Local routes revisited: the space and time dependence of the Ca2+ signal for phasic transmitter release at the rat calyx of Held

Graphical Abstract Abstract figure legend HCN channels play an evolutionarily conserved pacemaker role in renal pelvic smooth muscle (RPSM) of lower and higher order mammals. The function of hyperpolarization-activated cation (HCN) channels in smooth muscle pacemakers remains controversial. Renal pelvic smooth muscle pacemakers trigger smooth muscle contractions that expel waste from the kidney, and HCN channels have been localized to these pacemaker tissues. To date, however, the mechanisms underlying RPSM pacemaker activity remain elusive. RPSM pacemaker activity was investigated in both lower (top left) and higher order (bottom left) mammalian models, which exhibit divergent upper urinary tract anatomies. We performed morphological and functional studies from the single-molecule to the whole-organ level and showed that HCN channels drive RPSM pacemaker activity. RPSM pacemakers (boxed regions) integrated into the muscular syncytium, expressed HCN channels on their plasmalemma and exhibited the Ih ‘funny’ pacemaker current conducted by HCN channels. Critically, HCN channel block abolished electrical pacemaker activity and peristaltic smooth muscle contractions in both lower and higher order mammalian upper urinary tracts. Thus, HCN channels play an evolutionarily conserved pacemaker role in RPSM pacemakers.