Immunoreactive erythropoietin studies in hypoxic rats and the role of the salivary glands.

Rat erythropoietin (Ep) cross-reacts in the radioimmunoassay (RIA) for human Ep developed in this laboratory. Immunoreactive Ep was measured in serum and tissues of male rats in response to short-term hypoxia (0.43 atm for 24 h). In the unstimulated rat all tissues examined had low levels of Ep, with the exception of the submaxillary or salivary gland (SG). Exposure to hypoxia for 24 h resulted in significant increases in kidney and serum levels of Ep, with no apparent change in SG content. Sialectomy immediately prior to exposure reduced renal Ep production and serum levels significantly after 4 h of exposure. Nephrectomy (N) confirmed previous results by others: Ep production after exposure to hypoxia was reduced but not abolished. The effect of N plus sialectomy was identical to that of N alone, thus excluding the SG as a source of extrarenal Ep in nephrectomized rats. The long-term effect of SG ablation to the same constant stimulus was a steady decline of the Ep response during the first week after surgery, both in renal production and serum levels. Thereafter, from one to six weeks the serum levels remained constant, being higher than in the unstimulated rat but significantly lower than in intact hypoxic animals. No cross-reactivity in the RIA was found with renin, renin substrate, nerve and epidermal growth factor, or somatomedins. If this Ep-like substance in the SG were the source of extrarenal Ep, it should have been possible to document an increase in serum concentration before an increase could be measured in renal content. It appears, however, that the presence of the SG is necessary for renal tissue to be able to synthesize Ep during hypoxia.

Immunoreactive erythropoietin concentrations in fetal and neonatal rats and the effects of hypoxia.

Immunoreactive erythropoietin (Ep) was measured in normoxic and hypoxic (0.5 atm; 18 hours) fetal rats from day 14 to day 21 of gestation and in neonatal rats from birth to weaning, and was compared to the adult rat. Amniotic fluid (AF) Ep was approximately 100 mU/mL on day 14 and 15, and decreased to 20 mU/mL on day 20, with no difference between the hypoxic and normoxic mothers. Only on day 21 did the Ep in the AF increase slightly in the hypoxic group, while the Ep in the control group continued to fall to 15 mU/mL on day 21, the last day of pregnancy. Before day 17 of gestation the rat fetus appears to have hypoxia-independent, extrahepatic Ep available which is followed by hepatic and renal Ep production, both of which become sensitive to maternal hypoxia during the last days of pregnancy. In the neonatal rat plasma and tissue, Ep levels varied greatly during the first three weeks of life regardless of whether the animals were hypoxic or not. With the exception of the first and ninth days of life, circulating Ep levels were higher than adult levels in normal newborn rats. Neonatal rats responded to hypoxia with increasing Ep levels, and the response increased with age such that during the third week of life the plasma Ep levels were significantly higher than in adult hypoxic rats. No sex difference in male and female response to hypoxia could be documented until sexual maturity (day 42). In the normoxic neonatal rat more Ep originated from the liver than the kidneys until day 10, while under hypoxic conditions the switch occurred as early as two days after birth.