Is KB‐R7943 a specific inhibitor of reverse‐mode NCX in the vasculature?

The Na + /Ca 2+ Exchanger (NCX) is a bi‐directional regulator of cytosolic Ca 2+ , causing Ca 2+ efflux in forward‐mode and Ca 2+ influx in reverse‐mode. We hypothesized that reverse‐mode NCX is a means of Ca 2+ entry in rat aorta (RA) and vena cava (RVC). After confirming the presence of NCX protein in RA and RVC by Western blot, isometric contraction to ET‐1 was measured in RA and RVC rings exposed to vehicle or the reverse‐mode NCX inhibitor KB‐R7943. KB‐R7943 (10 μM) significantly attenuated maximal contraction to ET‐1 in RVC (53±9% of control), but not RA (91±1% of control). The NCX inhibitor SN‐6 (10 μM), however, had no effect on ET‐1 contraction in RA or RVC. To test if KB‐R7943 inhibited contraction to other agonists, isometric contraction to NE and KCl was also measured. KB‐R7943 (10 μM) attenuated maximal contraction to NE in RA (80±3%) and RVC (52±7%). Maximal contraction to KCl was attenuated in RA (48±5%) and nearly abolished in RVC (9±2%), suggesting KB‐R7943 inhibited voltage‐mediated Ca 2+ entry. In PGF‐2α (10 μM)‐contracted tissues, KB‐R7943 relaxed RA and RVC (−6±1% and −68±2% of PGF‐2α contraction, respectively). This was attenuated by 1 mM TEA in RVC, suggesting that KB‐R7943 activates K + channels. These data suggest that KB‐R7943 attenuates RA and RVC contraction by inhibiting voltage‐dependent Ca 2+ influx and activating K + efflux, and not reverse‐mode NCX inhibition. Supported by NIH P01HL70687.