Platelet Aggregation Inhibitors
The 5' nucleotidase protein apyrase is found in the saliva of all blood feeding arthropods (and an analogue in Vampire bats and leeches). Apyrases are enzymes that degrade ATP and ADP to AMP and phosphate. ATP and ADP are powerful platelet aggregators while AMP has no effect on platelet aggregation. Injured blood vessel endothelial cells and degranulated platelets release ADP and serotonin (a vasoconstrictor), thus removing a powerful and initial host response to vessel damage. Interestingly, AMP along with ATP and ADP, are strong phagostimulants in some insects (Culex pipiens). It is thought that by inhibiting platelet aggregation the apyrase allowed accessible haematomas to form quickly which facilitates quicker, less noticeable feeding episodes. Because there are parallel platelet aggregation pathways (thromboxane) it is expected that other (redundant) anti-platelet aggregation compounds will eventually be found in the saliva of blood feeding arthropods. It is also interesting to note that, with some minor differences (Ca+2 or Mg+2 cofactors), salivary apyrase activity is conserved across all the arthropod classes, possibly indicating a common ancestral blood feeder (Beaty and Marquardt, 1996).
Apyrases also seem to have an effect on probing time of selenophagic feeders. Ribeiro et al (1985), and Ribeiro and Garcia (1981), removed the salivary glands from the mosquito Aedes aegypti and the kissing bug Rhodnius prolixus and found that, in both cases, they fed normally through a membrane, but on a live host they probed more often, relocated more often, and obtained less food per feeding than sham operated controls. Salivary apyrase was also found to be significantly reduced and it was hypothesized that without the ATP degrading action of apyrase, platelet aggregation was proceeding normally in the host and inhibiting blood feeding by the mosquito.
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