The metabolism of pantothenic acid (Pa) by cardiac muscle was studied in normal and diabetic rats. Tissue levels of Coenzyme A (CoA) are elevated in the heart during early (6 to 12 h) diabetes, remains at a high level for several days, and then returns to normal or below normal levels. The increase in total tissue CoA mainly occurs in myocytes as indicated by isolation of cardiac myocytes from control and diabetic animals and measuring their content of CoA. The CoA concentration increased from 37 to 93 microM in the cytosolic compartment and from 2.0 to 2.6 mM in the mitochondrial matrix. These effects of diabetes were reversed by insulin treatment. CoA synthesis in hearts removed from control rats and perfused in vitro was stimulated by including in the perfusate Pa, cysteine and dithiothreitol, but no exogenous energy substrate. This stimulated in vitro rate of CoA synthesis was reduced in hearts removed from diabetic animals, and the reduction increased with duration of diabetes. The reduced rate in diabetic hearts resulted from both a decreased rate of Pa phosphorylation and decreased Pa transport. Transport of Pa into myocytes was decreased by as much as 80% in hearts from diabetic animals. The low transport rate was due to a decrease in Vmax with no apparent change in Km. Treatment of the isolated heart with insulin did not correct the diabetic-induced reduction in Pa transport. The transport rate in normal and diabetic hearts was not influenced by the type of energy substrate provided to the heart.(ABSTRACT TRUNCATED AT 250 WORDS)