Abstract
Intact human platelets can accumulate serotonin (5HT) in compartments which are vesicular (thrombin-releasable) or extra-vesicular (non-thrombin-releasable). 1. The net accumulation of extracellular 5HT in either compartment is not coupled in a reciprocal fashion to the presence of appropriate sodium gradients across the cell plasma membrane, and 5HT already present in either compartment is not removed when trans-membrane sodium gradients are abolished or reversed. 2. Net counter-transport of 5HT in either compartment is not detectable. 3. The data are not consistent with transport models such as that for glucose in which appropriate sodium gradients produce gradients of a co-transported molecule, or with models in which counter-transport can occur.