Periodization of the early postnatal development in the rat with particular attention to the weaning period.

The early postnatal period is characterized by a great plasticity with critical windows in which any inadequate insult or intervention may be able to influence both positively and adversely postnatal growth and development. After birth the rat littermates enter the presuckling period (initial 6 hours terminated by the first nursing), characterized by transition from the amniotic fluid to the air, by the changes of the ambient temperature, by the termination of placental nutrition and by oxidative stress. After this stage the suckling period initiates and the littermates start to consume milk of their mothers. Comsumption of milk culminates on day 15, then decreases and terminates on postnatal day 28. The end of the suckling period and the onset of physiological weaning is determined by the moment when the youngs for the first time consume the solid food together with milk (postnatal day 17 in rats). On day 19 the first intake of drinking water occurs. The weaning period terminates by the last consumption of maternal milk - on postnatal day 28. It is necessary to stress that the duration of early postnatal periods is independent of the changes of body weight. The precise knowledge of individual ontogenetic periods critical for further development is crucial for the prediction and explanation of reactions to various pathogenetic stimuli both under experimental conditions and in clinical medicine.

Cardiac tolerance to ischemia in neonatal spontaneously hypertensive rats.

Hypertension is the risk factor of serious cardiovascular diseases, such as ischemic heart disease and atherosclerosis. The aim of the present study was to analyze the development of cardiac tolerance to ischemia in neonatal spontaneously hypertensive rats (SHR) and possible protective effect of ischemic preconditioning (IP) or adaptation to intermittent high-altitude hypoxia (IHAH). For this purpose we used 1- and 10-day-old pups of SHR and their normotensive control Wistar Kyoto rats (WKY). Isolated hearts were perfused in the Langendorff mode with Krebs-Henseleit solution at constant pressure, temperature and rate. Cardiac tolerance to ischemia was expressed as a percentage of baseline values of developed force (DF) after global ischemia. IP was induced by three 3-min periods of global ischemia, each separated by 5-min periods of reperfusion. IHAH was simulated in barochamber (8 h/day, 5000 m) from postnatal day 1 to 10. Cardiac tolerance to ischemia in 1-day-old SHR was higher than in WKY. In both strains tolerance decreased after birth, and the difference disappeared. The high cardiac resistance in 1- and 10-day-old SHR and WKY could not be further increased by both IP and adaptation to IHAH. It may be concluded that hearts from newborn SHR are more tolerant to ischemia/reperfusion injury as compared to age-matched WKY; cardiac resistance decreased in both strains during the first ten days, similarly as in Wistar rats.

Ontogenetic changes of lipofuscin-like pigments in the rat heart.

Increased generation of reactive oxygen species results in the formation of fluorescent end-products of lipid peroxidation - lipofuscin-like pigments (LFP). LFP increased up to six-fold from the fetal value in the rat heart immediately after birth. In the experimental design of this study the fetuses were sampled 1 day before birth, and then the samples were collected on postnatal days 1, 4, 7, 10, 15, 30, and 60. Males and females were compared on day 30 and 60 when the difference between right and left ventricle was studied as well. Four LFP fluorophores were analyzed: F355/440, F310/470, F350/450, F315/450 (excitation/emission, nm). All fluorophores decreased on day 4 relative to day 1, subsequent transient increases ended in a significant decrease on day 60. However, the LFP levels on day 60 are still about threefold higher than those in fetuses. Differences between male and female hearts were observed on day 30. The corresponding male ventricles contained by one third higher concentration of LFP than the female counterparts. The increase in LFP concentration in male ventricles on day 30 was only transient, no difference between corresponding male and female ventricles was found on day 60. The most distinguished feature in the male heart was a sharp LFP decrease in the right ventricle on day 60.

Biological effects of selenium compounds with a particular attention to the ontogenetic development.

Selenium is a trace element that is essential for living organism. Its beneficial effect is, however, expressed in a very narrow dosage range: the high and low doses of selenium are connected with pathological manifestations. The toxicity depends on the chemical form of selenium, state of organism, interactions with heavy metals and on the stage of ontogenetic development. Whereas one dose of sodium selenite (20 micromol/kg b.w.) is lethal in adult rats, suckling rats are entirely resistant. However, within one week after administration of the same dose, cataract of eye lens developed. The highest incidence of cataract was observed in 10-day-old animals and it decreased until day 20. From postnatal day 20 to day 40 the rats were resistant to both the lethal and cataractogenic effects of selenium. The incidence of cataract may be suppressed by premature weaning, lower hydration of suckling, change of water soluble/water insoluble lens protein ratio, thyroxine treatment, and by interaction with mercury. By means of its oxidative and reduction properties, selenium is involved in the maintenance of the cell redox homeostasis. Typical example is its possible cardioprotective effect: selenium decreased number of arrhythmias, reduced infarct size and improved the contractile recovery after ischemia/reperfusion injury. Selenium supplementation may thus increase cardiac tolerance to ischemic damage.

Inotropic effect of low extracellular sodium on perfused perinatal rat heart.

The purpose of the present study was to estimate the development of the inotropic response to low extracellular sodium (LES) during the perinatal period. The effect of LES (35 mmol.L-1) was measured in isolated perfused control and ryanodine-pretreated rat hearts on prenatal day 20 and postnatal days 1, 2, 4, and 7. The effect of LES on the developed force (DF) of control hearts changes significantly day by day: whereas a persisting increase of magnitude of contractions was recorded in the prenatal hearts, this increase was only transient on postnatal day 1 and 2. Starting from day 4, the initial signs of a triphasic response, typical for adult hearts, appeared (an initial increase of DF, followed by a decrease of DF and a rise of resting force, and finally a delayed increase of DF); this trend was more pronounced on day 7. The LES-induced increase of resting force was recorded only in 2-, 4-, and 7-day-old hearts. The negative inotropic effect of ryanodine (10(-6) mol.L-1) was observed already prenatally (60% of the controls) and continued during the whole period of investigation; in contrast, a ryanodine-induced increase of resting force was recorded only postnatally. However, pretreatment with ryanodine abolished the day-by-day changes in the response to LES: in all the hearts studied, the first phase (initial increase of DF) was followed by a severe depression of the magnitude of contractions, together with increased resting force. Our data show significant age-dependent differences in the cardiac contractile response to LES.(ABSTRACT TRUNCATED AT 250 WORDS)