Acetylcholine modulation of high‐voltage‐activated calcium channels in the neurones acutely dissociated from rat paratracheal ganglia

The modulation of high‐voltage‐activated (HVA) Ca 2+ channels by acetylcholine (ACh) was studied in the paratracheal ganglion cells acutely dissociated from 2‐week‐old Wistar rats by use of the nystatin perforated patch recording configuration under voltage‐clamp conditions. ACh inhibited the HVA Ca 2+ currents in a concentration‐ and voltage‐dependent manner. The inhibition was mimicked by a muscarinic agonist, oxotremorine. Pirenzepine and methoctramine produced parallel shifts to the right in the ACh concentration‐response curves. Schild analysis of the ACh concentration‐ratios yield pA 2 values for pirenzepine and methoctramine of 6.85 and 8.57, respectively, suggesting the involvement of an M 2 receptor. Nifedipine, ω‐conotoxin‐GVIA and ω‐conotoxin‐MVIIC reduced the HVA I Ca by 16.8, 59.2 and 6.3%, respectively. A current insensitive to all of these Ca 2+ antagonists, namely ‘R‐type’, was also observed. The results indicated the existence of L‐, N‐, P/Q‐, and R‐type Ca 2+ channels. The ACh‐sensitive current component was markedly reduced in the presence of ω‐conotoxin‐GVIA, but not with both nifedipine and ω‐conotoxin‐MVIIC. ACh also inhibited the R‐type HVA I Ca remaining in saturating concentrations of nifedipine, ω‐conotoxin‐GVIA and ω‐conotoxin‐MVIIC. The inhibitory effect of ACh was prevented by pretreatment with pertussis toxin. It was concluded that ACh selectively reduces both the N‐ and R‐type Ca 2+ channels, by activating pertussis toxin sensitive G‐protein through the M 2 muscarinic receptor in paratracheal ganglion cells.British Journal of Pharmacology (1998) 123 , 1441–1449; doi: 10.1038/sj.bjp.0701753