Platelet aggregation in response to adenosine diphosphate (ADP) was evaluated in 16 healthy cats and in 10 cats with cardiomyopathy. The minimum threshold concentration of ADP required to induce irreversible (2nd-phase) aggregation was determined in each cat. The minimum ADP concentration needed for 2nd-phase aggregation in platelets from healthy cats ranged from 1 μM to 100 μM ADP, with 56% (9/16) requiring 100 μM ADP. Of the remaining seven normal cats, three had platelets responding irreversibly to 10 μM ADP, and four had platelets responding to 1 μM ADP. In cats with cardiomyopathy, the threshold concentrations ranged from 0.01 μM ADP to 10 μM ADP. Two cats had platelets responding irreversibly to 0.01 μM ADP, whereas another cat had a threshold response at 0.1 nM ADP. Platelets from the remaining seven cats with cardiomyopathy exhibited 2nd-phase aggregation in response to 1 μM ADP (five cats) or 10 μM ADP (two cats).
Platelet counts ranged from 210,000/mm3 to 630,000/mm3 in healthy cats and from 218,000/mm3 to 624,000/mm3 in cats with cardiomyopathy. There was no apparent correlation between the platelet count and the magnitude of the threshold aggregation response, as measured by lag phase and slope of the aggregation curves.
The results indicate that some cats with cardiomyopathy have platelets that are hyperaggregable to ADP in vitro.