“…15 However, APDshortening resulting from low rates of stimulation, as demonstrated in hearts of both the rabbit 16 and rat, 17 has been correlated with a slow reactivation of I TO1 , the 4-aminopyridine (4-AP)-sensitive component of the transient outward K + current, I TO . 14,18,19 The reactivation kinetics of I TO is species-dependent, 20 which may explain the lack of APD-shortening at low rates in human, 12 in contrast to rabbit myocardium. Moreover, in the rabbit, the contribution of I TO to the APD rate-relation in the AV node might be expected to be less than in the atrium, due to the relatively small I TO in mid-nodal cells.…”