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I Atwater

Publications and source records attributed to I Atwater.

84 records · Page 5Linked to original sources
PubMed1976

Dynamic characteristics of electrical activity in pancreatic beta-cells. I. - Effects of calcium and magnesium removal.

1. Electrical activity in beta-cells of mouse islets of Langerhans was studied using ultra-fine micro-electrodes. 11.1 mM glucose stimulated electrical activity in bursts of 12 to 24 sec duration, including an active phase, characterized by depolarization and rapid fluctuations of the potential (spikes), and a silent phase when the membrane repolarized. Analysis of the spike frequency distribution during the active phase supports the hypothesis that the spikes result from exocytosis. 2. Calcium removal, in the presence of magnesium, did not alter membrane potential. Burst activity in 11.1 mM glucose was inhibited, and spike activity appeared in some cells. The frequency of these spikes increased as the magnesium was reduced. Re-introduction of calcium induced depolarization and increased spike frequency. 3. Omission of calcium and magnesium reversibly induced depolarization. In 11.1 mM glucose, there was further depolarization, and an altered pattern of electrical activity. Bursts were reversed in polarity, the silent phase being depolarized with respect to the active phase, and duration of reversed bursts was increased while the number of spikes during the active phase was decreased. Caffeine, when added, increased the number of spikes. Re-introduction of calcium and magnesium induced a transient hyperpolarization with suppression of spike activity. 4. Simultaneous reduction of calcium and magnesium to one-half or one-tenth of the control concentration enhanced the overall spike activity in 11.1 mM glucose by shortening or blocking the silent phase between bursts. 5. Quinidine, 0.14 mM, irreversibly inhibited the silent phases of bursts and modified the biphasic electrical response to glucose.

Animals↗
PubMed1974

Calcium influxes and tension development in perfused single barnacle muscle fibres under membrane potential control.

1. Single giant barnacle muscle fibres from Megabalanus psittacus (Darwin) were used to measure the Ca entry and the development of tension in the fibres under membrane potential control.2. Fibres bathing in 60 mm-MgCl(2) sea water, free of Ca, did not develop tension with sudden displacements of the membrane potential towards more positive values. This failure to develop tension with depolarizations was observed with and without the internal application of Ca buffers.3. Fibres bathing in artificial sea water with either 10, 20, 60 or 100 mm-CaCl(2) developed tension with depolarization even after 60 min of internal perfusion of the fibres with solution containing no Ca buffers. In this case the maximum tension recorded during a voltage clamp run decreased with time from nearly 2.5 to 0.2 kg/cm(2). However, addition of 10 mm-Tris-EGTA (ethyleneglycol-bis (beta-aminoethyl ether) N, N' - tetraacetic acid) to the perfusing solution rapidly eliminated the development of tension; after 10 min of internal perfusion with Ca buffers no tension could be elicited by electrical stimulation.4. Ca-influx determinations were carried out only in the fibres in which the outward K(+) currents were blocked by internal application of TEA (tetra-ethylammonium). The ratio of ;measured extra Ca influx/computed ionic flux of divalent cations during the inward current' was 1.06 +/- 0.41.5. For fibres bathed in either natural sea water or in artificial sea water with various concentrations of Ca, the temporal course of development of isometric tension was similar to the temporal course of the integral of the inward current due to Ca(2+).6. In a fibre from M. psittacus bathing in natural sea water the calculated extra entry of Ca required to increase its internal concentration to about 50 mum was 500 p-mole per depolarization (60 mV); while the corresponding average influx calculated from the inward current record in natural sea water is 474 p-mole.7. Evidence was obtained for the accumulation of Ca in an internal compartment.

Acetates↗
PubMed1970

Time course of the sodium permeability change during a single membrane action potential.

1. A method for measuring the time course of ionic fluxes during a non-propagated membrane action potential is described; the technique combines intracellular perfusion of the squid giant axon with radioactive tracer methods and with a method for controlling the membrane potential during small time intervals.2. The method is used to determine the temporal course of the sodium extra influx during an action potential.3. The results agree with the time course of the permeability change to sodium ions calculated with the Hodgkin & Huxley equations.4. An average extra sodium influx of 7.13 p-mole/cm(2) per action potential was determined at 12 degrees C and of 5.23 p-mole/cm(2) per action potential at 15 degrees C.

Action Potentials↗
PubMed1969

Analysis of the effects of calcium or magnesium on voltage-clamp currents in perfused squid axons bathed in solutions of high potassium.

Isolated axons from the squid, Dosidicus gigas, were internally perfused with potassium fluoride solutions. Membrane currents were measured following step changes of membrane potential in a voltage-clamp arrangement with external isosmotic solution changes in the order: potassium-free artificial seawater; potassium chloride; potassium chloride containing 10, 25, 40 or 50, mM calcium or magnesium; and potassium-free artificial seawater. The following results suggest that the currents measured under voltage clamp with potassium outside and inside can be separated into two components and that one of them, the predominant one, is carried through the potassium system. (a) Outward currents in isosmotic potassium were strongly and reversibly reduced by tetraethylammonium chloride. (b) Without calcium or magnesium a progressive increase in the nontime-dependent component of the currents (leakage) occurred. (c) The restoration of calcium or magnesium within 15-30 min decreases this leakage. (d) With 50 mM divalent ions the steady-state current-voltage curve was nonlinear with negative resistance as observed in intact axons in isosmotic potassium. (e) The time-dependent components of the membrane currents were not clearly affected by calcium or magnesium. These results show a strong dependence of the leakage currents on external calcium or magnesium concentration but provide no support for the involvement of calcium or magnesium in the kinetics of the potassium system.

Animals↗
PubMed1969

Sodium influxes in internally perfused squid giant axon during voltage clamp.

1. An experimental method for measuring ionic influxes during voltage clamp in the giant axon of Dosidicus is described; the technique combines intracellular perfusion with a method for controlling membrane potential.2. Sodium influx determinations were carried out while applying rectangular pulses of membrane depolarization. The ratio ;measured sodium influx/computed ionic flux during the early current' is 0.92 +/- 0.12.3. Plots of measured sodium influx and computed ionic flux during the early current against membrane potential are very similar. There was evidence that the membrane potential at which the sodium influx vanishes is the potential at which the early current reverses.

Animals↗