AC	Adenylyl cyclase
Gi	Inhibitory G protein is coupled to the muscarinic acetylcholine (ACh) G protein receptor (M2); when activated by ACh, it activates the G protein coupled inward rectifying potassium current (GIRK) channel to inhibit AC, cAMP, and HCN current, decrease the action potential duration and reduce the intrinsic sinus node rate
Gs	Stimulatory G protein acts conversely to Gi, stimulating AC, cAMP and increasing pacemaker depolarization and rate
HCN	Channels, hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels; HCN4 is the major isoform present solely in the cardiac conducting tissues in the healthy heart
ICa, T	Low-voltage activated (T-type) calcium channel/current contributes to sinoatrial node activity; inactivation of ICa, T reduces the intrinsic heart rate slowing pacemaker cell activity
ICa, L	L-type calcium current, the most prominent calcium current in mammalian ventricular heart cells
IK	Delayed rectifier K+ current modulates sinoatrial pacemaking activity and action potential repolarization; even partial blockade of IKr (sinoatrial cells’ rapidly activating IK) results in prolongation of final repolarization of action potentials and a slowing of intrinsic heart rate; its blockade slows down the diastolic depolarization; it is augmented by beta-adrenergic signaling shortening the action potential duration during high heart rates
If	“Funny” current, a mixed Na+ - K+ inward HCN current activated by hyperpolarization and modulated by the ANS