RESUMEN

We have studied the effects of thyroidectomy and the in vivo administration of different triiodothyronine (T3) doses in thyroidectomized female rats on electrophysiological properties, measured in vitro, of the anterior and posterior papillary muscle fibers from the right ventricle. In each thyroid state, the action potential duration (APD) measured by stimulating at 1 Hz was shorter for the posterior papillary muscle. APD from both preparations was found significantly lengthened in thyroidectomized animals in comparison to euthyroid controls. APD was shortened owing to treatment of thyroidectomized rats with T3 doses up to 10 micrograms/100 g body weight every second day. Treatment with larger doses of T3 tended to restore the values of APD found for ventricular fibres from both controls and thyroidectomized animals treated with substitutive T3 doses (5 micrograms/100 g body weight every second day). As the stimulation rate was increased from 1 to 5 Hz, APD increased in both preparations of all groups. The changes were of different amounts but the APD difference between the rat groups, which were significant at 1 Hz, remained significant at 5 Hz, while the differences between anterior and posterior preparations were cancelled in animals treated with 50 micrograms of T3 and reversed in those treated with 100 micrograms.

Asunto(s)

RESUMEN

Male rats exposed to the cold (4 degrees C) for five or ten days exhibited modifications in their thyroid state, as documented by increases in serum thyroid hormone levels, to which differently graded modifications of heart weight/body weight ratio, heart rate, and resting metabolic rate were associated. The values of the above mentioned thyroid state indicators returned to those of the control when the animals, kept at cold for ten days, were re-exposed to room temperature (24 degrees C) for an additional 10 days. The configuration of action potentials, recorded in vitro at 26 degrees C from fibres of anterior papillary muscles, was different in control rats of different age and was affected by prolonged cold exposure. In fact, the action potential duration (APD) increased after ten days of cold exposure. In the re-exposed group the APD was not different from that of the controls. Such a pattern was not significantly modified when the stimulation frequency increased from 1 Hz to 5 Hz. The above results suggest that in cold exposure, as in experimental hyperthyroidism, thyroid hormone might exert a cardiac chronotropic effect by modifying heart electrophysiological properties. Thus thyroid hormone should play a basic role during the exposure to cold environment, stimulating the body metabolism and increasing heart rate as a response to the requirement for greater tissue perfusion.

Asunto(s)

RESUMEN

We investigated the effects on the electrophysiological properties of ventricular muscle fibres from lizards kept at 20 degrees C of mild and severe hyperthyroidism. The hyperthyroidism was induced by a 4-day treatment with either 0.025 or 1.0 microgram triiodothyronine g-1 body weight, documented by increased serum levels of thyroid hormone. Triiodothyronine treatment did not modify the duration of the action potential recorded in vitro at 25 degrees C from ventricular muscles stimulated at 1 Hz. Recordings at higher temperatures were associated with a faster repolarization phase and a decrease of of action potential duration in both euthyroid and hyperthyroid animals. However, in lizards treated with 1.0 microgram triiodothyronine . g-1 body weight, the 90% repolarization recovery times at 30 and 35 degrees C (95.6 +/- 14.9 ms and 53.0 +/- 6.0 ms, respectively), were significantly shorter than normal (177.6 +/- 29.2 and 107.2 +/ 18.1 ms, respectively). Action potential duration was also dependent on stimulation frequency of the preparations. Increased frequency led to significant decrease of the duration of action potentials recorded at 25 degrees C. In euthyroid preparations the reductions in 90% repolarization recovery time, owing to increase in stimulation frequency to 2.5 and 5 Hz, were 19.3 +/- 1.7 and 35.6 +/- 2.0 ms, respectively. In hyperthyroid preparations, the reductions in the 90% recovery time due to stimulus frequency increases varied from 35.4 +/- 1.9 and 58.1 +/- 2.1 ms at low hormone doses to 38.9 +/- 2.0 and 58.2 +/- 2.1 ms at high hormone doses. As a result of these differences, the action potential durations recorded from the two hyperthyroid preparations at high stimulation rates were shorter than from euthyroid preparations. The results obtained suggest that lizard cardiac tissue is responsive to hormone action at low environmental temperature, but the effects of such action become evident when the temperature and heart rate increase.

Asunto(s)

RESUMEN

A simple approach to quantitative determination of antioxidant capacity of rat liver homogenate is proposed. It consists of measuring chemiluminescence generated by a suitable system "detector" for .OH radicals produced from sodium perborate. The system generating the light signal contained luminol and compounds producing enhancement of light emission, such as sodium benzoate and indophenol. Two different methods, utilizing the same technique of enhanced luminescence, were set up. In a previous work, a parameter b, contained in the equation, which best describes the dependence of the intensity of light emission (E) on liver homogenate concentration (C) (E = a.C/exp(b.C), was found to be related to the level of antioxidants in the homogenate. Therefore, in the first method, the light emission from several dilutions of both liver homogenates, and homogenate and antioxidant mixtures, stressed with sodium perborate, was detected by a luminometer. The best fitting of data to theoretical equation provided b values, which were introduced in a system of equations relating such values to the antioxidant concentration. The solution of above system supplied the antioxidant concentration in the homogenate in terms of the equivalent concentration of the antioxidant used. In the other method, evaluations of the antioxidant capacity of liver homogenates were obtained by the determination of the ability of 10% homogenates to quench the light emission induced by either peroxidase or cytochrome c in comparison to the ability of antioxidant solutions. Both methods are able to evidence the decrease of the antioxidant concentration of liver homogenates after oxidative stress with ter-butylhydroperoxide. The value of both concentration changes and standard errors indicates that the method using a standard curve obtained with peroxidase, such as catalyst of radical reaction, and deferoxamine, such as antioxidant, is to be preferred.

Asunto(s)

RESUMEN

The serum thyroid hormone levels [total (TT3) and free (FT3) triiodothyronine] and the heart rates were determined in neonatal rats of different ages (1-5-10 days). Thyroid hormone levels increase gradually in the first 10 days of age. The heart rate, tested at a body temperature of 37 degrees C, also increases during the same period. As the increase in heart rate in this phase of rat life is not due to the catecholamines, it is suggested that such an increase might depend on the increased thyroid hormone activity. On the other hand in congenitally hypothyroid rats the levels of both hormones and heart rates are lower than in normal animals of the same age. The electrophysiological properties of ventricular muscle fibres include a longer action potential, irrespective of stimulation frequency, in younger, naturally hypothyroid animals. The duration of action potential is greater in the congenitally hypothyroid animals, at all ages. These data demonstrate that, as in young and adult rats, the age-related modifications in heart rate, found in neonatal rats, might be due to thyroid dependent modifications of cardiac electrophysiology.

Asunto(s)

RESUMEN

We have studied the effects of in vivo administration of different T3 doses to thyroidectomized rats on electrophysiological properties, measured in vitro, of papillary muscle fibers. The treatment with increasing T3 doses was associated with a significant reduction of the action potential duration up to a dose as large as 25 micrograms/100 g body weight every second day. The treatment with larger doses of T3 tended to restore the values of the action potential duration present in animals treated with physiological doses (5 micrograms/100 g body weight every second day). Action potential duration is frequency dependent. As the stimulation rate was increased from 1 to 5 Hz, this duration increased in all groups. However the difference between the rat groups remained significant. The cardiac frequency measured in unanaesthetized rats increased as the T3 doses. Furthermore the intrinsic frequency showed a similar increase, indicating a direct effect of T3 on the pacemaker cells in all thyroid states. The mechanism of this action of the thyroid hormone is not, however clear.

Asunto(s)

RESUMEN

1. PTU treatment-induced hypothyroidism is associated with a significant decrease in the chick heart rate. 2. Hypothyroidism produces a slow onset of bradycardia, indicating a late effect of thyroid hormone decrease. 3. After 15 days treatment, an increase of action potential duration, similar to that reported for several mammal species, has been found. 4. Action potential duration is frequency and temperature dependent, but it has been found to be significantly different in the euthyroid and hypothyroid chicks.

Asunto(s)

RESUMEN

The authors have determined the serum thyroid hormone levels [total (TT3) and free (FT3) triiodothyronine], the heart weight/body weight ratio and the heart rate of differently aged male rats. The variations of these parameters show a modification of thyroid state as a function of ageing. The authors have also recorded, at about 26 degrees C, resting and action potentials from single cells of papillary muscles isolated from the same groups of rats. The animals in the higher thyroid state exhibited a repolarization speed higher than the other animals. The thyroidectomy, performed on 50 day old rats, and T3 treatment of the thyroidectomized rats give rise to modifications of repolarization speed and then of action potential duration analogous to ones obtained in previous study for animals thyroidectomized at 30 days of age. These data demonstrate that the modifications of heart electrophysiological properties with age, are due fundamentally to thyroid state modifications. The results suggest also that the cardiac chronotropism modifications which the rat undergoes as a function of ageing are due to the changes of levels of thyroid hormone which might exert its effect by modifying the ion channel kinetics as well as the cardiac receptors.

Asunto(s)

RESUMEN

We have studied the effects of in vivo alterations of thyroid state on the electrophysiological properties, measured in vitro, of rat papillary muscle fibres. The duration of the action potential, recorded from single cells of papillary muscle obtained from 60 day old rats, thyroidectomized at weaning, was larger than in preparations of euthyroid animals of same age. The treatment of thyroidectomized rats with T3 physiological doses restored the values of action potential duration present in euthyroid animals of same age. The data dealing with the time course of these variations indicate a late effect of thyroidectomy or T3 treatment. The hyperthyroid state, obtained by treatment of thyroidectomized rats with larger doses of T3, was associated with a significant reduction in the action potential. Action potential duration is frequency dependent. As the stimulation rate was increased from 1 to 5 Hz, this duration increased in all groups. However the difference found among the rat groups remained significant. Propranolol had virtually no effect on action potentials from fibres of euthyroid animals and those in altered thyroid state.

Asunto(s)

RESUMEN

Confirming the literature data the authors describe that the heart rate is smaller in the newborn rats than in adult ones and increases until the adult values during the first two weeks of life. On the other hand, the blood thyroid hormone exhibits the same pattern, showing an early postnatal increment. As, according the Adolph's data (1967), the heart rate enhancement is not due to the catecholamines, the authors suppose that such enhancement might conceivably depend on thyroid hormone increment.

Asunto(s)

RESUMEN

To establish whether thyroid hormone modifies the heart rate directly or through an action on other neuroendocrine modulators, the authors have examined several animals models differing in the plasma levels of such compounds. Induction of the hypothyroid state in rats produced a slow onset of bradycardia, which may be removed by a prolonged triiodothyronine treatment. The involvement of TSH was excluded as, by comparing thyroidectomized, hypophysectomized and cold exposed rats, the heart rate was found to vary according to the thyroid levels and not to the TSH levels. Moreover growth hormone, corticotropin and gonadotropins do not influence the heart rate, as the bradycardia induced by hypophysectomy was fully removed by triiodothyronine treatment. The lack of influence by ACTH and GnH was confirmed by treatment of thyroidectomized rats with corticosteroids or testosterone, respectively. Finally, thyroid hormone did not act on the heart rate by changing the norepinephrine output at the sympathetic nerve endings in the heart. In fact, thyroidectomy produced a more intense bradycardia than sympathectomy, and such bradycardia was equally removed by triiodothyronine treatment in thyroidectomized rats and in thyroidectomized and then sympathectomized ones. The authors suggest that the direct effect of the thyroid hormone on cardiac chronotropism is due to an early enhancement of beta-adrenoceptors, followed by a late modification of the electrophysiological properties of the myocardium.

Asunto(s)

RESUMEN

The effects of the thyroid hormone on the number and protein content of mitochondria were investigated in rat heart. The specific mitochondrial population, determined by direct counting, was estimated to be about 3.7 X 10(11) mitochondria per g wet weight in young hypothyroid male rats (T) and about 2.4 X 10(11) (65%) in euthyroid animals, sex- and age-matched. Triiodothyronine (T3) treatment of T animals restored the levels to normal values. The protein content per mitochondrion, on the contrary, was higher in euthyroid animals or hypothyroid animals, following T3 treatment, compared to T. Finally, thyroid hormone enhanced the heart mass and, therefore, the cardiocytal and mitochondrial populations of the whole organ. These results differ from previous data from our laboratory indicating that liver weight is increased by thyroid treatment, albeit at a slower rate, while the number of mitochondria per g tissue increases and the protein content per mitochondrion decreases. In conclusion, the effects of the thyroid hormone on mitochondria are different in the hepatocyte and in the cardiocyte. It appears that the hormone differently modulates the machinery of mitochondrial protein synthesis in the two target cells according to the physiological role of mitochondria in liver and heart cells, i.e. heat production or mechanical energy output, respectively.

Asunto(s)

Asunto(s)

Asunto(s)