Histamine is an inflammatory mediator that can be released from mast cells to induce airway remodeling and cause persistent airflow limitation in asthma. In addition to stimulating airway smooth muscle cell constriction and hyperplasia, histamine promotes pulmonary remodeling by inducing fibroblast proliferation, contraction, and migration. It has long been known that histamine receptor 1 (H1R) mediates the effects of histamine on human pulmonary fibroblasts through an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i), but the underlying signaling mechanisms are still unknown. Herein, we exploited single-cell Ca²⁺ imaging to assess the signal transduction pathways whereby histamine generates intracellular Ca²⁺ signals in the human fetal lung fibroblast cell line, WI-38. WI-38 fibroblasts were loaded with the Ca²⁺-sensitive fluorophore, FURA-2/AM, and challenged with histamine in the absence and presence of specific pharmacological inhibitors to dissect the Ca²⁺ release/entry pathways responsible for the onset of the Ca²⁺ response. Histamine elicited complex intracellular Ca²⁺ signatures in WI-38 fibroblasts throughout a concentration range spanning between 1 µM and 1 mM. In accord, the Ca²⁺ response to histamine adopted four main temporal patterns, which were, respectively, termed peak, peak-oscillations, peak-plateau-oscillations, and peak-plateau. Histamine-evoked intracellular Ca²⁺ signals were abolished by pyrilamine, which selectively blocks H1R, and significantly reduced by ranitidine, which selectively inhibits H2R. Conversely, the pharmacological blockade of H3R and H4R did not affect the complex increase in [Ca²⁺]_i evoked by histamine in WI-38 fibroblasts. In agreement with these findings, histamine-induced intracellular Ca²⁺ signals were initiated by intracellular Ca²⁺ release from the endoplasmic reticulum through inositol-1,4,5-trisphosphate (InsP₃) receptors (InsP₃R) and sustained by store-operated Ca²⁺ channels (SOCs). Conversely, L-type voltage-operated Ca²⁺ channels did not support histamine-induced extracellular Ca²⁺ entry. A preliminary transcriptomic analysis confirmed that WI-38 human lung fibroblasts express all the three InsP₃R isoforms as well as STIM2 and Orai3, which represent the molecular components of SOCs. The pharmacological blockade of InsP₃ and SOC, therefore, could represent an alternative strategy to prevent the pernicious effects of histamine on lung fibroblasts in asthmatic patients.

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