Age-related difference in the response of plasma erythropoietin to hemorrhage in ovariectomized rats

The effect of aging on plasma erythropoietin (EPO) levels after acute hemorrhage was investigated in ovariectomized (Ovx) rats. Old (22 months), middle (mid)-aged (14 months) and adult (5-6 months) Ovx rats were studied. Rats were anesthetized with ether and bled by heart puncture (5 ml/kg). They were bled again after 1 h. The hematocrit and concentration of plasma EPO were determined for both bleedings. The prehemorrhage level of hematocrit was not altered by aging. The hematocrit level after hemorrhage was greater in old than in mid-aged (p < 0.05) and adult (p < 0.01) rats. The basal level of plasma EPO in old rats was higher than that in the adult rats (p < 0.05), but not in the mid-aged rats. The concentration of plasma EPO in response to hemorrhage was increased in all rats (p < 0.01). The hemorrhage-induced increase in plasma EPO was significantly greater in adult rats than in both mid-aged and old rats (p < 0.05). Our findings indicate that the basal level of plasma EPO is increased, but the hemorrhage-induced EPO secretion is diminished in ovariectomized rats during aging. These data then suggest that the tolerance to hemorrhage and the secretory function of EPO are changed by age.