Erythropoietin (EPO) is mainly produced in the kidneys and is regulated by blood oxygen availability. Studies with isolated perfused kidneys have established that an oxygen-sensing system exists intrarenally but the mechanisms involved are poorly understood. Using a quantitative RNase protection assay, we have demonstrated oxygen-dependent EPO mRNA production in isolated perfused rat kidneys, with EPO mRNA levels increasing 30-fold when perfusate pO2 was reduced from 474 to 25 mm Hg. To determine if the high amplitude changes in EPO mRNA levels in response to hypoxia are mediated by cyclic AMP, four agents, which activate the cyclic AMP system in different ways, were administered to isolated kidneys perfused over a range of perfusate pO2. Salbutamol and N6-ethyl carboxamidoadenosine, which activate adenylate cyclase, dibutyryl cyclic AMP (a cyclic AMP analogue) and forskolin did not augment EPO mRNA production, and no significant differences in the regression of log (EPO mRNA) on perfusate pO2, were found between experimental groups exposed to each of these compounds and controls. We conclude that the rapid increase in EPO mRNA levels in response to hypoxia is not mediated or substantially modulated by a cyclic AMP-dependent mechanism.
