Vasopressin: a possible link between hypoxia and hypertension

Cardiovascular and respiratory diseases are frequently associated with transient and prolonged hypoxia, whereas hypoxia exerts pro-hypertensive effects, through stimulation of the sympathetic system and release of pressor endocrine factors. This review is focused on the role of arginine vasopressin (AVP) in dysregulation of the cardiovascular system during hypoxia associated with cardiovascular disorders. AVP is synthesized mainly in the neuroendocrine neurons of the hypothalamic paraventricular nucleus (PVN) and supraoptic nucleus (SON), which send axons to the posterior pituitary and various regions of the central nervous system (CNS). Vasopressinergic neurons are innervated by multiple neuronal projections releasing several neurotransmitters and other regulatory molecules. AVP interacts with V1a, V1b and V2 receptors that are present in the brain and peripheral organs, including the heart, vessels, lungs, and kidneys. Release of vasopressin is intensified during hypernatremia, hypovolemia, inflammation, stress, pain, and hypoxia which frequently occur in cardiovascular patients, and blood AVP concentration is markedly elevated in cardiovascular diseases associated with hypoxemia. There is evidence that hypoxia stimulates AVP release through stimulation of chemoreceptors. It is suggested that acting in the carotid bodies, AVP may fine-tune respiratory and hemodynamic responses to hypoxia and that this effect is intensified in hypertension. There is also evidence that during hypoxia, augmentation of pro-hypertensive effects of vasopressin may result from inappropriate interaction of this hormone with other compounds regulating the cardiovascular system (catecholamines, angiotensins, natriuretic peptides, steroids, nitric oxide). In conclusion, current literature indicates that abnormal mutual interactions between hypoxia and vasopressin may significantly contribute to pathogenesis of hypertension.

Vasopressin and Breathing: Review of Evidence for Respiratory Effects of the Antidiuretic Hormone

Vasopressin (AVP) is a key neurohormone involved in the regulation of body functions. Due to its urine-concentrating effect in the kidneys, it is often referred to as antidiuretic hormone. Besides its antidiuretic renal effects, AVP is a potent neurohormone involved in the regulation of arterial blood pressure, sympathetic activity, baroreflex sensitivity, glucose homeostasis, release of glucocorticoids and catecholamines, stress response, anxiety, memory, and behavior. Vasopressin is synthesized in the paraventricular (PVN) and supraoptic nuclei (SON) of the hypothalamus and released into the circulation from the posterior lobe of the pituitary gland together with a C-terminal fragment of pro-vasopressin, known as copeptin. Additionally, vasopressinergic neurons project from the hypothalamus to the brainstem nuclei. Increased release of AVP into the circulation and elevated levels of its surrogate marker copeptin are found in pulmonary diseases, arterial hypertension, heart failure, obstructive sleep apnoea, severe infections, COVID-19 due to SARS-CoV-2 infection, and brain injuries. All these conditions are usually accompanied by respiratory disturbances. The main stimuli that trigger AVP release include hyperosmolality, hypovolemia, hypotension, hypoxia, hypoglycemia, strenuous exercise, and angiotensin II (Ang II) and the same stimuli are known to affect pulmonary ventilation. In this light, we hypothesize that increased AVP release and changes in ventilation are not coincidental, but that the neurohormone contributes to the regulation of the respiratory system by fine-tuning of breathing in order to restore homeostasis. We discuss evidence in support of this presumption. Specifically, vasopressinergic neurons innervate the brainstem nuclei involved in the control of respiration. Moreover, vasopressin V1a receptors (V1aRs) are expressed on neurons in the respiratory centers of the brainstem, in the circumventricular organs (CVOs) that lack a blood-brain barrier, and on the chemosensitive type I cells in the carotid bodies. Finally, peripheral and central administrations of AVP or antagonists of V1aRs increase/decrease phrenic nerve activity and pulmonary ventilation in a site-specific manner. Altogether, the findings discussed in this review strongly argue for the hypothesis that vasopressin affects ventilation both as a blood-borne neurohormone and as a neurotransmitter within the central nervous system.

Steroidogenesis in human aldosterone-secreting adenomas and adrenal hyperplasias: effects of hypoxia in vitro

The synthesis of adrenal steroids requires molecular oxygen. Because arterial hypoxemia is a common clinical condition, the purpose of the present study was to examine steroidogenesis in vitro under physiological changes in O(2) tension (Po(2)) in cells from human adrenal glands with aldosterone-secreting adenomas (ASA; n=3) or with bilateral adrenal hyperplasia causing Cushing's syndrome (n=4). A decrease in Po(2) from 150 mmHg (mild hyperoxia) to 80 mmHg had minimal effect on steroid production. A reduction to 40 mmHg (still well within the physiological range) significantly inhibited cAMP- and ACTH-stimulated aldosterone, cortisol, and dehydroepiandrosterone (DHEA) production from ASA. Furthermore, cortisol and DHEA production in cells from histologically normal tissue, adjacent to ASA and from bilateral adrenal hyperplasias, was also inhibited under a Po(2) of 40 mmHg. We conclude that physiological decreases in Po(2) to levels typical for adrenal venous Po(2) under mild hypoxia inhibit steroidogenesis. These studies may have implications for oxygen therapy in critically ill patients with functional adrenal insufficiency, as well as for therapeutic options in patients with adrenal neoplasms.