Relationships between glucose, insulin and glucagon during fasting in late gestation in the rat

Plasma glucose, insulin and glucagon were measured in pregnant and age-matched virgin rats in the fed state and after fasting 6, 48 or 120 hours during day 16-21 of gestation. The fed state in pregnancy was characterized by a metabolic setting favoring anabolism. The lower plasma glucose in the fed pregnant rats was associated with higher insulin, slightly lower glucagon and higher insulin/glucose and insulin/glucagon ratios than in virgin rats. During fasting, glucose fell to sustained hypoglycemic levels in the pregnant animals whereas glucose declined but did not achieve hypoglycemia at any point in the virgins. Despite the hypoglycemia, greater levels of plasma insulin persisted in the pregnant throughout the 120 hours of fasting and insulin/glucagon ratios did not differ significantly from the euglycemic virgins. Thus, "accelerated starvation" in pregnancy cannot be ascribed to relative glucagon excess. Rather, the preservation of normal insulin/glucagon ratios despite prevailing hypoglycemia, may provide a mechanism during fasting in pregnancy for restraining maternal protein catabolism in the face of the added fuel demands of the conceptus.