α₁-adrenoceptor-activated cation currents in neurones acutely isolated from rat cardiac parasympathetic ganglia

The noradrenaline (NA)-induced cation current was investigated in neurones freshly isolated from rat cardiac parasympathetic ganglia using the nystatin-perforated patch recording configuration. Under current-clamp conditions, NA depolarized the membrane, eliciting repetitive action potentials. NA evoked an inward cation current under voltage-clamp conditions at a holding potential of -60 mV. The NA-induced current was inhibited by extracellular Ca²⁺ or Mg²⁺, with a half-maximal concentration of 13 μM for Ca²⁺ and 1.2 mM for Mg²⁺. Cirazoline mimicked the NA response, and prazosin and WB-4101 inhibited the NA-induced current, suggesting the contribution of an α₁-adrenoceptor. The NA-induced current was inhibited by U73122, a phospholipase C (PLC) inhibitor. The membrane-permeable IP₃ receptor blocker xestospongin-C also blocked the NA-induced current. Furthermore, pretreatment with thapsigargin and BAPTA-AM could inhibit the NA response while KN-62, phorbol 12-myristate 13-acetate (PMA) and staurosporine had no effect. These results suggest that NA activates the extracellular Ca²⁺- and Mg²⁺-sensitive cation channels via α₁-adrenoceptors in neurones freshly isolated from rat cardiac parasympathetic ganglia. This activation mechanism also involves phosphoinositide breakdown, release of Ca²⁺ from intracellular Ca²⁺ stores and calmodulin. The cation channels activated by NA may play an important role in neuronal membrane depolarization in rat cardiac ganglia.