Syntaxin-1A inhibition of P-1075, cromakalim, and diazoxide actions on mouse cardiac ATP-sensitive potassium channel

Aims Syntaxin (Syn)-1A binds sulfonylurea receptor (SUR) nucleotide binding folds of cardiac myocyte (SUR2A) and islet β-cells (SUR1) to inhibit ATP-sensitive potassium (KATP) channels. We further reported that Syn-1A reduced the potency and efficacy of β-cell-specific KATP channel openers (KCOs). Here, we examined whether Syn-1A would influence non-specific (diazoxide) and SUR2-specific KCOs [ N -cyano- N ′-(1,1-dimethylpropyl)- N ″-3-pyridylguanidine (P-1075) and cromakalim] on cardiac myocyte KATP channels activation. Methods and results Confocal microscopy and Western blotting verified the presence of both Syn-1A and -1B expressions on rodent cardiac ventricular myocytes. Inside-out patch–clamp electrophysiology was utilized to examine the effects of these syntaxins on KATP macroscopic currents activated by various KCOs from a stable cell line expressing the potassium inward rectifier 6.2 (Kir6.2)/SUR2A and from C57BL/6 male mouse ventricular myocytes. Syn-1A inhibited the current amplitude activated by P-1075, cromakalim and diazoxide via its H3 but not Habc domain. Syn-1B exhibited similar inhibitory effects on P-1075 activation of KATP currents. In examining for direct effects of Syn-1A on the KCO binding to cardiac SUR2 receptors, we found that Syn-1A did not directly affect [3H]-P-1075 binding to rat cardiac membrane SUR2A at maximum binding capacity, but was able to mildly reduce the affinity of cold P-1075 and cromakalim to displace [3H]-P-1075 binding. Conclusion In conclusion, Syn-1A (and Syn-1B) could inhibit KATP currents activated by SUR2A-acting KCOs. Potential fluctuations in the levels of these syntaxins in the myocardium may affect the therapeutic effectiveness of cardiac KCOs.