Intracellular calcium does not directly modulate cardiac pacemaker (if) channels.

A study on the cardiac pacemaker current if in inside-out macro-patches of sino-atrial (SA) node cells has recently demonstrated that if channels are directly activated by intracellular cAMP. Using the same preparation, here we investigate the possibility that internal Ca2+ ions play a role in the modulation of if channels. Our results indicate that Ca2+ ions do not have a direct activating effect on if.