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Insulin Resistance and Laminitis 77 for Equine Practitioners Appendix 3 Oral Domperidone Test Dr.

Nicholas Frank Testing procedure This is a new test for pituitary pars intermedia dysfunction (PPID; also called equine Cushing’s disease) that was developed by researchers at Purdue University.60 Domperidone is a D2 dopamine receptor antagonist used for the treatment of agalactia in mares that have been exposed to endophyte on fescue grass (fescue toxicity).

This drug is available in a gel form that is packaged in a dose syringe (Equidone®, Equi-Tox Inc., Central, South Carolina) for oral administration, and it costs approximately $50 per tube.

Sample handling This test involves the measurement of plasma ACTH concentrations, so special sample handling is required.

Sampling conditions: Plastic tubes containing EDTA must be used and samples should be centrifuged within two hours of collection.

Samples should be kept standing (to allow separation) in a rack inside a refrigerator or cooler with ice packs.

Veterinarians in ambulatory practice should consider enlisting the help of the horse owner to transport samples back to the office.

Storage and shipping conditions: A minimum of 1 mL plasma should be harvested from each blood sample and transferred to plastic storage tubes.

Plasma samples must remain cool at all times when they are handled.

Samples should be sent via overnight mail the same day or frozen (-20º C) overnight and then shipped.

On the day of shipping, samples should be packaged in a cooler with ice packs and sent to the laboratory via courier or overnight mail.

It is very important to include enough ice packs to ensure that samples remain cold at all times.

Values obtained from plasma samples that have warmed above 4º C (refrigerator temperature) are likely to be inaccurate.

Laboratories that measure ACTH in equine plasma include: Cornell University College of Veterinary Medicine Animal Health Diagnostic Center (www.diaglab.vet.cornell.edu); telephone: ph (607) 253-3900 Michigan State University Diagnostic Center for Population and Animal Health (www.animalhealth.msu.edu); ph (517) 353-1683 Guide to 78 Insulin Resistance and Laminitis 79 for Equine Practitioners Procedure 1. Testing should begin in the morning between 8:00 AM and 10:00 AM. 2. The first blood sample is collected before domperidone is administered. 3. One tube of Equidone® containing 25 mL gel (2.75 grams domperidone in total) should be administered by mouth to a 500-kg horse.

This is equivalent to 5.5 mg domperidone/kg body weight and the appropriate dose should be calculated for smaller horses and ponies.

Each milliliter of gel contains 110 mg domperidone. 4. A second blood sample should be collected four hours later if testing is performed from January to August (middle and eastern United States) and samples should be collected two and four hours after domperidone is administered if testing is performed in the fall.

Principle of the test Horses with PPID secrete ACTH from hyperplastic or neoplastic tissue within the pars intermedia of the pituitary gland.

This region of the pituitary gland does not secrete ACTH in the healthy animal.

The pars intermedia is under tonic inhibition from dopamine produced by dopaminergic neurons originating from the hypothalamus and terminating in this region of the pituitary gland.

Domperidone is a D2 receptor blocker, so administration of this drug blocks inhibition by dopamine, which allows increased activity within the pars intermedia.

This increase in activity causes more ACTH to be secreted into circulation when PPID is present.

Plasma ACTH concentrations significantly increase after domperidone administration if the horse or pony suffers from PPID.

One confusing aspect of this test is that PPID results from degeneration of dopaminergic neurons, yet this test is based upon domperidone blocking the actions of dopamine produced by these neurons.

It must therefore be recognized that dopaminergic neurons may be reduced in number in horses with PPID, but dopamine is still being secreted.

Domperidone blocks the inhibitory actions of dopamine, which allows activity within the pars intermedia to increase.

In horses with PPID, abnormal melanotropes within the pars intermedia are secreting ACTH and they produce more of this hormone after domperidone is administered. Interpretation 1. A twofold or higher increase in plasma ACTH concentration after four hours is a positive result for PPID between January and August.

The ACTH response to domperidone is exaggerated in the autumn, so a greater than twofold increase at two hours is a positive result between September and December.

A horse that shows a less than twofold increase at two hours in the fall, but a positive result at four hours should be retested. 2. An elevated pre-domperidone (baseline) ACTH concentration is indicative of PPID and a cut-off value of 45 pg/mL (10 pmol/L) is generally used.

Plasma ACTH concentrations are reported in pg/mL or pmol/L.

Values reported in pg/mL can be converted to pmol/L using a multiplication factor of 0.22. 3. The ODT can be modified by instructing the horse owner to administer the domperidone four hours prior to the appointment time (two hours prior when testing is performed in the fall).

When this single-sample approach is used, a plasma ACTH concentration greater than 100 pg/mL (22 pmol/L) is considered positive for PPID (personal communication; Dr.

Janice Sojka; April 2008).

ReferenceS 1. Miller, M.A., Pardo, I.D., Jackson, L.P ., Moore, G.E., Sojka, J.E., 2008, Correlation of pituitary histomorphometry with adrenocorticotrophic hormone response to domperidone administration in the diagnosis of equine pituitary pars intermedia dysfunction.

Vet Pathol 45, 26-38.

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