Pregnancy alters nitric oxide synthase and natriuretic peptide systems in the rat left ventricle

Cyclic guanosine monophosphate (cGMP), which is implicated in cardiac cell growth and function, is synthesized by cytoplasmic soluble guanylyl cyclase (GC) stimulated via nitric oxide (NO) and by particulate membrane-bound GC activated via natriuretic peptides. We investigated possible cGMP elevation in the left ventricle (LV) of rats developing physiologic LV hypertrophy during gestation. Furthermore, expression of estrogen receptors (ER) and oxytocin receptors (OTR) was evaluated because their activation stimulates NO and atrial natriuretic peptide (ANP) release from the heart. Compared with nonpregnant controls, Sprague-Dawley rats on day 7 of gestation had similar heart weights, but, on days 14 and 21, ventricular mass increased by 12% and 28% respectively (P< 0.05). LV cGMP concentration was elevated at day 14 of gestation (3.25 +/- 0.12 vs 4.65 +/- 0.17 pmol/g wet weight, P< 0.01) but decreased at day 21 (2.45 +/- 0.09 pmol/g, P< 0.05) to increase again on postpartum day 1 (6.01 +/- 0.15 pmol/g) and day 4 (9.21 +/- 1.79 pmol/g). Changes in endothelial nitric oxide synthase (eNOS), inducible NOS (iNOS), OTR and ERalpha, but not ERbeta, proteins paralleled the pregnancy-related cGMP changes in the LV. In contrast, ANP mRNA of the LV remained at control level throughout gestation but increased postpartum, whereas brain natriuretic peptide (BNP) expression declined at term and increased postpartum. The particulate GC natriuretic peptide receptors (GC-A and GC-B) transcripts were already lower at day 14 of gestation. Natriuretic peptide clearance receptor (NPR-C) transcript was not altered on days 7 and 14, but increased at term. We conclude that cGMP concentration in the rat LV is influenced by both NOS and natriuretic peptide systems and may be involved in the changes of LV contractility and hypertrophy that occur during rat gestation.