The effect of high [K(+)](o) on spontaneous Ca(2+) waves in freshly isolated interstitial cells of Cajal from the rabbit urethra

Interstitial cells of Cajal (ICC) act as putative pacemaker cells in the rabbit urethra. Pacemaker activity in ICC results from spontaneous global Ca(2+) 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 [Ca(2+)] 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 [Ca(2+)](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 [Ca(2+)](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 NiCl(2) but not by the T‐type Ca(2+) channel blocker mibefradil (300 nmol/L). However, inhibition of Ca(2+) 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 Ca(2+) influx through reverse‐mode NCX.