Glucose metabolism in the newborn rat. Temporal studies in vivo.

1. The concentrations of plasma d-glucose, l-lactate, free fatty acids and ketone bodies and of liver glycogen were measured in caesarian-delivered newborn rats at time-intervals up to 4h after delivery. Glucose and lactate concentrations decreased markedly during the first hours after delivery, but there was a delay of 60-90min before significant glycogen mobilization occurred. 2. The specific radioactivity of plasma d-glucose was measured as a function of time for up to 75min after the intraperitoneal injection of d-[6-(14)C]glucose and d-[6-(3)H]glucose into caesarian-delivered rats at 0, 1 and 2h after delivery. Calculations revealed that there was an appreciable rate of glucose formation at all ages studied, but immediately after delivery this was exceeded by the rate of glucose utilization. Around 2h post partum the rate of glucose utilization decreased dramatically and this coincided with a reversal of the immediately postnatal hypoglycaemia. 3. The specific radioactivity of plasma l-lactate and the incorporation of (14)C into plasma d-glucose and liver glycogen was measured as a function of time after the intraperitoneal injection of l-[U-(14)C]lactate into rats immediately after delivery. The logarithm of the specific radioactivity of plasma l-[U-(14)C]lactate decreased linearly with time for at least 60min after injection and the calculated rate of lactate utilization exceeded the rate of lactate formation. 4. (14)C incorporation into plasma d-glucose was maximal from 30-60min after injection of l-[U-(14)C]lactate and the amount incorporated at 60min was 23% of that present in plasma lactate. Although (14)C was also incorporated into liver glycogen the amount was always less than 3% of that present in plasma glucose. 5. The results are discussed in relationship to the adaptation of the newly born rat to the extra-uterine environment and the possible involvement of gluconeogenesis at this time before feeding is established.