Changing basal and stimulated activity of the renin-angiotensin system during the second half of gestation in the anaesthetized piglet.

Concentrations of components of the renin-angiotensin system were studied in eighty anaesthetized pig fetuses aged between 57 and 112 d of gestation (term = 114 +/- 1 d). Plasma renin concentration rose from a mean value of 3.2 +/- 0.4 ng X ml-1 X h-1 (mean +/- S.E.M.) at 57 d to 17.5 +/- 2.7 ng X ml-1 X h-1 at 93 d of gestation and thereafter declined to 5.0 +/- 1.6 ng X ml-1 X h-1 just prior to term. There was no distinct pattern of change in plasma renin substrate concentrations during gestation; the concentration near term (0.33 +/- 0.10 micrograms X ml-1) was not different from that at 57 d (0.50 +/- 0.18 micrograms X ml-1) or that at 93 d (0.31 +/- 0.08 micrograms X ml-1) of gestation. Plasma angiotensin II concentrations increased from 33 (log10 (mean +/- S.E.M.) = 1.554 +/- 0.050) pg X ml-1 at 60 d to 232 (2.365 +/- 0.225) pg X ml-1 at 105 d of gestation, after which it decreased to 50 (1.699 +/- 0.100) pg X ml-1 in 20-d-old neonatal piglets. Haemorrhage of 10-20% estimated blood volume produced no consistent changes in plasma renin substrate concentrations, and inconsistent changes in renin and angiotensin II concentrations at 85-90 d of gestation, but a consistent increase in both renin and angiotensin II concentrations thereafter. These results indicate that the components of the renin-angiotensin system are present in the circulation of the pig fetus, and that their responses to stimuli develop during the second half of gestation.