Altered regulation of natriuretic peptides in the rat heart by prenatal exposure to morphine

Abstract

1. Both endogenous and exogenous opioids modulate blood pressure and cardiac function by stimulating cardiac synthesis of atrial natriuretic factor (ANF) and brain natriuretic peptide (BNP). Since morphine crosses the placental barrier, it could alter the ANF-BNP system in the fetal heart. The aim of this study was to characterize cardiac natriuretic peptides in normal rat development and in rats prenatally exposed to morphine. 2. Female rats received either saline or morphine (10 or 20 mg kg-1 day-1) via osmotic minipumps during gestation. The effects of this treatment were investigated in offspring at 1, 4 and 22 days of age. 3. During maturation, atrial ANF and ANF mRNA increased by 3-fold from birth to 3 weeks of age, but BNP and BNP mRNA tended to decrease. In the ventricles, both ANF and BNP content decreased at 3 weeks after birth, from 25.11 +/- 3.6 to 0.81 +/- 0.1 ng (mg protein)-1 (P < 0.001), and from 3.36 +/- 0.33 to 0.19 +/- 0.01 ng (mg protein)-1 (P < 0.001), respectively. However, whereas ventricular ANF mRNA decreased, BNP mRNA levels did not change during maturation. Prenatal exposure to morphine significantly increased ANF content in the left atria of 22-day-old rats, and in the right atria of 1-, 4- and 22-day-old rats compared with age-matched saline controls. In contrast, prenatal exposure to 20 mg kg-1 day-1 morphine significantly inhibited BNP and BNP mRNA in the ventricles at all ages studied. 4. These observations suggest that alterations in mRNA synthesis or stability and/or post-translational processing of ANF and BNP occur in the heart during maturation, and that prenatal exposure to morphine alters cardiac production, and possibly release, of both peptides.

Figure 1. Effect of prenatal treatment with morphineon ventricular levels of atrial natriuretic factor (ANF) and brain natriuretic peptide (BNP)

Rats were treated prenatally with morphine (20 mg kg⁻¹ day⁻¹ and 10 mg kg⁻¹day⁻¹) or saline. Subsequent experiments were performed after birth in 1-, 4- and 22-day-old rats (n= 16 in each group). *P < 0.05, **P < 0.001, 4- or 22-day-old vs. 1-day-old rats; †P < 0.05, ††P < 0.001, morphine-treated vs. corresponding saline-treated controls.

Figure 2. Northern blot of ANF mRNA and BNP mRNA from left atria of rats prenatally treated with morphine (M; 20 mg kg⁻¹ day⁻¹) or saline (S) vehicle

Experiments were performed on 1-, 4- and 22-day-old rats (n= 4 in each group). Representative bands of the mRNA hybridized to ³²P-labelled cDNA probes are shown vs. electrophoretic position of ribosomal 18 S RNA. The PhosphorImager scan of 0.9 kb ANF or 0.9 kb BNP bands was normalized to the signal obtained with 1.6 kb tubulin bands.

Figure 3. Northern blot analysis of ventricular ANF and BNP mRNA

A and B show the effect of prenatal treatment of rats with morphine (20 mg kg⁻¹ day⁻¹ and 10 mg kg⁻¹ day⁻¹) or saline vehicle on ANF and BNP mRNA, respectively. Representative bands of hybridized mRNA are presented in C. Experiments were performed after birth in 1-, 4- and 22-day-old rats (n= 8 in each group). Values represent means ±s.e.m.; *P < 0.05, **P < 0.001, 4- or 22-day-old vs. 1-day-old rats; †P < 0.05, ††P < 0.001, morphine treatment vs. corresponding saline-treated controls.