Objective: Centrally released arginine vasopressin (AVP) and atrial natriuretic peptide (ANP) have been shown to participate in brain volume regulation. The aim of the present study was to evaluate the effects of centrally administered AVP and ANP on the time course of development of brain edema in vivo in hyponatremic rats, using diffusion-weighted magnetic resonance imaging.
Methods: We performed intracerebroventricular (ICV) administration of 120 microg AVP, 20 microg ANP, or physiological saline into the right lateral ventricle in 18 rats. Twenty-five minutes after the treatment, we induced systemic hyponatremia by the intraperitoneal administration of 140 mmol/L dextrose solution. Serial diffusion-weighted imaging scans were obtained up to 96 minutes after the start of the hyponatremia. Changes in the brain extra-to intracellular volume fraction ratio were estimated as changes in the apparent diffusion coefficient (ADC).
Results: No change in the ADC was observed after the ICV injection of saline or AVP. The onset of hyponatremia induced a rapid and marked ADC reduction in both groups, indicating an increased intracellular space. However, the ADC decrease became significantly more pronounced in the ICV AVP group (83.3+/-4.7% of baseline level, mean +/- standard deviation) than in the saline group (93.7+/-3.3% of baseline, P < 0.001) after 78 minutes of hyponatremia. The ICV injection of ANP induced a prompt ADC increase to 111.5+/-10.0% (P < 0.05) of the baseline level, indicating a rapid reduction in the intracellular compartment. In the initial phase of hyponatremia, the ADC values in the ANP group were consistently higher than those in the saline group, decreasing finally to 86.9+/-9.6% after 96 minutes of hyponatremia.
Conclusion: Our findings demonstrate the opposite effects of AVP and ANP on the intracellular volume fraction of the brain during the development of cellular brain edema, with an immediate effect on ANP and a delayed effect on AVP. The results emphasize the direct effects of these hormones on the cellular volume regulatory mechanisms in the brain during the development of cerebral edema.