Fetal hearts use glucose as their primary metabolic substrate. Adult hearts use free fatty acids, which are less efficient (require more $O_2$ to synthesize the same amount of ATP); however, during cardiac failure, the heart can switch to glucose utilization. Why doesn't it just use glucose all the time?
Striated muscles (primarily skeletal muscle) have a higher concentration of mitochondria compared to cardiac smooth muscle cells. Cardiac myocytes do not store and create glucose at nearly an efficient enough rate to utilize glucose exclusively.
