Mountain climbers tend to acclimatize to high altitudes faster during a second ascent than during the first. Red blood cells have a role in this, 'remembering' how they initially adapted to the low-oxygen conditions.

Yang Xia at the University of Texas Health Science Center in Houston and her colleagues studied mice exposed to low oxygen, and healthy volunteers raised in lowland areas who climbed Mount Chacaltaya in Bolivia, which has an elevation of more than 5,000 metres. Altitude raised blood levels of adenosine, which dilates blood vessels, boosting blood flow. The team found that adenosine also helps to degrade a protein called eENT1 in red blood cells in humans and mice, causing increased production of adenosine during the first exposure to low oxygen.

Red blood cells exposed to low oxygen levels a second time had maintained the low levels of eENT1, allowing them to make adenosine faster, and in larger amounts, than during the first exposure.

Nature Commun. 8, 14108 (2017)