Biomedical subjects

E Rojas

Publications and source records attributed to E Rojas.

At least 181 records · Page 10Linked to original sources

PubMed1986

TEA-insensitive K-channels in the crab giant axon.

TTX and TEA-insensitive permeabilities were studied in the crab giant axon under voltage-clamp. Membrane currents in the presence of internal TEA (40 mmol/l) and external TTX (300 nmol/l) may be analyzed as the sum of two components: a linear component, identified as the so-called leakage current, and a non-linear component, identified as a TEA-insensitive potassium channel. Ion permeability ratio of the TTX and TEA insensitive cation channel calculated from reversal potential shows the following sequence pK+:pNa+:pCs+:pRb+:pNH+4 = 1.00:0.16:0.16:0.09:0.06. TEA-insensitive outward currents, carried mainly by Cs+, may be recorded in the presence of different external solutions. Voltage-dependence and equilibrium potential of this channel in physiological conditions allows to postulate its contribution to maintain the cell depolarized during repetitive firing.

Animals↗

PubMed1985

Real-time measurements of acetylcholine-induced release of ATP from bovine medullary chromaffin cells.

The luminescent oxidation of luciferin has been used to monitor acetylcholine-induced ATP release from cultured bovine chromaffin cells. Acetylcholine (1-100 microM) evoked ATP release of up to 30% of the total cellular ATP. This secretion required external free calcium and could also be elicited by K+-induced membrane depolarization. The size of the cytosolic ATP compartment was estimated as 5% of the ATP in the cell by solubilising the cell membrane using digitonin (20 microM) or by application to the cells of brief pulses (2 microseconds) of high electric field (2000 V/cm). Blockers of the voltage-gated Ca2+ channel effectively blocked K+-induced ATP release, while the acetylcholine antagonists d-tubocurarine and beta-bungarotoxin inhibited the acetylcholine-induced release of ATP. These data support the concept that ATP is released together with the catecholamines by exocytosis of chromaffin granule contents.

Acetylcholine↗

PubMed1985

Monovalent cation permeabilities of the potassium systems in the crab giant axon.

Permeability ratios for pairs of monovalent cations permeating the two potassium systems proposed for the giant axon of the crab Carcinus maenas (M.E. Quinta-Ferreira, E. Rojas & N. Arispe, J. Membrane Biol. 66:171-181, 1982b) were estimated from measurements of the reversal potential of the currents under voltage-clamp conditions. With K+ inside the axon, permeability ratios from the reversal potential of the currents through the late channel are: PRb/PK = 0.9, PNH4/PK less than 0.2 and PCS/PK = 0.18. With CS+ inside the ratios are: PK/PCS = 8.7, PRb/PCS = 7.1 and PNH4/PCS = 2.4. The analysis of the inward currents carried by Rb+ or NH+4 showed similar reversal potentials for the early transient component and the late sustained component. Whence, the sequence of permeabilities for the two types of potassium channels is: PK greater than PRb greater than PNH4 greater than PNa = PCS. The time constants for the activation of the two components recorded either in K-, Rb-, or NH4-artificial seawater are twice as large as the corresponding time constants measured in Na-artificial seawater.

Ammonia↗

PubMed1985

Differential blockage of two types of potassium channels in the crab giant axon.

Measurements were made of the kinetic and steady-state characteristics of the potassium conductance in the giant axon of the crabs Carcinus maenas and Cancer pagirus. The conductance increase during depolarizing voltage-clamp pulses was analyzed assuming that two separate types of potassium channels exist in these axons (M.E. Quinta-Ferreira, E. Rojas and N. Arispe, J. Membrane Biol. 66:171-181, 1982). It is shown here that, with small concentrations of conventional K+-channel blockers, it is possible to differentially inhibit these channels. The potassium channels with activation and fast inactivation gating (m3h, Hodgkin-Huxley kinetics) were blocked by external application of 4 amino-pyridine (4-AP). The potassium channels with standard gating (n4, Hodgkin-Huxley kinetics) were preferentially inhibited by externally applied tetraethylammonium (TEA). The differential blockage of the two types of potassium conductance changes suggests that they represent two different populations of potassium channels. It is further shown here that blocking the early transient conductance increase leads to the inhibition of the repetitive electrical activity induced by constant depolarizing current injection in fibers from Cardisoma guanhumi.

4-Aminopyridine↗

PubMed1985

A comparative analysis of two methods for the study of alcohol intake in Mexico.

Two different methodologies were studied in a semi-rural community in Mexico city to investigate alcohol-intake patterns and attitudes towards drinking and drunkenness. One of the methods consisted of a household survey. The other used the anthropological informant's technique. These methods were compared only at those points where were found similarities in the data collected. We conclude that both methods give comparable estimates of what goes on in the community. Nevertheless the informant method is cheaper, results are obtained in less time and the community members are engaged in this problem, thus permitting future action to be taken.

Alcohol Drinking↗

PubMed1985

Glucose-induced oscillatory changes in extracellular ionized potassium concentration in mouse islets of Langerhans.

Liquid membrane [K+]-sensitive microelectrodes (1-2 micron tip diameter) were used to measure the extracellular ionized potassium concentration in mouse pancreatic islets of Langerhans. With the tip of the microelectrode at the surface of the islet, the time course of the [K+]-sensitive electrode potential changes in response to the application of rapid changes in [K+]o (from 1.25 to 5 mM), could be reproduced by the equation for K+-diffusion through a 100-micron-thick unstirred layer around the islet (diffusion coefficient for K+ at 27 degrees C, DK,o, taken as 1.83 X 10(-5) cm2/s). The time to reach 63% of the steady-state electrode response with the tip in the chamber at the surface of the islet was from 5 to 6 s. When the tip of the [K+]-sensitive electrode was placed in the islet tissue, the time for the response to reach 63% of the steady-state level increased. The time course of the [K+]-sensitive electrode response could be reproduced using the same diffusion model assuming that K+ diffusion into the islet tissue takes place in a tortuous intercellular path with an apparent diffusion coefficient, DK,I, about half of DK,o, in series with the unstirred layer around the islet. In the absence of glucose the potassium concentration in the extracellular space, [K+]I, was found to be higher than the concentration in the external modified Krebs solution, [K+]o. The difference in concentration [K+]I - [K+]o was greater when [K+]o was smaller than 2 mM. In the presence of glucose (between 11 and 16 mM), under steady-state conditions, small oscillatory changes in the [K+], (1.48 +/- 0.94 mM) were detected. Simultaneous recording of membrane potential from one B-cell and [K+], in the same islet indicated that the potassium concentration increased during the active phase of the bursts of electrical activity. Maximum concentration in the intercellular was reached near the end of the active phase of the bursts. We propose that the space between islet cells constitutes a restricted diffusion system where potassium accumulates during the transient activation of potassium channels.

Animals↗

PubMed1985

Membrane potential measurements in islets of Langerhans from ob/ob obese mice suggest an alteration in [Ca2+]i-activated K+ permeability.

High-resistance micro-electrodes were used to measure membrane potentials in beta-cells from islets of Langerhans of ob/ob obese mice (Norwich colony). In the presence of glucose the burst pattern of electrical activity recorded in ob/ob beta-cells, although similar to the burst pattern recorded from normal beta-cells, presents important differences. The membrane potential of the ob/ob beta-cells in the presence of 11 mM glucose in the modified Krebs solution oscillates between a silent-phase level at -48 mV and an active-phase level at -36 mV, similarly to normal mouse islet beta-cells. However, the average active-phase duration is 20 s in ob/ob beta-cells compared with 5 s in normal beta-cells. The average burst frequency is 1.8 bursts/min in ob/ob beta-cells compared with 3 bursts/min in normal beta-cells. While normal beta-cells show continuous spike activity above 16 mM glucose, ob/ob beta-cells often exhibit a burst pattern of electrical activity at glucose concentrations as high as 33 mM. Compared with normal beta-cells, the relationship between spike frequency and glucose concentration is shifted towards lower concentrations in ob/ob beta-cells. Thus, the concentration for half-maximal spike frequency is 6.9 mM for the ob/ob beta-cells and 10.2 mM for the normal beta-cells. In ob/ob beta-cells, the mitochondrial inhibitor carbonyl-cyanide m-chlorophenylhydrazone induces hyperpolarization of the membrane, consistent with its effect of stimulating K+ permeability in normal islets. However, quinine and the sulphonylurea glibenclamide did not block the silent phase between the bursts of electrical activity. Both drugs block the [Ca2+]i-activated K+ permeability thought to control the membrane potential at the silent phase in normal beta-cells. The modified pattern of response to glucose and decreased sensitivity to quinine and glibenclamide suggest that the beta-cell membrane of the ob/ob islet of Langerhans has a modified [Ca2+]i-activated K+ permeability.

Animals↗

PubMed1984

Electrical coupling between cells in islets of Langerhans from mouse.

Two microelectrodes have been used to measure membrane potentials simultaneously in pairs of mouse pancreatic islet cells. In the presence of glucose at concentrations between 5.6 and 22.2 mM, injection of current i into cell 1 caused a membrane potential change in this cell, V1, and, provided the second microelectrode was less than 35 micron away, in a second impaled cell 2, V2. This result establishes that there is electrical coupling between islet cells and suggests that the space constant of the coupling ratio within the islet tissue is of the order of a few beta-cell diameters. The current-membrane potential curves i-V1 and i-V2 are very similar. By exchange of the roles of the microelectrodes, no evidence of rectification of the current through the intercellular pathways was found. Removal of glucose caused a rapid decrease in the coupling ratio V2/V1. In steady-state conditions, the coupling ratio increases with the concentration of glucose in the range from 0 up to 22 mM. Values of the equivalent resistance of the junctional and nonjunctional membranes have been estimated and found to change with the concentration of glucose. Externally applied mitochondrial blockers induced a moderate increase in the junctional resistance possibly mediated by an increase in intracellular Ca2+.

Animals↗

PubMed1984

The response of pancreatic beta-cell membrane potential to potassium-induced calcium influx in the presence of glucose.

Membrane potential measurements were made in pancreatic beta-cells from microdissected islets from normal mice. In the presence of 11 mM glucose, depolarization of the membrane for 1 min with 50 mM potassium is followed by an inhibition of electrical activity before the normal burst pattern resumes. This inhibitory period, called the recovery time, is steady for each beta-cell after three consecutive pulses of 50 mM potassium. The mean recovery time is 109 s. During the recovery time, the membrane is hyperpolarized and the input resistance is decreased, indicating that potassium permeability is high over this period. The recovery time is dependent on the size of the depolarization: 1 min exposure to potassium concentrations below 50 mM reduces the recovery time with a half-maximal effect at 38.5 mM potassium, corresponding to -27 mV. Also, increasing the extracellular calcium concentration lengthens the recovery time. Increasing the glucose concentration, however, shortens the recovery time. It is postulated that the recovery time represents activation of the calcium-gated potassium permeability and is a reflexion of the time taken for the beta-cell to buffer the increased intracellular calcium resulting from the potassium depolarization.

Animals↗

PubMed1984

Electrophysiological evidence for the inhibition of potassium permeability in pancreatic beta-cells by glibenclamide.

The effects of glibenclamide on the electrical activity of the beta-cells of the islets of Langerhans of normal mice have been investigated in the absence and presence of glucose (11.1 mM). Glibenclamide depolarized the cell membrane and this has been interpreted in terms of an increase in the ratio of the Na+ and K+ permeabilities, PNa/PK. This ratio increased from 0.05 to 0.24 in the presence of 4 microM glibenclamide and zero glucose. The input resistance of the beta-cells also increased. These observations indicate a decrease in K+ permeability. The effect is only slowly reversed after removal of glibenclamide. Uncouplers of oxidative phosphorylation do not reverse the depolarization induced by glibenclamide. It is suggested that glibenclamide is acting directly to inhibit the [Ca2+]i-gated K+ permeability in the beta-cell membrane.

Animals↗

PubMed1984

Cooling dissociates glucose-induced insulin release from electrical activity and cation fluxes in rodent pancreatic islets.

Insulin release and beta-cell membrane potentials in response to glucose at 37 and 27 degrees C have been measured simultaneously in single, micro-dissected, perifused islets of Langerhans from normal mice. Insulin release and 45Ca outflow in response to glucose at 37 and 27 degrees C have been measured simultaneously from perfused islets isolated by collagenase digestion from normal rats. The effect of cooling on beta-cell membrane potassium permeability was assessed by changes in measured membrane potential and input resistance (in the mouse) and by changes in 86Rb outflow (in the rat). Resting and active beta-cell membrane parameters (i.e. membrane potential, spike frequency, input resistance, 45Ca outflow and 86Rb outflow), in both mouse and rat islets, were affected only slightly by cooling to 27 degrees C, with temperature coefficients of 2 or lower. At 27 degrees C glucose-stimulated insulin release was inhibited completely in mouse islets and almost completely in rat islets. The temperature coefficients in both preparations were greater than 5. It is concluded that beta-cell electrical activity and changes in membrane permeability induced by glucose are not consequences of insulin release.

Animals↗

PubMed1984

Effects of Zn2+ on glucose-induced electrical activity and insulin release from mouse pancreatic islets.

The effects of Zn2+ and CO2+ on glucose-induced beta-cell electrical activity and on insulin release from microdissected mouse pancreatic islets were studied. In 11 mM glucose the electrical activity is characterized by a burst pattern with a bimodal distribution of spike amplitudes along the plateau phase. Zn2+ at 0.05 mM induced a reduction in the number of spikes during the bursts and preferentially blocked the large action potentials. Zn2+ at 0.1 mM and CO2+ at 1.0 mM completely inhibited the electrical activity in response to glucose. Zn2+ inhibition of electrical activity was poorly reversible, whereas CO2+ inhibition was rapidly and completely reversible. Zn2+ and CO2+ inhibited the glucose-stimulated insulin release from microdissected perifused islets. Half-maximal inhibition occurred at about 0.3 mM for both metals. Zn2+ also inhibited K+-induced insulin release in the absence of glucose, indicating that Zn2+ inhibition does not involve glucose metabolism. It is proposed that Zn2+ blocks the voltage-gated Ca2+ channels in pancreatic beta-cells.

Action Potentials↗

PubMed1984

The topography of electrical synchrony among beta-cells in the mouse islet of Langerhans.

beta-Cells in microdissected islets of Langerhans produce rhythmical bursts of electrical activity. This was monitored with two micro-electrodes simultaneously and the frequency and phase (collectively referred to as synchrony) of the two signals was investigated. At any instant two impaled cells produced bursts of the same frequency even when separated by up to 400 micron. When the electrode tips were separated by less than about 20 micron and current injection showed the cells to be ionically coupled the two signals were in phase and had almost identical shape. The phase relations between cells further apart were variable, the leading cell usually being located deeper within the islet than the other impaled cell. Increasing the glucose concentration increased electrical activity, reduced any phase lags and made the shape of the bursts more similar. There was less lag between the responses from two cells when the glucose concentration was suddenly reduced, than when it was suddenly increased. Qualitatively similar observations were made in glibenclamide-treated mice, a treatment previously shown to increase dye coupling between islet cells. However, the response to increasing glucose concentrations showed less phase lag; likewise the phase lag between bursts was reduced. Furthermore the response to current injected into one cell could be detected at much larger distances (up to 80 micron) than in control islets. This suggests that electrical coupling of beta-cells was improved in sulphonylurea-treated mice. Electron microscopy of both control and glibenclamide-treated mouse islets fixed at the end of each electrophysiological experiment showed the region impaled by the electrodes to be well preserved and, whenever the electrodes penetrated at least 20 micron into the islet, to contain a large proportion of beta-cells. The data support the view that, within an islet, most but not necessarily all cells are electrically synchronized, and that the coupling can be modulated by natural and pharmacological secretagogues.

Action Potentials↗

PubMed1983

Local effects induced by coral snake venoms: evidence of myonecrosis after experimental inoculations of venoms from five species.

The local effects induced by intramuscular inoculations of venoms from six species of coral snakes were studied in mice. Venoms of Micrurus nigrocinctus nigrocinctus, M. n. mosquitensis, M. alleni, M. frontalis, M. carinicauda and M. surinamensis induced prominent myonecrosis which was observed histologically. From a morphological point of view all these venoms induced a similar pattern of myonecrosis, characterized by a conspicuous alteration of the intracellular structure. This myotoxic activity was corroborated by an increase in plasma creatine kinase levels 3 hr after i.m. injection of M. n. nigrocinctus, M. n. mosquitensis, M. frontalis and M. carinicauda venoms. M. mipartitus venom did not induce myonecrosis. None of the venoms induced edema or hemorrhage at the site of injection.

Animals↗

PubMed1983

Properties of the Ca-activated K+ channel in pancreatic beta-cells.

The existence of [Ca2+]i-activated K+-channels in the pancreatic beta-cell membrane is based in two observations: quinine inhibits K+-permeability and, increasing intracellular Ca2+ stimulates it. The changes in K+-permeability of the beta-cell have been monitored electrically by combining measurements of the dependence of the membrane potential on external K+ concentration and input resistance. The changes in the passive 42K and 86Rb efflux from the whole islet have been measured directly. Intracellular Ca2+ has been increased by various means, including increasing extracellular Ca2+, addition of the Ca2+-ionophore A23187 or noradrenaline and application of mitochondrial uncouplers and blockers. In addition to quinine, many other substances have been found to inhibit or modulate the [Ca2+]i-activated K+-channel. The most important of these is the natural stimulus for insulin secretion, glucose. Glucose may inhibit K+-permeability by lowering intracellular Ca2+. Glibenclamide, a hypoglycaemic sulphonylurea, is about 25 times more active than quinine in blocking the K+-channel in beta-cells. The methylxanthines, c-AMP, various calmodulin inhibitors and Ba2+ also inhibit K+-permeability. Genetically diabetic mice have been studied and show an alteration in the [Ca2+]i-activated K+-channel. It is concluded that the [Ca2+]i-activated K+-channel plays a major role in the normal function of the pancreatic beta-cell. The study of its properties should prove valuable for the understanding and treatment of diabetes.

Animals↗

PubMed1983

Beta cell membrane potential and insulin release; role of calcium and calcium:magnesium ratio.

Glucose-induced insulin release from perfused rat pancreas was compared with glucose-induced changes in membrane potential of beta cells from mouse islets. Extracellular concentrations of Ca and Mg were varied as steps, simultaneously or separately, from 10% to 200% of normal in the presence of 11.1 mM glucose. A change in Ca induced a transient change in electrical activity paralleled by a transient change in insulin release. If the Ca/Mg ratio was maintained, steady-state insulin release remained constant between 10% and 200% Ca, while electrical activity showed alterations. Analysis of burst parameters indicated that increased or decreased Ca entry was balanced by decreased or increased excitability. It is postulated that the beta cell contains a compensator mechanism for the regulation of Ca influx.

Animals↗

PubMed1983

Effects of adrenaline and noradrenaline on glucose-induced electrical activity of mouse pancreatic beta cell.

The effects of adrenaline and noradrenaline on membrane potential and glucose-induced electrical activity were studied in micro-dissected mouse Islets of Langerhans. Both catecholamines induced hyperpolarization and blocked electrical activity in the presence of 11.1 mM glucose. Phentolamine, but not propranalol, blocked these effects, indicating predominantly alpha receptor action. Quinine, but not tetraethylammonium ions, antagonized the inhibitory effects of the catecholamines. The data are consistent with the hypothesis that alpha receptor activation induces a transient increase in intracellular Ca2+ concentration which in turn leads to an increase in K+ permeability.

Drug Antagonism↗

PubMed1983

Valinomycin inhibition of the electrical activity of mouse pancreatic beta-cells.

The effect of the K-ionophore valinomycin (VAL) on the electrical activity of single mouse pancreatic beta-cells was measured using the glass micro-electrode technique. In the presence of 11.1 mM glucose, after 4 min of exposure to 10 nM VAL the spike activity was abolished, bursts became irregular and after 10 min the membrane hyperpolarized 4 mV. The electrical activity was completely blocked by 100 nM VAL within 3 min and the membrane hyperpolarization averaged 10 mV. The effect of VAL was irreversible. VAL (100 nM) also inhibited the electrical activity induced by 11.1 mM glucose plus 10 mM tetraethylammonium chloride (TEA), a specific blocker of the voltage sensitive PK. Total inhibition was observed 6-7 min after VAL addition and the cell hyperpolarized 14 mV. With the simultaneous application of 10 mM TEA and 100 microM quinine, a specific blocker of the [Ca2+]i-dependent PK, in the presence of 16.7 mM glucose, the membrane showed continuous activity and progressive depolarization from -45 to -5 mV within 23 min. The subsequent addition of 100 nM VAL also hyperpolarized the cell, with the membrane potential reaching -17 mV after 7 min. On the basis of these data, we suggest that VAL affects the glucose-dependent depolarization of the beta-cell membrane by increasing K+ permeability.

Animals↗