Fetal nucleated red blood cells in a rat model of intrauterine growth restriction induced by hypoxia and nitric oxide synthase inhibition.

Abstract

Nucleated red blood cells (NRBCs) in fetal circulation have been proposed as a marker of chronic hypoxia in fetuses with intrauterine growth restriction (IUGR). We sought to determine the effects of chronic hypoxia, chronic nitric oxide inhibition with N(G)-nitro-L-arginine methyl ester (L-NAME), or both on NRBC counts, erythropoietin levels, and pathologic changes in an animal model of IUGR. We assigned timed pregnant adult Sprague Dawley rats to the following groups: (1) 21% oxygen + saline solution (n = 7); (2) 21% oxygen + L-NAME (n = 8); (3) 10% oxygen + saline solution (n = 6); and (4) 10% oxygen + L-NAME (n = 6). We inserted osmotic pumps that were prefilled with saline solution or L-NAME subcutaneously on day 17 of gestation. The animals were placed in a Plexiglas hypoxic chamber, which ensured a constant hypoxic environment. The animals were killed on day 21 of gestation before the onset of spontaneous labor. We collected maternal and fetal blood for measurement of NRBC and erythropoietin levels. The results were interpreted in relationship to maternal arterial blood gases and hemoglobin and hematocrit levels. Fetuses were examined for gross abnormalities and histological abnormalities that are characteristic of vascular disruptions by a blind examiner to experimental manipulation. Nitric oxide inhibition induced IUGR with maximal effect when both L-NAME and hypoxia treatments were combined. Inhibition of nitric oxide synthesis, but not chronic hypoxia, increased the number of fetal NRBCs and generalized hemorrhagic diathesis in utero. These features were aggravated significantly when the treatments were combined. Moreover, chronic hypoxia induced significant maternal metabolic acidosis and increased hematocrit and erythropoietin levels in maternal and fetal blood. Nitric oxide inhibition increased maternal hematocrit levels while decreasing maternal erythropoietin levels without significantly altering the maternal acid-base status. In contrast with chronic hypoxia, nitric oxide inhibition increased fetal NRBCs without affecting erythropoietin levels. Our findings indicate that the number of NRBCs in fetal circulation does not serve as a specific marker of chronic hypoxia that accompanies IUGR or of elevated erythropoietin levels but are an epiphenomenon that is related to the inhibition of nitric oxide.