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

Interstitial cells of C ajal ( 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 ( SMC s) and ICC using a N ipkow spinning disk confocal microscope. Increasing [K + ] o to 60 mmol/L caused an increase in [Ca 2+ ] i accompanied by contraction in SMC s. 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  μ mol/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  μ mol/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  μ mol/L SEA0400 or 5  μ mol/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 .