Abstract Body: Background. Inflammation is a crucial mechanism driving kidney disease. One of the key mediators of inflammation is histamine; limited data are available regarding the renal histaminergic system (HiS). We have reported that rat and human kidneys express all components of the HiS, including histamine receptors (HRs). Although HRs are known to elicit Ca²⁺ signaling in the lung epithelium and other tissues, there is a gap in knowledge regarding the effects of histamine in the podocyte - the key cell of the glomerular filtration barrier. We hypothesize that histamine elicits acute Ca²⁺ transients in podocytes and influences podocyte structure and function.

Methods. Glomeruli freshly isolated from male Sprague-Dawley rats were loaded with Fluo8/FuraRed for confocal imaging; live Ca²⁺ responses from podocytes were recorded in Ca²⁺-free or 2 mM CaCl₂-containing solution to quantify intracellular vs extracellular Ca²⁺ contributions after histamine application. Transients were quantified for peak response, AUC, and the t (decay constant). Antagonists for HRs – 10 μM olopatadine (H1R), 10 μM tiotidine (H2R), 10 μM JNJ-7207852 dihydrochloride (H3R), and 10 μM JNJ-7777120 (H4R), and 1 μM BI-749327 (TRPC6 antagonist) were used to characterize the Ca²⁺ signaling pathway. OriginPro was used for statistical analysis (1-way ANOVA; p < 0.05). Values are reported as the mean ± the standard error of the mean (SEM).

Results. We detected acute robust Ca²⁺ transients after histamine application in rat glomerular podocytes. The peak responses in 2 mM CaCl₂ vs 0 mM CaCl₂ were similar, 2.1 ± 0.1 vs 2.2 ± 0.2 A.U. (p > 0.05). The AUC in 2 mM CaCl₂ vs 0 CaCl₂ was 11.7 ± 2.0 vs. 7.2 ± 1.8 A.U., respectively (p > 0.05). The t decay constant between 2 mM CaCl₂ and 0 CaCl₂ solution was 7.6 ± 2.0 s vs. 2.7 ± 0.5 s, respectively (p > 0.05). Of note, inhibiting TRPC6 channels did not change Ca²⁺ transient sizes or kinetics. Pilot experiments show that an H2R antagonist, but not other HR inhibitors, dampens the peak Ca²⁺ response vs. 2 CaCl₂ controls (p < 0.05).

Conclusions. Histamine elicits Ca²⁺ transients in podocytes which is primarily mediated through intracellular stores and not TRPC6 channels. Future studies will define the specific HR involvement, and mechanistic details of intracellular Ca²⁺ store signaling. There is a translational impetus to unveil HiS’s role in the kidneys as dose and frequency adjustments of antihistamines may be required in CKD patients.