Background— Homozygous mutant mice with a defect of klotho gene expression (kl/kl) show multiple age-related disorders and premature death from unknown causes.

Methods and Results— The kl/kl mice subjected to 20-hour restraint stress showed a high rate (20/30) of sudden death, which was associated with sinoatrial node dysfunction (conduction block or arrest). Heart rate and plasma norepinephrine of kl/kl mice, unlike those of wild-type (WT) mice, failed to increase during the stress. Intrinsic heart rate after pharmacological blockade of autonomic nerves in kl/kl mice was significantly lower than that in WT mice (380±33 versus 470±44 bpm; n=7). The sinus node recovery time after an overdrive pacing (600 bpm, 30 seconds) in kl/kl mice was significantly longer than in WT mice (392±37 versus 233±24 ms; n=6). In isolated sinoatrial node preparations, the positive chronotropic effect of isoproterenol was significantly less, whereas the negative chronotropic effect of acetylcholine was significantly greater in kl/kl than in WT mice. There was no degenerative structural change in the sinoatrial node of kl/kl mice. The precise localization of klotho was analyzed in newly prepared klotho-null mice with a reporter gene system (kl^−geo). Homozygous kl^−geo mice showed characteristic age-associated phenotypes that were almost identical to those of kl/kl mice. In the kl^−geo mice, klotho expression was recognized exclusively in the sinoatrial node region in the heart in addition to parathyroid, kidney, and choroid plexus.

Conclusions— In the heart, klotho is expressed solely at the sinoatrial node. klotho gene expression is essential for the sinoatrial node to function as a dependable pacemaker under conditions of stress.