Plasma and platelet levels of 5-hydroxytryptamine (5 HT) may be altere
d in essential hypertension. To establish the determinants and correla
tes of 5 HT in plasma and platelets, we studied 53 untreated subjects
with essential hypertension (26 men; 30 whites; mean supine blood pres
sure 172/101 mm Hg; mean age 49.3 +/- 1.5 years) and 61 normotensive s
ubjects (37 men; 47 whites; mean supine blood pressure 128/78 mm Hg; m
ean age 42.8 +/- 1.6 years). Plasma and platelet 5 HT were assayed by
reverse-phase high performance liquid chromatography with electrochemi
cal detection. No significant difference was found in platelet-poor pl
asma or platelet 5 HT levels in hypertensive or normotensive subjects
(plasma: 43.0 +/- 4.2 and 39.6 +/- 4.4 nmol/L; platelet: 1.65 +/- 1.22
and 1.70 +/- 1.39 nmol/10(9) cells in hypertensive and normotensive s
ubjects, respectively). No significant correlation was found between p
lasma or platelet 5 HT and systolic or diastolic blood pressure (plasm
a: r =0.01 and 0.01 in normotensive subjects and r = 0.01 and -0.14 in
hypertensive subjects; platelet: r = 0.12 and 0.13 in normotensive su
bjects and r = 0.02 and -0.09 in hypertensive subjects). However, plas
ma 5 HT was associated with supine and standing pulse rates (supine: r
= 0.27, p = 0.05 in normotensive subjects and r = 0.54, p < 0.001 in
hypertensive subjects; standing: r = 0.19 and r = 0.46, p < 0.001, res
pectively). Significant relations were also found between platelet 5 H
T levels and supine and standing heart rate in the subjects mentioned
above (supine: r = 0.28, p = 0.05 in normotensive subjects and r =0.64
, p < 0.001 in hypertensive subjects; standing: r =0.24 and r = 0.51,
p < 0.001, respectively). These associations were stronger in the hype
rtensive group as a whole, and they held when adjustment was made for
differences in age and total blood cholesterol. The present study show
ed that plasma and platelet 5 HT are nor significantly altered in hype
rtensive subjects. However, plasma and platelet 5 HT levels showed a s
ignificant association with supine and standing pulse rate predominant
ly in hypertensive subjects. This is consistent with experimental evid
ence of a positive chronotropic effect of 5 HT on perfused hearts and
it suggests a possible role of plasma serotonin in the regulation of h
eart rate.