EFFECTS OF INSULIN, INSULIN-LIKE GROWTH FACTOR-I (IGF-I) AND PROGESTERONE ON GLUCOSE AND AMINO-ACID-UPTAKE IN XENOPUS-LAEVIS OOCYTES

In Xenopus oocytes, insulin or IGF-I activated glucose transport and maturation, but not amino acid transport, as measured by the uptake of alanine. Glucose transporters identical or closely related to the mammalian erythroid/brain transporter (Glut-1/EGT) were found in oocyte membranes. The EC50 for stimulation of glucose uptake and of maturation were similar (1-5 x 10(-8) M for insulin and 2-8 x 10(-10) M for IGF-I), confirming that these effects were mediated through IGF-I receptors. Other agents, such as phorbol 12-myristate 13-acetate (TPA) (0.5-mu-M) and vandate (2 mM) evoked only part of the insulin effect on glucose uptake (50% and 65%, respectively), without being additive to insulin. In contrast, progesterone at 1-mu-M, a potent inducer of maturation, inhibited at least partially the insulin-induced glucose uptake. Uptake of alanine and glucose was decreased after prolonged incubations (3-6 h) with agents that trigger maturation, and was dramatically reduced in oocytes that have undergone maturation (unfertilized eggs). Maturation is thus accompanied by a reduction in glucose and amino acid transports. These results further document the validity of Xenopus oocytes as a model to study insulin and IGF-I signalling.