Dr. Peng-Sheng Chen, Indiana University School of Medicine, Indiana, USA, discussed the latest evidence on the mechanisms that interact to initiate the heart beat. Noble et al (1979) proposed that the mechanism of sinoatrial node (SAN) depolarization involves the pacemaker current (If), which activates on hyperpolarization, causes diastolic depolarization, and then deactivates. This process occurs in cycles, producing the sinus beat.

The human SAN is located next to the terminal crest and surrounds the sino-atrial artery. The cardiac pacemaker current is recorded in isolated SAN cells during hyperpolarizations at voltages from -40/-50 mV to -100/-110 mV, which is the range where depolarization occurs. The pacemaker currentcarries Na and K, flowing into the cell to initiate depolarization. This current is controlled by catecholamines and acetylcholine.

In an experiment in a rabbit SAN, catecholamine stimulation caused earlier diastolic depolarization and shortened P wave to P wave interval. Acetylcholine had the opposite effect. In a patch clamp study, acetylcholine reduced the pacemaker current, while catecholamines increased it. These experiments show that pacemaker current is controlled by catecholamines and acetylcholine, which correspond to sympathetic stimulation, which accelerates sinus rate, and vagostimulation, which reduces sinus rate.

HCN4, a gene in the hyperpolarization-activated, cyclic nucleotide-gated (HCN) family, controls the pacemaker current and is highly expressed in the SAN. HCN4 is most abundant in the superior portion of the SAN and is not present in the surrounding atrium, showing that it is uniquely expressed in the SAN.

Members of a family with autosomal dominant reduced HCN4 had reduced pacemaker current, resulting in asymptomatic bradycardia. Those with the mutation had heart beats ranging from 25 to 36 beats/minute.  They had no syncope or dizziness, normal QT interval, and a normal chronotropic response. During exercise, they developed normal heart beats (118-165 beats/minute), explaining why they were asymptomatic. If the pacemaker current is the only mechanism of automaticity in the SAN, the heart rate should not increase during exercise. Therefore, another mechanism must play a role in generating the heart beat during exercise.

The pacemaker current is activated by hyperpolarization and deactivated by depolarization, working like a clock. Importantly, reduced pacemaker current does not prevent heart rate acceleration.

Cardiac electrical contraction coupling starts with an action potential and depolarization, causing calcium to enter the cell. Calcium interacts with the ryanodine receptor (RyR) on the sarcoplasmic reticulum (SR), which opens, releasing calcium into the cytosol. The calcium’s actions on the sarcomere initiates a contraction, and the calcium exits the cell via the sodium calcium exchange (NCX). This process is regulated by catecholamine actions on the calcium current, RyR, and phospholambam (PLB).

In reverse EC coupling, calcium is spontaneously released from the SR prior to the action potential, activating the NCX and initiating an action potential. Maltsev et al studied this phenomenon in rabbit SAN cells and argued that spontaneous SR calcium release is important in generating sinus rhythm. Honjo et al published a study on the effects of ryanodine and isoproterenol on isolated SAN cells, concluding that SR calcium release is not a dominating factor in SAN pacemaker activity.

Dr. Chen explored this issue in an experiment on the intact canine SAN and right atrium. At baseline the spontaneous phase 4 depolarization was observed only in the SAN. Isoproterenol infusion caused the pacemaking site to shift upward to the superior SAN, where the calcium leaks from the SR prior to the action potential, resulting in a depolarization and sinus rate acceleration. The same experiment showed that the heart rate increases with increasing doses of isoproterenol.

When a small amount of ryanodine is infused into the SAN, the RyR opens, allowing some calcium to leak from the SR, resulting in a 30% to 50% sinus rate increase. When a large amount of ryanodine is added, the RyR closes, keeping the calcium in the SR. This is similar to what happened in the Honjo experiment. Adding isoproterenol causes an 80% increase in the sinus rate. It appears that the maximal sinus rate (sinus tachycardia) is primarily driven by the calcium clock, while the baseline sinus rate is maintained by both mechanisms. The calcium clock and the membrane clock work synergistically to maintain the baseline heart beat. The calcium clock begins when the SR is full and ends when it is empty. The membrane clock begins when the membrane potential is hyperpolarized, which activates the pacemaker current. It ends when the membrane potential is depolarized, which deactivates the pacemaker current.

Ogawa et al demonstrated that a similar phenomenon works in the ventricle. He induced ventricular fibrillation in a rabbit ventricle, during which the intracellular calcium levels were very high. Upon termination of the fibrillation, the SR was still full of calcium. The calcium was released before the next action potential, causing a delayed afterdepolarization (DAD) in the epicardial cells. The endocardial cells had trigger activity, which propagated transmurally to reach the epicardium and start a heart beat.

A pause in the heart beat can occur after atrial fibrillation (AF), resulting in bradycardia. A 1996 study showed that pacing-induced AF impairs SAN function in dogs, which involves remodeling of the calcium handling proteins. The RyR is hyperphosphorylated, changing calcium dynamics. This results in SAN dysfunction when the AF stops. This study explains the transient SAN dysfunction that occurs in patients with AF.

Hocini et al performed AF ablation in 20 patients with paroxysmal AF and prolonged sinus pauses. After ablation, the patients had reverse remodeling of the SAN and improved SAN function, demonstrating that SAN dysfunction is reversible.

In conclusion, the calcium clock and membrane ionic clock work synergistically to generate sinus rhythm. The calcium clock works by reverse excitation contraction coupling and by activation of the NCX current. The membrane clock works via the pacemaker current, which is activated by hyperpolarization and deactivated by depolarization. The calcium clock and membrane clock work together to generate sinus rhythm. An abnormal calcium clock is the underlying mechanism of SAN dysfunction in AF. According to Dr. Chen, in the future the calcium clock will probably be found to be important in other conditions of SAN dysfunction, such as heart failure.