Introduction hydroxytryptamine HT is found

Introduction 5-hydroxytryptamine (5-HT) is found throughout the body and influences smooth muscle activity in various tissues including the intestine, vasculature and urinary Probenecid synthesis (Kim and Camilleri, 2000, Klarskov and Horby-Petersen, 1986, Mohammad-Zadeh et al., 2008). The current classification of 5-HT receptors include 5-HT1 (1A, 1B, 1D, 1E, 1F) which mediate responses via the inhibition of adenylate cyclase, 5-HT2 (2A, 2B, 2C) which lead to activation of phospholipase C, 5-HT3 which act as ligand-gated ion channels and 5-HT4 which are positively coupled to adenylate cyclase. In addition, there are three subtypes that are not so well-defined, 5-HT5 which mediate inhibition of adenylate cyclase and 5-HT6 and 5-HT7 receptors that lead to activation of adenylate cyclase (Roth, 1994). While the mammalian urinary bladder and lower urinary tract have been shown to contract in response to 5-HT, in many studies in the human and pig (Todd and Mack, 1969, Ambache and Zar, 1970, Long and Nergardh, 1978, Sellers et al., 2000), relaxation has also been reported (Klarskov and Horby-Petersen, 1986). In the ureter however, in all species examined thus far, 5-HT has been found to produce an excitatory action only. In isolated strips of human ureter, 5-HT induces concentration-dependent contractions (Barnes, 2001, Gidener et al., 1999) that are antagonised by the 5-HT2A receptor antagonist ketanserin and the mixed 5-HT1/5-HT2 receptor antagonist methysergide, but are not altered by 5-HT3 and 5-HT4 receptor antagonists (Gidener et al., 1995). In addition, the 5-HT2A antagonist ritanserin inhibits contraction of the isolated pig intravesical ureter (Hernandez et al., 2003), whilst the 5-HT2A agonist 2,5-dimethoxy-4-iodoamphetamine has been reported to increase the frequency of ureteral contractions in pig ureter in vivo in a dose-dependent manner (Hauser et al., 2002). Ageing has been shown to increase the risk of ureteral calculus development (Hess, 2003). Whilst this could be related to changes in ureteral function, the effects of age on ureteral contractility are not yet known. Age-related changes in 5-HT-mediated contractile mechanisms have been observed in other systems, including the gastrointestinal system (Keating et al., 2015), and in the rat vas deferens the inhibitory effect of 5-HT2 receptor stimulation was found to decrease with increasing age (Moritoki et al., 1986). The aim of this study was to identify the receptor subtype mediating responses to 5-HT in the porcine ureter and to determine whether age has any effect on these responses.
Materials and methods
Results
Discussion Whilst not clearly understood, the ureters have an efferent and afferent innervation that includes adrenergic, cholinergic, and non-adrenergic non-cholinergic components (Gray, 1918). Previously, it has been demonstrated that 5-HT stimulates contraction of the smooth muscle of isolated ureteral strips in a number of species (Gidener et al., 1999, Gidener et al., 1995, Hernandez et al., 2003). In the current study, similarly, 5-HT induced concentration-dependent increases in contractile activity, increasing both the frequency and magnitude of phasic contractions in distal ureters from older and younger pigs. This is in agreement with most other findings in tissues from pig and human (Gidener et al., 1999, Gidener et al., 1995, Hernandez et al., 2003), but contrasts with a study of isolated rabbit tissues that failed to find any ureteral response to 5-HT, suggesting that these responses are species dependent and that the pig represents a good model for human (Yalcin et al., 2013). In our present porcine study, 5-HT produced concentration-dependent increases in overall phasic contractile activity (AUC) and frequency. A similar finding has been reported by Hernandez et al. (2003) for the porcine intravesical ureter. However, in that study, 5-HT preferentially increased contractile tone, but the frequency of these phasic contractions was not related to 5-HT concentration (Hernandez et al., 2003). These authors also found that phasic contractions were abolished during 5-HT-evoked increases in ureteral tone (Hernandez et al., 2003). In our studies no increases in baseline tone were observed upon addition of either 5-HT or α-methyl-5-HT at any concentration. This observed difference can be explained by the difference in the ureteral region being studied. The intravesical ureter region utilised in their experiments acts as a sphincter, controlling movement of urine into the bladder (Blok et al., 1985, Hernandez et al., 1999, Prieto et al., 1997), whilst the distal ureter was examined in the present study. It therefore appears that 5-HT plays different, region specific roles in the urinary tract.