Interactions of antagonists with subtypes of inositol 1,4,5-trisphosphate (IP ₃) receptor

Abstract

Background and Purpose

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.

Experimental Approach

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.

Key Results

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.

Conclusions and Implications

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.