Capa GPCR and NMUR in Fluid Homeostasis Figure 4. Functional role of NMU in vivo. (A) Fluid transport by Drosophila wild-type tubule is significantly increased after application of the peptides Drm-capa-1 and NMU-25 at 1027 M but not with Drm-PK-1, hugg, Drm-PK-2. (B) [Ca2+]i levels in NMU-25-stimulated (1027 M) transgenic tubules expressing UAS-NMUR 2 and apoaequorin transgenes driven by c42-GAL4 (grey) compared with a typical control response (black, c42aeq). The calcium trace in blue represent a typical biphasic capa-1 (1027 M) response in tubule principal cells. Pooled cytosolic [Ca2+] data from separate experiments are shown where data are nM [Ca2+] 6 SEM, N = 6, where P,0.05. (C) NMU-25 (1027 M) stimulates increased fluid transport in transgenic tubules, in which UAS-NMUR 2 was driven by actin-GAL4. (D) capa-1 (1027 M) stimulates increased fluid transport in transgenic tubules, in which UAS-NMUR 2 was driven by actin-GAL4. Data are expressed as mean fluid transport rate (nl/min) 6 SEM, N = 6–10. The level of significance in A, C and D was determined using a Student’s t-test (* P,0.05) and in C and D, statistical analysis was confined to the comparison between the parental and progeny response. doi:10.1371/journal.pone.0029897.g004 possible; this could be achieved by releasing the capa peptides at the right physiological moment (e.g., during feeding) and once feeding is over, the diuretic action of the capa peptides needs to be terminated for the insect to conserve water. The rapid desensi- tization of the capaR may limit the responsiveness of the receptor to repeat agonist challenge and the physiological consequence would be to limit water loss. Is Neuromedin U a putative functional homolog of capa peptide in vivo? The amino acid sequence of human neuromedinU-25 (NMU- 25) shows evident homology to capa peptides [12] (Table 1). We thus investigated the possibility that mammalian NMU can act as an agonist for capaR. To validate functional human NMU receptor 2 (NMUR 2) in S2 cells, [Ca2+]i was measured in human PLoS ONE | www.plosone.org 8 January 2012 | Volume 7 | Issue 1 | e29897