Adrenaline diminishes K+ contractures and Ba2+-current in chicken slow skeletal muscle fibres

The effects of adrenaline and the beta-adrenergic agonist isoprenaline on K+-evoked tension (K+-contracture) and Ba2+ current were investigated in chicken slow (anterior latissimus dorsi (ald)) muscle using isometric-tension measurements and current recording. Addition of adrenaline (10(-7) - 10(-5) M) or isoprenaline (10(-6) - 10(-5) M) to the bath reduced the amplitude of the K+-contractures. These effects were blocked by the beta-antagonist propranolol (5 x 10(-6) M). External application of a cAMP analogue (8-bromo cyclic AMP; 1 x 10(-4) M) also decreased the amplitude of the K+-contractures. To analyze the possible relationship between the induced tension reduction and effects on sarcolemmal Ca2+ channels, a slow action potential and a slow inward membrane current were studied in intact ald chicken muscle fibres. When the ald muscle was immersed in a Na+- and Cl--free solution containing Ba2+ and depolarizing pulses were delivered from a -80 mV holding potential, the muscle fibres exhibited a small, slow Ba2+-dependent potential (observed at about -26 mV, peak amplitude, around -10 mV). The response was blocked by the addition of Co2+ (5 mM) or Cd2+ (2 mM). Using the three-microelectrode voltage-clamp technique, a slow inward membrane current underlying the Ba2+ potential could be discerned. The current had a mean threshold of -60 mV, reached maximum at about -5 mV and ranged from ca. 9 to 19 pA/cm2 (depending on the external Ba2+ concentration). It had a mean reversal potential of +45 mV. The Ba2+ inward current was diminished when adrenaline or isoprenaline was added to the bath (1 x 10(-5) M); however, this decrease did not occur when propranolol was present (5 x 10-6 M). These results suggest that the decreases in the tension of K+-contractures induced by adrenaline and isoprenaline may occur through beta-adrenergic effects on sarcolemmal Ca2+ channels in ald chicken slow muscle fibres.

Effect of Dimethyl Sulfoxide on Excitation-Contraction Coupling in Chicken Slow Skeletal Muscle

To evaluate the suitability of dimethyl sulfoxide (DMSO) as a solvent for muscle-contraction studies in the chicken, its effect on the slow muscle contracture induced by high-K(+) solution was explored using the anterior latissimus dorsi (ALD) muscle from one-week-old chicks. Measurements were made of isometric tension and various characteristics of the contractures [peak tension, total tension (area under the curve), duration of contraction, drop in tension from peak to plateau, and resting tension], in the presence and absence of DMSO (20 mM). Exposure to DMSO led to a concentration-dependent reduction in resting tension of up to 9 +/- 1.8% (n = 4) with respect to the control. The threshold concentration was 10 mM, and the maximum effect was reached between 20 and 30 mM. The drop in tension from peak to plateau was three times larger in the presence of DMSO (20 mM) than in its absence. At the same concentration, there was a 10 +/- 2.3% increase in the time constant of activation. No significant changes were observed in peak tension or in total tension in the presence of 20 mM DMSO. As a consequence, this type of biological preparation is not suitable for research on muscle contractures involving drugs that must be dissolved in DMSO (at the DMSO concentrations tested here).

Frequency of, indications for and clinical epidemiological characteristics of first time cesarean section, compared with repeated cesarean section

The aim of this study was to determine the frequency of, indications for and clinical epidemiological characteristics in patients having their first cesarean section (FCS) and then to compare the data with that found in patients with repeated cesarean section (RCS).

PATIENTS AND METHODS

A cross-sectional study was carried out. 493 pregnant patients who gave birth by cesarean section or vaginal delivery were seen. Some of the variables analyzed were: age, prenatal care consultations, gyneco-obstetric antecedents, cesarean section indication and neonate weight. Statistical analysis included ANOVA, chi(2) and OR, with a 95% CI. Significance was p<0.05.

RESULTS

66% of the patients had vaginal births (VB) and 33.4% had cesarean sections, with a FCS frequency of 61%. The three most frequent indications for FCS were dystocias and cephalopelvic disproportion (45%), fetal distress (12.8%), and pelvic presentation (9.9%). Meanwhile, those for RCS were previous cesarean section (51%), dystocias (20%) and pelvic presentation (6.2%). The variables significantly associated with FCS were: first pregnancy, antecedent of labor room induction and a neonate weight above 3500 g. The remaining variables were not associated with FCS. Percentages of nulliparity, secondgravidity and multigravidity were greater in RCS patients.

CONCLUSION

The frequency of FCS is still high in Mexico. Adequate following of programs to diminish the percentage of FCS and increase the number of VB, would significantly reduce the prevalence of cesarean section.

[White-coat hypertension and risk factors in recently diagnosed hypertensive patients]

OBJECTIVE

To determine the frequency of white-coat hypertension (WCH) and associated risk factors in recently diagnosed hypertensive patients.

MATERIAL AND METHODS

A cross-sectional study was carried out from January 1997 to December 1998. The reference population consisted of 195 patients presenting with systemic arterial hypertension (SAH) of less than 1-month evolution and receiving no medical treatment. According to Joint National Committee (JNCVI) criteria, 100 individuals with Stages I, II, and III SAH underwent the following procedures: clinical history; physical examination; electrocardiogram, serum creatinine; urianalysis, and ambulatory monitoring of arterial pressure (AMAP).

RESULTS

A total of 54% of patients were WCH carriers and 46% presented sustained hypertension (SH); 37% of WCH patients and 63% of SH patients had a family history of SAH, while 19.7% of WCH patients and 39.2% of SH patients presented with stage 1 retinopathy. Left ventricular hypertrophy was absent in all WCH patients and present in 21.7% of SH patients. A total of 31% of WCH patients and 50% of SH patients presented electrocardiographic changes.

CONCLUSIONS

A total of 54% of patients presented WCH. Clinical/epidemiologic profiles of WCH carriers and SH patients are different.

Furosemide plus albuterol compared with albuterol alone in children with acute asthma

Several reports have shown that inhaled furosemide protects patients with asthma from different bronchoconstrictor agents. However, the effect of this widely used diuretic in acute exacerbation in adults is unproven. There are no reports of furosemide's therapeutic effect in acute asthma in children; thus, the objective of this study was to determine the effectiveness of the combined treatment of furosemide and albuterol in pediatric patients. Using a double-blind design, 20 emergency room patients with an asthmatic exacerbation were studied and randomly assigned to one of the following treatments: (1) furosemide + albuterol (1 and 0.15 mg/kg, respectively) or (2) albuterol (0.15 mg/kg). The forced expiratory volume in one second (FEV1) was measured in each patient before medication and then 30 and 60 minutes after inhalation of the individual drug or drug combination. Neither group differed in age or baseline FEV1. An increase in FEV1 of 22.8 +/- 4.3% (mean +/- SE) in the drug combination group was noted at 60 minutes, and an increase in FEV1 of 18.0 +/- 2.6% in the albuterol group was obtained at the same time. Although the increase in FEV1 was greater in the first group after 1 hour of treatment, this was not significant. These results suggest that inhaled furosemide does not have a synergistic effect with albuterol in the treatment of asthmatic exacerbations in children.

Measurement of Ca(2+) currents in intact slow skeletal muscle fibers of the frog by the three-microelectrode technique

The recording of currents passing through calcium channels in intact skeletal muscle fibers presents several difficulties. However, use of the three-microelectrode voltage-clamp technique at the end of the fiber provided us with a good approximation of current values in such fibers. Using this technique, we were able to measure the calcium-channel current in slow skeletal muscle fibers of the frog (Rana pipiens) and to quantify the effects of denervation on this current.

Effects of denervation on Ca2+ channels in slow skeletal muscle fibers of the frog

Effects of denervation on calcium channels in slow skeletal muscle fibers in the frog (Rana pipiens) were studied using the three-microelectrode voltage-clamp technique in intact fibers. Ca2+, Ba2+, and Sr2+ currents were all significantly reduced in amplitude during the first 2 weeks after denervation. After nerve section the selectivity sequence Ba congruent with Ca > Sr was changed to Ba > Sr > Ca and the values for relative ratio increased from 1.04 to 2.65 for Ba2+ and from 0.58 to 1.20 for Sr2+ (with respect to Ca2+). Barium current saturation was more obvious in denervated fibers than in non-denervated fibers. The values obtained with the Michaelis-Menten type expression, I = Imax/(1+Kd/[Ba]e) were Kd = 2.7 mM and Imax = 20 microA/cm2 in fibers 2 weeks after nerve section compared with the values Kd = 4.4 mM and Imax = 60 microA/cm2 obtained in non-denervated fibers. Additionally, the effects of two calcium channel blockers (cobalt and nifedipine) were greater by a factor of two in denervated fibers than in non-denervated fibers. Three weeks or so after nerve section, all the biophysical properties studied began to show a tendency to recover toward the values obtained in non-denervated muscles (controls). These results suggest that calcium channels are modified or that there is a change in the types of calcium channels present in frog slow skeletal muscle fibers after denervation.

Folding-unfolding of immunoglobulin domains in titin: a simple two-state model

The folding-unfolding reaction rate process in the giant protein titin is studied within a simple two-state model. The molecule is assumed to be stretched by an external force which modulates the potential barrier associated with the folded state. A two-state model for this process is assumed (i.e., the immunoglobulin domains are considered to be either folded or unfolded, with no intermediate states at all). Simple calculations yield a relation between the force and the pulling speed that agrees fairly well with data from experiments and Monte Carlo simulations performed recently. Moreover, in a regime involving ultrafast pulling, the results show that the detailed form of the potential barrier is irrelevant, a conclusion that agrees with the current theoretical work on molecular dynamics.

Passive forces in mammalian skeletal muscle: a freely-jointed and worm-like chain

The passive mechanical properties of whole muscle in active and nonactive states are compared. The experimental results are presented as stress-strain curves, which are analyzed in the framework of the current theoretical background [viz. the freely-jointed chain model (FJCM) and the worm-like chain model (WLCM)] in a semi-quantitative fashion. This analysis shows that both models can explain the mechanical behavior of whole muscle in non-active state. In the active state, the presence of crossbridges alters the mechanical response, leading to a markedly different behavior, as expected. A discussion of the mechanisms involved and the interpretation of the parameters required for the fitting of the stress-strain curves is also presented.

beta-Adrenergic modulation of Ba2+ currents and K+ contractures in frog slow skeletal muscle fibers

beta-Adrenergic modulation of the Ba2+ current (IBa) and K+ contracture in slow skeletal muscle fibers of the frog (Rana pipiens) were investigated in intact fibers with the three-microelectrode voltage-clamp technique and isometric tension measurements. Application of epinephrine (10(-6) to 10(-5) M) to the bath increased the amplitude of IBa. This increase was blocked by the beta-antagonist propranolol (3 microM), and a similar increase was observed with the beta-specific agonist isoproterenol (1 microM). Thus the epinephrine effect was mediated mainly by beta-adrenergic receptors. External application of permeable 8-bromoadenosine 3',5'-cyclic monophosphate (0.5 mM) increased the amplitude of both IBa and K+ contractures. The present results suggest that beta-adrenergic modulation of IBa in slow skeletal muscle fibers could reflect a modulation of Ca2+ channels via adenosine 3',5'-cyclic monophosphate (cAMP). cAMP (0.5 mM) also potentiated the K(+)-evoked tension in these slow fibers. The physiological contribution made by the modulation of slow skeletal muscle Ca2+ channels to the increase in tension is still not completely understood.

Electrostatic forces as a possible mechanism underlying skeletal muscle contraction

A possible mechanism is put forward to explain the sliding of thin filaments during muscle contraction. In our model, repulsion due to electrostatic forces is the mechanism which triggers crossbridges to cause the thin filaments to slide. The mechanism proposed could operate regardless of whether the myosin heads rotate or bend, although recent experimental evidence seems to confirm the latter action. In spite of its simplicity, the model prediction of the velocity of sliding of the thin filaments agrees well with experimental values from in vitro motility assays.

Caffeine and theophylline contractures in tonic skeletal muscle fibers of the frog

Caffeine and theophylline evoke maintained tension in tonic skeletal muscle fibers of the frog. Their effects depended upon concentrations ranging from 0.5 to 14 mM. The resting potential of tonic skeletal fiber was unaffected by caffeine or theophylline (4 mM). Caffeine and theophylline contractures have a peak tension followed by a sustained tension, which recovered spontaneously when returned to normal solution. The peak tension and total tension (the area under tension-time curve) were reduced when the fibers were soaked in calcium-free solution. In addition the tension was reduced by calcium channel blockers (cadmium). The sustained tension was increased when external calcium was raised. These results suggest that caffeine and theophylline promote the calcium release from sarcoplasmic reticulum and probably the entry of calcium from external medium.

Sodium/calcium exchange in tonic skeletal muscle fibers of the frog

The Na/Ca exchange system was investigated by contractile response to alteration of the extracellular sodium concentration in tonic skeletal muscle fibers of the frog. Contractures were evoked when the extracellular sodium was reduced or withdrawn in the normal solution. This effect was not associated with membrane depolarizations. In the presence of d-tubocurarine, the amplitude and time course of sodium withdrawal contractures were modified, except when sodium was replaced by TEA. When external calcium was omitted from the solution, the tension of sodium withdrawal contracture was greatly reduced. This effect was reversible. These results suggest that Na/Ca exchange is present in the membrane of tonic skeletal muscle fibers of the frog. This conclusion is further supported by the effect of veratridine and strophantidin, which increase the tension of the low-sodium contractures.

Adrenergic modulation of the K+ contractures in tonic skeletal muscle fibers of the frog

The effects of adrenaline and isoprenaline on K+ contractures of curarized tonic skeletal fibers were investigated. The K+ contractures of tonic fibers have a peak tension followed by a sustained tension. The peak tension and total tension (the tension-time integral--area--of K+ contractures) were increased by adrenaline and isoprenaline. The resting potential of tonic skeletal fibers were unaffected by adrenaline. The calcium channel blocker (cadmium and nifedipine) greatly blocked the effects of adrenaline on the peak and total tension of K+ contractures. On the other hand, the peak and sustained tensions of K+ contractures were greatly reduced in Ca(2+)-free solution, but, the peak tension recovered when the fibers were pre-incubated in adrenaline. It is proposed that adrenergic modulation of tension in tonic skeletal muscle fibers could be related with the modulation of Ca2+ channels and/or Ca2+ release from the sarcoplasmic reticulum.