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Biomedical subjects

H G Ferreira

Publications and source records attributed to H G Ferreira.

At least 19 recordsLinked to original sources
PubMed2002

A minimal mathematical model of calcium homeostasis.

A mathematical model of calcium homeostasis is presented in which the controlling factors are the plasma concentrations of calcium, PTH, and calcitriol, and the effector organs are the parathyroids, bone, kidney, and intestine. Other factors can be added as the need arises. The model is aimed at simulating what happens in a single individual, but its parameters and variables were adjusted to the corresponding published average values. Simulations of published observations in humans undergoing the infusion of calcium or its chelators are presented. With a single exception, these simulations provided a good fit to the data. The response of the system to extrinsic perturbations was characterized by simulating chronic infusions of calcium, PTH, and calcitriol. Finally, the steady state response to perturbations in some of its parameters (the secretory mass of the parathyroids and the affinity and/or sensitivity of the calcium, PTH, and calcitriol receptors) and to renal failure were also investigated in an attempt to analyze the pathogenesis of clinical hypo- or hypercalcemias. In its present form the model cannot be used to base clinical decisions in individual cases. However, it requires modest computational resources, and clinicians with a modest mathematical background can manipulate it. It is a useful tool for the analysis of general mechanisms of the diseases of calcium metabolism and for the design of clinical experiments aimed at characterizing these diseases. The model can also be the core of future autoadaptive extensions to be used in individual patients.

Calcitriol↗
PubMed1997

[Relationships between basic sciences and medicine revisited].

1. The role of basic research in medical institutions and in particular in Medical Schools is discussed. 2. Clinical practice is characterized, in its technical aspects, as a branch of biological technology. 3. It is suggested that the main functions of basic research practiced in medical institutions is to promote the interaction between physicians and scientists in other fields and to train physicians in the practices of other scientists. 4. It is suggested that the introduction of basic research in medical institutions implies: the recruitment of scientists from non-medical disciplines; a policy of funding that does not favour exclusively the clinical sector; the setting up of a regular graduate program, based on the scientifically active groups and adequately regulated.

Clinical Medicine↗
PubMed1996

The effect of bile acid on the activation of amiloride-sensitive sodium channels in frog skin.

The effect of the bile acid, deoxycholic acid (DOC), on the transport properties of isolated frog skin was studied under short-circuit conditions. The addition of DOC (1 mM) to the apical side of the frog skin induced a threefold increase in the short-circuit current (Iac). This effect was inhibited by amiloride. DOC also increased the conductance of the preparation by two different mechanisms. At low concentrations (< 2.5 mM) it activated amiloride-sensitive sodium channels. At higher concentrations of DOC, basolateral-apical unindirectional fluxes, measured with 22Na+, 36Cl-. [14C]mannitol and [14C]inulin, showed a selective increase in the permeability to Na+, Cl- and mannitol in relation to [14C]inulin. These data suggest that sodium and chloride ions use the same diffusional pathway across the preparation. This pathway discriminates between NaCl and mannitol, and discriminates even more in relation to inulin. The effects of DOC are additive to those of cAMP (1 mM). ADH (20 mU ml-1), prostaglandin E2 (0.1 microM) or forskolin (10 microM). It is concluded from our study that the final effect of DOC in stimulating the Isc in frog skin is through the activation of amiloride-sensitive sodium channels. However, since DOC is liposoluble, a direct activation of an adenylate cyclase or of phosphokinase A cannot be excluded.

Amiloride↗
PubMed1994

[Evaluation of medical schools and universities. Not postponable options, unavoidable risks].

1. The evolution of the relations between the Portuguese universities and the State is presented. It is considered that over the last 30 years the most important policies were the creation of new universities and the legal recognition of Ph.D.s obtained abroad. These measures influenced engineering much more than medical schools. 2. Multiple mechanisms of assessment covering the quality of teaching in conformity with international standards, research and post-graduate activity should be established by medical schools themselves. It is also proposed that Medical Schools should embark on an ambitious post-graduate program so as to establish closer links with the medical profession. 3. It is considered that the assessment method under consideration by the Ministry of Education is dangerous, inadequate and unjustified. 4. It is considered that the present level of investment in tertiary education will cause our standards, in that area, to fall even more when compared with those of other European countries.

History, 15th Century↗
PubMed1993

[Role of formal education in the training of researchers: post-graduate education, masters, doctorates, and other standards].

The social function of Ph.D. graduates in the portuguese universities and scientific institutions is analyzed. Portuguese Ph.D degrees are compared with those obtained in British and American universities. American Graduate Schools are briefly described. It is proposed that, in Portugal, the Ph.D studies should start much earlier, than what is usual in portuguese universities. The impact on Portuguese universities of the very large number of Ph.D graduates produced as result of the Science I Program, is discussed. It is proposed that MSc programs should consist, almost exclusively, of professional refresher courses. The implications of formal graduate programs for Portuguese universities are discussed.

Education, Graduate↗
PubMed1993

[Medical research in Portugal: the geniuses and the structures, the myths and common sense].

There was recently a growth in the funds allocated directly to scientific programs and projects and in particular to infrastructures (buildings and large pieces of equipment). It is very likely that this investment will continue to grow as a result of the development policies of the EEC applied to its less developed members. The recent experience with the Science Program, the main factor in this evolution, has demonstrated that the bottleneck in our scientific development is at the level of the allocation of funds (management). Hence, the nonrealistic Regulations of the Program, the allocation of funds to ghost institutions and the inability to distribute the funds allocated to the Program. The Program was conceived based on problem oriented and not on research oriented strategies. In the case of medical research this choice is not supported by the recent history of research in this area or by the experience of countries such as the UK or the USA. Funding of medical research in this country should be based on the identification of a talented scientist with a good research program and on an inventory of the productivity, potentialities and needs of existing institutions. We should seek the support of foreign scientists and of Portuguese scientists living abroad for the technical evaluation of projects but they should not be asked to provide global strategies of investment in Portuguese science.

Portugal↗
PubMed1992

Ion transport across the epithelium of the rabbit caecum.

The isolated rabbit caecum was studied in vitro. Under our experimental conditions, the rabbit caecum secreted potassium and chloride and absorbed sodium. To characterize the transport properties of the apical and the basolateral barriers, transepithelial electrical and flux (22Na, 36Cl and 86Rb) measurements and their sensitivity to transport inhibitors (furosemide, DIDS, ouabain and barium) are presented together with intracellular measurements with double-barrelled microelectrodes of intracellular electrical potentials and ionic activities. The fluxes of sodium and chloride were insensitive to DIDS and furosemide. The secretion of potassium and the absorption of sodium were both inhibited by ouabain, indicating that they are coupled through the sodium pump. Ouabain induced a slow fall in the chloride net fluxes, suggesting that these fluxes are also driven by the sodium pump, albeit indirectly. The basolateral to apical fluxes of potassium are insensitive to barium added to the apical side, but are accelerated by the replacement of chloride by gluconate on the apical side, suggesting the presence of a K+/Cl- symport in the apical barrier.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1991

A mathematical model of rabbit cortical thick ascending limb of the Henle's loop.

A mathematical version of the cell model of the cortical thick ascending limb of the rabbit proposed by Greger and Schlatter ((1983) Pfügers Arch. 396, 325-334) is described. Available data are sufficient to compute the most important parameters. Simulations of experiments with perfused tubules in which the transepithelial voltage and conductance, and the intracellular electrical potential were measured in the course of ionic substitutions in the perfusing baths or treatment with ouabain or furosemide are in good agreement with the experimental results with the exception of those relating to dilution potential experiments. The model can be used in the analysis and planning of experiments and is capable of predicting the instantaneous values of ionic fluxes and intracellular concentrations and of cell volume.

Animals↗
PubMed1989

Determination of ionic permeability coefficients of the plasma membrane of Xenopus laevis oocytes under voltage clamp.

1. A method of estimating absolute ionic permeability coefficients which does not depend on the use of impermeant substitutes is reported. 2. The method is based on a pump leak model of the Xenopus laevis oocyte membrane. The procedure consists of measuring, in the same experiment, the pump current and the currents generated under voltage clamp by the partial substitution of one or two ions at a time. For each experimental condition, the measured currents are substituted in a Goldman-Hodgkin-Katz type equation with two unknowns (the permeability coefficients). The set of equations thus generated enables the computation of all the ionic permeability coefficients. 3. The Xenopus oocyte membrane (stages IV and V, Dumont, 1972) has been found to be permeable to conventional ion substitutes such as N-methyl-D-glucamine (NMG), sulphate, isethionate and gluconate. 4. The values for sodium, potassium and chloride permeability coefficients obtained from sixty-eight pooled experiments were, respectively, 5.44, 17.41 and 1.49 x 10(-8) cm s-1. 5. The diffusional currents for sodium, potassium and chloride computed from the experiments referred to above were, respectively, -1.16, 0.69 and -0.038 microA cm-2. 6. A stoichiometry of the Na+-K+ pump exchange of 3/1.8 was computed. 7. The intracellular concentrations of sodium, potassium and chloride ions, as determined by ion-selective microelectrodes, were, respectively, 10.1 +/- 0.66 mM (n = 12), 109.5 +/- 3.3 mM (n = 13) and 37.7 +/- 1.18 mM (n = 19), corresponding to equilibrium potentials of 61, -95 and -28 mV. 8. Since chloride is not at equilibrium across the membrane, we propose that there is an inward uphill Cl- transport.

Animals↗
PubMed1989

The coupled movements of sodium and chloride across the basolateral membrane of frog skin epithelium.

1. When frog skin epithelium was exposed to a chloride-free solution bathing the basolateral side of the frog skin preparation the short-circuit current fell and there was a simultaneous loss of chloride and water from its cells. This effect was partially blocked by furosemide when this drug was added to the basolateral bathing solution. 2. Under control conditions and when added to the solution bathing the basolateral side of the preparation furosemide had no effect on the ion and water contents of the frog skin epithelium. 3. Furosemide but not SITS (4-acetamide-4'-isothiocyanate-stilbene-2,2'-disulphonic acid) or amiloride blocked the recovery of short-circuit current and the reuptake of chloride and water by preparations pre-incubated with chloride-free solution on the basolateral side. The recovery of the short-circuit current was also blocked by the replacement of basolateral potassium by sodium. 4. The effect on the short-circuit current of graded replacements by impermeant ions of sodium or chloride did not show saturation for concentrations of these ions up to their control values. 5. Replacement of basolateral potassium by sodium inhibited the short-circuit current and the recovery observed when potassium was reintroduced in the basolateral bathing solution was blocked by furosemide. 6. The replacement of basolateral sodium or chloride by impermeant ions induced an immediate fall in the intracellular concentrations of both sodium and chloride suggesting that the transport system coupling the movements of the two ions across the basolateral membrane is operative under control conditions. 7. It is proposed that the coupled movements of sodium and chloride across the basolateral membrane of the frog skin epithelium are mediated by a sodium-potassium-2 chloride co-transport system which under control conditions is very near equilibrium.

4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfo↗
PubMed1987

Effects of cell volume changes on membrane ionic permeabilities and sodium transport in frog skin (Rana ridibunda).

1. Membrane potential and conductances and short-circuit current were continuously measured with microelectrodes and conventional electrophysiological techniques in a stripped preparation of frog skin epithelium. The effects of the removal of chloride or sodium ions and the concentration or dilution of the serosal (inner) bathing solution were studied. 2. Chloride- or sodium-free solutions produced a cell depolarization of about 30 mV in parallel with a fall in the short-circuit current. Mucosal and serosal membrane conductances both decreased and the sodium permeability of the mucosal barrier was calculated to fall to about one-half its value in standard Ringer solution. The observed decrease in the short-circuit current is probably related to the combined effect of the decrease in sodium permeability and the decrease in the driving force across the mucosal membrane. 3. The removal of chloride or sodium ions reduced the depolarization caused by serosal perfusion with high-potassium solutions (50 mM-KCl). The ratio of the change in cell membrane potential under short-circuit conditions to the change in the potassium equilibrium potential (delta Ec(s.c.)/delta EK), was 0.59 in standard Ringer solution and 0.26 and 0.24 after the removal of chloride or sodium respectively. The depolarizing effect of barium-containing solutions (2 mM-BaCl2) was also markedly reduced in chloride- or sodium-free solutions, suggesting a decrease of the potassium selectivity of the serosal membrane in these conditions. 4. Increasing the osmolality of the serosal bathing solution produced similar effects, i.e. cell depolarization, fall in the short-circuit current and membrane conductances and reduction of the depolarizing effect of high-potassium and barium solutions. On the contrary, dilution of the serosal bath produced the opposite effects, consistent with an increase in the serosal permeability to potassium. 5. The effects of chloride- or sodium-free solutions were reversed by the dilution of the serosal bath. Cells repolarized when exposed to low-osmolality solutions after being in the absence of serosal chloride or sodium. The repolarization ran in parallel with the restoration of the short-circuit current and the potassium selectivity of the serosal membrane. 6. The results show that the effects produced by the removal of sodium or chloride ions from the serosal bathing solution are most probably mediated by a reduction in cell volume. Cell volume changes would lead to changes in the serosal membrane selectivity to potassium and thus to changes in cell membrane potential and sodium transport.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1983

Epithelial transport parameters: an analysis of experimental strategies.

A set of experiments was simulated on a computer version of the Koefoed-Johnsen & Ussing model for high-resistance epithelia. The results obtained were analysed according to procedures commonly applied to the analyses of experimental data and interpreted in terms of the model parameters. Although the computer model encodes a stoichiometry of 3:2 for Na-K exchange through the Na pump, the simulation of published experimental procedures yields different figures in almost every case. We show that ENa as originally defined by Ussing & Zerahn (Acta physiol. scand. 23, 110-127 (1951)) and as obtained from flux-ratio experiments has different values under different experimental conditions with unchanged system parameters and that it is distinct from ENa measured by other methods. We also show that unless the pump is saturated with internal Na an increase in the rate of pumping cannot cause a substantial increase in the rate of transepithelial Na transport.

Animals↗
PubMed1981

The regulation of volume and ion composition in frog skin.

1. Chemical determinations of Cl- in frog skin indicate that the intracellular concentration of this ion is above that corresponding to an equilibrium distribution. 2. Published data on Na+ efflux from the cell compartments into the two external bathing solutions, and on unidirectional fluxes across the whole preparation suggest that there are large unidirectional fluxes across the basolateral membranes. 3. Cl- uptake from the mucosal barrier is very small, and the removal of Cl- from the mucosal bathing solution does not affect the intracellular Cl- concentration. 4. Removal of Cl- from the serosal bathing solution produces a drastic decrease in cell Cl- together with a loss of water. 5. This is accompanied by a less marked effect on Na+ and K+ content of the cells. 6. The removal of Na+ from the serosal bathing solution produces also a decrease in Cl-, Na+ and K+ content of the cells with a marked loss of water. 7. It is suggested that the basolateral membrane of the frog skin is the site of mechanisms able to regulate volume and ion composition of the epithelial cells and that part of these mechanisms consists of a coupling between the movements of Na+ and Cl-.

Acetazolamide↗
PubMed1979

The behaviour of transporting epithelial cells. I. Computer analysis of a basic model.

We analyse the non-steady state behaviour of a computer model representing functional epithelial cells. The results show that a simple model of an epithelium, containing the essential ion transport asymmetries of the original Koefoed-Johnsen-Ussing model, predicts much of the observed behaviour of 'tight-type' epithelia under various well characterized experimental conditions.

Amiloride↗