Dysfunction of the endothelium and constriction of the isolated rat's middle cerebral artery in low sodium environment in the presence of vasopressin

Role of the endothelial reverse mode sodium-calcium exchanger in the dilation of the rat middle cerebral artery during hypoosmotic hyponatremia

A decrease in serum sodium ion concentration below 135 mmol L^-1 is usually accompanied by a decrease in plasma osmolality (hypoosmotic hyponatremia) and leads to the disorder of intracranial homeostasis mainly due to cellular swelling. Recently, using an in vitro model of hypoosmotic hyponatremia, we have found that a decrease in sodium ion concentration in the perfusate to 121 mmol L^-1 relaxes the isolated rat middle cerebral artery (MCA). The aim of the present study was to explore the mechanism responsible for this relaxation. Isolated, pressurized, and perfused MCAs placed in a vessel chamber were subjected to a decrease in sodium ion concentration to 121 mmol L^-1. Changes in the diameter of the vessels were monitored with a video camera. The removal of the endothelium and inhibition of nitric oxide-dependent signaling or the reverse mode sodium-calcium exchanger (NCX) were used to study the mechanism of the dilation of the vessel during hyponatremia. The dilation of the MCA (19 ± 5%, p < 0.005) in a low-sodium buffer was absent after removal of the endothelium or administration of the inhibitor of the reverse mode of sodium-calcium exchange and was reversed to constriction after the inhibition of nitric oxide (NO)/cGMP signaling. The dilation of the middle cerebral artery of the rat in a 121 mmol L^-1 Na⁺ buffer depends on NO signaling and reverse mode of sodium-calcium exchange. These results suggest that constriction of large cerebral arteries with impaired NO-dependent signaling may be observed in response to hypoosmotic hyponatremia.

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-Induced Hyponatremia in Patients With Aneurysmal Subarachnoid Hemorrhage: A Case Series and Literature Review

OBJECTIVE

Vasopressin may be administered to treat vasospasm following aneurysmal subarachnoid hemorrhage (aSAH). The objectives of this study were to describe five cases of suspected vasopressin-induced hyponatremia after aSAH and to review the literature.

DESIGN

Single-center, observational case series of intensive care unit (ICU) patients.

SETTINGS

Ten-bed neurological ICU at Maine Medical Center in Portland, Maine.

PATIENTS

Convenience sample of patients with aSAH treated with a vasopressin for symptomatic, radiologically confirmed vasospasm.

RESULTS

A total of five patients were included in the case series with a median age of 57 (51, 65) years and all were women. The median Glasgow coma scale score was 15 (11, 15) on admission, and the Hunt and Hess scale score was 3, (3, 4). All patients were treated with endovascular coiling of their aneurysm. Vasopressin was administered to treat symptomatic, radiographically confirmed vasospasm on median post-bleed day (PBD) 10 (10, 15) at a fixed-dose of .03 units/min. Serum sodium at baseline was 140 (140, 144) mEq/L and decreased to 129 (126, 129) mEq/L within 26 (17, 83) hours of vasopressin initiation for a median change of -16 (-10, -16) mEq/L. Serum sodium returned to baseline within 18 (14, 22) hours of stopping the infusion.

CONCLUSIONS

Vasopressin use in vasospasm after aSAH may be associated with clinically significant hyponatremia within 24 hours of starting the infusion. Hyponatremia appears to resolve within 24 hours of stopping the infusion. Additional study in a larger sample size is needed to determine if a causal relationship exist.

Compromised regulation of the rat brain parenchymal arterioles in vasopressin-associated acute hyponatremia

OBJECTIVE

In this study, we examined the effect of acute hyponatremia associated with vasopressin (AVP) on the responses of the isolated rat's MCAs and PAs to acidosis, nitric oxide donor (SNAP) and to endothelium-dependent vasodilator ATP.

METHODS

The studies were performed on isolated, perfused and pressurized MCAs and PAs in control conditions and during AVP-associated hyponatremia. Hyponatremia was induced in vitro by lowering Na⁺ concentration from 144 to 121 mmol/L in intra- and extravascular fluid in the presence of AVP.

RESULTS

Parenchymal arterioles showed greater response to an increase in H⁺ and K⁺ ions concentration and to ATP in comparison with MCAs in control normonatremic conditions. Both PAs and MCAs constricted in response to acute hyponatremia associated with AVP. Interestingly, disordered regulation of vascular tone was observed in PAs but not in MCAs. The abnormalities in the regulation comprised a significant reduction of PA response to acidosis and the absence of the response to the administration of SNAP or ATP.

CONCLUSIONS

Arginine vasopressin-associated hyponatremia leads to constriction and dysregulation of PAs which may impair neurovascular coupling.