Researchers from Purdue University are unlocking possible ways to extend the benefits of a feed additive that makes pigs meatier.

Ractopamine hydrochloride is an effective and popular supplement, but it begins to lose its effectiveness seven to 10 days after the initial feeding. Understanding how ractopamine speeds growth in hogs and makes them leaner may enable scientists to make it more valuable for producers by increasing the additive’s effective period or creating new, similar drugs, said Scott Mills, Purdue associate professor of animal science.

Mills, in a paper soon to be posted online in the Journal of Animal Science E-supplement 1, writes that hogs apparently become desensitized to ractopamine when it’s fed as a daily ration. This results in the fleeting growth benefits of the metabolism-modifying drug. Mills and his research team are investigating molecular pathways involved when the supplement redirects nutrients from fat to muscle tissue.

He explained: “We’re looking for ways to improve efficiency of meat animals through enhanced growth rate and body composition. We want to know what we can do to prolong the benefits of ractopamine and to circumvent desensitization.”

Pigs have a built-in safeguard to prevent overstimulation by growth enhancers and other supplements, Mills said. This is a biochemical process that can be likened to a key and a lock. The key opens the lock, but after the lock is used too many times, it wears out so the key no longer opens it. The key is a molecule called a ligand, in this case ractopamine, and the lock is a molecule in the body called a receptor.

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When ractopamine couples with its corresponding receptor over a sustained period, something in the body determines that enough is enough, halting the supplement’s effectiveness.

With ractopamine, Mills said not only do the receptors stop reacting to the additive but also the pigs actually lose receptors. As the receptors disappear, the key (ractopamine) has fewer locks to open that will trigger the chain reaction leading to faster growth and more muscle.

“The net result is that chronic exposure leads to fewer receptors, so then there is a loss of response to, and function of, the drug,” he said.

Ractopamine, sold commercially by Eli Lilly and Co. as Paylean, is one of a group of compounds called beta-adrenergic agonists. Forms of these substances have been used for years to treat asthma and other diseases in people. Ractopamine is the first of this class to be FDA approved for swine growth, but studies have been done in other animals with similar compounds in this category.

For instance, other researchers found that feeding mice a daily ration of a drug similar to ractopamine also resulted in only short-term benefits. But they discovered feeding it intermittently to the rodents lengthened the enhancement period. In addition, the animals needed 50% less of the supplement to produce the same positive results as feeding it every day.

A second method is to increase the dose incrementally to compensate for the decline in receptor numbers. Purdue scientists are now investigating this feeding method with ractopamine in pigs.

Although both feeding approaches are possibilities, Mills said feeding the supplement “on and off”, or intermittently, would be too labor-intensive in most pork operations to be practical. Feeding it incrementally is showing some benefit.

“The receptors in pigs are different than in mice and humans,” he said.

“More potential may be gained by designing drugs that target receptors in fat and muscle tissue of pigs.”

The US Department of Agriculture funded this study.

Related Web sites:

Journal of Animal Science, Vol. 80, E-supplement 1:

Purdue University beta agonist site: