For women in developing countries where parenteral oxytocics are not available, nipple stimulation might reduce the incidence of postpartum hemorrhage. A larger trial now seems warranted.
The effect of infused atrial natriuretic peptide (ANP) on sodium excretion (UNa), glomerular filtration rate (GFR), and effective renal plasma flow (ERPF) was studied in 12 normotensive primigravidae at 32 wk gestation [late pregnancy (LP)] and again 4 mo postpartum [nonpregnant (NP)]. Three 20-min steady-state (renal) clearances of inulin and p-aminohippurate were used to measure GFR and ERPF, respectively, before and after infusion of ANP at 2 pmol.kg-1.min-1. Basal plasma ANP (pANP) was increased in LP compared with NP [7.8 +/- 0.6 vs. 3.3 +/- 0.4 pmol/l (P < 0.0001), respectively]. In LP, infusion of ANP increased pANP from 7.8 +/- 0.6 to 21.8 +/- 1.4 pmol/l (P < 0.00001), which produced a natriuresis [UNa of 0.18 +/- 0.02 vs. 0.25 +/- 0.03 mmol/min (P = 0.03), respectively], with no change in GFR (153 +/- 13 vs. 142 +/- 8 ml/min, P = 0.16) but a significant reduction in ERPF (766 +/- 52 vs. 660 +/- 31 ml/min, P = 0.002). In NP, ANP infusion increased pANP from 3.3 +/- 0.4 to 27.7 +/- 2.5 pmol/l (P < 0.00001), which produced no significant natriuresis [UNa of 0.22 +/- 0.07 vs. 0.26 +/- 0.09 mmol/min (P = 0.15), respectively] and no change in GFR (87 +/- 3 vs. 89 +/- 3 ml/min), but again a reduction in ERPF (486 +/- 17 vs. 414 +/- 9 ml/min, P < 0.001).
To further elucidate the role of atrial natriuretic peptide (ANP) in preeclampsia, its metabolic clearance (MCRANP) was determined concomitantly with its effects on sodium excretion (UNa), glomerular filtration rate (GFR), and effective renal plasma flow (ERPF). Ten untreated preeclamptic primigravidae (PET) were studied at 29-37 wk gestation and again 4 mo postpartum (PP). Basal plasma concentration of ANP was significantly increased in PET compared with PP (14.8 +/- 1.9 vs. 4.1 +/- 0.5 pmol/l, respectively; P < 0.0001). MCRANP in PET and PP was 5.0 +/- 0.8 and 4.9 +/- 0.5 l/min [not significant (NS)], respectively. In PET, infusion of ANP produced (basal vs. ANP) a natriuresis (UNa 0.14 +/- 0.02 vs. 0.28 +/- 0.04 mmol/min, P < 0.001) and an increase in GFR (97 +/- 7 vs. 106 +/- 8 ml/min, P < 0.05), with ERPF unchanged (609 +/- 24 vs. 634 +/- 29 ml/min, NS). In PP, ANP infusion also produced a natriuresis (UNa 0.20 +/- 0.02 vs. 0.25 +/- 0.02 mmol/min, P = 0.01), no significant change in GFR (109 +/- 7 vs. 102 +/- 4 ml/min), and a significant reduction in ERPF (514 +/- 22 vs. 409 +/- 18 ml/min, P < 0.0001). Analysis of variance demonstrated a greater natriuretic effect of ANP in PET compared with PP (P < 0.05), similarly a significant difference in the effect of ANP on ERPF (P < 0.01) and GFR (P < 0.05) was seen but not on filtration fraction (P = 0.35).
This study confirms that suckling is a potent stimulus to thirst in the mother, and is not associated with vasopressin release or dependent on any measurable alterations in osmoregulation. What actually stimulates thirst during breast feeding remains unknown, but there are two potential explanations for these observations: (1) suckling sends nerve impulses to the paraventricular and supraoptic nuclei in the hypothalamus which may have afferents within the central nervous system which stimulates a thirst response simultaneous with oxytocin release; (ii) a learned anticipation of thirst may be occurring in a situation associated with expectant fluid loss to preserve homeostasis.
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