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Heterogeneous expression of the delayed-rectifier K⁺ currents i_K,r and i_K,s in rabbit sinoatrial node cells

M. Lei *, H. Honjo †, I. Kodama † and M. R. Boyett

1. The electrical activity of sinoatrial node cells is heterogeneous. To understand the reasons for this, the density of the delayed-rectifier K⁺ current and its two components, i_K,r and i_K,s, as a function of the size (as measured by cell capacitance) of rabbit sinoatrial node cells was investigated using the whole-cell voltage-clamp technique at 35 °C.
2. i_K,r and i_K,s were isolated using E-4031 and 293B. Features of the E-4031-sensitive and 293B-insensitive currents corresponded well to those of i_K,r, while features of the E-4031-insensitive and 293B-sensitive currents corresponded well to those of i_K,s.
3. The densities of the outward current under control conditions and the drug-sensitive and -insensitive currents were significantly (P < 0.01) correlated with cell capacitance, with current densities being greater in larger cells.
4. The effects of partial blockade of i_K,r by 0.1 µM E-4031 on spontaneous action potentials were greater in smaller cells.
5. It is concluded that there are cell size-dependent differences in the density of the i_K,r and i_K,s components, and these may be involved in the heterogeneity of the electrical activity of single sinoatrial node cells as well as that of the intact sinoatrial node.

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