mRNA expression of the acute phase protein serum amyloid A (SAA) in histologically normal tissues derived from horses (n=13) and cattle (n=4) was investigated by quantitative reverse-transcriptase real-time polymerase-chain reaction. As expected, high constitutive SAA mRNA expression was demonstrated in hepatic tissue in both species. In horses, moderate (>1% of the hepatic expression) SAA mRNA expression was detected in the lung, mammary gland, pancreas, synovial membrane, thymus, thyroid gland and uterus. Other equine tissues and organs sampled included adipose tissue, adrenal gland, aorta, brain, different gastro-intestinal tissues, heart, kidney, lymph node, ovary, testis, prostate, skeletal and cardiac muscle, skin and spleen; all showed low (<1% of the hepatic expression) SAA mRNA expression. In cattle, SAA mRNA was expressed in moderate levels in adipose tissue, colon, jejunum, mammary gland, skeletal muscle, synovial membrane, thymus, thyroid gland, and uterus; expression was low in the remainder of the samples (same tissue panel as horses). The results confirm the liver as the main site of SAA production. Even though there was some inter-species variation in tissues expressing SAA mRNA, several organs communicating with the external environment (lung, mammary gland, uterus, and certain parts of the gastro-intestinal tract) showed SAA mRNA expression, which supports the hypothesis that SAA might possess a role in the innate defence against invading pathogens. The results of the study thus warrant further studies into functions of hepatically and extrahepatically produced SAA isoforms.
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