Hormonal regulation of renal inorganic phosphate transport in the winter flounder, Pleuronectes americanus

Intestinal uptake and renal excretion are the primary determinants of inorganic phosphate (P_i) balance in teleosts. In general, teleost kidneys may either reabsorb filtered P_i or secrete excess P_i into the urine. Primary monolayer cultures of flounder (Pleuronectes americanus) renal proximal tubule epithelium (PTCs) have helped identify several hormones that may participate in conservation or excretion of P_i. Mounted in Ussing chambers, the monolayer cultures can be used to assay transepithelial P_i transport. Several factors, including metabolic acidosis, elevation of plasma P_i], salmon stanniocalcin, salmon somatolactin and mammalian prolactin, have now been shown to alter transepithelial P_i transport in winter flounder PTCs. Salmon stanniocalcin (STC) stimulated P_i luminal-to-peritubular transport (reabsorption) at a dosage of 12.5–50 ng/ml (0.25–1.0 nM). Net P_i transport changed within 30 min and progressively increased from slight net secretion in untreated controls to net reabsorption after 3 h. The target and function of somatolactin have been uncertain. In our hands salmon somatolactin (sSL) also stimulated P_i reabsorption by flounder PTCs in a dose-dependent manner at physiological levels of the hormone (12.5 ng/ml). Net P_i transport was significantly altered by sSL within 2 h after the initial exposure. Neither sSL nor STC had any effect on transepithelial Ca²⁺ transport. The effects of both sSL and STC were mimicked by forskolin, whereas H-89, a highly specific protein kinase A inhibitor, significantly decreased the effects of the hormones as well as forskolin-induced P_i reabsorption. Furthermore, the production and release of cAMP were increased more than two-fold following exposure to STC or sSL. The data indicate that STC and sSL directly stimulate net renal P_i reabsorption by a cAMP-dependent pathway. In addition, mammalian prolactin greatly, and salmon growth hormone slightly, increased net P_i reabsorptive flux, whereas salmon prolactin had no effect. These results appear to be related to the location of the cysteine disulfide bonds within the molecular structure. Although somatolactin and stanniocalcin may both stimulate renal P_i conservation, their actions may be related to different physiological conditions.