pH-sensitive expression of calcium-sensing receptor (CaSR) in type-B intercalated cells of the cortical collecting ducts (CCD) in mouse kidney.

BACKGROUND: The localization and role of the calcium-sensing receptor (CaSR) along the nephron including the collecting ducts is still open to debate. METHODS: Using the quantitative, highly sensitive in situ hybridization technique and a double-staining immunohistochemistry technique, we investigated the axial distribution and expression of CaSR along the nephron in mice (C57B/6J) treated for 6 days with acid or alkali diets. RESULTS: Under control condition, CaSR was specifically localized in the cortical and medullary thick ascending limb of Henle's loop (CTAL and MTAL), macula densa (MD), distal convoluted tubule (DCT), and CCD (TALs, MD > DCT, CCD). Along the CCD, CaSR was co-localized with an anion exchanger type 4 (AE4), a marker of the basolateral membrane of type-B intercalated cell (IC-B) in mice. On the contrary, CaSR was not detected either in principal cells (PC) or in type-A intercalated cell (IC-A). CaSR expression levels in IC-B significantly (P < 0.005) decreased when mice were fed NH4Cl (acid) diets and increased when animals were given NaHCO3 (alkali) diets. As expected, cell heights of IC-A and IC-B significantly (P < 0.005) increased in the above experimental conditions. Surprisingly, single infusion (ip) of neomycin, an agonist of CaSR, significantly (P < 0.005) increased urinary Ca excretion without further increasing the hourly urine volume and significantly (P < 0.05) decreased urine pH. CONCLUSION: CaSR, cloned from rat kidney, was localized in the basolateral membrane of IC-B and was more expressed during alkali-loading. Its alkali-sensitive expression may promote urinary alkali secretion for body acid-base balance.

Osmoregulation in elephant fish Callorhinchus milii (Holocephali), with special reference to the rectal gland.

Osmoregulatory mechanisms in holocephalan fishes are poorly understood except that these fish are known to conduct urea-based osmoregulation as in elasmobranchs. We, therefore, examined changes in plasma parameters of elephant fish Callorhinchus milii, after gradual transfer to concentrated (120%) or diluted (80%) seawater (SW). In control fish, plasma Na and urea concentrations were about 300 mmol l(-1) and 450 mmol l(-1), respectively. These values were equivalent to those of sharks and rays, but the plasma urea concentration of elephant fish was considerably higher than that reported for chimaeras, another holocephalan. After transfer to 120% SW, plasma osmolality, urea and ion concentrations were increased, whereas transfer to 80% SW resulted in a fall in these parameters. The rises in ion concentrations were notable after transfer to 120% SW, whereas urea concentration decreased predominantly following transfer to 80% SW. In elephant fish, we could not find a discrete rectal gland. Instead, approximately 10 tubular structures were located in the wall of post-valvular intestine. Each tubular structure was composed of a putative salt-secreting component consisting of a single-layered columnar epithelium, which was stained with an anti-Na(+),K(+)-ATPase serum. Furthermore, Na(+),K(+)-ATPase activity in the tubular structures was significantly increased after acute transfer of fish to concentrated SW (115%). These results suggest that the tubular structures are a rectal gland equivalent, functioning as a salt-secreting organ. Since the rectal gland of elephant fish is well developed compared to that of Southern chimaera, the salt-secreting ability may be higher in elephant fish than chimaeras, which may account for the lower plasma NaCl concentration in elephant fish compared to other chimaeras. Since elephant fish have also attracted attention from a viewpoint of genome science, the availability of fish for physiological studies will make this species an excellent model in holocephalan fish group.