Kidney is one of the organs with a clear circadian functional organization. It is proved that various external and internal factors can influence its functioning. A chronological structure of the kidney’s acid, ionic and excretion functions is being changed in course of ontogenesis and the ionic regulation function is the one that undergoes most significant changes, which manifest themselves through a sharp decrease of the sodium ions excretion because of activation of their proximal transportation. It has been found that the night tubular reabsorption of water is more active than the day one. Investigation of the rhythms of urea osmomolarity, electrolytes excretion, hydrogen ions concentration, glomerular filtration and tubular reabsorption of water proves that the daily rhythms depend on interconnection between the glomerular and tubular systems. This conclusion has been drawn from the results of investigation carried out in the group of children suffering of enuresis and hypercalcinuria. A response of kidney to the water and salt load depends on the content of sodium in meals and this is an evidence of the stable adaptive changes formed in the rat’s organism and directed onto homeostasis maintenance. The kidney functional reserve plays an important role in this process. This is the main physiological mechanism governing depth of the kidney adjustment towards the water/salt homeostasis regulation. On the other hand, tubular reabsorption and secretion process is responsible for accuracy and specificity of this adjustment. All experiments were carried out with white pubertal nonlinear male rats, weight 180±10 g. All experimental animals were divided into three groups. First group (6 animals) – control, no load; second group (6 animals) – 5 % water load calculated as 5 ml of water per 100 g weight and third group (6 animals) – 3 % salt load in the form of infusion of the 3 % NaCl solution calculated as 2.56 mmoles (59 mg) of Na per 100 g weight. All types of the exertion were applied during 2 hours before euthanasia by the metal intragastric probe. The ion regulation function was evaluated by the sodium ions excretion values as well as by Na concentration in urea, its reabsorption value and filtration charge while pH of urea, excretion of the titrable acids and ammonia were used to evaluate the acid regulation function. Folin’s method was employed to measure concentration of creatinine in urea while its concentration in the blood plasma was determined by photocolorimetry. It has been shown that both types of the load cause changes in the kidney functional activity indexes. For instance, it is proved by investigation of the ion-adjusting function of the rat kidneys that the 3 % salt load causes increase in sodium concentration in urea by 82 % comparing to the control group while this concentration rises by 28 % at the 0.75 % load. Excretion of sodium ions has also risen twice at the 3 % salt load and by 43 % at the 0.75 % load. Finally, it was found that the 0.75 % salt load caused a 27 % increase in the sodium filtration charge. A rise in sodium concentration and excretion was provided mainly by decrease in its tubular reabsorption. On the other hand, changes in creatinine excretion and calculated glomerular filtration for the animals exposed to NaCl infusion prove that the filtration was experiencing increase simultaneously with rise in the sodium filtration charge. These changes in the kidneys functional activity under salt exertion can be caused by rise of sodium ions concentration in the organism and higher osmomolarity after exposition to the salt exertion. It can also be noted that almost no changes in pH values after the salt load were registered except shifting pH for 16 % down after the 0.75 % exertion. Ammonia excretion lifted by 24 % and 15 % while excretion of the titrable acids also increased by 19 % and 32 % for the 3 % and 0.75 % salt load simultaneously. It is interesting that its concentration in urea also stays almost unchanged as it can be found from the water reabsorption index. Therefore, it can be stated that the increase of osmomolarity of urea is caused by changes in the reabsorption values for different ions, not by water reabsorption in the distal kidney tubules. Increased excretion of sodium is primarily caused by its lower reabsorption, and yet retarded transportation in the proximal tubules can also be influential for this process. Thus, sodium excretion and concentration are rising 1.5 times averagely under the 3 % salt load. Experimental investigations involved various groups of animals at 8:00 and 20:00 and almost no difference in the kidney functional conditions was registered. However, the Na+ ions concentration has lifted by 2.24 times comparing to the control group and this index was a little higher in the 8:00 experiment. This is also proved by decrease of the filtration fraction of Na+. The salt load results in increase of concentration and excretion of potassium and sodium, ammonia and titrable acids. Changes in the kidney functionality affect tubular reabsorption and activate secretion while they do not depend on the kidney damage.
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