Effect of the stable thromboxane derivative, carbocyclic thromboxane A 2 , on membrane potential of rat myenteric neurones in culture

  The effects of carbocyclic thromboxane A 2 (cTXA 2 ; 10 −6  mol L −1 ) on membrane potential and cytosolic Ca 2+ concentration were measured with the whole‐cell patch‐clamp or the fura‐2 method, respectively, at rat myenteric ganglia. cTXA 2 caused a hyperpolarization of myenteric neurones from −19.3 ± 2.5 to −29.3 ± 2.3 mV. In addition, the eicosanoid potentiated the carbachol‐induced depolarization from 4.2 ± 1.0 mV under control conditions to 11.1 ± 1.1 mV in the presence of the cTXA 2 ( n  = 9). The hyperpolarization was abolished by internal application of CsCl (140 mmol L −1 ), a non‐selective blocker of K + channels, or EGTA (11 mmol L −1 in the pipette solution), a chelator of intracellular Ca 2+ . A similar inhibition was observed in the presence of charybdotoxin (10 −7  mol L −1 ). Fura‐2 imaging experiments revealed a cTXA 2 ‐evoked increase in the intracellular Ca 2+ concentration as indicated by a rise in the fura‐2 ratio signal. This response was mediated by a release of Ca 2+ from intracellular stores as sarcoplasmic‐endoplasmic reticulum Ca 2+ ‐ATPase blockade with cyclopiazonic acid (5 × 10 −5  mol L −1 ) completely abolished the response to cTXA 2 . A similar inhibition was observed after blockade of phospholipase C with U‐73122 (10 −5  mol L −1 ). These results suggest an activation of Ca 2+ ‐activated K + channels by cTXA 2 after stimulation of phospholipase C.