Platelet aggregation induced by ADP in vivo: continuous and quantitative measurements in different vascular beds

Effect of intravascular platelet aggregation on blood recirculation following prolonged ischemia of the cat brain

Reversible complete cerebro-circulatory arrest was produced in normothermic, anesthetized cats for 1 h by clamping the innominate and subclavian arteries, followed by blood recirculation of the brain for 30 min to 4 h. Blood volume of tissue samples was measured by intravenous injection of 125I-albumin and the platelet number by labelling of autologous platelets with 51Cr. The number of platelets trapped in the tissue was determined by subtracting from the total platelet count of the tissue sample those contained in the streaming blood, calculated from the blood volume by 125I-albumin. In sham-operated animals, after recirculation of autologous 51Cr labelled platelets for 60 min, no platelets were entrapped in the brain vasculature and total platelet number and fibrinogen concentration in the venous blood did not change. In animals submitted to complete cerebral ischemia for 1 h the number of trapped platelets increased significantly in the whole brain after 30 min of recirculation, i.e. the period of maximal postischemic vasodilatation and hyperemia, and highest concentrations were found in the border zone with 270 +/- 60 x 10(4) platelets/g tissue and in the brainstem with 240 +/- 70 x 10(4) platelets/g tissue. Blood platelets and fibrinogen concentration in the venous blood decreased significantly from 288 +/- 37 x 10(6)/ml and 299 +/- 50 mg% to 186 +/- 25 x 10(6)/ml platelets and 112 +/- 22 mg+ of fibrinogen, respectively. Platelets were nearly completely washed out of the brain after 4 h of recirculation. The highest increase of platelets was found in the kidney, lung and liver after 30 min of recirculation, indicating that intravascular coagulation following prolonged cerebral ischemia affects the extracerebral organs more than the brain itself.