Contribution of reverse Na+-Ca2+ exchange to spontaneous activity in interstitial cells of Cajal in the rabbit urethra

Interstitial cells of Cajal (ICC) isolated from the rabbit urethra exhibit regular Ca2+ oscillations that are associated with spontaneous transient inward currents (STICs) recorded under voltage clamp. Their frequency is known to be very sensitive to external Ca2+ concentration but the mechanism of this has yet to be elucidated. In the present study experiments were performed to assess the role of Na+-Ca2+ exchange (NCX) in this process. Membrane currents were recorded using the patch clamp technique and measurements of intracellular Ca2+ were made using fast confocal microscopy. When reverse mode NCX was enhanced by decreasing the external Na+ concentration [Na+](o) from 130 to 13 mM, the frequency of global Ca2+ oscillations and STICs increased. Conversely, inhibition of reverse mode NCX by KB-R7943 and SEA0400 decreased the frequency of Ca2+ oscillations and STICs. Application of caffeine (10 mM) and noradrenaline (10 mu M) induced transient Ca2+-activated chloride currents (I-ClCa) at -60 mV due to release of Ca2+ from ryanodine- and inositol trisphosphate (IP3)-sensitive Ca2+ stores, respectively, but these responses were not blocked by KB-R7943 or SEA0400 suggesting that neither drug blocked Ca2+-activated chloride channels or Ca2+ release from stores. Intact strips of rabbit urethra smooth muscle develop spontaneous myogenic tone. This tone was relaxed by application of SEA0400 in a concentration-dependent fashion. Finally, single cell RT-PCR experiments revealed that isolated ICC from the rabbit urethra only express the type 3 isoform of the Na+-Ca2+ exchanger (NCX3). These results suggest that frequency of spontaneous activity in urethral ICC can be modulated by Ca2+ entry via reverse NCX.