Acute effects of insulin-like growth factor I on glucose and amino acid metabolism in the awake fasted rat. Comparison with insulin.
To elucidate the acute metabolic actions of insulin-like growth factor I (IGF-I), we administered a primed (250 micrograms/kg), continuous (5 micrograms/kg.min) infusion of human recombinant (Thr 59) IGF-I or saline to awake, chronically catheterized 24-h fasted rats for 90 min. IGF-I was also infused while maintaining euglycemia (glucose clamp technique) and its effects were compared to those of insulin. IGF-I infusion caused a twofold rise in IGF-I levels and a 75-85% decrease in plasma insulin. When IGF-I alone was given, plasma glucose fell by 30-40 mg/dl (P less than 0.005) due to a transient twofold increase (P less than 0.05) in glucose uptake; hepatic glucose production and plasma FFA levels remained unchanged. IGF-I infusion with maintenance of euglycemia produced a sustained rise in glucose uptake and a marked stimulation of [3-3H]glucose incorporation into tissue glycogen, but still failed to suppress glucose production and FFA levels. IGF-I also produced a generalized 30-40% reduction in plasma amino acids, regardless of whether or not hypoglycemia was prevented. This was associated with a decrease in leucine flux and a decline in the incorporation of [1-14C]leucine into muscle and liver protein (P less than 0.05). When insulin was infused in a dosage that mimicked the rise in glucose uptake seen with IGF-I, nearly identical changes in amino acid metabolism occurred. However, insulin suppressed glucose production by 65% and FFA levels by 40% (P less than 0.001). Furthermore, insulin was less effective than IGF-I in promoting glycogen synthesis. We conclude that (a) IGF-I produces hypoglycemia by selectively enhancing glucose uptake; (b) IGF-I is relatively ineffective in suppressing hepatic glucose production or FFA levels; and (c) IGF-I, like insulin, lowers circulating amino acids by reducing protein breakdown rather than by stimulating protein synthesis. Thus, IGF-I's metabolic actions in fasted rats are readily distinguished from insulin.
Jacob
R
R
Department of Medicine, Yale University School of Medicine, New Haven, Connecticut 06510, USA.
Barrett
E
E
Plewe
G
G
Fagin
K D
KD
Sherwin
R S
RS
D000596:Amino Acids; D000818:Animals; D001786:Blood Glucose / Q000378:metabolism*; D005215:Fasting; D006801:Humans; D007003:Hypoglycemia / Q000139:chemically induced; D007262:Infusions, Intravenous; D007328:Insulin / Q000008:administration & dosage*; D007334:Insulin-Like Growth Factor I / Q000008:administration & dosage*; D007700:Kinetics; D007930:Leucine; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D011994:Recombinant Proteins / Q000008:administration & dosage; D013002:Somatomedins / Q000008:administration & dosage*
D000596:Amino Acids; D001786:Blood Glucose; D007328:Insulin; D011994:Recombinant Proteins; D013002:Somatomedins; D007334:Insulin-Like Growth Factor I; D007930:Leucine