On the 5-hydroxytryptamine transport across the plasma membrane of rabbit platelets and its inhibition by imipramine.
1. The carrier-mediated uptake of labelled 5-hydroxytryptamine (3H-5-HT) in rabbit platelets (defined as the difference between uptake observed in the absence and presence of 10 mumol l-1 imipramine) was studied after inhibition of monoamine oxidase and after a 1:13 dilution of the platelet-rich plasma (PRP) with Tris-containing buffer. 2. Irrespective of whether the rabbits were pretreated with reserpine or not, initial rates of 3H-5-HT uptake were maintained for at least 15 s. 3. Analysis of the saturation kinetics of 3H-5-HT uptake using Hill's equation yielded Km, Vmax and nH values of 130 nmol l-1, 116 pmol 10(8) platelets-1 min-1 and 1.40, respectively. Pretreatment of the animals with reserpine did not affect any of these kinetic parameters, but depleted more than 99% of the platelets' 5-HT stores. 4. The nH value remained greater than unity when the duration of incubation with 3H-5-HT was extended from 15 to 30 s and when the uptake of 3H-5-HT was inhibited by the presence of imipramine (10-40 nmol l-1). However, it was reduced to unity (with a consequential increase in Km) when 300 nmol l-1 ketanserin was present. This concentration of ketanserin did not affect 3H-5-HT uptake at substrate concentrations far below Km. 5. Imipramine inhibited 3H-5-HT uptake by increasing the Km for 3H-5-HT without changing Vmax. The Ki for this interaction was 18 nmol l-1.(ABSTRACT TRUNCATED AT 250 WORDS)
Wölfel
R
R
Institut für Pharmakologie und Toxikologie, Universität Würzburg, Federal Republic of Germany.
Böhm
W
W
Halbrügge
T
T
Bönisch
H
H
Graefe
K H
KH
D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D001792:Blood Platelets / Q000378:metabolism*; D001798:Blood Proteins / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D005260:Female; D007099:Imipramine / Q000494:pharmacology*; D066298:In Vitro Techniques; D007650:Ketanserin / Q000494:pharmacology; D008297:Male; D011817:Rabbits; D012701:Serotonin / Q000378:metabolism*; D013997:Time Factors
D001798:Blood Proteins; D012701:Serotonin; D007650:Ketanserin; D007099:Imipramine