Characterization of distinct receptors for the peptidyl leukotrienes LTC4 and LTD4/LTE4 coupled to the same signaling pathway in isolated gastric muscle cells
DeLegge, M.; Murthy, K.S.; Grider, J.R.; Makhlouf, G.M.
Journal of Pharmacology and Experimental Therapeutics 266(2): 857-863
1993
ISSN/ISBN: 0022-3565 PMID: 8394921 Document Number: 410252
Receptors for the peptidyl leukotrienes, (LT)C-4, LTD-4 and LTE-4, and the signaling pathways to which they are coupled were characterized in isolated guinea pig gastric muscle cells. The three LTs were equipotent contractile agonists (EC-50 values = 0.10-0.12 nM), but they elicited their responses by interacting with distinct receptors. The contractile responses to LTD-4 and LTE-4, but not LTC-4, were inhibited by the LTD-4 antagonist, SKF 104353 (2-(S)-hydroxy-3-(R)-((2-carboxyethyl)thiol)-3-(2-(8phenyloctyl)phenyl)-propanoic acid). Similar K-i estimates for SKF 104353 suggested interaction of LTD-4 and LTE-4 with a common receptor. Decisive evidence for distinct LTC-4 and LTD-4/LTE-4 receptors was obtained by applying a receptor protection technique. Cells in which LTC-4 was used as a receptor protective agent while other receptors were inactivated by N-ethylmaleimide retained their responses to LTC-4 only. Cells in which LTD-4, LTE-4 or SKF 104353 were used as a receptor protective agent retained their responses to LTD-4 and LTE-4 only. Both LTC-4 and LTD-4/LTE-4 receptors were coupled to PI hydrolysis: all three LTs stimulated similar increases in inositol 1,4,5-trisphosphate (IP-3) levels (3.9-4.3 pmol/10-6 cells), protein kinase C activity (85-94 pmol/mg/min) and cytosolic-free Ca++ ((Ca++)-i) (278-306 nM). Contractile responses were abolished: 1) when PI hydrolysis was inhibited by neomycin and 2) when Ca++ stores were depleted by pretreatment of muscle cells with caffeine in Ca++-free medium, but not when muscle cells were incubated in Ca++-free medium or with Ca++ channel blockers, suggesting that contraction and (Ca++), were mediated by IP-3-dependent Ca++ release. Indomethacin and meclofenamic acid did not affect contraction, implying that it did not involve activation of the cyclooxygenase pathway. In conclusion, distinct LTC-4 and LTD-4/LTE-4 receptors coupled separately to PI hydrolysis and IP-3-dependent Ca++ release and contraction are present in gastric muscle cells.