Reduced serotonin uptake by spontaneously hypertensive rat platelets

5-Hydroxytryptamine and platelets: uptake and aggregation in hypoxic pulmonary hypertensive rats

Pulmonary hypertension is associated with various alterations in 5-hydroxytryptamine (5-HT) physiology. In this study in platelets from hypoxic pulmonary hypertensive rats (10% O(2); 1 week) and normoxic rats (room air), (i) initial rates of specific [3H]5-HT uptake were measured and (ii) potentiation of collagen- and ADP-induced aggregation by 5-HT was quantified. The platelet count was almost halved in hypoxic rats. In uptake experiments, there was a decrease in 5-HT uptake in platelets from hypoxic compared with normoxic rats, due to a 36% reduction in the maximal initial rate of uptake. The aggregation experiments showed that 5-HT (1-100 microM) increased the magnitude of responses to collagen and the duration of responses to ADP, but there was no difference between hypoxic and normoxic rats. Abnormalities in platelet function may conceivably lead to increases in plasma 5-HT levels in hypoxic pulmonary hypertension, but are unlikely to aggravate pulmonary thromboembolism.

Serotonin transporter overexpression is responsible for pulmonary artery smooth muscle hyperplasia in primary pulmonary hypertension

Hyperplasia of pulmonary artery smooth muscle cells (PA-SMCs) is a hallmark pathological feature of primary pulmonary hypertension (PPH). Here we found that PA-SMCs from patients with PPH grow faster than PA-SMCs from controls when stimulated by serotonin or serum and that these effects are due to increased expression of the serotonin transporter (5-HTT), which mediates internalization of indoleamine. In the presence of 5-HTT inhibitors, the growth stimulatory effects of serum and serotonin were markedly reduced and the difference between growth of PA-SMCs from patients and controls was no longer observed. As compared with controls, the expression of 5-HTT was increased in cultured PA-SMCs as well as in platelets and lungs from patients with PPH where it predominated in the media of thickened pulmonary arteries and in onion-bulb lesions. The L-allelic variant of the 5HTT gene promoter, which is associated with 5-HTT overexpression and increased PA-SMC growth, was present in homozygous form in 65% of patients but in only 27% of controls. We conclude that 5-HTT activity plays a key role in the pathogenesis of PA-SMC proliferation in PPH and that a 5HTT polymorphism confers susceptibility to PPH.

Platelet serotonergic mechanisms in patients with cancer of the urinary bladder

Platelet reactivity of hypertensive patients and patients with cancer of the urinary bladder was examined in vitro in comparison with normotensive patients. A significant increase in free serotonin in the plasma of patients with cancer of the bladder and a less pronounced increase in hypertensive subjects was observed. The platelets in neoplastic disease and hypertension showed a significant reduction in the kinetics of serotonin uptake. However, the release reaction was markedly higher, compared with healthy volunteers. Enhancement of platelet aggregation with serotonin was much greater in patients with cancer of the bladder than in control groups and in hypertensive patients. These data indicate increased sensitivity to serotonin within the platelets of patients with cancer of the urinary bladder.

Attenuated hypoxic pulmonary hypertension in mice lacking the 5-hydroxytryptamine transporter gene

Hypoxia is a well-recognized stimulus for pulmonary blood vessel remodeling and pulmonary hypertension development. One mechanism that may account for these effects is the direct action of hypoxia on the expression of specific genes involved in vascular smooth muscle cell (SMC) proliferation. Previous studies demonstrated that the serotonin (5-hydroxytryptamine; 5-HT) transporter (5-HTT) mediates the mitogenic activity of 5-HT in pulmonary vascular SMCs and is overexpressed during hypoxia. Thus, 5-HT-related mitogenic activity is increased during hypoxia. Here, we report that mice deficient for 5-HTT (5-HTT(-/-)) developed less hypoxic pulmonary hypertension and vascular remodeling than paired 5-HTT(+/+) controls. When maintained under normoxia, 5-HTT(-/-)-mutant mice had normal hemodynamic parameters, low blood 5-HT levels, deficient platelet 5-HT uptake, and unchanged blood levels of 5-hydroxyindoleacetic acid, a metabolite of 5-HT. After exposure to 10% O(2) for 2 or 5 weeks, the number and medial wall thickness of muscular pulmonary vessels were reduced in hypoxic 5-HTT(-/-) mice as compared with wild-type paired controls. Concomitantly, right ventricular systolic pressure was lower and right ventricle hypertrophy less marked in the mutant mice. This occurred despite potentiation of acute hypoxic pulmonary vasoconstriction in the 5-HTT(-/-) mice. These data further support a key role of 5-HTT in hypoxia-induced pulmonary vascular SMC proliferation and pulmonary hypertension.

Central serotonergic uptake mechanisms in hypertensive rats: effects of clonidine and centhaquin

The binding of a highly specific ligand for serotonin (5-HT) uptake sites, [3H]paroxetine, was studied in brain regions of normotensive Wistar-Kyoto rats (WKY) and spontaneous hypertensive rats (SHR). [3H]Paroxetine bound to a single, high affinity binding site in the brain. In midbrain, the density (Bmax values) of [3H]paroxetine binding were significantly reduced (27.16%) in SHR as compared to WKY. The affinity (Kd values) were found to be similar in SHR and WKY. The Kd and Bmax values of [3H]paroxetine binding were found to be similar in spinal cord, pons and medulla and cerebral cortex of WKY and SHR. The effect of centrally acting hypotensive agents, clonidine and centhaquin, on [3H]paroxetine binding was also determined and compared with imipramine, a known 5-HT uptake inhibitor. Clonidine did not displace [3H]paroxetine binding at any concentration (10(-4) to 10(-7) M). On the other hand, centhaquin, which produces hypotension similar to clonidine, could displace [3H]paroxetine binding in a concentration dependent manner. In cerebral cortex and brainstem (midbrain, pons and medulla) membranes, the IC50 values of imipramine and centhaquin for [3H]paroxetine binding were found to be similar in WKY and SHR. The IC50 of centhaquin in displacing paroxetine from 5-HT uptake sites, was 10 times lower in the cerebral cortex and 4 times lower in the brainstem membranes when compared to imipramine. Clonidine had no effect on 5-HT uptake sites. The results indicate that (1) the density of 5-HT uptake sites is reduced in the midbrain of hypertensive rats, and (2) centhaquin, a centrally acting hypotensive agent, acts on 5-HT transporter sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic and environmental influences on behavioral and neurochemical aspects of emotionality in rats

Three pairings of rats (two derived from divergent, selective breeding and one from divergent environmental conditions) were compared with regard to behavioral and hormonal parameters. Striking differences were observed: results obtained in our own laboratory as well as those found in a review of the literature pointed to higher emotionality (e.g., increased defecation and corticosterone secretion, etc.) in Roman low-avoidance, Wistar-Kyoto and group-housed rats, as compared to their respective counterparts, Roman high-avoidance, spontaneously hypertensive, and individually housed Wistar rats. Concomitant receptor binding studies reviewed here (3H-diazepam- and 3H-imipramine-binding sites) have revealed, however, less consistent intrapair differences.

Platelet serotonin content and uptake in spontaneously hypertensive rats

Platelet serotonin (5-HT) content and uptake were studied in male SHR and WKY at various ages. Blood was withdrawn from the carotid artery under anesthesia and 5-HT levels determined from platelet rich plasma (PRP) using a HPLC technique coupled with an electrochemical detection method. Platelet 5-HT uptake was studied by incubating PRP at 37 degrees C for 10 sec with increasing concentrations of 3H-5HT. Lineweaver- Burk plots of 3H-5HT uptake were linear suggesting simple Michaelis- Menten uptake kinetics. The SHR had more platelets than age-matched controls and consequently a higher blood circulating pool of 5-HT. Nevertheless, the 5-HT platelet levels were similar to those of their age-matched rats. The 5 week-old SHR and WKY had greater numbers of platelets and higher 5-HT platelet levels than the older rats of both strains. The affinity constants (Km) and the maximal velocities (Vmax) of platelet 5-HT uptake did not differ significantly between the 12 week- and the 6 month-old SHR and WKY. These data suggest that the SHR do not show the same impairment in platelet 5-HT metabolism as observed in essential hypertension in man.

Especial considerations for neurotoxicological research

In recent years, there has been much impetus toward a definition of behavior in terms of underlying biological events. Such correlations have been attempted in several areas ranging from learning and memory to neurological disease. Increased information concerning the relation between behavior and neurobiological mechanisms is especially important in the area of neurotoxicology. It is often abnormal behavior that is a first sign of exposure to a neurotoxic agent and such changes may give clues as to the anatomical or chemical sites of attack on the nervous system. These clues might also lead to the development of a therapeutic treatment as to the development of tests designed to reveal exposure to a toxic agent at levels below those causing gross behavioral change. Unfortunately, there is a relatively small amount of literature reporting on both behavioral and biological disturbances caused by a toxic agent in the same experimental animal. However, a variety of methodological advances combined with a growing interest in neurotoxicology is gradually changing this. Increased information concerning the role of defined nerve pathways and the means of action of their chemical constituents offers an opportunity to bring about a deepening understanding of neurotoxic events. This review will suggest how new pharmacological findings can be applied to neurotoxicology. Examples of human and animal exposure to toxic materials will be used and current problems will be shown to be major determinants of future research directions.

Decreased uptake of 3H-serotonin and endogenous content of serotonin in blood platelets in hypertensive patients

The uptake and content of serotonin in blood platelets were studied in patients with essential hypertension and in five families in which at least one member was hypertensive. Blood was obtained from male and female normotensive volunteers and hypertensive patients who were free of medication. Lineweaver-Burk plots of 3H-serotonin uptake from both control subjects and hypertensive patients were linear, which suggested simple Michaelis-Menten uptake kinetics. The maximal uptake velocity (Vmax) in hypertensive patients was significantly lower than in control subjects (control = 41.7 +/- 3.3 pmol/min/10(8) platelets, n = 17; hypertensive = 26.6 +/- 3.0 pmol/min/10(8) platelets, n = 16; p less than 0.005). The affinity constant (Km) was slightly but significantly lower in hypertensive patients (control = 0.70 +/- 0.08 microM; hypertensive = 0.46 +/- 0.08 microM; p less than 0.05). The serotonin content in blood platelets determined by high pressure liquid chromatography with electrochemical detection was significantly lower in hypertensive patients (control = 165.0 +/- 12.9 nmol/10(11) platelets, n = 29; hypertensive = 105.9 +/- 10.4 nmol/10(11) platelets, n = 27; p less than 0.001). In the five families investigated, the lowered serotonin content was observed in some normotensive members. The reduced number of carriers of serotonin uptake and the slight decrease in the affinity constant observed in platelets of patients with essential hypertension suggest that serotonin metabolism is altered in essential hypertension and that blood platelets may be a useful model in studying the serotonergic modifications at the molecular level.

Studies on the mechanism of the enhanced cold-induced TSH secretion in spontaneously hypertensive rats

Various noradrenergic and tryptaminergic antagonists as well as pinealectomy significantly inhibited cold-induced TSH secretion in SHR as in control rats.

Plasma protein changes in primary hypertension in humans and rats

To determine whether plasma protein changes may be associated with primary hypertension, we analyzed plasma proteins from essential hypertensive (EHT) patients and genetically hypertensive rats using two-dimensional electrophoresis. An additional plasma protein, having a molecular weight of 13,000 daltons and an isoelectric point of 4.5, was found in 82% of the patients with borderline or moderate hypertension (n = 29) and in all permanently hypertensive patients (n = 12). This protein was detected in 36% of normotensive (NT) subjects (n = 50). In the latter, the influence of family history, sex, and secondary hypertension were studied. Plasma proteins were also studied in spontaneously hypertensive rats (SHR). In all plasma from young male (n = 10) and female (n = 6) SHR, two additional proteins (molecular weight = 16,000 daltons, pHi = 4.7 and 5.1) were detectable. These plasma proteins were not detectable in male Wistar Kyoto rats (WKY) and in 50% of female WKY, and their frequency was 10% (n = 10) and 0% (n = 3) in normal male WKY and in male WKY rendered hypertensive by methylprednisolone, respectively. We conclude that these alterations of plasma proteins may be considered a biochemical feature of primary hypertension.