The Effect of Moderate Heat on Rat Pituitary ACTH Cells: Histomorphometric, Immunofluorescent and Hormonal Study

Abstract

In areas with moderate continental climate, increased average ambient temperature during the summer represents a stressogenic factor that affects the hypothalamo-pituitaryadrenocortical axis in mammals. Therefore, we wanted to examine the effects of 4 days of constant exposure to moderately elevated ambient temperature (35 ± 1 o C) on the histomorphometric and immunofl uorescent characteristics, as well as on the hormonal secretion of pituitary corticotropes (ACTH) cells in adult male rats. In comparison with the controls kept at 20 ± 2 o C, a signifi cant increase (p<0.05) of the absolute and relative pituitary weight (23.1% and 36.1%, respectively) was registered after exposure to heat. The localization, as well as the shape of the ACTH cells in the heat exposed group was not signifi cantly altered, but their immunopositivity was weaker. After 4 days of heat exposure, a weaker signal confi rmed the relative fl uorescence intensity of the ACTH cells (15.3%, p<0.05). In heat exposed rats, an increase of the cellular and nuclear volumes of immunolabelled ACTH cells and decrease of their volume density (6.9%, 14.3% and 20.0%, respectively; p<0.05) was registered. Observed histomorphometric and immunofl uorescent features of the pituitary ACTH cells were in accordance with the increased (p<0.05) value of plasma adrenocorticotropic hormone (ACTH) by 23.7% compared to the control rats. It can be concluded that the 4-day exposure to moderately elevated ambient temperature intensifi es pituitary ACTH secretion in adult male rats.

The Effects of Long-Term Exposure to Moderate Heat on Rat Pituitary ACTH Cells: Histological and Hormonal Study

“…On the other hand, our earlier research suggested reduced immuno-histomorphometric parameters of ACTH cells, with concurrently decreased blood ACTH and corticosterone concentrations in rats after one-day exposure to moderate heat (35 ± 1°C) [17]. Four days of continuous exposure to the same ambient temperature (short-term exposure) caused an active resistance of rat ACTH cells, manifested by their weaker immuno-positivity/fl uorescence, increased cellular and nuclear volumes and intensifi ed ACTH secretion [9,18]. Furthermore, the adrenocortical volume and number of cells in all cortical zones, as well as the serum levels of aldosterone and corticosterone, were signifi cantly increased in the rats subjected to elevated temperature (35 ± 1°C) for four days [4].…”

“…The animals in the control group were kept at room temperature (20 ± 2°C), while the rats continuously exposed to elevated ambient temperature were placed in a special heat chamber, with regulated air temperature and air humidity at 30-40%, as previously described [17,19]. The specific temperature for the experimental groups (35 ± 1°C) was exploited in our previous studies and established as a moderately high environmental temperature [4,9,17]. In line with this, the mode of continuous exposure to the elevated temperature was proposed in other studies [19,20].…”

“…In line with this, the mode of continuous exposure to the elevated temperature was proposed in other studies [19,20]. In order to further support the chosen temperature range, as an experimental design for thermal exposure, it should be mentioned that the climate region of Western Balkans we belong to is well known to have similar air temperatures during the summer months [4,9,17,19]. After 7 days, 14 days, 21 days, 30 days and 60 days of exposure to moderately high temperature (refl ecting the exposure to a heatwave), the animals were weighed and subsequently sacrificed by a laparotomic procedure under ether narcosis (Diethyl ether stabil.…”

“…Ambient temperature is one of the essential environmental factors that primarily determines the physiological processes in living organisms. Thus, the temperature extremes may strongly infl uence animal homeostasis and trigger numerous physiological, autonomic, and behavioral responses [4,9]. The "animal thermal comfort zone" is an ambient temperature range in which animal metabolic processes are stable and directed to the storage of carbohydrates, proteins and fat [10].…”

“…Temperatures above this range induce thermal stress in the organism and the activation of the sympatho-adrenomedullary system, as the fi rst response to the thermal stressor, or in the case of a persistent stimulus, the activation of the hypothalamic-pituitary-adrenal (HPA) axis [11]. An increase in the plasma adrenocorticotropic hormone (ACTH) and circulating glucocorticoids, as part of the HPA axis activation upon exposure to high ambient temperature, stimulates numerous metabolic processes (glycogenolysis, lipolysis, and proteolysis) aimed to increase energy availability [4,9,12,13]. The majority of the studies, focused on the effect of short exposure of rats to high ambient temperature (one hour at 38 o C), have found increased blood ACTH and corticosterone concentrations [13][14][15][16].…”