Effect of fasting on myocardial substrates in male and female rats.

Adult male and female rats were fasted for 1, 2, or 3 days to determine its effect on circulating and endogenous fuels available to the heart. Liver glycogen was depleted within the first 24 h of food restriction. Plasma glucose decreased approximately 2.5 mM in both sexes during the 3 days. Fasting significantly increased plasma beta-hydroxybutyrate to approximately the same level in female and male rats. Plasma free fatty acid (FFA) increased approximately 0.2 mM in both groups during the first 24 h without food and remained elevated over the next 2 days. FFA concentrations were higher in fed female than in fed male rats and remained significantly higher in female rats throughout the experimental period. Myocardial glycogen increased 64% during the first 2 days of fasting in the male rats and stayed elevated on the third day of fasting. In contrast, heart glycogen of female rats remained unchanged from an initial value of 3.13 mg/g throughout the 3-day fasting period. Endogenous triglyceride (TG) of male rats decreased from 2.14 +/- 0.09 to 1.41 +/- 0.21 mumol/g during the first 24 h without food and remained at that level during the second and third days. Heart TG in female rats fell progressively from 2.36 +/- 0.19 to 1.02 +/- 0.12 mumol/g during the fasting period. Cardiac FFA were higher in female than in male animals throughout the entire experiment. These data indicate that quantitative and qualitative metabolic differences exist between male and female rats stressed by fasting.(ABSTRACT TRUNCATED AT 250 WORDS)