Starvation (48 h) decreases
fructose 2,6-bisphosphate (Fru-2,6-P2) concentrations and the ratio of free to acylated
carnitine in hearts of euthyroid rats. These decreases, which are indicative of increased
lipid fuel oxidation, are accompanied by decreased rates of
glucose uptake and phosphorylation, assessed by using radioactive
2-deoxyglucose. Cardiac concentrations of acylated carnitines were increased at the expense of free
carnitine even in the fed state in response to experimental
hyperthyroidism, but neither Fru-2,6-P2 concentrations nor rates of
glucose utilization were suppressed.
Starvation (48 h) did not further increase the proportion of acylated
carnitine in the heart in
hyperthyroidism, and suppression of Fru-2,6-P2 concentrations and
glucose utilization rates by
starvation was attenuated. Although
glucose utilization rates were decreased,
starvation did not decrease immunoreactive
GLUT 4 protein concentrations. Furthermore, although
hyperthyroidism was associated with a statistically significant (30-40%) increase in relative abundance of GLUT 4
mRNA, the amount of
GLUT 4 protein was not increased by
hyperthyroidism in either the fed or the starved state. The results demonstrate a significant effect of
hyperthyroidism to enhance cardiac
glucose utilization in
starvation by a mechanism which does not involve changes in GLUT 4 expression but may be secondary to changes in
glucose-
lipid interactions at the tissue level.