sentences:  Cyclic adenosine monophosphate (cAMP) molecules bind directly to f-channels and increase their open probability.cAMP dependence is a particularly relevant physiological property, since it underlies the If-dependent autonomic regulation of heart rate.Sympathetic stimulation raises the level of cAMP-molecules which bind to f-channels and shift the If activation range to more positive voltages; this mechanism leads to an increase of the current at diastolic voltages and therefore to an increase of the steepness of DD and heart rate acceleration.