Marked variation in responses to long-term nitric oxide inhibition during pregnancy in outbred rats from two different colonies

OBJECTIVE: Some but not all studies have shown that long-term nitric oxide synthase inhibition during pregnancy induces symptoms similar to those of preeclampsia that include hypertension, proteinuria, and intrauterine growth restriction. This study was undertaken to compare the effects of long-term nitric oxide synthase inhibition during pregnancy on blood pressure and fetal weight between Sprague-Dawley rats from outbred colonies of two different suppliers. STUDY DESIGN: Osmotic minipumps were inserted on day 10 or day 17 of pregnancy in Sprague-Dawley rats obtained from Charles River Laboratories, Inc, Wilmington, Mass, or Harlan Sprague Dawley, Inc, Indianapolis, Ind. The pumps were set to deliver vehicle only (control group) or Nw-nitro-L-arginine methyl ester (a nitric oxide synthase inhibitor) at a rate of 50 mg/d until postpartum day 7. Systolic blood pressures were measured daily with the tail-cuff method. Neonatal weights and survival were recorded. RESULTS: Nw-nitro-L-arginine methyl ester infusion initiated on gestational day 10 increased blood pressure relative to control levels in all rats studied. Harlan rats were much more sensitive to the hypertensive effect of Mw-nitro-L-arginine methyl ester. When Nw-nitro-L-arginine methyl ester infusion was initiated on gestational day 17, blood pressure increased only in Harlan rats. Pups born to Harlan rats treated with N omega -nitro-L-arginine methyl ester had lower birth weights and a higher stillbirth rate than did pups of Charles River rats. The degree of hypertension was significantly correlated with the deleterious effects of Nw-nitro-L-arginine methyl ester on the fetuses. CONCLUSION: Within the same strain of rats the effects of long-term nitric oxide synthase inhibition on blood pressure and fetal outcome depended on the original animal colony, with animals from Harlan Sprague Dawley being more sensitive than those from Charles River Laboratories. This difference in response between animals from different suppliers is most likely caused by genetic differences inbred into the strain. In addition to explaining some of the reported inconsistencies between laboratories, these results may also provide insights into the genetic basis of preeclampsia.