If and SR Ca2+ release both contribute to pacemaker activity in canine sinoatrial node cells

Increasing evidence suggests that cardiac pacemaking is the result of two sinoatrial node (SAN) cell mechanisms a 'voltage clock' and a Ca2+ dependent process, or 'Ca2+ clock' The voltage clock initiates action potentials (APs) by SAN cell membrane potential depolarization from inward currents, of which the pacemaker current (I-f) is thought to be particularly important A Ca2+ dependent process triggers APs when sarcoplasmic reticulum (SR) Ca2+ release activates inward current carried by the forward mode of the electrogenic Na+/Ca2+ exchanger (NCX) However, these mechanisms have mostly been defined in rodents or rabbits, but are unexplored in single SAN cells from larger animals Here, we used patch-clamp and confocal microscope techniques to explore the roles of the voltage and Ca2+ clock mechanisms in canine SAN pacemaker cells We found that ZD7288, a selective I-f antagonist, significantly reduced basal automaticity and induced irregular, arrhythmia-like activity in canine SAN cells In addition, ZD7288 impaired but did not eliminate the SAN cell rate acceleration by isoproterenol In contrast, ryanodine significantly reduced the SAN cell acceleration by isoproterenol, while ryanodine reduction of basal automaticity was modest (similar to 14%) and did not reach statistical significance Importantly, pretreatment with ryanodine eliminated SR Ca2+ release, but did not affect basal or isoproterenol-enhanced I-f Taken together, these results indicate that voltage and Ca2+ dependent automaticity mechanisms coexist in canine SAN cells, and suggest that I-f and SR Ca2+ release cooperate to determine baseline and catecholamine-dependent automaticity in isolated dog SAN cells (C) 2010 Elsevier Ltd All rights reserved