Inositol 1,4,5-trisphosphate receptors (IP 3Rs) are intracellular Ca 2+ channels. Interactions of the commonly used antagonists of IP 3Rs with IP 3R subtypes are poorly understood.
IP 3-evoked Ca 2+ release from permeabilized DT40 cells stably expressing single subtypes of mammalian IP 3R was measured using a luminal Ca 2+ indicator. The effects of commonly used antagonists on IP 3-evoked Ca 2+ release and 3H-IP 3 binding were characterized.
Functional analyses showed that heparin was a competitive antagonist of all IP 3R subtypes with different affinities for each (IP 3R3 > IP 3R1 ≥ IP 3R2). This sequence did not match the affinities for heparin binding to the isolated N-terminal from each IP 3R subtype. 2-aminoethoxydiphenyl borate (2-APB) and high concentrations of caffeine selectively inhibited IP 3R1 without affecting IP 3 binding. Neither Xestospongin C nor Xestospongin D effectively inhibited IP 3-evoked Ca 2+ release via any IP 3R subtype.
Heparin competes with IP 3, but its access to the IP 3-binding core is substantially hindered by additional IP 3R residues. These interactions may contribute to its modest selectivity for IP 3R3. Practicable concentrations of caffeine and 2-APB inhibit only IP 3R1. Xestospongins do not appear to be effective antagonists of IP 3Rs.