The effect of oscillatory flow on the release reaction and aggregation of human platelets

The effect of variable shear rate on the thrombin-induced release of [¹⁴C]serotonin from human platelets was investigated by subjecting washed suspensions containing 5 × 10⁵ cells/μl to laminar oscillatory flow at 37°C in 1-meter lengths of 1.57-mm polyethylene tubing. The suspensions were prepared at 37°C in Tyrodes-albumin containing apyrase to avoid desensitization of the platelets by released ADP. After flow, the sheared sample and a control, incubated at rest at 37°C, were stirred with thrombin at concentrations of 0.04 units/ml (threshold at which aggregation just occurred) and 0.02 units/ml, and the ¹⁴C-activity in the supernatant measured after centrifugation.At time-averaged wall shear rates $\text{G}\text{(R}{}_0\text{) = 2350}\text{sec}{}^{\mathrm{?1}}$ and 20 min shear, there was no significant release of [¹⁴C]serotonin due to flow alone. However, in 53 experimental runs the mean release at 0.02 and 0.04 units/ml exceeded that in the paired controls by 6.1%(±1.4%, SE) and 10.1%(±1.3%) of the total activity in the suspension. The increase in serotonin release was not significant at $\text{G}\text{(R}{}_0\text{) < 1300}\text{sec}{}^{\mathrm{?1}}$.The synergistic effects of flow were also noted when [Ca²⁺] was increased from 4 to 12 mEq, and in the presence of ASA (1 mg/ml), release being less inhibited in the sheared samples. When adrenaline (2.5 μM) was added, the serotonin release, which increased by 26.4%(±3.8%) in 17 controls at the lower thrombin level, was further enhanced by 10.9%(±2.6%) in the paired sheared samples. However, when adrenaline was added after oscillatory flow, the reverse occurred, the sheared samples releasing 8.5% less than the controls.Studies in the aggregometer showed that the rate and extent of aggregation in the sheared samples was a little lower than that in the controls.