Muscarinic inhibition of L-type Ca2+ channels in guinea-pig tracheal smooth muscle cells.

Abstract

Modulation of L-type Ca2+ channels by acetylcholine (ACh) was studied in enzymatically isolated guinea-pig tracheal smooth muscle cells (TSMCs). ACh reversibly inhibited whole cell L type Ca2+ current measured with Ba2+ ions as charge carriers (I(Ba)). With pipette solution containing 0.1 mM EGTA, 1 microM ACh induced transient inhibition of I(Ba) followed by sustained inhibition (67.0+/-3.7% of the control, n=19). When intracellular Ca2+ concentration ([Ca2+]i) was fixed at 50 nM by BAPTA-Ca2+ buffer in the pipette, the transient inhibition was abolished whereas the sustained inhibition (66.0+/-7.8%, n=6) still occurred, suggesting that the transient inhibition was attributed to inactivation of the channels induced by increase in [Ca2+]i. The sustained inhibition was abolished when [Ca2+]i was fixed at zero. The sustained inhibition of I(Ba) by 1 microM ACh was observed in the presence of 10 microM AF-DX 116, whereas it was not observed in the presence of 1 microM 4 DAMP. ACh did not inhibit I(Ba) in the presence of 1 mM GDP-beta-S in the pipette, whereas the drug irreversibly inhibited the current in the presence of 0.1 mM GTP-gamma-S in the pipette. Pretreatment of TSMCs with pertussis toxin did not altered the effects of ACh. Application of neither 1-oleoyl-2-acetyl sn-glycerol (1 microM) nor phorbol 12-myristate 13-acetate (1 microM) reduced I(Ba). These results suggest that the sustained inhibition of I(Ba) by ACh is mediated by Ca2+ requiring and protein kinase C-independent mechanisms existing in the downstream of G-protein coupled with M3 receptors.