Failure of autoresuscitation in weanling mice: significance of cardiac glycogen and heart rate regulation.

"Autoresuscitation" (AR) is the spontaneous recovery from hypoxic apnea by gasping. We examined aspects of heart function in two situations: 1) the maturationally acquired failure of AR that is characteristic of SWR, but not BALB/c, weanling mice and 2) AR failure in BALB/c mice induced by repeated exposures to anoxia. We determined maturational changes in heart and liver glycogen. Unlike liver glycogen levels, heart glycogen levels in SWR mice differed from those in BALB/c mice. They were consistently much lower throughout maturation and reached a nadir during the brief period when SWR weanling mice are vulnerable to AR failure. Also, rate of cardiac glycogen utilization in vulnerable SWR mice was lower than that of same-aged BALB/c mice and was nil during the latter one-half of the gasping stage when heart function is critical for AR success. Therefore, because glycogen utilization reflects cardiac work, heart failure could explain AR failure in SWR weanlings. Additionally, the increase in hypoxic heart rate that occurs with maturation is developmentally delayed in SWR mice, and this may contribute to their AR failure. Cardiac glycogen was not fully depleted in BALB/c mice during repeated anoxic exposures, indicating other reasons for AR failure. We view these findings as a potential model for the age-related peak in incidence of sudden infant death syndrome.