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Biomedical subjects

G Suarez-Kurtz

Publications and source records attributed to G Suarez-Kurtz.

10 recordsLinked to original sources
PubMed1992

Effects of phenytoin on the ventricular tachyarrhythmias of chronic Chagas' disease.

The effects of phenytoin on the ventricular tachyarrhythmias of 11 patients with chronic chagasic myocarditis were investigated, and correlated with the serum phenytoin levels. Physical examination, laboratory tests and 24- to 48-h ambulatory electrocardiographic recordings were performed before, during (7-14 days) and after treatment with phenytoin (4-6 mg/kg/day, orally, in three divided doses). Significant (greater than 90%) reduction of couplets, bigeminy and runs of ventricular tachycardia were observed in 50-67% of the patients, whereas the frequency of isolated premature ventricular contractions was significantly (greater than 70%) reduced in only 2 patients (18%). Proarrhythmic activity was not observed and adverse side effects were of mild intensity and usually transient, except in one patient, who developed pruritus and skin rash in the presence of toxic phenytoin serum levels (27 micrograms/ml). It is suggested that phenytoin may be useful for the control of repetitive forms of ventricular tachyarrhythmias in selected patients with chronic chagasic myocarditis.

Adult↗
PubMed1992

Comparative pharmacokinetics of different oral nifedipine preparations in healthy Brazilian volunteers.

1. The pharmacokinetics of different pharmaceutical preparations of oral nifedipine--Adalat (capsule), Oxcord and Cardalin (tablets)--was determined after administration of single oral doses of 10 mg to nine healthy young Brazilian volunteers (7 men). 2. There were no significant changes in heart rate or systolic and diastolic blood pressure measured in the sitting position within 8 h of nifedipine administration to these normotensive volunteers. No side effects were reported by the volunteers or observed by the attending physicians during the study. 3. No significant differences were observed among the three preparations in relation to the following pharmacokinetic parameters obtained from the plasma concentration-time curves: area under the curve (AUC), slope (beta) and half-life (T1/2) of the elimination phase, volume of distribution (Vd/F) and total body clearance (CL/F), both expressed as functions of the oral bioavailability (F) of nifedipine. 4. The peak plasma concentration of nifedipine (Cmax) and the time to reach Cmax (Tmax) were not different for the two tablet preparations. However, Cmax was significantly higher, and Tmax was significantly shorter for the capsule. These data indicate that the capsule and the tablet preparations are not bioequivalent.

Administration, Oral↗
PubMed1979

Dissociation of action potentials from contraction in single crab muscle fibers.

In single crab fibers (Callinectes danae) bathed in Ca2+-free media, Ba2+ action potentials did not elicit tension. In contrast, Sr2+ spikes evoked twitches similar in amplitude to those accompanying the control Ca2+ spikes. Tension development in these fibers, therefore, depends on the ionic species carrying the inward current during membrane excitation. The Ca2" or Sr2+ influx appears insufficient to evoke the observed twitch tensions, and it seems necessary to postulate mobilization of an intracellular source of Ca1+. Procaine, which suppresses release of Ca2+ from sarcoplasmic reticulum, depressed twitch tension but did not reduce the overshoot or duration of Ca2+ or Sr2+ spikes. This finding is compatible with the suggestion that the contractions results from release of Ca2+ from the sarcoplasmic reticulum initiated by signals given by the influx of Ca2+ or Sr2+, but not Ba2+.

Action Potentials↗
PubMed1979

The depolarizing afterpotential of crab muscle fibres. A sodium-dependent process mediated by intracellular calcium.

1. A study was made of the depolarizing afterpotential (d.a.p.) which follows the initial graded electrogenesis of crab muscle fibres. 2. Increasing the strength, duration or amplitude of the stimulating current pulses enhanced both the d.a.p. and the local contractions. 3. Arsenazo III was injected intracellularly and changes in light absorbance by the dye were used to monitor the increase in free sarcoplasmic Ca concentration during excitation-contraction (E-C) coupling. The onset of the absorbance changes occurred during the depolarizing phase of the initial electrogenesis and the maximum value coincided with the peak of the d.a.p. An exponential decay of the absorbance signal occurred during the repolarizing phase of the d.a.p. 4. Ionophoretic injection of EGTA into the sarcoplasm did not affect the initial electrogenesis but did reduce changes in dye absorbance, blocked tension development and abolished the d.a.p. 5. Caffeine (0.1--0.4 mM) markedly enhanced both the d.a.p. and the local contractions, but had no effect on the initial electrogenesis. 6. Replacement of extracellular Na ions with Li, Tris or choline abolished the d.a.p. The initial electrogenesis was enhanced in the choline-containing medium, but was not affected by Li or Tris. The rate of relaxation of the local contractions and the rate of decay of the light absorbance changes were slowed in Na-free saline. 7. Tetrodotoxin (10(-5) g/ml.) had no effect on either the membrane responses or tension development. 8. For initial graded responses of comparable peak amplitude a threefold reduction of [Ca-a1o shortened the d.a.p., but had little effect onits peak amplitude. A fivefold increase in [Ca]o reduced both the amplitude and duration of the d.a.p. 9. Changes in [Mg]o had little effect on the d.a.p., but both Mn (4--10 mM) and La (0.1 mM) blocked the initial electrogenesis and the d.a.p. 10. It is concluded that distinct ionic mechanisms give rise to the initial electrogenesis and the d.a.p. While the former is due to activation of Ca conductance, the d.a.p. is a Na-dependent phenomenon that is tetrodotoxin-insensitive, and mediated by the rise of intracellular Ca concentration during E--C coupling.

Animals↗
PubMed1979

Inactivation of creatine kinase in physiological saline solutions.

To determine the optimal conditions for measuring the activity of creatine kinase released from isolated skeletal muscle preparations, we investigated the stability of the enzyme during storage in saline solutions of varying composition. Parameters studied were temperature, presence of albumin or dithiothreitol, calcium concentration and anionic composition. It is concluded that maximal activity is retained when samples are kept at 4 degrees C and analyzed within 24 hours in the presence of a thiol-protective compound. If longer storage periods are anticipated, inactivation is significantly retarded by addition of albumin or by decreasing the free calcium concentration of the storage salines.

Animals↗
PubMed1978

Competitive antagonism between calcium and aminoglycoside antibiotics on guinea-pig intestinal smooth muscle.

Streptomycin and related aminoglycoside antibiotics and magnesium ions antagonize in a competitive manner the contractile response elicited by calcium ion on the potassium-depolarized guinea-pig ileum. The inhibitory potency decreases in the following way: gentamicin greater than magnesium greater than neomycin greater than streptomycin=streptidine greater than kanamycin. Streptomycin was also found to reduce the uptake and to increase the efflux of 45Ca. To explain these effects two alternative mechanisms are proposed: 1) a competition of the antibiotics with calcium ions for binding sites of a transport system which carries the activator ion to the myoplasm, and 2) an interaction of the antibiotics with membrane sites that regulate calcium permeability with a consequent reduction of calcium passage to the cell.

Aminoglycosides↗
PubMed1977

Effects of verapamil on excitation-contraction coupling in single crab muscle fibers.

The effects of verapamil and its optical isomers on the electrical and mechanical characteristics of single muscle fibers of Callinectes danae were studied. Verapamil (10-20 microng/ml) blocked the procaine- and TEA-induced spikes; the blockade was preceded by reduction in the rate of rise of the upstroke and increase in the duration of the action potentials. Inhibition of Ba-spikes required higher concentrations of verapamil (greater than 50 microng/ml). These concentrations reduced the amplitude of the normally occurring graded electrogenic membrane responses and reduced the rate of development of the current-induced tensions. With lower concentrations (10-30 microng/ml) verapamil enhanced the negative afterpotentials and the peak amplitude of the local contractions elicited by depolarizing current pulses, while the graded membrane responses were not markedly modified. Verapamil (1-100 microng/ml) did not affect the resting membrane potential but increased the effective membrane resistance. Determination of the cable characteristics by DC pulses indicated that verapamil (1-10 microng/ml) shortens the membrane length constant, increases the specific resistivity of the sarcoplasm and, in most cases, increases the membrane time constant. Verapamil (10 microng/ml) induced tension in these crab fibers. The contractions were potentiated in Na-deficient media, by increase in [Ca]0, and by membrane depolarization; "Ca-free" salines depressed, and procaine abolished these contractions. The results suggest that verapamil affects both Ca and K conductances and interferes with the Ca-sequestering mechanisms of these fibers. The (-)-isomer of verapamil was more effective than the (+)-isomer with respect to tension development, prolongation and subsequent blockade of procaine-spikes and enhancement of current-induced after-potentials and contractions.

Animals↗
PubMed1977

Inhibition by hypertonic solutions of Ca-dependent electrogenesis in single crab muscle fibers.

This study describes the effect of hypertonic solutions on isolated muscle fibers of Callinectes danae. Solutions of twice normal tonicity (2.0 T) inhibit both the normal graded membrane responses and the spikes induced by procaine, tetraethylammonium, or barium. The inhibition is maintained throughout exposure to hypertonic solutions prepared by addition of impermeant solutes such as NaCl, sucrose, or Tris-propionate, but is reversible on their withdrawal. In the presence of permeant solutes such as glycerol or acetamide, the inhibition is transient. In both cases the onset of inhibition of the depolarizing Ca electrogenesis is correlated with shrinkage of the fiber. In the case of permeant solutes, the time course of recovery of the graded responses or the spikes follows the recovery of the fiber volume. Changes in the passive electrical characteristics of the fibers due to hypertonic solutions were unrelated to the blockade of membrane Ca activation. The current-voltage relationship in hypertonic sollution revealed no increase in depolarizing K activation. Inhibition of the graded membrane responses and spikes appears to be associated with depression of Ca conductance. Hypertonic solutions might affect the activation of Ca conductance through reduction of the electric field generated by fixed negative surface charges and/or morphological changes in the T tubules. Membrane depolarization elicited little or no tension in 2.0 T solutions while caffeine contracture (10 mM) with an ampliture of 76% of the maximal contractile ability could still be elicited. This indicates that direct effects of hypertonic solutions on the contractile apparatus were not responsible for loss of tension. The latter is attributed to the inhibition of the transmembrane Ca currents.

Acetamides↗
PubMed1976

A comparison of effects of SKF 525-A and procaine on excitation-contraction coupling in single crayfish muscle fibers.

The effects of beta-diethylaminoethyldiphenylpropylacetate hydrochloride (SKF 525-A) on excitation-contraction coupling and Ca-dependent electrogenesis are compared to those of procaine. At pH 7.2, SKF 525-A and procaine occur essentially (greater that 97%) as a free base and as a cation, respectively. At this pH, SKF 525-A elicited tension development, blocked K-induced contractures and the K-induced repriming of caffeine contractions, potentiated caffeine-induced tensions, inhibited the procaine-induced spikes and twitches and, depending on the concentration, either potentiated (25-50 muM) or depressed (greater than 100 muM) the tensions associated with the graded membrane electrogenesis. At the same pH, procaine blocked the contractions elicited by SKF 525-A, by high K media, by the graded electrogenesis and by caffeine, and converted the graded membrane responses into all-or-none spikes. It is proposed that SKF 525-A as a free base 1) inhibits membrane Ca activation more effectively than it depresses K conductance and 2) is synergistic with caffeine in reducing the effectiveness of Ca sequestration by the sarcoplasmic reticulum. Procaine as a cationic molecule is thought to depress K activation more than Ca activation during depolarization and to block the release of Ca ions from the sarcoplasmic reticulum.

Animals↗