Figure 9.

Interspecies differences in AP repolarization allow for enhancement of inotropy and lusitropy during the fight-or-flight response. Sympathetic stimulation transiently prolongs mouse APD and shortens rabbit APD. In mouse, the longer APD allows for slightly greater Ca²⁺ influx through I_CaL and more outward NCX, whereas in rabbit, Ca²⁺ influx is enhanced with shorter APDs due to a faster increase in the driving force for I_CaL as the membrane potential falls. Increased Ca²⁺ influx enhances Ca²⁺ transient amplitude (inotropy), which in turn increases SERCA activity that hastens Ca²⁺ transient decay in both species (lusitropy). Increased inward NCX during the shorter APD also contributes to shorter Ca²⁺ transients in rabbit.