title: Tachyphylaxis to the inhibitory effect of L-type channel blockers on ACh-induced [Ca2+]i oscillations in porcine tracheal myocytes.
abstract: Discrepancies about the role of L-type voltage-gated calcium channels (VGCC) in acetylcholine (ACh)-induced [Ca(2+)](i) oscillations in tracheal smooth muscle cells (TSMCs) have been seen in recent reports. We demonstrate here that ACh-induced [Ca(2+)](i) oscillations in TMCS were reversibly inhibited by three VGCC blockers, nicardipine, nifedipine and verapamil. Prolonged (several minutes) application of VGCC blockers, led to tachyphylaxis; that is, [Ca(2+)](i) oscillations resumed, but at a lower frequency. Brief (15-30 s) removal of VGCC blockers re-sensitized [Ca(2+)](i) oscillations to inhibition by the agents. Calcium oscillations tolerant to VGCC blockers were abolished by KB-R7943, an inhibitor of the reverse mode of Na(+)/Ca(2+) exchanger (NCX). KB-R7943 alone also abolished ACh-induced [Ca(2+)](i) oscillations. Enhancement of the reverse mode of NCX via removing extracellular Na(+) reversed inhibition of ACh-induced [Ca(2+)](i) oscillations by VGCC blockers. Inhibition of non-selective cation channels using Gd(3+) slightly reduced the frequency of ACh-induced [Ca(2+)](i) oscillations, but did not prevent the occurrence of tachyphylaxis. Altogether, these results suggest that VGCC and the reverse mode of NCX are two primary Ca(2+) entry pathways for maintaining ACh-induced [Ca(2+)](i) oscillations in TSMCs. The two pathways complement each other, and may account for tachyphylaxis of ACh-induced [Ca(2+)](i) oscillations to VGCC blockers.
authors.affiliation: Department of Pharmacology and Toxicology, University of Mississippi Medical Center, Jackson, MS 39216-4505, USA.
authors.forename: Wen-Shuo
authors.initials: WS
authors.lastname: Chung
authors.forename: Jerry M
authors.initials: JM
authors.lastname: Farley
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D020746:Calcium Channels, L-Type / Q000378:metabolism*; D020013:Calcium Signaling / Q000187:drug effects*; D002478:Cells, Cultured; D032389:Myocytes, Smooth Muscle / Q000378:metabolism*; D013552:Swine; D014132:Trachea / Q000166:cytology / Q000378:metabolism*
chemical_list: D002121:Calcium Channel Blockers; D020746:Calcium Channels, L-Type; D002118:Calcium