Role of K⁺ channels in maintaining the synchrony of spontaneous Ca²⁺ transients in the mural cells of rat rectal submucosal arterioles

Abstract

Mural cells in precapillary arterioles (PCAs) generate spontaneous Ca²⁺ transients primarily arising from the periodic release of Ca²⁺ from sarcoendoplasmic reticulum (SR/ER). The Ca²⁺ release induces Ca²⁺-activated chloride channel (CaCC)-dependent depolarisations that spread to neighbouring mural cells to develop the synchrony of their Ca²⁺ transients. Here, we explored the roles of K⁺ channels in maintaining the synchrony of spontaneous Ca²⁺ transients. Intracellular Ca²⁺ dynamics in mural cells were visualised by Cal-520 fluorescence Ca²⁺ imaging in the submucosal PCAs of rat rectum. Increasing extracellular K⁺ concentration ([K⁺]_o) from 5.9 to 29.7 mM converted synchronous spontaneous Ca²⁺ transients into asynchronous, high-frequency Ca²⁺ transients. Similarly, the blockade of inward rectifier K⁺ (K_ir) channels with Ba²⁺ (50 μM) or K_v7 voltage-dependent K⁺ (K_v7) channels with XE 991 (10 μM) disrupted the synchrony of spontaneous Ca²⁺ transients, while the blockers for large-, intermediate- or small-conductance Ca²⁺-activated K⁺ channels had no effect. K_ir2.1 immunoreactivity was detected in the arteriolar endothelium but not mural cells. In the PCAs that had been pretreated with XE 991 or Ba²⁺, nifedipine (1 μM) attenuated the asynchronous Ca²⁺ transients but failed to restore their synchrony. In contrast, levcromakalim, an ATP-sensitive K⁺ channel opener, restored the synchronous Ca²⁺ transients. Thus, constitutively active K_v7 and K_ir channels appear to be involved in maintaining the relatively hyperpolarised membrane of mural cells. The hyperpolarised membrane prevents depolarisation-induced ‘premature’ Ca²⁺ transients to ensure sufficient SR/ER Ca²⁺ refilling that is required for regenerative Ca²⁺ release resulting in synchronous Ca²⁺ transients amongst the mural cells.