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.
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.
figures)The serum thyroid hormone levels [total ('IT3) and free (m;) 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"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 .
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.
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