added, is defined as ground.) Under high amplification, the anion conductance shows single channel behavior with a voltage-independent, single channel conductance of 13.9 +/- 0.1 pmho in 0.1 M Cl-. Furthermore, the anion channel, but not the background conductance, is inhibited by submillimolar concentrations of SITS and DIDS, two well known anion transport inhibitors. The inhibition is seen only when SITS or DIDS is added to the cis side. No cholinergic agents tested have any effect on the channel.

4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfo↗
PubMed1976

Evaluation of stroke disability.

The disabilities resulting from a stroke are not well understood from the epidemiologic or functional point of view. The stroke may impair mental status, perception, sensation, communication and motor ability; the total resulting disability is related to the extent of impairment in each of these areas. A complete evaluation in all these areas has to be done to determine the degree of disability before any rehabilitation program is planned. A comprehensive approach to evaluating stroke disability is presented that includes correlating the degree of impairment in each of the above-mentioned areas with the overall functional ability of the patient.

Activities of Daily Living↗