title: Interactions of antagonists with subtypes of inositol 1,4,5-trisphosphate (IP3) receptor.
abstract: OBJECTIVE
  Inositol 1,4,5-trisphosphate receptors (IP3 Rs) are intracellular Ca(2+) channels. Interactions of the commonly used antagonists of IP3Rs with IP3R subtypes are poorly understood.
  
  
  METHODS
  IP3-evoked Ca(2+) release from permeabilized DT40 cells stably expressing single subtypes of mammalian IP3R was measured using a luminal Ca(2+) indicator. The effects of commonly used antagonists on IP3-evoked Ca(2+) release and (3) H-IP3 binding were characterized.
  
  
  RESULTS
  Functional analyses showed that heparin was a competitive antagonist of all IP3R subtypes with different affinities for each (IP3R3 > IP3R1 ≥ IP3R2). This sequence did not match the affinities for heparin binding to the isolated N-terminal from each IP3R subtype. 2-aminoethoxydiphenyl borate (2-APB) and high concentrations of caffeine selectively inhibited IP3R1 without affecting IP3 binding. Neither Xestospongin C nor Xestospongin D effectively inhibited IP3-evoked Ca(2+) release via any IP3R subtype.
  
  
  CONCLUSIONS
  Heparin competes with IP3, but its access to the IP3-binding core is substantially hindered by additional IP3R residues. These interactions may contribute to its modest selectivity for IP3R3. Practicable concentrations of caffeine and 2-APB inhibit only IP3R1. Xestospongins do not appear to be effective antagonists of IP3Rs.
authors.affiliation: Department of Pharmacology, University of Cambridge, Cambridge, UK.
authors.forename: Huma
authors.initials: H
authors.lastname: Saleem
authors.forename: Stephen C
authors.initials: SC
authors.lastname: Tovey
authors.forename: Tedeusz F
authors.initials: TF
authors.lastname: Molinski
authors.forename: Colin W
authors.initials: CW
authors.lastname: Taylor
mesh_terms: D000818:Animals; D001896:Boron Compounds / Q000494:pharmacology; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002460:Cell Line; D002645:Chickens; D006493:Heparin / Q000494:pharmacology*; D053496:Inositol 1,4,5-Trisphosphate Receptors / Q000037:antagonists & inhibitors*; D047028:Macrocyclic Compounds / Q000494:pharmacology; D010080:Oxazoles / Q000494:pharmacology
chemical_list: D001896:Boron Compounds; D053496:Inositol 1,4,5-Trisphosphate Receptors; D047028:Macrocyclic Compounds; D010080:Oxazoles; C511704:xestospongin C; C108642:xestospongin D; D002110:Caffeine; D006493:Heparin; C109986:2-aminoethoxydiphenyl borate; D002118:Calcium
keywords: 2-APB; Ca2+ signal; DT40 cell; IP3 receptor; Xestospongin; antagonist; caffeine; heparin; inositol 1,4,5-trisphosphate; structure-activity relationship