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

Abstract

1. The modulation of high-voltage-activated (HVA) Ca²⁺ 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.

2. ACh inhibited the HVA Ca²⁺ currents in a concentration- and voltage-dependent manner.

3. 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 pA₂ values for pirenzepine and methoctramine of 6.85 and 8.57, respectively, suggesting the involvement of an M₂ receptor.

4. 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 Ca²⁺ antagonists, namely `R-type', was also observed. The results indicated the existence of L-, N-, P/Q-, and R-type Ca²⁺ channels.

5. 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.

6. The inhibitory effect of ACh was prevented by pretreatment with pertussis toxin.

7. It was concluded that ACh selectively reduces both the N- and R-type Ca²⁺ channels, by activating pertussis toxin sensitive G-protein through the M₂ muscarinic receptor in paratracheal ganglion cells.

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