July - September 2006, Volume 2, Issue 3

Popa M, Stefanescu AM, Procopiuc C

Increased number and affinity of platelet serotoninergic receptors in short, GH-deficient children

Acta Endo (Buc) 2006, 2 (3): 259-267
doi: 10.4183/aeb.2006.259

Serotonergic 5HT2A receptors constitute the sole subtype identifiable in platelets, their sole location outside CNS. They may intervene in intra-CNS pathways involved in GHRH and GH release, mainly during sleep. To gain information about such a subtype receptor in GH deficiency and, indirectly, on its role in GH release, studies on the platelets membrane binding sites of labelled LSD were undertaken in dwarf, GH-deficient children, assuming that the platelets sites number is parallel to their number in the brain. Five dwarf (Dw) children (3 boys) aged 7-13, having no signs of puberty, with a peak GH level under 5 ng/ml during ITT, no tumor in the hypothalamic and pituitary area and no previous rhGH therapy were compared with ten normally statured, non-obese children serving as controls (C). Fifty mL of platelet membrane preparation of pooled samples were incubated at 25?C with radioiodinated lysergic acid diethylamide ([125I] LSD) in concentrations of 0.35-3.5 nM/L. The reaction kinetics was followed up within 60 min weekly for 4 weeks. Bmax and Kd were calculated as means of 4 repetitions. Competitive inhibition curves were also drawn by using ketanserin (KET), mianserin (MIA) and cyproheptadin (CYP) in concentrations of 10-4 mM- 1nM/L and the inhibition constant (Ki) was calculated. The results showed that Bmax was (mean ? SEM) 33.0 ? 3.06 fmol/mg protein in C group versus 64.06 ? 13.82 fmol/mg protein in Dw group (F test in covar p<.05). Kd was 0.76? 0.166 nM in C and 2.0? 0.48 nM in Dw (t test p<0.01). The earliest time of 100% binding (Tmin) was 20 min in C and 5 min in Dw groups. Ki in C was 0.1 nM for KET, 18 nM for MIA and <0.1 nM/L for CYP. In Dw children Ki was 1.85 nM for KET, 18 nM for MIA and <0.1 nM for CYP. The results indicated that the number of 5HT2A receptors in platelets was significantly greater in GH-deficient children than in controls, as well as Kd. Tmin indicated an earlier steady state in Dw patients. Ki values pleaded to some extent in favour of the presence of excess 5HT2A receptors. In conclusion, excessive binding of labelled LSD and its inhibition by specific antagonists proves excess of 5HT2A receptors in platelets preparations collected from dwarf children.

Keywords: 5HT2A receptors, platelets, children, dwarfism, GH deficiency

Correspondence: Mircea Popa, &#8220;C. I. Parhon&#8221; Institute of Endocrinology, 34-36 Aviatorilor Blvd., 011863 Bucharest, Romania, email growthing@yahoo.com