What keeps us ticking: a funny current, a calcium clock, or both?

On the basis of prolific experimental evidence supported by novel numerical modeling, it is reasonable to conclude that the tightly coupled system of SR Ca²⁺ cycling and surface membrane proteins is the clock that controls SANC normal automaticity, leading to their mutual functional entrainment [115]. The clock is robust because the same factors that regulate SR Ca²⁺ cycling, i.e., Ca²⁺ and PKA and CaMKII-dependent protein phosphorylation, also regulate sarcolemmal ion channel function and thereby couple SR Ca²⁺ cycling to the surface membrane. G protein-coupled receptor signaling ensures pacemaker flexibility by affecting rate regulation by impacting on the very same factors that ensure pacemaker fail-safe operation or robustness in a given steady state. Intimately intertwined robustness and flexibility of the heart's pacemaker ensure a wide range of stable heart rates. Rebuttal: DiFrancesco. Ed, you concentrate on the existence of a complex cellular machinery underlying Ca²⁺ cycling. I have no problem with this, but we are discussing here the function of Ca²⁺ cycling and that of the funny current. Clearly, none of the mechanisms working in a pacemaker cell can be functionally removed without major impact on the whole behaviour of the cell. Normal physiological pacemaking depends on the integrity of all participating cellular processes, and pointing out that one mechanism is essential for rate control does not mean that mechanism is responsible for that control. The questions we need to address here are simpler ones: which is the physiological process selected to generate spontaneous activity (or, better, discriminating between pacing and silent cardiomyocytes)? Also, when a change in rate is required, which is the physiological process selected, typically by the autonomic nervous system, to produce it? A change in the rate of Ca²⁺ fluctuations, or a change in I_f?