Abstract
Cardiac rhythm is triggered by the activity of the sinus node. A mixed Na+/K+ current identified as If controls the frequency of depolarization. If is activated at hyperpolarized membrane potentials. Recently, a family of hyperpolarization-activated cyclic nucleotidegated cation (HCN) channels has been cloned. The four members of this family exhibit the general features of If channels. HCN2 and HCN4 are expressed in mouse sinus node. In addition, the mRNA of a T-type low voltage activated and a L-type high voltage activated calcium channel is present in the sinus node. These results support the hypothesis that cardiac pacemaking involves HCN, T-type and L-type calcium channels.
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