“Serotonin mobilized intracellular Ca++ in a dose-dependent fashion … for 5-HT of 0.2 microM. Ketanserin, … Ca++ increase in a dose-dependent manner with an IC50 value of 2 nM. An initial treatment …, suggesting that 5-HT evoked an acute desensitization of 5-HT2 receptors in human platelets.”