Interactions of antagonists with subtypes of inositol 1,4,5-trisphosphate (IP3) receptor

BACKGROUND AND PURPOSE

Inositol 1,4,5-trisphosphate receptors (IP₃Rs) are intracellular Ca²⁺ channels. Interactions of the commonly used antagonists of IP₃Rs with IP₃R subtypes are poorly understood.

EXPERIMENTAL APPROACH

IP₃-evoked Ca²⁺ release from permeabilized DT40 cells stably expressing single subtypes of mammalian IP₃R was measured using a luminal Ca²⁺ indicator. The effects of commonly used antagonists on IP₃-evoked Ca²⁺ release and ³H-IP₃ binding were characterized.

KEY 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 Ca²⁺ release via any IP₃R subtype.

CONCLUSIONS AND IMPLICATIONS

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.