Tricle (e.g. Ohya et al. 2001; Rosati et al. 2001).these muscle tissues and contain transcripts that could influence absolute quantification. To supply further assistance for the measurements of transcriptional expression and to address the challenge of contamination from nonmuscle cells, we investigated myocytespecific expression of Kv4.two and Kv4.three channels with immunohistochemistry. Robust Kv4.3like immunoreactivity was observed in colonic myocytes, whereas Kv4.2like immunoreactivity was reasonably weaker. Kv4.2 and Kv4.3like immunoreactivities have been also substantially weaker in jejunal myocytes. To additional test these observations we also characterized the existing density of IA in dispersed colonic and jejunal myocytes. The stronger Kv4.3like immunoreactivity in the colon correlated with 2fold greater present density than in jejunal myocytes. There was a discrepancy involving the levels of Kv4 transcript expression as well as the levels of Kv4 protein and IA density in colonic and jejunal myocytes. We regarded the possibility that this discrepancy could be resulting from differential expression of KChIP proteins in these cells. KChIPs, which belong for the neuronal calcium sensor (NCS) family of proteins, are good modulators of native and heterologously expressed Kv4derived currents (An et al. 2000; Decher et al. 2001; Liss et al. 2001). These auxiliary proteins improve Kv4 present density by rising expression with the channels in the plasma membrane (An et al. 2000; Bahring et al. 2001). KChIPs also modify the kinetic behaviour of Kv4 channels (Beck et al. 2002). Kv4 channels underlie the Atype present (ITO) in ventricular myocytes (Xu et al. 1999; see Nerbonne, 2000), plus the pattern of KChIP2 expression has lately been shown to mirror the transmural gradient of ITO in canine and human ventricles (Rosati et al. 2001). In 3-Oxo-5��-cholanoic acid medchemexpress transgenic mice harbouring a targeted nullKChIP2 allele, heterozygotes displayed ventricular ITO that was lowered by around half in the current in wildtype myocytes (Kuo et al. 2001). Homozygote nullKChIP2 mice did not express functional ITO. By analogy with cardiac muscle, we suggest that similar regulation of functional Kv4 channels by KChIPs might take place in gastrointestinal smooth muscle tissues and clarify the disparity among transcriptional expression of Kv4 isoforms and existing density in colonic and jejunal muscle tissues. We detected transcripts encoding KChIPs in colonic and jejunal myocytes and, in agreement with our hypothesis, total KChIP transcripts were 2.6fold greater in colon than in jejunum. In these tissues KChIP1 was the dominant isoform. Our data recommend that in gastrointestinal smooth muscle tissues, functional expression of Kv4 may well be regulated by the pattern of KChIP expression. Yet another member of the NCS protein family, frequenin (NCS1), has been shown to act as a good modulator of Kv4 currents (Nakamura et al. 2001b). Although examination of other NCS family members in gastrointestinal smooth muscle is warranted,Journal of PhysiologyWe also developed primers for an unrelated K channelassociated protein, KChAP, the coexpression of that is also identified to improve Kv4 current density (Kuryshev et al. 2000, 2001). Immediately after 35 amplification cycles, RTPCR detected KChAP transcripts in cDNA from mouse ventricle and brain, but did not detect KChAP transcripts in colonic or jejunal cDNA (n = three; information not shown).DISCUSSIONPreviously, we characterized an Atype current (IA) in murine colonic myocytes that dampens excitability and may perhaps participa.