Impaired cerebral microcirculation induced by ammonium chloride in rats is due to cortical adenosine release

BACKGROUND: Liver failure results in hyperammonaemia, impaired regulation of cerebral microcirculation, encephalopathy and death. However, the key mediator that alters cerebral microcirculation remains unidentified. In this study we show that topical ammonium significantly increases periarteriolar adenosine tone on the brain surface of healthy rats and is associated with a disturbed microcirculation.

METHODS: Cranial windows were prepared in anaesthetized Wistar rats. The flow velocities were measured with speckle contrast imaging and compared before and after 30 min of exposure to 10 mM ammonium chloride applied on the brain surface and compared to control groups exposed to artificial cerebrospinal fluid or ammonium + an adenosine receptor antagonist. A flow preservation curve was obtained by analysis of flow responses to a haemorrhagic hypotensive challenge and during stepwise exsanguination. The periarteriolar adenosine concentration was measured with enzymatic biosensors inserted in cortex.

RESULTS: After ammonium exposure the arteriolar flow velocity increased by 21.7 (23.4)% vs. controls 7.2 (10.2)% (median (IQR), N=10 and 6, respectively p<0.05) and the arteriolar surface area increased. A profound rise in the periarteriolar adenosine concentration was observed. During the hypotensive challenge the flow decreased by 27.8 (14.9)% vs. 9.2 (14.9)% (p<0.05). The lower limit of flow preservation remained unaffected: 27.7 (3.9) mmHg vs. 27.6 (6.4) mmHg whereas the autoregulatory index increased: 0.29 (0.33) flow units/mmHg vs. 0.03 (0.21) flow units/mmHg (p<0.05). When ammonium exposure was combined with a topical adenosine receptor antagonist the autoregulatory index was normalized.

CONCLUSION: Vasodilatation of the cerebral microcirculation during exposure to ammonium chloride is associated with an increase in the adenosine tone. Application of a specific adenosine receptor antagonist restores the regulation of the microcirculation. This indicates that adenosine could be a key mediator in the brain dysfunction seen during hyperammonemia and represents a potential therapeutic target.

LAY SUMMARY: In patients with liver failure disturbances in the brain function is caused in part by ammonia toxicity. In our project we have studied how ammonia, through adenosine release, affects the blood flow in the brain of rats. In our experimental model we demonstrated that the detrimental effect of ammonia on blood flow regulation was counteracted by blocking the adenosine receptors in the brain. With this observation we have identified a novel potential treatment target. If we can confirm our findings in a future clinical study it might help patients suffering from liver failure and the severe condition called hepatic encephalopathy.