Changes in blood and red cell volume in the neonatal lamb and the effect of insulin-like growth factor I.

1. Blood volume was measured weekly using [51Cr]-labelled red cells in 10 lambs from 3 to 10 weeks of age. Red cell and plasma volumes were calculated using the measured blood volume and haematocrit. Other parameters, including plasma erythropoietin, urea, creatinine and glucose, were measured twice weekly. The results were compared to a group of five lambs that received an infusion of insulin-like growth factor I (IGF-I). 2. In control lambs, plasma volume increased linearly by 47 +/- 7 mL/week over the experimental period. Red cell volume only increased by 10 +/- 2 mL/week during weeks 3-7, but then increased by 25 +/- 2 mL/week over weeks 7-10. Haematocrit declined from 28.0 +/- 1.6 to 24.7 +/- 1.7% over weeks 3-7 and then increased to 30.7 +/- 1.1% by week 10. 3. In 10 control lambs infused for 8 days (starting at 22-26 days of age) with 10 mmol/L HCl, there was a decrease in plasma IGF-I concentrations, 3 days after the start of infusion. In five lambs infused for 8 days with IGF-I (6 microg/kg per h) plasma IGF-I concentration was maintained significantly (P <0.01) higher than that of the controls. 4. There was no significant difference in haematocrit, red cell or plasma volumes between the treatment groups and no reticulocytosis was observed. Plasma erythropoietin concentrations did not change over the infusion period in either group. 5. Serum urea decreased significantly in the IGF-I infused group but serum creatinine did not change in either group during the infusion period. In both the groups, there was a significant decrease in glucose, urea and creatinine over weeks 3-10 after birth. There was no difference in growth rates between the two groups. 6. Thus, it appears that the observed changes in haematocrit are due to a constant increase in plasma volume with varying rates of red cell volume increases. 7. IGF-I infused at a dose that maintains physiological concentrations and alters protein metabolism does not result in increased erythropoietin or erythropoiesis during the neonatal period of the lamb.