Differential Desensitization of Ca²⁺ Mobilization and Vasoconstriction by ET_A Receptors in the Gerbil Spiral Modiolar Artery

Abstract.

Endothelins are known to be among the most potent endogenous vasoconstrictors. Vasoconstriction of the spiral modiolar artery, which supplies the cochlea, may be implicated in hearing loss and tinnitus. The purpose of the present study was to determine whether the spiral modiolar artery responds to endothelin, whether a change in the cytosolic Ca²⁺ concentration ([Ca²⁺]_i) mediates the response and which endothelin receptors are present. The vascular diameter and [Ca²⁺]_i were measured simultaneously by videomicroscopy and microfluorometry in the isolated spiral modiolar artery from the gerbil. ET-1 induced a transient [Ca²⁺]_i increase and a strong and long-lasting vasoconstriction. The transient [Ca²⁺]_i increase underwent rapid desensitization, was independent of extracellular Ca²⁺ and inhibited by the IP₃-receptor blocker (75 μm) 2-aminoethoxydiphenyl borate (2-APB) and by depletion of Ca²⁺ stores with 10⁻⁶ m thapsigargin. In contrast, the vasoconstriction displayed no comparable desensitization. The initial vasoconstriction was independent of extracellular Ca²⁺ but maintenance of the constriction depended on the presence of extracellular Ca²⁺. The half-maximal concentration values (EC ₅₀) for the agonists ET-1, ET-3 and sarafotoxin S6c were 0.8 nm, >10 nm and >100 nm, respectively. Affinity constants for the antagonists BQ-123 and BQ-788 were 24 nm and 77 nm, respectively. These observations demonstrate that ET-1 mediates a vasoconstriction of the gerbil spiral modiolar artery via ET_A receptors and an IP₃ receptor-mediated release of Ca²⁺ from thapsigargin-sensitive Ca²⁺ stores. The marked difference in desensitization between Ca²⁺ mobilization and vasoconstriction suggests that Ca²⁺ mobilization is not solely responsible for the vasoconstriction and that other signaling mechanisms must be present.