Traditional animal models of severe asthma do not recapitulate defining asthma characteristics, including persistent airway hyper-responsiveness, and chronic neutrophilic inflammation. This is problematic because moderating airway hyper-responsiveness decreases asthma frequency and severity, making it a paramount pharmacological goal in asthma research. Employing a spontaneous equine asthma model (equine pasture asthma, EPA), we first confirmed reversible airway obstruction in eight diseased horses during asthma exacerbations in response to ß2renergic agonist stimulation. Next, non-specific airway hyper-responsiveness was confirmed using methacholine bronchoprovocation to identify the provocative concentration causing a 40% increase in baseline lung resistance (PC40RL)- a threshold similarly employed in evaluating human asthmatics unable to mount forced expiration. The PC40RL of ten EPA horses was consistently <1mg/ml of methacholine, which is a cutoff that has been used to diagnose severe human asthma. Like non-asthmatic humans, ten control horses did not respond to methacholine doses up to 16 mg/ml. Finally, persistence of AHR was documented during absence of seasonal aeroallergen triggers in five horses that were evaluated between 3 and 31 months following the initial methacholine bronchoprovocation. This unique ability of EPA horses to model AHR attributes that are not addressed by other animal models points to the suitability of EPA horses to decipher the mechanistic basis of airway hyper-responsiveness. Building on knowledge that 2renergic receptor (AR) signaling is required to develop the asthma phenotype in a murine model, differentially expressed genes from serial lung biopsies of two EPA affected and two controls were filtered to identify genes that interact with the 2-AR. Hyperpolarization Activated Cyclic Nucleotide-Gated Potassium Channel 4 (HCN4) was prioritized because of its interactions with the 2-AR. Relative to control horses, HCN4 was constitutively expressed in airway smooth muscle of EPA horses during remission and increased during seasonal disease exacerbation. Agonism of airway contraction by HCN4 was proven using the specific HCN4 antagonist, ivabradine, which caused dose dependent decreases in carbachol induced contractile responses in both EPA and control bronchi in vitro. These findings highlight utility of EPA as a model of severe asthma and HCN4 as a mediator of airway contraction that warrants further investigation in severe human asthma.