In the article entitled "Feline Bronchial Asthma: Pathophysiology and Diagnosis" (June 2005, p. 418), the second sentence of the second paragraph on the right column of page 420 states, "The β₂-adrenergic receptor is adenyl cyclase on bronchial smooth muscle cell membranes." Perhaps the authors intended to state that the β₂-adrenergic receptor is coupled with adenyl cyclase?

The authors are correct in stating that the β₂-adrenergic receptor is found on bronchial smooth muscle membranes; however, the β₂-adrenergic receptor is anatomically distinct from adenyl cyclase. The β₂-adrenergic receptor is a protein receptor that activates neighboring adenyl cyclase (an enzyme) via the heterotrimeric G_s-protein. In this sense, adenyl cyclase is considered the effector enzyme for the β₂-adrenergic receptor, but the two should not be considered the same cell"membrane component. The distinction between the β₂-adrenergic receptor and adenyl cyclase is important because various drugs are available for treating feline asthma that work specifically on either of these two cell-membrane components (β-agonists versus phosphodiesterase inhibitors versus inhaled glucocorticoids, which up-regulate β₂-adrenergic receptor populations), and clinicians are responsible for deciding the appropriate mechanistic approach for their patients (e.g., if there is decreasing response to a β-agonist, homologous desensitization of the receptor is likely occurring and a different mechanistic approach should be considered).

I otherwise enjoyed the article, as it provided an excellent overview of this multifactorial, frustrating disease, and have recommended it to my colleagues.

Tim LaBranche, DVM, PHD
University of Georgia

We thank Dr. LaBranche for his comments regarding adenyl cyclase and β₂-adrenergic receptors on bronchial smooth muscle. He is correct in his assertion that our statement should convey that adenyl cyclase is coupled with β₂-adrenergic receptors. As Dr. LaBranche appropriately mentioned, this anatomic distinction is truly important when selecting medications for feline patients with bronchial asthma.

Christopher G. Byers, DVM
VCA Veterinary Referral Associates, Inc.
Gaithersburg, MD

Nishi Dhupa, BVM, MRCVS, DACVIM, DACVECC
Cornell University