Molecular cloning of otoconin-22 complementary deoxyribonucleic acid in the bullfrog endolymphatic sac: effect of calcitonin on otoconin-22 messenger ribonucleic acid levels.

Anuran amphibians have a special organ called the endolymphatic sac (ELS), containing many calcium carbonate crystals, which is believed to have a calcium storage function. The major protein of aragonitic otoconia, otoconin-22, which is considered to be involved in the formation of calcium carbonate crystals, has been purified from the saccule of the Xenopus inner ear. In this study, we cloned a cDNA encoding otoconin-22 from the cDNA library constructed for the paravertebral lime sac (PVLS) of the bullfrog, Rana catesbeiana, and sequenced it. The bullfrog otoconin-22 encoded a protein consisting of 147 amino acids, including a signal peptide of 20 amino acids. The protein had cysteine residues identical in a number and position to those conserved among the secretory phospholipase A(2) family. The mRNA of bullfrog otoconin-22 was expressed in the ELS, including the PVLS and inner ear. This study also revealed the presence of calcitonin receptor-like protein in the ELS, with the putative seven-transmembrane domains of the G protein-coupled receptors. The ultimobranchialectomy induced a prominent decrease in the otoconin-22 mRNA levels of the bullfrog PVLS. Supplementation of the ultimobranchialectomized bullfrogs with synthetic salmon calcitonin elicited a significant increase in the mRNA levels of the sac. These findings suggest that calcitonin secreted from the ultimobranchial gland, regulates expression of bullfrog otoconin-22 mRNA via calcitonin receptor-like protein on the ELS, thereby stimulating the formation of calcium carbonate crystals in the lumen of the ELS.

Responses of the ultimobranchial gland to vitamin D3 treatment in freshwater mud eel, Amphipnous cuchia, kept in different calcium environments.

Freshwater mud eel, Amphipnous cuchia, were injected intraperitoneally daily with 100 ng of vitamin D3/100 g body weight and maintained in media containing either no calcium or different calcium concentrations. The eels were killed after 1, 3, 5, 10 and 15 days following the treatment and their serum calcium levels were measured. The ultimobranchial glands were fixed and processed using the routine paraffin method for histological studies. The results of the present study indicate that vitamin D3 can induce hypercalcaemia in eels kept in different calcium environments. Also, the ultimobranchial glands became hyperactive following vitamin D3 treatment. It is concluded that in mud eels, the gland has a calcium-regulating function.

Hypocalcemic potency of the ultimobranchial gland in some urodelan amphibians.

The hypocalcemic potencies of the ultimobranchial glands of two urodelans, Onychodactylus japonicus and Hynobius nigrescens, were studied; according to the rat bioassay, their calcitonin values (MRC) were 30 and 18 mU/kg body wt, respectively. Various organs other than the ultimobranchial gland were also assayed in rats to see whether they had any hypocalcemic potency. However, the ultimobranchial gland was the only organ examined with detectable hypocalcemic potencies in these urodelans. The hypocalcemic potencies of urodelan ultimobranchial glands are one order lower than those reported in the other vertebrate classes, contrasting with the potencies of anuran ultimobranchial glands. The biological significance of this low potency is discussed.

Relationships between dietary and plasma concentrations of calcium and phosphorus in intact and ultimobranchialectomized chickens.

Two experiments were conducted with cockerels to determine whether the presence or absence of the ultimobranchial glands would influence the relationship between dietary and plasma calcium and phosphorus. Broiler type cockerels, 16 weeks of age which had been sham operated (SHAM) or ultimobranchialectomized (UBX) 1 to 3 weeks earlier, were fed diets containing 0.8 or 2.4% calcium and 0.13 to 0.33% phosphorus. The SHAM cockerels fed diets containing 0.8% Ca and 0.13% P did not develop hypercalcemia whereas the UBX cockerels fed this diet developed slight significant hypercalcemia after 17 to 21 days. In Experiment I, SHAM cockerels fed the diet containing 2.4% Ca and 0.13% P developed mild, chronic hypercalcemia (12.7 mg./100 ml.) with a plasma phosphorus of 3.03 mg. P/100 ml., whereas the UBX cockerels fed the same diet developed severe hypercalcemia (16.0 mg./100 ml.) and hypophosphatemia, 1.68 mg. P/100 ml. In Experiment 2 the following plasma values were observed after 17 days of consuming the experimental diets: SHAM fed 2.4% Ca and 0.13% P had 10.6 mg. Ca/100 ml. and 3.59 mg. P/100 ml., whereas UBX fed the same diet had 12.8 mg. Ca/100 ml. and 2.24 mg. P/100 ml. The UBX fed 2.4% Ca and 0.33% P for 17 days had plasma values of 10.8 mg. Ca/100 ml. and 4.48 mg. P/100 ml. It is concluded that the presence of the ultimobrancial glands are essential to the regulation of plasma calcium and phosphorus in chickens which consume high calcium-low phosphorus diets.