Mystixin-7 and mystixin-11 increase cytosolic free Ca2+ and inositol trisphosphates in human A-431 cells

Mystixin-7 and mystixin-11, small peptides structurally related to corticotropin-releasing factor (CRF), have been shown to attenuate vascular leakage in injured skin. The goal of this study was to characterize changes in cytosolic Ca²⁺ concentration ([Ca²⁺]_i) in human epidermoid A-431 cells treated with these two peptides and to investigate the mechanisms by which these changes occurred. The resting [Ca²⁺]_i in A-431 cells at 37°C was 76±2 nM (n = 373). When cells were treated with either peptide, [Ca²⁺]_i increased immediately. The increase depended on the peptide concentration, with a median effective concentration of 299 ± 9 pM for mystixin-7 and 2.23 ± 0.04 pM for mystixin-11. The increases also depended on extracellular Ca²⁺ and were blocked by Cd²⁺, Co²⁺, verapamil, and nifedipine. α-Helical CRF-(9–41), a synthetic CRF receptor antagonist, and pertussis toxin also blocked the increase in [Ca²⁺]_i induced by the two peptides. Taken together, these results suggest that mystixin-7 and mystixin-11 interact with CRF receptors to activate pertussis-sensitive G proteins coupled to L-type Ca²⁺ channels that allow an uptake of extracellular Ca²⁺. Because U-73122, an inhibitor of 1,4,5-inositol trisphosphate production, partially inhibited the increase in [Ca²⁺]_i, we measured inositol trisphosphates in cells stimulated by the two peptides. Both increased inositol triphosphate levels within 1 min. The increase was inhibited by the removal of extracellular Ca²⁺ or treatment with U-73122. The results suggest that the Ca²⁺ influx stimulated by mystixin-11 induces an increase in inositol trisphosphates, resulting in a mobilization of Ca²⁺ from 1,4,5-inositol trisphosphate-sensitive Ca²⁺ pools.