Inhibition of depolarization-induced [3H]noradrenaline release from SH-SY5Y human neuroblastoma cells by some second-generation H1 receptor antagonists through blockade of store-operated Ca2+ channels (SOCs)

In the present study, the effect of the blockade of membrane calcium channels activated by intracellular Ca²⁺ store depletion on basal and depolarization-induced [³H]norepinephrine ([³H]NE) release from SH-SY5Y human neuroblastoma cells was examined. The second-generation H₁ receptor blockers astemizole, terfenadine, and loratadine, as well as the first-generation compound hydroxyzine, inhibited [³H]NE release induced by high extracellular K⁺ concentration ([K⁺]_e) depolarization in a concentration-dependent manner (the ic₅₀s were 2.3, 1.7, 4.8, and 9.4 μM, respectively). In contrast, the more hydrophilic second-generation H₁ receptor blocker cetirizine was completely ineffective (0.1-30 μM). The inhibition of high [K⁺]_e-induced [³H]NE release by H₁ receptor blockers seems to be related to their ability to inhibit Ca²⁺ channels activated by Ca_i ²⁺ store depletion (SOCs). In fact, astemizole, terfenadine, loratadine, and hydroxyzine, but not cetirizine, displayed a dose-dependent inhibitory action on the increase in intracellular Ca²⁺ concentrations ([Ca²⁺]_i) obtained with extracellular Ca²⁺ reintroduction after Ca_i ²⁺ store depletion with thapsigargin (1 μM), an inhibitor of the sarcoplasmic-endoplasmic reticulum calcium ATPase (SERCA) pump. The rank order of potency for SOC inhibition by these compounds closely correlated with their inhibitory properties on depolarization-induced [³H]NE release from SH-SY5Y human neuroblastoma cells. Nimodipine (1 μM) plus ω-conotoxin (100 nM) did not interfere with the present model for SOC activation. In addition, the inhibition of depolarization-induced [³H]NE release does not seem to be attributable to the blockade of the K⁺ currents carried by the K⁺ channels encoded by the human Ether-a-Gogo Related Gene (I_HERG) by these antihistamines. In fact, whole-cell voltage-clamp experiments revealed that the ic₅₀ for astemizole-induced hERG blockade is about 300-fold lower than that for the inhibition of high K⁺-induced [³H]NE release. Furthermore, current-clamp experiments in SH-SY5Y cells showed that concentrations of astemizole (3 μM) which were effective in preventing depolarization-induced [³H]NE release were unable to interfere with the cell membrane potential under depolarizing conditions (100 mM [K⁺]_e), suggesting that hERG K⁺ channels do not contribute to membrane potential control during exposure to elevated [K⁺]_e. Collectively, the results of the present study suggest that, in SH-SY5Y human neuroblastoma cells, the inhibition of SOCs by some second-generation antihistamines can prevent depolarization-induced neurotransmitter release.