Objectives: Hyponatremia is diagnosed in approximately 30% of neurosurgical patients after head trauma, subarachnoid or intracerebral haemorrhage. It is well documented that hyponatremia deteriorates the general state of the patients, aggravates brain damage and increases mortality. Its influence on the regulatory mechanisms of cerebral blood vessels is unknown. The aim of our experiments was to study the impact of acute and chronic hyponatremia on the regulation of the isolated middle cerebral artery (MCA) of the rat. Material and Methods: Seventy five vessels were studied in the organ chamber. Acute hyponatremia (AH, 120mM Na+) was induced in vitro 1 hour prior to the study of MCA reactivity. Chronic hyponatremia (CH) was induced in vivo with a help of vasopressin and liquid diet (AIN-76). Vasopressin was delivered continuously from subcutaneously implanted ALZET osmotic minipumps. After 3.5 days, plasma Na+ concentration ranged from 114 to 122 mM in these rats. MCAs were isolated and studied in the organ chamber containing 120 mM Na+ buffer. MCAs placed in normonatremic bath (Na+=144 mM) served as a reference group. The reactivity tests comprised responses to acidosis (pH=7.0), to hyperkalemia (20 mM K+) and to changes in perfusion pressure. Results: In normonatremia MCA dilated on average by 19±2% (p<0.05) during decrease of pH from 7.4 to 7.0 and by 30±3% (p<0.05) during increase of K+ concentration in the bath from 3.5 to 20 mM. Hyponatremia impaired reactivity of the MCA to both acidosis and hyperkalcemia. There were differences in the severity of this impairment depending on the duration of hyponatremia. During AH decrease of pH induced constriction of the MCA by 13±3% (p<0.05) whereas during CH constriction was only 4±1% (p<0.05 vs. AC and normonatremia). The impairment of the response to 20 mM K+ (constriction by 18±2%, p<0.05 vs. normonatremia) was observed only during AH. The response to changes in perfusion pressure was well preserved both during AH and CH. Conclusion: Our results demonstrate for the first time that hyponatremia selectively disturbs the regulatory mechanisms of cerebral blood vessels. They also show that acute hyponatremia impairs regulation of the MCA more than chronic hyponatremia. In conclusion, vascular impairment may be an important component of intracranial pathology during hyponatremia. Acknowledgements: This study was financially supported by the grant N401 19032/3924 from the Ministry of Science and Higher Education.
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