Mature three-spined stickleback males use spiggin threads secreted from their kidney to glue together nest material. This requires strongly hypertrophied renal proximal tubular cells, which compromises renal osmoregulatory function during the breeding period. Experimental evidence suggests that the intestine takes over hypotonic fluid secretion at that stage but the mechanism is unexplored. To unravel the molecular mechanism we analyzed and compared transcript levels of several membrane proteins involved in water and salt transport in intestinal and renal tissues, in non-mature males (NM), mature males (MM), and mature females (MF). Aquaporin paralogs aqp1a, -3a, -8aa, -8ab, -10a, and -10b, two Na(+),K(+)-ATPase alpha-1 subunit isoforms (nka547, nka976), Na(+),K(+),2Cl(-)-, and Na(+),Cl(-)-cotransporters (nkcc1a, nkcc2, ncc), the cystic fibrosis transmembrane conductance regulator (cftr) and two claudin isoforms (cldn2, cldn15a) were expressed in the intestine and kidney in all groups. There were no differences in aqp and cldn expression between intestines of NM and MM; nkcc2 was lower and nka levels tended to be higher in intestines of MM than in NM. In the kidney, aqp1 and aqp8ab levels were lower in MM than in NM, whereas aqp3a, nkcc1a, cldn15a, and spiggin were markedly elevated. This was accompanied by marked hypertrophy of kidney tubules in MM. The data support an altered kidney function in terms of water handling in mature males, whereas there was no support for modified trans-epithelial water permeability or salt-secretory activity in the intestine of mature males. Salt-absorptive activity in the intestine may, however, be down-regulated during male maturation.

When euryhaline fish move between fresh water (FW) and seawater (SW), the intestine undergoes functional changes to handle imbibed SW. In Japanese medaka, the potential transcellular aquaporin-mediated conduits for water are paradoxically downregulated during SW acclimation, suggesting paracellular transport to be of principal importance in hyperosmotic conditions. In mammals, intestinal claudin-15 (CLDN15) forms paracellular channels for small cations and water, which may participate in water transport. Since two cldn15 paralogs, cldn15a and cldn15b, have previously been identified in medaka, we examined the salinity effects on their mRNA expression and immunolocalization in the intestine. In addition, we analyzed the drinking rate and intestinal water handling by adding non-absorbable radiotracers, 51-Cr-EDTA or 99-Tc-DTPA, to the water. The drinking rate was >2-fold higher in SW than FW-acclimated fish, and radiotracer experiments showed anterior accumulation in FW and posterior buildup in SW intestines. Salinity had no effect on expression of cldn15a, while cldn15b was approximately 100-fold higher in FW than SW. Despite differences in transcript dynamics, Cldn15a and Cldn15b proteins were both similarly localized in the apical tight junctions of enterocytes, co-localizing with occludin and with no apparent difference in localization and abundance between FW and SW. The stability of the Cldn15 protein suggests a physiological role in water transport in the medaka intestine.