The effect of high [K⁺ ]_o on spontaneous Ca²⁺ waves in freshly isolated interstitial cells of Cajal from the rabbit urethra

Abstract

Interstitial cells of Cajal (ICC) act as putative pacemaker cells in the rabbit urethra. Pacemaker activity in ICC results from spontaneous global Ca2+ waves that can be increased in frequency by raising external [K+]. The purpose of this study was to elucidate the mechanism of this response. Intracellular [Ca2+] was measured in fluo‐4‐loaded smooth muscle cells (SMCs) and ICC using a Nipkow spinning disk confocal microscope. Increasing [K+]o to 60 mmol/L caused an increase in [Ca2+]i accompanied by contraction in SMCs. Raising [K+]o did not cause contraction in ICC, but the frequency of firing of spontaneous calcium waves increased. Reducing [Ca2+]o to 0 mmol/L abolished the response in both cell types. Nifedipine of 1 μmol/L blocked the response of SMC to high [K+]o, but did not affect the increase in firing in ICC. This latter effect was blocked by 30 μmol/L NiCl2 but not by the T‐type Ca2+ channel blocker mibefradil (300 nmol/L). However, inhibition of Ca2+ influx via reverse‐mode sodium/calcium exchange (NCX) using either 1 μmol/L SEA0400 or 5 μmol/L KB‐R7943 did block the effect of high [K+]o on ICC. These data suggest that high K+ solution increases the frequency of calcium waves in ICC by increasing Ca2+ influx through reverse‐mode NCX.

The role of Ca2+ influx in spontaneous Ca2+ wave propagation in interstitial cells of Cajal from the rabbit urethra

“…; Drumm et al . b , c ), only Ca 2+ waves could be adequately resolved in isolated rabbit urethral ICC at low image acquisition rates (5–15 fps). A typical example of such a recording is shown in Fig.…”

“…; Drumm et al . b ). When Ca 2+ influx was blocked with the specific reverse mode NCX blocker KB‐R 7943 (3 μ m , Fig.…”

“…; Drumm et al . b , c ). In the current study, using confocal microscopy with fast acquisition rates (50 fps), we observed a range of Ca 2+ events that have not been reported previously in this cell type.…”

“…; Drumm et al . b ). However, the precise mechanism by which Ca 2+ influx contributes to spontaneous Ca 2+ wave propagation in urethral ICC remains unclear.…”

“…Ca 2+ influx is also vital for the generation of Ca 2+ waves in urethral ICC (Johnston et al 2005) and subsequent studies indicated that reverse mode sodium-calcium exchange (NCX) was the primary pathway by which this Ca 2+ influx occurred (Bradley et al 2006;Drumm et al 2014b). However, the precise mechanism by which Ca 2+ influx contributes to spontaneous Ca 2+ wave propagation in urethral ICC remains unclear.…”