Neurokinin receptors in recurrent airway obstruction: a comparative study of affected and unaffected horses

Abstract

The purpose of the study was to compare in vitro airway responses to neurokinin A & B (NKA and NKB) and expression of NK-2 receptors in airways of horses affected and unaffected with recurrent airway obstruction (RAO). Neurokinin-A, an inflammatory mediator belonging to the tachykinin family of neuropeptides, causes bronchoconstriction by binding to NK-2 receptors. Neurokinin-B is a lesser-known neuropeptide that acts on NK-3 receptors. Horses were placed into RAO-affected and RAO-unaffected groups based on their history, clinical scoring, and pulmonary function testing. Lung tissue from each lobe was collected for immunohistochemical staining for NK-2 receptors. Cumulative concentration-response relationships were determined on bronchial rings (4-mm wide) collected and prepared from the right diaphragmatic lung lobe to graded concentrations (half log molar concentrations 10(-7)M to 10(-4)M) of NKA and NKB. The results showed that NKA caused significantly greater contraction than NKB in both groups. In RAO-affected horses, both agents produced significantly greater bronchial contractions than those in the RAO-unaffected horses. Immunohistochemical staining showed that the overall NK-2 receptor distribution was significantly increased in bronchial epithelium and smooth muscles of bronchi and pulmonary vessels of RAO-affected than RAO-unaffected horses. The findings indicate that NK-2 receptors are up-regulated in RAO, suggesting that NK-2 receptor antagonists may have some therapeutic value in controlling the progression of airway hyperreactivity in horses affected with RAO.

Figure 1

A and B — Concentration-response (CR) relationships of the bronchial rings of RAO-affected (▪)and unaffected (□) horses to half-log molar concentrations of neurokinin-A and neurokinin-B, respectively. The response is depicted as mg of tension per mg of dry tissue weight. The responses of the affected horses were significantly greater (*) than the unaffected horses. RAO — recurrent airway obstruction. (P ≤ 0.05).

Figure 2

Immunohistostaining of rat and horse brain tissues (20× magnification). A — immunohistostaining of rat brain showing positive for NK-2 receptors; B — staining where primary antibody was replaced with rabbit gamma globulin as a negative control; C — immunohistostaining of horse brain showing positive for NK-2 receptors; and D — staining where the primary antibody was replaced with rabbit gamma globulin as a negative control.

Figure 3

Immunohistostaining of RAO-affected and unaffected horse lung tissues (20× magnification). A — immunohistostaining of equine lung tissue from unaffected horse showing staining for NK-2 receptors; B — lung tissue from the same sample where the primary antibody was replaced with rabbit gamma globulin as a negative control; C — immunostaining of NK-2 receptors in the lung tissue of a RAO-affected horse; and D — lung tissue from the same sample where the primary antibody was replaced with rabbit gamma globulin as a negative control.

Figure 4

Immunohistochemical staining intensity scores for NK-2 receptors in the bronchial smooth muscle, bronchial epithelium, vascular smooth muscle, and vascular endothelium of different lobes of lungs from RAO-affected (black bars) and RAO-unaffected (white bars) horses. AC — accessory lobe, LA — left apical lobe, LD — left diaphragmatic lobe, RA — right apical lobe, and RD — right diaphragmatic lobe. *Significantly greater intensity of staining (P ≤ 0.05)