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Biomedical subjects

A Urbina

Publications and source records attributed to A Urbina.

15 recordsLinked to original sources
PubMed2003

Quantum conductance steps in solutions of multiwalled carbon nanotubes.

We have prepared solutions of multiwalled carbon nanotubes in Aroclor 1254, a mixture of polychlorinated biphenyls. The solutions are stable at room temperature. Transport measurements were performed using a scanning-tunneling probe on a sample prepared by spin coating the solution on gold substrates. Conductance steps were clearly seen. A histogram of a high number of traces shows maximum peaks at integer values of the conductance quantum G(0)=2e(2)/h, demonstrating ballistic transport at room temperature along the carbon nanotube over distances longer than 1.4 microm.

Journal Article↗
PubMed2002

Stretched exponential relaxation for growing interfaces in quenched disordered media.

We study the relaxation for growing interfaces in quenched disordered media. We use a directed percolation depinning model introduced by Tang and Leschhorn for 1+1 dimensions. We define the two-time autocorrelation function of the interface height C(t('),t) and its Fourier transform. These functions depend on the difference of times t-t(') for long enough times, this is the steady-state regime. We find a two-step relaxation decay in this regime. The long time tail can be fitted by a stretched exponential relaxation function. The relaxation time tau(alpha) is proportional to the characteristic distance of the clusters of pinning cells in the direction parallel to the interface and it diverges as a power law. The two-step relaxation is lost at a given wavelength of the Fourier transform, which is proportional to the characteristic distance of the clusters of pinning cells in the direction perpendicular to the interface. The stretched exponential relaxation is caused by the existence of clusters of pinning cells and it is a direct consequence of the quenched noise.

Journal Article↗
PubMed1999

Treadmill exercise-induced stress causes a rise of blood histamine in normotensive but not in primary hypertensive humans.

We have previously shown an interaction between noradrenergic and histamine-containing neurons in the rat vas deferens. As a generalized phenomenon, this interaction is involved in a novel peripheral reflex that, in an inhibitory way, modulates sympathetic activity and arterial pressure. Consistent with this, an activation of postganglionic sympathetic neurons causes a rise in rat blood histamine. In the present study, we showed that enhanced sympathetic activity due to treadmill exercise in normotensive humans, is accompanied by a rise in blood histamine, suggesting the presence of a similar neuronal interaction in humans. In contrast, the rise in blood histamine does not occur in primary hypertensive humans during the same degree of physical exercise, suggesting that this interaction is faulty in such hypertensives and could be involved in the pathophysiology of the disease.

Adult↗
PubMed1997

Regulatory differences among avian ecto-ATPases.

Extracellular ATP is found to produce a variety of important biological responses. Ecto-ATPases are located on numerous cell types in many different species, regulate extracellular ATP levels and can be a key step in generating adenosine. Studies conducted on chicken ecto-ATPases from liver and cardiac and smooth muscle show a variety of differing properties including (1) different apparent Km's, (2) lectin sensitivity, (3) responses to detergents, (4) responses to lipid mediators, and (5) responsiveness to nucleotide-mimetic affinity labels. These results suggest that although each enzyme hydrolyzes extracellular ATP, they should each be viewed as a distinct subtype of the whole ecto-ATPase family due to their differential responses, largely linked to proposed regulatory phenomenon.

Adenosine Triphosphatases↗
PubMed1992

Comparative effects of dopaminergic agonists on cardiovascular, renal, and renin-angiotensin systems in hypertensive patients.

The role of dopaminergic receptors on renal function has been extensively studied. Recently dopaminergic receptor has been classified in two subtypes D1 and D2, which seem to have different modulatory function. However, the role of dopaminergic receptors on cardiovascular function and more specifically the potential role of dopaminergic agonists as antihypertensive agents has not yet been clarified. Nine outpatients with mild and moderate hypertension were studied in the Cardiology Service of Vargas Hospital with a D1 agonist, piribedil, at 50-100 mg/day, orally, for 8 weeks, and with a D2 agonist, bromocriptine, at 2.5 - 5 mg/day, orally, for an another 8 weeks by using a placebo comparative crossover design. Piribedil reduced blood pressure with a modest increase in heart rate, plasma renin activity, and of plasma aldosterone, and an important increment of renal function. Bromocriptine reduced blood pressure with a decrease in heart rate and plasma aldosterone without altering renal function. There was no orthostatic hypotension with either agent. The authors conclude that activation of dopaminergic D1 receptor induces a vasodilatory and antihypertensive effect with a reflex increase in sympathetic tone, whereas activation of dopaminergic D2 receptor induces a decrease in sympathetic tone, probably due to a decrease in norepinephrine release at adrenergic endings. The potential effect of these compounds as antihypertensive agents is of great interest because blood pressure reduction can be induced by a new mechanism, i.e. activation of dopaminergic receptors which results in a decrease of the renin angiotensin system or a vasodilatory action.(ABSTRACT TRUNCATED AT 250 WORDS)

Aldosterone↗
PubMed1991

A double-blind, parallel, comparative evaluation of amlodipine vs. captopril in the monotherapeutic treatment of mild and moderate essential hypertension.

A double-blind, parallel, comparative study of the antihypertensive efficacy and safety of once-daily oral doses of amlodipine (5-10 mg/day) vs. twice-daily oral doses of captopril (25-50 mg twice daily) in adult patients with mild or moderate essential hypertension (supine diastolic blood pressure of 95-115 mm Hg) was undertaken in two hospital centers. Interim analysis of data from 40 patients shows that amlodipine and captopril both significantly (p < 0.05) reduced blood pressure compared with baseline. Nineteen of 21 (90.5%) amlodipine-treated patients and 15 of 19 (78.9%) captopril-treated patients had their diastolic blood pressure "normalized" (< 90 mm Hg) with mean final doses of 8.2 and 76.7 mg/day, respectively. At the final visit, there were no statistically significant differences between the two treatment groups in either mean supine or standing blood pressure. Minor nonsignificant changes in standing heart rate were observed in both treatment groups. Seven of the 21 amlodipine-treated patients and 3 of the 19 captopril-treated patients reported adverse experiences. No patients in either treatment group discontinued due to adverse events; one patient in the amlodipine group required a dose reduction. These interim data indicate that the overall efficacy and safety profiles of these two drugs are comparable.

Adult↗
PubMed1987

Antihypertensive properties of tiapamil (RO 11-1781)--a new calcium antagonist--in patients with mild and moderate essential hypertension.

An open, non-comparative study, with a new calcium antagonist-tiapamil, was undertaken in 22 patients with mild and moderate essential hypertension (stage I-II WHO). After a two-week placebo period, patients were treated with tiapamil, 300-600 mg twice daily during a period of six weeks (Dose-finding period). Thereafter patients were continued on tiapamil during a 54 week period (Long-term follow-up). In some patients it was necessary to add the diuretic hydrochlorothiazide to obtain adequate control of the arterial hypertension. Monotherapy with tiapamil normalized supine diastolic blood pressure after the first six weeks in 17 of the 21 evaluable patients, and reduced it by greater than or equal to 10 mmHg (1.3 kPa) from baseline without normalization in two patients. In the two remaining cases the decrease was less than 10 mmHg. The optimal dose administered at six weeks in those patients who responded to treatment (normalization or decrease by greater than or equal to 10 mmHg.) was 600 mg/day in 32% of the cases, 750-900 mg/day in 47% and more than 900 mg/day in 21%. After completion of the dose-finding part, 19 patients continued treatment for a further 54 weeks. In 16 out of 19 patients hydrochlorothiazide was added to enhance the antihypertensive effect. All three patients who received tiapamil monotherapy throughout the trial, had normalized supine diastolic blood pressure on completing the study. In the 16 patients with the combination therapy, the addition of hydrochlorothiazide led in two patients to no further decrease in supine diastolic blood pressure, to an additional decrease by less than 10 mmHg in ten patients and by greater than or equal to 10 mmHg in four in comparison with the values obtained before starting combination therapy. At the end of the study 11 of these 16 patients had normalized supine diastolic blood pressure. The mean daily dose was 900 +/- 45 mg of tiapamil and 39 +/- 4 mg of hydrochlorothiazide. Both monotherapy and the combination regimen were well tolerated, and no effects attributable to drug interactions were observed. It may be concluded that tiapamil in oral doses of 300-600 mg twice daily is an effective antihypertensive agent with an excellent tolerance when administered for a period of 54 weeks.

Adolescent↗
PubMed1985

Haemodynamic effects of enalapril, a new converting enzyme inhibitor, in hypertensive patients.

The haemodynamic effects of enalapril (EN), a new, long-acting, nonsulphhydryl converting enzyme inhibitor, were evaluated by non-invasive methods in 10 adult patients with mild to moderate essential hypertension (EH). Patients were randomly assigned, double blind to 2 treatment groups (EN 20 mg o.d. or 10 mg b.d.) for 4 weeks, and were crossed over to the other dosage regimen after a 2-week washout period. Measurements included mean arterial pressure (MAP), heart rate (HR), cardiac output (CO), limb blood flow (LBF), plasma aldosterone (ALD), plasma renin activity (PRA) and systolic time intervals (STI). Both regimens (b.d. and o.d.) significantly reduced MAP (15.3% and 16.3%, respectively), total peripheral resistance (20.3% and 21.8%, respectively), limb vascular resistance (24.1% and 24.9%) and ALD (33.5% and 36.9%) and increased CO (7.8% and 8.7%), LBF (10.9% and 11.6%) and PRA (10.4% and 9.5%). No significant change was observed in HR or STI. EN 20 mg o.d. or 10 mg b.d. reduced arterial pressure to a similar extent through a fall in total peripheral resistance. An increase in CO was also observed.

Adult↗
PubMed1981

Pulmonary capillary wedge pressure, as inferred from lung areas in gated blood-pool scintigrams: concise communication.

To determine whether the apex-to-base distribution of pulmonary blood volume, as obtained from gated cardiac blood-pool scans, could be used as a noninvasive method to estimate mean pulmonary capillary wedge pressure (PCWP), gated blood-pool scans were analyzed in 77 patients who also had PCWP measurements at cardiac catheterization. Ten of these patients had gated cardiac blood-pool scans and PCWP measurements both at rest and during exercise. The apex-to-base distribution of pulmonary blood volume was determined from the end-systolic frame of the left anterior oblique view by placing equal-sized regions of interest over the apex and base of the right lung. The ratio of apex counts over base counts (A/B ratio) was considered abnormal if greater than unity. The mean A/B ratio was 1.15 +/- 0.27 (1 s.d.) for the 32 studies associated with an abnormal mean PCWP (greater than 12 mm Hg). The mean A/B ratio was 0.85 +/- 0.23 for the 55 studies associated with a normal mean PCWP (p less than 0.01 comparing normal group with abnormal). The sensitivity of the A/B ratio for a mean PCWP greater than 12 mm Hg was 81%R (26/32). The specificity of the A/B ratio for a mean PCWP greater than or equal to 12 mm Hg was 89% (49/55). Thus, noninvasive determination of the pulmonary apex-to-base ratio from gated cardiac blood-pool scans appears to differentiate subjects with normal and abnormal mean pulmonary capillary wedge pressures.

Adult↗
PubMedDate not supplied

Zymodeme and serodeme characterization of Leishmania isolates obtained from Costa Rican patients.

Thirty-four Leishmania isolates obtained from Costa Rican patients, from different geographical areas, were characterized by isoenzyme electrophoresis and indirect immunofluorescense with monoclonal antibodies. Thirty-two were characterized as L. panamensis strains and two were L. braziliensis variants. We confirm the evident predominance of L. panamensis as the main etiological agent of leishmaniasis in Costa Rica and the existence of L. braziliensis in the country.

Adolescent↗
PubMedDate not supplied

[Bacterial translocation in a model of intestinal obstruction. II. Bacteriological study and role of cellular immunity].

Bacterial translocation (Bt) from the gastrointestinal (GI) tract to systemic organs creates the possibility of Infection and sepsis in a great number of pathologic entities. In a mouse model of Intestinal Obstruction (IO), we evaluated the type of micro-organisms and the organs that bacteria frequent translocated. At 24 hours post-10, positive cultures where obtained at the MLN, portal, systemic circulation and peritoneal cavity, establishing that the translocation is bi-directional. The more frequent bacteria isolated were the Streptococcus group D, Proteus mirabilis, Escherichia coli, Pseudomonas sp., an clostridium. BT occurs at 24 hour post-OI and was due to increased intestinal permeability, at 48 hrs BT increased and related to the physical disruption of the mucosal barrier in the intestinal mucosa. Cell mediated immunity (CMI) response in this model was not altered, although a progressive decrease was observed at 48 hrs it was not significant, suggesting that the CMI play no role in the pathogenesis of BT. In the Control-Laparotomy group, CMI response was increased significantly at 48 hours, suggesting that a simple laparotomy boost the immune defense response.

Animals↗