COMPARISON OF THE HYPERPOLARIZATION-ACTIVATED AND DELAYED RECTIFIER CURRENTS IN RABBIT ATRIOVENTRICULAR NODE AND SINOATRIAL NODE

The properties of the hyperpolarization-activated inward current (I-f) and the delayed rectifier K(z)z= current (I-K) in isolated single cel ls from rabbit atrioventricular (AV) and sinoatrial (SA) nodes were st udied using nystatin-permeablized whole-cell recordings. The current d ensity for I-f is 3-5 times smaller in the AV node than in the SA node . No significant differences between AV and SA node cells were observe d in: (i) the voltage-dependence of I-f activation, (ii) voltage-depen dent blockage of I-f by Cs+, and (iii) modulation of I-f by isoprenali ne. The current density for I-K is also much smaller in the AV node th an in the SA node. This difference appears to be mainly due to the abs ence of the slow-type I-K (I-K,I-slow) in the AV node. In contrast, th e size of the L-type Ca2+ current is similar in these two regions of t he rabbit heart. The small net currents in the AV node provide an inte resting example of heterogeneous channel distribution between primary and subsidiary pacemaker tissue.