Content and Subcellular Localization of Catecholamines and 5‐hydroxytryptamine in Human and Animal Blood Platelets: Monoamine Distribution Between Platelets and Plasma

Abstract

The content of adrenaline (Ad), noradrenaline (NA) and dopamine was measured in human, guinea‐pig, cat, rabbit and rat blood platelets by a highly sensitive and specific radioenzymatic method. In all platelet specimens analyzed, the content of the three catecholamines (CA) was several thousand times lower than that of 5‐hydroxytryptamine (5‐HT). In basal conditions, the NA concentration in platelets and plasma always exceeded that of Ad and dopamine. In rat and rabbit platelets, Ad, NA and dopamine were present only in the free (unconjugated) form. Platelets of rats with storage pool deficiency (Fawn‐hooded) contained much less 5‐HT and CA than normal rat platelets. Following restraint stress, platelets of Fawn‐hooded rats, in contrast to normal rat platelets, did not accumulate CA in spite of a dramatic rise in plasma CA. Reserpine, a monoamine depletor, released CA as well as 5‐HT from rabbit platelets in vivo. Subcellular fractionation experiments with rabbit platelets indicate that both CA and 5‐HT are most concentrated in the fraction consisting of pure 5‐HT organelles. Both in humans and rabbits the concentration gradient between platelets and plasma was much lower for CA than for 5‐HT, indicating that a high affinity transport mechanism operates in vivo for 5‐HT but not for CA. In conclusion, the present data show that both human and animal platelets contain Ad, NA and dopamine. The bulk of the CA seems to be stored as unconjugated amines together with 5‐HT, histamine and p‐octopamine in a multitransmitter storage site, namely the 5‐HT organelle.

Evidence for a Control of Plasma Serotonin Levels by 5-Hydroxytryptamine2BReceptors in Mice

“…Blood serotonin, which is concentrated in the plateletdense granules (Ͼ99%), is almost absent from plasma (nanomolar range) (Da Prada and Picotti, 1979). Defective platelet uptake (Awabdy et al, 2003) or release from activated platelets could generate increased plasma serotonin as in platelet storage diseases (Hervé et al, 1990).…”

Dissipation mechanisms for 5‐hydroxytryptamine in the coronary circulation of the isolated perfused heart of the rat

“…(Snedecor & Cochran, 1980). (Stahl & Meltzer, 1978;Da Prada & Picotti, 1979) or in vesicles of either 5-hydroxytryptaminergic neurones (Aghajanian & Bloom, 1967) or noradrenergic neurones (Thoa et al, 1969;Verbeuren et al, 1983;McCulloch et al, 1984;Paiva et al, 1984). Alternatively, the 5-HT may be metabolized by MAO, e.g.…”

Serotonin in Neurological Diseases

“…Moreover, 5-HT in the blood is concentrated mainly in platelets what underlines their significant function in the serotonergic system. The 5-HT level in platelets is 24,000 times higher than in plasma [86] and the platelet 5-HT accounts for 98% of its total circulating amount [87].…”