Vasopressin Antagonism

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

Hyponatraemia, a water-electrolyte disorder diagnosed in patients with subarachnoid haemorrhage (SAH), increases a risk of persistent vasospasm. In majority of cases, hyponatraemia results from inappropriate secretion of vasopressin (AVP). The effect of AVP-associated hyponatraemia on cerebral vasculature is unknown. The present study aimed to elucidate the role of AVP in the response of the middle cerebral artery (MCA) of the rat to hyponatraemia. Isolated, cannulated, and pressurized rat MCAs were perfused/superfused with physiological (Na⁺  = 144 mmol/L) buffer or low-sodium (Na⁺  = 121 mmol/L) buffer containing either AVP or angiotensin II (ANG II). ANG II was used to check if the effect of low plasma sodium concentration combined with AVP on the MCA tone is unique to vasopressin. At physiological Na⁺ concentration, vasopressin (1.4 × 10^-11  mol/L) or angiotensin II (10^-9  mol/L) resulted in relaxation of the MCA. Substitution of low-sodium for the normal sodium buffer with the same concentration of AVP, resulted in the constriction of the MCA. This effect was absent after removal of the endothelium, administration of vasopressin V₁ receptor antagonist or concomitant inhibition of endothelin-1 receptors and synthesis of thromboxane A_2. In contrast, no constriction of the MCA in low-sodium buffer was observed when AVP was replaced with ANG II. Our data suggest that presence of vasopressin and low sodium ion concentration results in the change of endothelium phenotype from pro-vasodilatory to pro-vasoconstrictory. This phenomenon may be an overlooked factor contributing to vasospasm in SAH patients with hyponatraemia caused by inappropriate antidiuretic hormone secretion (SIADH).

Effect of vasopressin-induced chronic hyponatremia on the regulation of the middle cerebral artery of the rat

Vasopressin (arginine vasopressin, AVP) plays a crucial role in maintaining body fluid homeostasis. Excessive release of vasopressin can lead to hyponatremia. Changes in cerebral circulation during vasopressin-induced chronic hyponatremia are not elucidated. The present study has been designed to investigate the effect of chronic vasopressin-induced hyponatremia on the regulation of the tone of the middle cerebral artery (MCA) of the rat. Chronic hyponatremia was induced in vivo with the help of vasopressin, released continuously from subcutaneously implanted ALZET mini-osmotic pumps, and a liquid diet. After 3.5 days of chronic hyponatremia, the plasma Na⁺ concentration decreased to 119 ± 3 mM. MCAs were isolated and placed in a MOPS-buffered saline solution containing 121 mM Na⁺. Chronic hyponatremia did not affect the response of the MCA to increased intravascular pressure, to the administration of acetylcholine (ACh) and nitric oxide (NO) donor (SNAP, S-nitroso-N-acetyl-DL-penicillamine), and to increased K⁺ concentration, but impaired the response of the MCA to increased extravascular H⁺ concentration. Disturbed response of the MCA to acidosis was associated neither with the impairment of K_ATP channels nor with the activation of vasopressin V₁ receptor. Correction of hyponatremia did not restore the response of the MCA to acidosis. These results indicate that cerebral blood vessels do not fully adapt to prolonged vasopressin-induced hyponatremia.

Restoration of the response of the middle cerebral artery of the rat to acidosis in hyposmotic hyponatremia by the opener of large-conductance calcium sensitive potassium channels (BKCa)

Hyposmotic hyponatremia (the decrease of extracellular concentration of sodium ions from 145 to 121 mM and the decrease of hyposmolality from 300 to 250 mOsm/kg H₂O) impairs response of the middle cerebral artery (MCA) to acetylcholine and NO donor (S-nitroso-N-acetyl-DL-penicillamine). Since acidosis activates a similar intracellular signaling pathway, the present study was designed to verify the hypothesis that the response of the MCA to acidosis is impaired during acute hyposmotic hyponatremia due to abnormal NO-related signal transduction in vascular smooth muscle cells. Studies performed on isolated, cannulated, and pressurized rat MCA revealed that hyposmotic hyponatremia impaired the response of the MCA to acidosis and this was associated with hyposmolality rather than with decreased sodium ion concentration. Response to acidosis was restored by the BK_Ca but not by the K_ATP channel activator. Patch-clamp electrophysiology performed on myocytes freshly isolated from MCAs, demonstrated that hyposmotic hyponatremia does not affect BK_Ca currents but decreases the voltage-dependency of the activation of the BK_Ca channels in the presence of a specific opener of these channels. Our study suggests that reduced sensitivity of BK_Ca channels in the MCA to agonists results in the lack of response of this artery to acidosis during acute hyposmotic hyponatremia.

Intensive care unit management of aneurysmal subarachnoid hemorrhage

The emergence of dedicated neurologic-neurosurgical intensive care units, advancements in endovascular therapies, and aggressive brain resuscitation and monitoring have contributed to overall improved outcomes for patients with aneurysmal subarachnoid hemorrhage (aSAH) over the past 20 to 30 years. Still, this feared neurologic emergency is associated with substantial mortality and morbidity. Emergency care for patients with aSAH focuses on stabilization, treatment of the aneurysm, controlling intracranial hypertension to optimize cerebral perfusion, and limiting secondary brain injury. This complex disorder can be associated with many neurologic complications such as acute hydrocephalus, rebleeding, global cerebral edema, seizures, vasospasm, and delayed cerebral ischemia in addition to systemic complications such as electrolyte imbalances, cardiopulmonary injury, and infections. Background routine intensive care practices such as avoidance of hyperthermia, venous thromboembolism prophylaxis, and avoidance of severe blood glucose derangements are additional important elements of care.

Hyponatremia in patients with neurologic disorders

The kidney and the brain play a major role in maintaining normal homeostasis of the extracellular fluid by neuroendocrine regulation of sodium and water balance. Therefore, disturbances of sodium balance are common in patients with central nervous system (CNS) disorders and clinicians should focus not only on the CNS lesion, but also on the potentially deleterious complications. Hyponatremia is the most common and important electrolyte disorder affecting patients with critical neurologic diseases. In these patients, the maladaptation to hyponatremia by impaired osmoregulation in pathologic lesions of brain may cause more aggressive cerebral edema and increased intracranial pressure due to hypoosmolality induced by hyponatremia. Furthermore, hyponatremia accompanied by CNS disorders has shown to increase delayed cerebral ischemia and mortality rates. Two main pathophysiologies of hyponatremia, excluding iatrogenic causes, are inappropriate secretion of antidiuretic hormone (SIADH) and cerebral salt wasting (CSW) syndrome. Differential diagnosis between these two entities can be difficult due to considerable overlap in the laboratory findings and clinical situations. SIADH is in a volume expanded status due to inappropriately secreted arginine vasopressin (AVP) and requires water restriction. However, CSW syndrome is characterized by renal sodium wasting mainly due to increased natriuretic peptides resulting in volume depletion and follows appropriate secretion of AVP. Therefore, maintenance of volume status and sodium replacement is the mainstay of treatment in CSW syndrome. In this review, we aimed to describe the regulation of sodium and water balance, and pathophysiology, diagnosis and treatment of hyponatremia in neurologic patients, especially focusing on SIADH and CSW syndrome.

Conivaptan bolus dosing for the correction of hyponatremia in the neurointensive care unit

INTRODUCTION

Hyponatremia frequently complicates acute brain injury and may precipitate neurological worsening by promoting cerebral edema. An increase in brain water may be better managed through water excretion than with fluid restriction or hypertonic fluids. Vasopressin-receptor antagonists such as conivaptan, which promote free water excretion, may be ideal agents to treat this common and potentially serious disorder.

METHODS

The efficacy of intermittent bolus doses of conivaptan to correct hyponatremia was examined in a consecutive series of patients treated in our neurointensive care unit. Patients were excluded if baseline sodium was over 135 mEq/l or if another conivaptan dose was given within 12 h. We assessed the proportion responding with a 4 or 6 mEq/l rise in sodium by 12 h, the change in sodium from baseline, and, in those not receiving another dose for at least 72 h, the long-term ability of a single dose to maintain sodium at least 4 mEq/l above baseline. We also recorded the effects of conivaptan on urine output and specific gravity, and noted any adverse events.

RESULTS

A total of 25 doses given to 19 patients were included (out of 44 total doses administered in the study period). Sodium rose by 5.8 +/- 3.2 mEq/l within 12 h, with 71% rising by at least 4 mEq/l and 52% manifesting at least a 6 mEq/l increase. In those receiving only a single dose, 69% maintained at least a 4 mEq/l rise up to 72 h. Conivaptan also consistently led to increased urine output and a significant drop in urine specific gravity (i.e., aquaresis). No cases of phlebitis were observed despite administration of conivaptan through peripheral IVs.

CONCLUSION

Intermittent dosing of conivaptan was effective in increasing free water excretion and correcting hyponatremia in neurologically ill patients. This supports its further evaluation for managing hyponatremia in this population.