Temporal pattern of erythropoietin titers in kidney tissue during hypoxic hypoxia

Plasma titers of erythropoietin (Ep) are known to increase initially during hypoxia and to return then towards prehypoxia values. To find out if this pattern of plasma Ep might be related to changes in the production of the hormone, I have compared plasma with kidney Ep titers in hypoxic rats. Rats were exposed to hypoxia in a hypobaric chamber at 0.42 atm for various time intervals for up to 4 days. Kidney Ep titers were assayed in extracts from kidneys that had been flushed free of blood in situ. It was found that kidneys of normal rats do not store significant amounts of Ep. Kidney Ep titers increased transiently during hypoxia. They reached maximum values after 6h and then declined to almost undetectable levels at continued hypoxia. In the plasma, maximum values were found after 12-18h of hypoxia. Additional studies were done on the effects of discontinuous hypoxia. It was found that, even after 3 days of previous hypoxia exposure, plasma and kidney Ep titers increased again in rats when these were maintained intermittently in normoxia for 18 h. It is concluded that the rise and fall in plasma Ep titers during hypoxia reflect similar changes in kidney Ep production.