HEPES may stimulate cultured endothelial cells to make growth-retarding oxygen metabolites.

Zwitterion buffers are often used to modulate the pH of cell culture medium but their effect on cultured cells is controversial. We found that addition of 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) caused superoxide dismutase (SOD) inhibitable increases in nitroblue tetrazolium dye reduction and SOD and catalase inhibitable decreases in the growth of cultured bovine pulmonary artery endothelial cells. The findings suggest that HEPES stimulates endothelial cells to make toxic oxygen metabolites that contribute to decreased cell growth.