A. Elizabeth Linder

Vascular reactivity, 5-HT uptake, and blood pressure in the serotonin transporter knockout rat

The handling of serotonin [5-hydroxytryptamine (5-HT)] depends on the serotonin transporter (SERT). A SERT knockout (KO) rat is a useful model to test the hypothesis that SERT is the primary mechanism for arterial 5-HT uptake and to investigate the impact of SERT removal on blood pressure. Wild-type (WT) and KO rats were used to measure 5-HT content (plasma, raphe, aorta, carotid, and mesenteric artery), aortic isometric contraction, and blood pressure. HPLC supported the lack of circulating 5-HT in plasma (ng/ml plasma, WT, 310 +/- 96; and KO, 1.0 +/- 0.5; P < 0.05). Immunohistochemistry and Western blot analyses validated the presence of the SERT protein in the WT rats and a lesser expression in the KO rat. The aorta isolated from KO rats had a normal contraction to phenylephrine and norepinephrine and a normal relaxation to the endothelium-dependent agonist acetylcholine compared with the aorta from WT. In contrast, the potency of 5-HT was increased in the aorta from KO rats compared with WT rats [-log EC(50) (M); WT, 5.71 +/- 0.08; and KO, 6.7 +/- 0.18] and maximum contraction was reduced [%phenylephrine (10 muM) contraction, WT, 113 +/- 6%; and KO, 52 +/- 12%]. 5-HT uptake was reduced but not abolished in arteries of the KO compared with the WT rats. Diurnal mean arterial blood pressure, heart rate, and locomotor activity level of the KO rats were similar to the WT rats. These data suggest that there are other mechanisms of 5-HT uptake in the arteries of the rat and that although the absence of circulating 5-HT and/or SERT function sensitizes arteries to 5-HT, SERT dysfunction does not impair normal blood pressure.

Serotonin (5-HT) in Veins: Not All in Vain

The circulatory system consists of veins and arteries. Compared with arteries, veins have been neglected in cardiovascular research. Although veins are significantly less muscular than similarly sized arteries, the contribution of veins to cardiovascular homeostasis cannot be left un-noted because veins accommodate 70% of the circulating blood. Circulating blood platelets contain the majority of systemic 5-HT (5-hydroxytryptamine; serotonin). Similar to venous function, the physiological role of 5-HT in the cardiovascular system is not well understood. Here, we present not only a review on 5-HT and veins but ways in which these two topics might intersect in a physiologically relevant manner. Here we show the novel findings that veins exhibit higher amounts of intracellular 5-HT than arteries. Moreover, we also show evidence that, similar to arteries, veins have the ability to uptake 5-HT. In this review, we introduce the venous system as a reservoir for 5-HT in the periphery, suggesting that veins, in addition to arteries, may represent an important target for drugs that interfere with the serotonergic system. In addition, the serotonergic system from synthesis to metabolism, 5-HT receptor activation and venous diseases will also be discussed.

A Serotonergic System in Veins: Serotonin Transporter-Independent Uptake

We hypothesized that the 5-hydroxytryptamine (5-HT; serotonin) system is present and functional in veins. In vena cava (VC), the presence of the 5-HT synthesis rate-limiting enzyme tryptophan hydroxylase-1 mRNA and accumulation of the 5-HT synthesis intermediate 5-hydroxytryptophan after incubation with tryptophan supported the ability of veins to synthesize 5-HT. The presence of 5-HT and its metabolite 5-hydroxyindole acetic acid was measured by high-performance liquid chromatography in VC and jugular vein (JV), and it was compared with similarly sized arteries aorta (RA) and carotid (CA), respectively. In rats treated with the monoamine oxidase-A (MAO-A) inhibitor pargyline to prevent 5-HT metabolism, basal 5-HT levels were higher in veins than in arteries. 5-HT uptake was observed after exposure to exogenous 5-HT in all vessels. The presence of MAO-A and the 5-HT transporter (SERT) in VC was observed by immunohistochemistry and Western analysis. However, 5-HT uptake was not inhibited by the SERT inhibitors fluoxetine and/or fluvoxamine in VC and JV, as opposed to the inhibition in RA and CA. Moreover, studies performed in VC from mutant rats lacking SERT showed no differences in 5-HT uptake compared with VC from wild type. These data suggest the SERT is not functional under physiological conditions in veins. The differences in 5-HT handling between veins and arteries may represent alternative avenues for targeting the 5-HT system in the peripheral circulation for controlling vascular tone.

Body distribution of infused serotonin in rats.

1 Our goal was to investigate the body distribution of serotonin (5‐hydroxytryptamine; 5‐HT) in rats infused with 5‐HT (25 µg/kg per min) for 7 days and the contribution of the 5‐HT transporter (SERT) for 5‐HT uptake into the tissues. 2 Mini‐osmotic pumps containing 5‐HT or vehicle were implanted in rats knocked out for SERT (SERT‐KO) or in wild‐type (WT) rats. On the 8th day, tissues were harvested for measurements of 5‐HT by high‐performance liquid chromatography (HPLC). The 5‐HT metabolite 5‐hydroxyindole acetic acid (5‐HIAA) was also measured by HPLC, because an increase in 5‐HIAA in tissues from rats receiving 5‐HT reflects 5‐HT uptake followed by metabolism. 3 In WT rats infused with 5‐HT, an increase in 5‐HT or 5‐HIAA was observed in the heart, pancreas, thyroid, adrenal gland, kidney, seminal vesicle, bladder, prostate, liver, oesophagus, stomach, femur, trachea, lung and spleen compared with vehicle‐infused rats. An increase in 5‐HT and 5‐HIAA was not observed in aorta, vena cava and jejunum. In tissues from SERT‐KO rats infused with 5‐HT, the content of 5‐HT or 5‐HIAA was decreased in most of the tissues studied compared with 5‐HT‐infused WT rats. Although 5‐HT uptake in the kidney, seminal vesicle, prostate, jejunum and trachea is SERT dependent, it is SERT independent in the pancreas. The remaining tissues display SERT‐dependent and ‐independent mechanisms for 5‐HT uptake. 4 Altogether, tissues from different systems, such as the cardiovascular, endocrine, genitourinary and gastrointestinal, accumulate 5‐HT mainly via SERT and, thus, these systems are potential targets for drugs that interfere with 5‐HT homeostasis.

Erectile Function in Two-Kidney, One-Clip Hypertensive Rats is Maintained by a Potential Increase in Nitric Oxide Production

INTRODUCTION Hypertension is closely associated with erectile dysfunction (ED) as it has been observed in many experimental models of hypertension. Additionally, epidemiological studies show that approximately a third of hypertensive patients have ED. AIM To test the hypothesis that the two-kidney, one-clip (2K-1C) rat model of hypertension displays normal erectile function due to increased nitric oxide (NO) production in the penis. METHODS Ganglionic-induced increase in intracavernosal pressure (ICP)/mean arterial pressure (MAP) ratio was used as an index of erectile function in 2K-1C and in normotensive sham-operated (SHAM) anesthetized rats. Cavernosal strips from hypertensive and normotensive rats were used for isometric tension measurement. The contraction induced by alpha-adrenergic agonist phenylephrine and the relaxation induced by the NO donor sodium nitroprusside (SNP) and by the Rho-kinase inhibitor Y-27632 were performed in the absence and in the presence of the NO synthase inhibitor N(omega)-nitro-L-arginine (L-NNA). RESULTS Changes in ICP/MAP induced by ganglionic stimulation were not different between 2K-1C and SHAM rats. The contractile response induced by phenylephrine as well as the relaxation induced by SNP or the Y-27632 were similar in cavernosal strips from both groups. However, in the presence of L-NNA, the relaxation induced by Y-27632 was significantly impaired in 2K-1C compared to SHAM. CONCLUSIONS These data suggest that hypertension and ED could be dissociated from high levels of blood pressure in some animal models of hypertension. Erectile function in 2K-1C hypertensive rats is maintained in spite of the increased Rho-kinase activity by increased NO signaling.

Function of the Serotonin Transporter in Vasculature of the Female Rat: comparison with the male

1. The serotonin transporter (SERT) handles serotonin (5‐hydroxytryptamine (5‐HT)) and is blocked by the antidepressant SERT inhibitors fluoxetine and fluvoxamine. Although the importance of SERT in the central nervous system is clear, SERT also functions in the peripheral vasculature. In the present study, we tested the hypothesis that the vasculature from female rats has increased SERT function compared with male rats because females are more responsive to SERT inhibitors.

Proceedings of the 2008 American Urological Association Foundation Summer Research Conference on Sexual Medicine, August 2008, Linthicum, Maryland, USA Sponsored by the AUA Foundation, NIDDK and SBURErectile Function in Two‐Kidney, One‐Clip Hypertensive Rats is Maintained by a Potential Increase in Nitric Oxide Production

INTRODUCTION Hypertension is closely associated with erectile dysfunction (ED) as it has been observed in many experimental models of hypertension. Additionally, epidemiological studies show that approximately a third of hypertensive patients have ED. AIM To test the hypothesis that the two-kidney, one-clip (2K-1C) rat model of hypertension displays normal erectile function due to increased nitric oxide (NO) production in the penis. METHODS Ganglionic-induced increase in intracavernosal pressure (ICP)/mean arterial pressure (MAP) ratio was used as an index of erectile function in 2K-1C and in normotensive sham-operated (SHAM) anesthetized rats. Cavernosal strips from hypertensive and normotensive rats were used for isometric tension measurement. The contraction induced by alpha-adrenergic agonist phenylephrine and the relaxation induced by the NO donor sodium nitroprusside (SNP) and by the Rho-kinase inhibitor Y-27632 were performed in the absence and in the presence of the NO synthase inhibitor N(omega)-nitro-L-arginine (L-NNA). RESULTS Changes in ICP/MAP induced by ganglionic stimulation were not different between 2K-1C and SHAM rats. The contractile response induced by phenylephrine as well as the relaxation induced by SNP or the Y-27632 were similar in cavernosal strips from both groups. However, in the presence of L-NNA, the relaxation induced by Y-27632 was significantly impaired in 2K-1C compared to SHAM. CONCLUSIONS These data suggest that hypertension and ED could be dissociated from high levels of blood pressure in some animal models of hypertension. Erectile function in 2K-1C hypertensive rats is maintained in spite of the increased Rho-kinase activity by increased NO signaling.