In vitro response of large colon arterial and venous rings to vasodilating drugs in horses.

This research investigated how arteries and veins in the large colons of horses respond to different vasodilating drugs, with and without endothelium, a layer lining blood vessels. It found that acepromazine and isoxsuprine were most effective, potentially improving blood flow after colon twisting events. However, dopamine was deemed unsuitable due to its confusing response and risk of reducing blood flow further.

Methodology

• Seven adult horses provided artery and vein rings from their large colons for this experiment, some with the endothelium intact, and others without.
• These samples were mounted in organ baths and attached to a transducer, before being contracted using norepinephrine (NE).
• The study used various drugs known for vasodilation: dopamine (DOP), dopexamine (DPX), acepromazine (ACE), isoxsuprine (ISX), and nifedipine (NFP), to see how the vessels responded. The vessels’ relaxation in reaction to the drugs was measured by monitoring the change in tension of each ring.
• They used varying concentration levels of each drug (ranging from 10(-8) to 10(-4)M) to gain a cumulative concentration-response relationship for each.
• They calculated the concentration of each drug needed to relax the NE-induced contracted tissue to 50% of its contracted state, known as the EC50 value, and the percentage maximal relaxation was measured.

Results

• The results showed significant differences among the drugs in both their EC50 values and their maximum possible relaxation. Dopamine had a unique biphasic response, meaning it first increased and then decreased, potentially posing a risk of reducing blood flow further.
• When the endothelium was removed from the veins, there were no significant changes noticed. However, the EC50 values increased and the maximum relaxation percentage decreased quite significantly in the removed-endothelium arteries compared to those left intact.

Conclusions

• The study concluded that ACE and ISX were the most potent and efficacious drugs evaluated, suggesting that they could potentially be used to improve blood flow after the correction of large-colon volvulus, a condition in which the colon twists around itself.
• They advised against the use of dopamine due to its confounding response and the potential for further reducing blood flow.
• Finally, they noted that the removal of endothelium altered the vasodilatory responses of colonic arterial rings, but not the venous rings, suggesting different reactions across different areas of the body’s vasculature.