PPADS, a P2X receptor antagonist, as a novel inhibitor of the reverse mode of the Na⁺/Ca²⁺ exchanger in guinea pig airway smooth muscle

The Na(+)/Ca(2+)exchanger (NCX) principal function is taking 1 Ca(2+) out of the cytoplasm and introducing 3 Na(+). The increase of cytoplasmic Na(+) concentration induces the NCX reverse mode (NCX(REV)), favoring Ca(2+) influx. NCX(REV) can be inhibited by: KB-R7943 a non-specific compound that blocks voltage-dependent and store-operated Ca(2+) channels; SEA0400 that appears to be selective for NCX(REV), but difficult to obtain and SN-6, which efficacy has been shown only in cardiomyocytes. We found that PPADS, a P2X Receptor Antagonist, acts as a NCX(REV) inhibitor in guinea pig tracheal myocytes. In these cells, we characterized the NCX(REV) by substituting NaCl and NaHCO(3) with LiCl, resulting in the increase of the intracellular Ca(2+) concentration ([Ca(2+)]i) using fura 2-AM. We analyzed 5 consecutive responses of the NCX(REV) every 10 min, finding no differences among them. To evaluate the effect of different NCX(REV) blockers, concentration response curves to KB-R7943 (1, 3.2 and 10 μM), and SN-6 (3.2, 10 and 30 μM) were constructed, whereas PPADS effect was characterized as time- and concentration-dependent (1, 3.2, 10 and 30 μM). PPADS had similar potency and efficacy as KB-R7943, whereas SN-6 was the least effective. Furthermore, KCl-induced contraction, sensitive to D600 and nifedipine, was blocked by KB-R7943, but not by PPADS. KCl-induced [Ca(2+)]i increment in myocytes was also significantly decreased by KBR-7943 (10 μM). Our results demonstrate that PPADS can be used as a reliable pharmacological tool to inhibit NCX(REV), with the advantage that it is more specific than KB-R7943 because it does not affect L-type Ca(2+) channels.