Regulation of transient receptor potential melastatin 4 channel by sarcoplasmic reticulum inositol trisphosphate receptors: Role in human detrusor smooth muscle function.

We recently reported key physiologic roles for Ca-activated transient receptor potential melastatin 4 (TRPM4) channels in detrusor smooth muscle (DSM). However, the Ca-signaling mechanisms governing TRPM4 channel activity in human DSM cells are unexplored. As the TRPM4 channels are activated by Ca, inositol 1,4,5-trisphosphate receptor (IPR)-mediated Ca release from the sarcoplasmic reticulum represents a potential Ca source for TRPM4 channel activation. We used clinically-characterized human DSM tissues to investigate the molecular and functional interactions of the IPRs and TRPM4 channels. With in situ proximity ligation assay (PLA) and perforated patch-clamp electrophysiology, we tested the hypothesis that TRPM4 channels are tightly associated with the IPRs and are activated by IPR-mediated Ca release in human DSM. With in situ PLA, we demonstrated co-localization of the TRPM4 channels and IPRs in human DSM cells. As the TRPM4 channels and IPRs must be located within close apposition to functionally interact, these findings support the concept of a potential Ca-mediated TRPM4-IPR regulatory mechanism. To investigate IPR regulation of TRPM4 channel activity, we sought to determine the consequences of IPR pharmacological inhibition on TRPM4 channel-mediated transient inward cation currents (TICCs). In freshly-isolated human DSM cells, blocking the IPRs with the selective IPR inhibitor xestospongin-C significantly decreased TICCs. The data suggest that IPRs have a key role in mediating the Ca-dependent activation of TRPM4 channels in human DSM. The study provides novel insight into the molecular and cellular mechanisms regulating TRPM4 channels by revealing that TRPM4 channels and IPRs are spatially and functionally coupled in human DSM.