Abstract
Vitamin C (ascorbate) and vitamin E (?-tocopherol) are highly concentrated in the adrenal gland. Being antioxidants, they protect adrenocortical cells from oxigen derived radicals produced by steroidogenic cytochrome P450 enzymes. In the adrenal cortex, there exists a gradient of lipidperoxidation that increases towards the inner zones, corresponding with high concentrations of ?-tocopherol in the outer and low concentrations in the inner zone. These gradients may contribute to the functional zonation of the adrenal cortex. Ascorbate may be, in addition, part of an electron transport system within the inner mitochondrial membran, supplying additional reduction aquivalents for aldosterone biosynthesis.
In the present study the in vivo influence of vitamin C or vitamin E depletion on cortisol and aldosterone secretion of the adrenal cortex was examined in guinea pigs. Subgroups of animals were sodium depleted or injected with depot-ACTH for three days. Besides the vitamin concentrations of serum and tissues, the plasma concentrations of cortisol and aldosterone and their most important regulators, ACTH, serum potassium, serum sodium and plasma renin activity were measured. Within the last two days of vitamin depletion, the daily urine excretion of sodium and potassium was determined.
In preliminary experiments the optimum duration of vitamin depletion was determined in order to find out the point of severe vitamin deficiency at which the pituitary-adrenal-axis was not yet significantly activated by scorbutic stress. In the main experiments, vitamin C depletion lasted 15 days while vitamin E depletion was carried out over 72 days.
Vitamin C depletion led to a slight rise in plasma cortisol without a concomitant rise in plasma ACTH. ACTH administration greatly stimulated cortisol secretion and suppressed aldosterone and renin secretion irrespective of the vitamin C status. Vitamin E depletion did not influence cortisol secretion, but plasma aldosterone rose slightly. All other parameters did not change.
15 days of sodium depletion stimulated aldosterone secretion threefold. The stimulation of aldosterone secretion was totally abolished by vitamin C depletion and reduced to fifty percent in vitamin E depleted animals. All other parameters, even plasma renin activity, were not affected by vitamin C or E depletion. A functional correlate of inhibited aldosterone secretion in sodium and vitamin C depleted animals was an impaired renal sodium conservation.
These results show that ascorbate and ?-tocopherol play a permissive role in aldosterone biosynthesis and secretion in the guinea pig. The antioxidant vitamins may also be important for the functional zonation of the adrenal cortex. |
