Table 1.

Main ionic currents and their proteins, gens, and function in cardiac SA node of human and small animals mediating excitable activity

Current/Transporter	Protein (α sub unit)	Gen	Ion	Direction	Function
If	HCN1, HCN4	hHCN1, hHCN4	Na+	Inward	Involves in Diastolic depolarization	Yes, HCN1, HCN4	Yes, HCN1 > HCN4	Yes, HCN4 > HCN1	Yes, HCN4  > HCN2  > HCN1 in mouse [31, 32, 35, 36, 39–41]
Ist	CaV1.3	CACNA1D	Na+	Inward	Involves in Diastolic depolarization	Yes?	Yes, CaV1.3	Yes, CaV1.3	Yes, CaV1.3 [31, 33, 34]
ICaT	Cav3.1, Cav3.3	CACNA1G, CACNA1I	Ca2+	inward	contributes to the final phase of diastolic depolarization	Yes, Cav3.1, Cav3.3	Yes, Cav3.1, Cav3.3	Yes, Cav3.1, Cav3.3	Yes, Cav3.1 > Cav3.2 [31, 33, 35, 36, 39]
ICaL	Cav1.3, Cav1.2	CACNA1D CACNA1C	Ca2+	Inward	Responsible for the SA node action potential upstroke	Yes, Cav1.3 > Cav1.2	Yes, Cav1.3 > Cav1.2	Yes, Cav1.3,Cav1.2	Yes, Cav1.2 > Cav1.3 [31, 33, 35, 36, 39, 41]
INCX	NCX1, NCX2	SLC8A1	Na+ , Ca2+	Inward/outward	contribute to the final phase of diastolic depolarization	Yes, NCX1, NCX2	Yes, NCX1, NCX2	Yes, NCX1, NCX2	Yes, NCX1, NCX2 [31, 35–39, 41]
IKr	Kv11.1	KCNH2	K+	Outward	Responsible for repolarization of the SAN AP	Yes, Kv11.1	Yes, ?	– ?	Yes, Kv11.1 [31, 35, 36, 39, 41]
IKs	Kv7.1	KCNQ1	K+	Outward	Responsible for repolarization of the SAN AP	Yes, Kv7.1	Yes, Kv7.1	Yes, Kv7.1	Yes Kv7.1 [31, 35, 36, 39]
IK1	Kir2.1/Kir2.2	KCNJ2	K+	Outward/inward	Responsible for end of repolarization of the SAN AP and Resting potential	Yes, Kir2.1/Kir2.2	Yes, Kir2.1	?	Yes, Kir3.4 [31, 35, 36, 39–41]
Connexins (Cx)	Cx45, Cx40, Cx43	GJC1, GJA5, GJA1	Non-specific ion channels	Bidirectional	Couples the neighboring cells as relatively weakly	Yes, Cx45, Cx40	Yes, Cx43, Cx45, Cx40	Yes, Cx43, Cx45, Cx40	Yes Cx45 [31, 35, 39, 41, 42]

INa, fast Na⁺ current; Ist, sustained inward Na⁺ current; IKur, ultra-rapid outward K⁺ current; ICaL, L-type Ca²⁺ current; IKr, rapid delayed rectifier K⁺ current; IKs, slow delayed rectifier K⁺ current; IK1, inward rectifier potassium current; INCX, Na⁺–Ca²⁺ exchanger current; NCX, Na⁺/Ca²⁺ exchanger protein; SAN, sinoatrial node; AP, action potential; ?, indistinctive