Characteristics of the in vitro hypoxic pulmonary vasoconstrictor response in isolated equine and bovine pulmonary arterial rings.

This research article explores the possible causes for horse’s poor tolerance to dorsal recumbency (laying on their back), particularly during general anesthesia. The study investigates the effect of hypoxic pulmonary vasoconstriction (HPV) in pulmonary arteries of horses and cattle, and how various biological substances like nitric oxide and nonsteroidal anti-inflammatory drugs may impact this response.

Investigating Pulmonary Vessel Reaction to Hypoxia

• The study investigated the physiological reactions associated with poor tolerance to dorsal recumbency in horses, especially the phenomenon of hypoxic pulmonary vasoconstriction (HPV), which could contribute to complications during anesthesia.
• To do this, the researchers compared the HPV responses in isolated pulmonary artery vessels from horses and cows.
• Pulmonary arteries were removed from horses and cattle that had been euthanized. These isolated rings of pulmonary vessels were then subjected to isometric tension measurements at a temperature of 37 degrees Celsius in Krebs’ saline solution.
• Hypoxia (low levels of oxygen) was induced by exposing the samples to a gas mixture with 0% oxygen.

Results and Findings

• The researchers found a noticeable HPV response above a baseline tension induced by two substances, namely, phenylephrine (PE; 0.3 microm) or 5-hydroxytryptamine (5-HT; 0.1 microm).
• The HPV response in horse pulmonary vessels was approximately 33% weaker than that observed in corresponding cow vessels.
• The study found that removing the endothelium (the inner lining of the blood vessels) significantly decreased, but did not completely eliminate, the HPV response. This suggested that a portion of the HPV response necessitates a functional endothelium.
• The use of certain inhibitors, such as N-nitro-l-arginine methyl ester (L-NAME; a nitric oxide synthase inhibitor), or COX-1/COX-2 inhibitor indomethacin, significantly weakened the HPV response in equine vessels. This suggests that nitric oxide and/or the products of cyclooxygenase (COX), enzymes that produce substances that aid in vessel constriction and dilation, play a role in modulating the HPV response.
• Furthermore, the study noted that a non-COX related action by the nonsteroidal anti-inflammatory drug, indomethacin, might contribute to this reduced HPV response.