ACE 2, a novel homologue of angiotensin converting enzyme, has recently been identi¢ed. This study used QRT-PCR to quantitatively map the transcriptional expression pro¢le of ACE 2 (and the two isoforms of ACE) in 72 human tissues. While con¢rming that ACE 2 expression is high in renal and cardiovascular tissues, the novel observation has been made that ACE 2 shows comparably high levels of expression in the gastrointestinal system, in particular in ileum, duodenum, jejunum, caecum and colon. Therefore, in probing the functional signi¢-cance of this novel peptidase, some consideration should be given to a role in gastrointestinal physiology and pathophysiology.

Quantitative gene expression data are often normalized to the expression levels of control or so-called "housekeeping" genes. An inherent assumption in the use of housekeeping genes is that expression of the genes remains constant in the cells or tissues under investigation. Although exceptions to this assumption are well documented, housekeeping genes are of value in fully characterized systems. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is one of the most commonly used housekeeping genes used in comparisons of gene expression data. To investigate the value of GAPDH as a housekeeping gene in human tissues, the expression of GAPDH mRNA was measured in a panel of 72 different pathologically normal human tissue types. Measurements were obtained from 371,088 multiplexed, quantitative real-time RT-PCRs with specific target genes. Significant differences in the expression levels of GAPDH mRNA were observed between tissue types and between donors of the same tissue. A 15-fold difference in GAPDH mRNA copy numbers was observed between the highest and lowest expressing tissue types, skeletal muscle and breast, respectively. No specific effect of either age or gender was observed on GAPDH mRNA expression. These data provide an extensive analysis of GAPDH mRNA expression in human tissues and confirm previous reports of the marked variability of GAPDH expression between tissue types. These data establish comparative levels of expression and can be used to add value to gene expression data in which GAPDH is used as the internal control.

1 5-Hydroxytryptamine (5-HT) is known to produce a number of di erent e ects in the gastrointestinal tract of various species, and has been proposed to play a key role in a number of intestinal disorders in man, including irritable bowel syndrome (IBS), although the receptors involved have yet to be established. The aim of the present study was to investigate the distribution and function of 5-HT 2B receptors in human colon, and to establish their possible role in the aetiology of IBS. 2 The distribution of 5-HT 2B receptor mRNA and protein were investigated by quantitative RT ± PCR, Western analysis and immunocytochemistry. High levels of both mRNA and protein for 5-HT 2B receptors were found throughout the human gastrointestinal tract, and in particular in colon, where 5-HT 2B receptors were found predominantly in the longitudinal and circular smooth muscle layers within the muscularis externa, and in the myenteric nerve plexus lying between these two layers.3 Electrical ®eld stimulation of longitudinal muscle preparations of human colon mounted in organ baths resulted in neuronally-mediated contractile responses, that were signi®cantly potentiated by application of 5-HT (up to 10 77 M), with a pEC 50 of 8.2+0.1 (n=49 donors). The response to 5-HT was inhibited by a number of selective 5-HT 2B receptor antagonists. 4 This study has shown for the ®rst time that, in contrast to animal studies, the excitatory e ects of 5-HT in human colon are mediated by 5-HT 2B receptors. It is proposed that these receptors contribute to the putative 5-HT-induced colonic smooth muscle hypersensitivity associated with IBS.

The physiological role of the duodenal peptide secretin is as a potent stimulant of electrolyte and water movement in pancreatic and biliary epithelium, via activation of G protein-coupled secretin receptors (hSCTR). However, the distribution and potential function of hSCTR in human lung has not previously been addressed. Using real-time quantitative reverse transcriptase-polymerase chain reaction profiling, in situ hybridization, and immunohistochemistry, we demonstrated that the hSCTR is abundantly expressed within the distal regions of human lung (tertiary bronchus and parenchyma), with negligible expression detected in more proximal regions (trachea, primary, and secondary bronchus). Expression was observed predominantly on the basolateral membrane of the bronchial epithelial layer, with some expression also observed in bronchial smooth muscle. In primary cultures of human tertiary bronchial epithelial cells, secretin was demonstrated to potently stimulate channel-mediated Cl- efflux in a concentration-dependent manner. Secretin was also shown to cause concentration-dependent relaxation of human tertiary bronchial smooth muscle. In summary, these data demonstrate that secretin receptors are present in human lung, and that activation of these receptors with human secretin potently stimulates concentration-dependent Cl- efflux from bronchial epithelial cells and bronchorelaxation.