Effect of U-73,122, an inhibitor of phospholipase C, on actions of parathyroid hormone in opossum kidney cells

Parathyroid hormone (PTH) decreases sodium-phosphate cotransporter type IIa (NpT2a) mRNA stability

The acute inhibitory effects of parathyroid hormone (PTH) on proximal tubule Na+-K+-ATPase (Na-K) and sodium-dependent phosphate (NaPi) transport have been extensively studied, while little is known about the chronic effects of PTH. Patients with primary hyperparathyroidism, a condition characterized by chronic elevations in PTH, exhibit persistent hypophosphatemia but not significant evidence of salt wasting. We postulate that chronic PTH stimulation results in differential desensitization of PTH responses. To address this hypothesis, we compared the effects of chronic PTH stimulation on Na-Pi cotransporter (Npt2a) expression and Na-K activity and expression in Sprague Dawley rats, transgenic mice featuring parathyroid-specific cyclin D1 overexpression (PTH-D1), and proximal tubule cell culture models. We demonstrated a progressive decrease in brush-border membrane (BBM) expression of Npt2a from rats treated with PTH for 6 h or 4 days, while Na-K expression and activity in the basolateral membranes (BLM) exhibited an initial decrease followed by recovery to control levels by 4 days. Npt2a protein expression in PTH-D1 mice was decreased relative to control animals, whereas levels of Na-K, NHERF-1, and PTH receptor remained unchanged. In PTH-D1 mice, NpT2a mRNA expression was reduced by 50% relative to control mice. In opossum kidney proximal tubule cells, PTH decreased Npt2a mRNA levels. Both actinomycin D and cycloheximide treatment prevented the PTH-mediated decrease in Npt2a mRNA, suggesting that the PTH response requires transcription and translation. These findings suggest that responses to chronic PTH exposure are selectively regulated at a posttranscriptional level. The persistence of the phosphaturic response to PTH occurs through posttranscriptional mechanisms.

Proximal tubular phosphate reabsorption: molecular mechanisms

Renal proximal tubular reabsorption of P(i) is a key element in overall P(i) homeostasis, and it involves a secondary active P(i) transport mechanism. Among the molecularly identified sodium-phosphate (Na/P(i)) cotransport systems a brush-border membrane type IIa Na-P(i) cotransporter is the key player in proximal tubular P(i) reabsorption. Physiological and pathophysiological alterations in renal P(i) reabsorption are related to altered brush-border membrane expression/content of the type IIa Na-P(i) cotransporter. Complex membrane retrieval/insertion mechanisms are involved in modulating transporter content in the brush-border membrane. In a tissue culture model (OK cells) expressing intrinsically the type IIa Na-P(i) cotransporter, the cellular cascades involved in "physiological/pathophysiological" control of P(i) reabsorption have been explored. As this cell model offers a "proximal tubular" environment, it is useful for characterization (in heterologous expression studies) of the cellular/molecular requirements for transport regulation. Finally, the oocyte expression system has permitted a thorough characterization of the transport characteristics and of structure/function relationships. Thus the cloning of the type IIa Na-P(i )cotransporter (in 1993) provided the tools to study renal brush-border membrane Na-P(i) cotransport function/regulation at the cellular/molecular level as well as at the organ level and led to an understanding of cellular mechanisms involved in control of proximal tubular P(i) handling and, thus, of overall P(i) homeostasis.

Phospholipase C in mouse oocytes: characterization of beta and gamma isoforms and their possible involvement in sperm-induced Ca2+ spiking

This study involved an investigation of the role of phospholipase C (PLC) in generating repetitive Ca2+ spikes at fertilization. Using a PCR-based strategy we have demonstrated that mouse oocytes have mRNA coding for PLC beta 1, PLC beta 3 and PLC gamma isoenzymes. Furthermore, immunodetection of PLC gamma 1 using monoclonal antibodies reveals that PLC gamma 1 protein is present in mature mouse oocytes, ruling out the possibility that mRNA was being transcribed but not expressed. We were unsuccessful at detecting the presence of PLC beta protein, but the presence of this isoform can be inferred from functional studies. The PLC inhibitor, U73122, exerted an inhibitory effect on oocytes activated by spermatozoa or acetylcholine at concentrations of 10 and 30 microM respectively, while its inactive analogue had no effect. The soluble tyrosine kinase inhibitors, genistein (100 microM), herbimycin (10 microM) and geldanamycin (0.6 microM) which could affect signalling through PLC gamma hindered but never completely inhibited Ca2+ spiking in response to fertilization. We conclude that the activation of PLC to generate InsP3 may play a critical role in fertilization.