A Parathyroid Hormone-Stimulated Transport Process in Avian Renal Proximal Tubule Cells. Characterization of a Novel Hormone Response.

University Of Delaware, Newark DE

Investigators

Abstract

The comparative physiology approach has provided many examples of discoveries of novel epithelial transport systems or of models in which known systems are more easily studied. In this proposal experiments will be done on a primary cell culture model of the chick renal proximal tubule (PT). In this model renal cells are grown on rigid, microporous filters, altering the tissue architecture from tubules to flat monolayers. The monolayers reach a confluent, polarized state and display transport properties consistent with the PT. An electrophysiological approach is used to measure ion transport currents under varying conditions and in the presence or absence of agonists or antagonists. Initial studies have revealed a novel ion current response to parathyroid hormone (PTH), a calcium-regulating hormone with known actions on the PT. The novel current response appears to be chloride ion secretion (transport into the apical space) and to be dependent on two separate, but linked transport systems, a cystic fibrosis transmembrane regulator (CFTR)-like chloride channel and a new, chloride-dependent member of the sodium hydrogen exchanger (NHE) family, recently cloned from rat colon. The hypothesis of this study is that this PTH response is part of an orchestrated change in the overall pattern of proximal tubule transport, and that, although never previously reported, it also exists in mammalian proximal tubules at much lower expression levels. Thus, the chick culture system may represent an ideal model for further investigating this PTH effect and the role of CFTR in the proximal tubule, both of which seem to be highly expressed in this particular system. A combination of molecular studies (PCR and immunoblotting), isotopic tracers, fluorescent assays of intracellular pH (an assay for NHE activity) and electrophysiological methods will be used to determine whether the PTH effect can also be detected in native (non-cultured) chick kidney and whether it is found in rat proximal tubule cultures. Potential physiological roles for the PTH response will also be examined, including effects on fluid and urate secretion in this segment. The potential role of CFTR as a regulator of other transport systems will also be specifically addressed, using known blockers and antibodies to the CFTR protein. This study will provide significant new information about the role of PTH and CFTR in the kidney and will be of interest to the fields of comparative, renal and epithelial transport physiology.