The combination of physiology and genetics affords opportunities to perform experiments with greater precision than heretofore possible (Dow & Davies, 2001). We illustrate this by selective manipulation of the second messengers cyclic AMP (cAMP), cyclic GMP (cGMP) and calcium (Ca²⁺) in the renal (Malpighian) tubule of the genetic model organism, Drosophila melanogaster (Dow & Davies, 2001). Pharmacological intervention typically involves bathing a whole tissue in a drug, or the use of agents with broad specificities that might confound interpretation. However, ectopic expression of receptors in a cell renders it sensitive to the cognate ligand, and – provided that the necessary internal machinery is present – gives it the potential to respond. In Drosophila, it is possible to target such transgenes with great precision, to specific cells in an organotypic context, using GAL4/UAS enhancer traps (Brand & Perrimon, 1993).

Flies were generated that were transgenic for the Drosophila 5-HT₇ receptor or the rat atrial natriuretic peptide (rANP) receptor, under control of UAS or heat-shock promoters. Flies transgenic for the Drosophila 5-HT_1A receptor (Saudou et al. 1992) were a gift from L. Maroteaux. Fluid secretion could be stimulated in tubules dissected from such flies by application of 5-HT or rANP, whereas control tubules showed no response. The 5-HT₇ receptor acted to raise cAMP levels, and the rat ANP receptor raised cGMP levels.

UAS-targeted aequorin to measure Ca²⁺ levels in intact tubules showed that 5-HT_1A increases Ca²⁺, with corresponding increases in fluid transport. Furthermore, 5-HT-and rANP-induced signal transduction also increased Ca²⁺ in only principal cells, consistent with the presence of cyclic-nucleotide gated Ca²⁺ channels in this cell-type (MacPherson et al. 2001).

The robust tubule transport phenotype makes this system an ideal test-bed for integrative physiology, for example, in investigations of the role of signalling and transport proteins in renal function, and of the in vivo function of novel vertebrate receptors. However, this generic technology has potential beyond this specific tissue: in principle, such transgenes can be expressed specifically in any population of cells that can be delineated by a GAL4 driver line.

This work was supported by the BBSRC.

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