Electrophysiology of the ageing rabbit and cat sinoatrial node--a comparative study.

Intrinsic properties of the human sinoatrial (SA) node have been shown to decline with age. In the present study we aimed at investigating the underlying mechanisms of age-dependent changes in intrinsic cycle length and sinoatrial conduction time. To this end, the cycle length and transmembrane potentials of the SA nodes of rabbits (2 days-5.6 years) and cats (6 weeks-18 years) were recorded and nodal conduction was reconstructed. The size of the SA nodes was measured in Sirius Red stained sections. Cycle length increases with age in both the rabbit and cat SA node, and in both species cycle length is dependent on diastolic depolarization rate and action potential duration. Nodal action potential duration increases with age in both rabbit and cat, whereas diastolic depolarization rate decreases in the cat only. The location of the primary pacemaker is not related to age. With age, sinoatrial conduction time increases in both species as a result of an enlargement of the area with low phase 0 upstroke velocities. The size of the SA node of adult animals does not increase with age.