Summary
Highlights
The video focuses on the zona fasciculata, the middle layer of the adrenal cortex, responsible for secreting cortisol, a glucocorticoid. The production begins in the hypothalamus, where the paraventricular nucleus releases corticotropin-releasing hormone (CRH). CRH stimulates corticotropes in the anterior pituitary to release adrenocorticotropic hormone (ACTH). ACTH then stimulates the zona fasciculata cells to initiate cortisol synthesis. Unlike protein hormones, steroid hormones like cortisol are synthesized from cholesterol through a series of enzymatic steps, including pregnenolone and progesterone intermediates, with 21-hydroxylase being a crucial enzyme in this pathway. Protein kinase A, activated by ACTH, phosphorylates enzymes involved in this synthesis.
Cortisol, being lipid-soluble, circulates in the blood bound to transport proteins: approximately 25% to albumin and 75% to corticosteroid-binding globulin (transcortin). Cortisol influences muscles and bone by stimulating protein catabolism. It enters muscle and bone cells, activates intracellular receptors, and promotes gene expression for proteases. These proteases break down proteins into amino acids, which are then released into the bloodstream and taken up by the liver.
In adipose tissue, cortisol stimulates the breakdown of triglycerides into glycerol and fatty acids. Fatty acids can be used by muscles or redistributed, while glycerol is transported to the liver. In the liver, cortisol promotes gluconeogenesis, converting amino acids, glycerol, and lactic acid into glucose, thereby increasing blood glucose levels. Additionally, cortisol has a direct and unusual effect on the liver: it stimulates glycogenesis, the conversion of glucose into glycogen for storage.
Cortisol enhances the sympathetic nervous system's effects by increasing the sensitivity of adrenergic receptors in smooth muscle cells of blood vessels. This increased sensitivity to norepinephrine leads to amplified vasoconstriction, resulting in elevated blood pressure. In the liver, cortisol indirectly boosts glycogenolysis by making adrenergic receptors more responsive to norepinephrine, which breaks down glycogen into glucose, further raising blood glucose levels.
Cortisol acts as an immunosuppressant, inhibiting immune cells like monocytes, lymphocytes, and basophils from releasing inflammatory mediators such as histamines, leukotrienes, prostaglandins, and interleukins. This suppression of the inflammatory response is significant, especially during conditions of prolonged stress, where a depressed immune system can increase susceptibility to infections.
The primary stimuli for cortisol release include hypoglycemia (low blood glucose) and chronic stress. Cortisol counteracts hypoglycemia through gluconeogenesis and indirectly by enhancing glycogenolysis. In response to long-term stress, cortisol causes protein catabolism, increased blood pressure, and immune system depression. High cortisol levels exert negative feedback on the hypothalamus and anterior pituitary, reducing CRH and ACTH release, respectively. Conversely, low cortisol levels diminish this negative feedback, leading to increased CRH and ACTH, thereby stimulating cortisol synthesis and maintaining homeostasis.