A1899, PK‐THPP, ML365, and Doxapram inhibit endogenous TASK channels and excite calcium signaling in carotid body type‐1 cells

Sensing of hypoxia and acidosis in arterial chemoreceptors is thought to be mediated through the inhibition of TASK and possibly other (e.g., BKCa) potassium channels which leads to membrane depolarization, voltage‐gated Ca‐entry, and neurosecretion. Here, we investigate the effects of pharmacological inhibitors on TASK channel activity and [Ca2+]i‐signaling in isolated neonatal rat type‐1 cells. PK‐THPP inhibited TASK channel activity in cell attached patches by up to 90% (at 400 nmol/L). A1899 inhibited TASK channel activity by 35% at 400 nmol/L. PK‐THPP, A1899 and Ml 365 all evoked a rapid increase in type‐1 cell [Ca2+]i. These [Ca2+]i responses were abolished in Ca2+‐free solution and greatly attenuated by Ni2+ (2 mM) suggesting that depolarization and voltage‐gated Ca2+‐entry mediated the rise in [Ca2+]i. Doxapram (50 μmol/L), a respiratory stimulant, also inhibited type‐1 cell TASK channel activity and increased [Ca2+]i.. We also tested the effects of combined inhibition of BKCa and TASK channels. TEA (5 mmol/L) slightly increased [Ca2+]i in the presence of PK‐THPP and A1899. Paxilline (300 nM) and iberiotoxin (50 nmol/L) also slightly increased [Ca2+]i in the presence of A1899 but not in the presence of PK‐THPP. In general [Ca2+]i responses to TASK inhibitors, alone or in combination with BKCa inhibitors, were smaller than the [Ca2+]i responses evoked by hypoxia. These data confirm that TASK channel inhibition is capable of evoking membrane depolarization and robust voltage‐gated Ca2+‐entry but suggest that this, even with concomitant inhibition of BKCa channels, may be insufficient to account fully for the [Ca2+]i‐response to hypoxia. A1899, PK‐THPP, and doxapram inhibit endogenous TASK channels in carotid body chemoreceptor cells and promote voltage‐gated Ca‐entry. TEA, Paxilline, and iberiotoxin only weakly enhance the Ca‐response to TASK channel inhibition. The Ca‐response to TASK channel inhibition, with or without concomitant inhibition of BKCa channels was generally less than that to hypoxia.