Human bronchial epithelial cells express K<sub>Ca</sub>3.1 mRNA and protein, and K<sub>Ca</sub>3.1 expression is upregulated in the asthmatic bronchial epithelium.

<p><b>(A)</b> K<sub>Ca</sub>3.1 mRNA (predicted PCR product size: 130 bp) was detected in monolayers of primary HBECs isolated from both patients with asthma (n = 10; denoted with “A”) and non-asthmatic healthy controls (n = 5; denoted with “NA”), alongside the housekeeping gene β-actin (predicted PCR product size: 146 bp). (<b>B</b>) qPCR revealed that K<sub>Ca</sub>3.1 mRNA was expressed at similar levels in primary HBECs isolated from patients with asthma (n = 10) and healthy controls (n = 5). (<b>C</b>) An immunoreactive band of the appropriate size (K<sub>Ca</sub>3.1: 48 kDa; β-actin: 42 kDa) was detected in lysates of primary HBECs isolated from two patients with asthma and one healthy control. (<b>D</b>) Quantification of K<sub>Ca</sub>3.1 immunostaining by threshold analysis revealed that K<sub>Ca</sub>3.1 expression was significantly elevated in asthmatic bronchial epithelium (*P = 0.007). (<b>E</b>) K<sub>Ca</sub>3.1 immunostaining was significantly increased in severe asthma compared to mild asthma (**P = 0.008) and compared with healthy controls (*P = 0.002). (<b>F</b>) K<sub>Ca</sub>3.1 and MUC5AC immunostaining co-localised in bronchial epithelial cells of sequential sections of biopsies from patients with asthma and healthy controls. (<b>G</b>) Quantification of MUC5AC immunostaining by threshold analysis revealed that MUC5AC expression was significantly increased in asthmatic bronchial epithelium (*P = 0.030), and (<b>H</b>) this was driven by a significant difference between the severe asthma and healthy control groups (*P = 0.034). (<b>I</b>) A significant correlation was found between K<sub>Ca</sub>3.1 and MUC5AC immunostaining across the different severities of asthma and the healthy control groups (P < 0.001; r<sub>s</sub> = 0.608). All data are plotted as median ± interquartile range; horizontal bars represent medians.</p>