Origin an importance of increased alkaline phosphatase activity in peritoneal fluids of horses with colic.

The origin of increased alkaline phosphatase (ALP) activity in peritoneal fluid (PF) of horses with clinical signs of abdominal pain was investigated to determine the usefulness of measuring ALP in PF in the diagnosis of small intestinal injury. The ALP isoenzymes in PF from 10 clinically normal horses and from 50 horses with clinical signs of acute abdominal pain were analyzed for their sensitivities to inhibition by L-phenylalanine, L-homoarginine, and levamisole and to inactivation by heat (56 C, 15 minutes). The enzymes also were discriminated by their patterns of migration during polyacrylamide gel disc electrophoresis. Of 50 horses with colic, 20 had ALP activity in PF at least 3 times the upper limit of normal. Of these 20 horses, 10 had marked increases of ALP activity in PF ranging from 10 to 150 times the mean value of activity as determined in the 10 normal horses. In the 50 horses with colic, ALP values in serum were within the normal range. In 19 of the 20 sick horses, the ALP in PF had properties different from small intestinal ALP. Of the 10 PF samples with markedly increased ALP activity, 9 had a group of properties that were unique for granulocytic ALP. The clinical diagnoses for the 10 horses with markedly increased ALP activity in PF included thromboembolic colic (4 horses), colonic torsion (2 horses), small intestinal volvulus (2 horses), peritonitis (1 horse), and salmonellosis (1 horse). Properties of the enzyme in the 10 PF samples with moderately increased ALP activity were compatible with a granulocytic origin, but insufficient enzyme concentration precluded electrophoretic confirmation of the source. The PF from 1 horse had a mixture of ALP isoenzymes derived from granulocytes and small intestinal mucosa. Of the 50 horses with colic, 6 had severe small intestinal disease without increased ALP activity in PF. Apparently, increased ALP activity in PF cannot be used as a reliable indicator of small intestinal injury in horses, because the ALP is predominantly granulocytic in origin.