Differential expression of natriuretic peptide receptors in primary cultures of rat and human proximal tubular cells : a role for the natriuretic peptides
The natriuretic peptide system consists primarily of the peptides, atrial natriuretic factor (ANF), brain natriuretic peptide (BNP) and c-type natriuretic peptide (CNP) and their actions are mediated via the natriuretic peptide receptors -A, -B and -C (NPR-A, -B, -C). Collectively, these peptides and receptors form an integrated hormonal system, which acts to regulate body fluid homeostasis and blood pressure control. This thesis aimed to confirm natriuretic peptide and NPR expression in freshly isolated and primary cultures of both rat and human proximal tubular (PT) cells, and to identify a possible role for local natriuretic peptide production. Local expression and production of the natriuretic peptides and NPRs was confirmed by RT-PCR, Northern analysis, cGMP response to ANF (NPR-A) and CNP (NPR-B) and RIA for the peptides A, B and C. This study demonstrated that PT cells in culture express both natriuretic peptides and NPRs, and that the process of culture results in increased natriuretic peptide expression and secretion. Furthermore, growth in culture results in a shift in NPR expression from the NPR-C in freshly isolated cells to the functional GC-linked NPR-A and -B at confleunce. Short-term incubation with exogenous natriuretic peptides suggested that natriuretic peptides could accelerate or induce changes in natriuretic peptide and NPR subtype expression, possibly via the NPR-C. The differential expression and secretion of natriuretic peptides and NPRs at different stages of culture strongly suggests a growth modulatory role for the natriuretic peptide system. These growth modulatory actions appear to be mediated by the NPR-C during the initial stages of culture and by the GC-linked NPR at later stages in culture. These results add further strength to the hypothesis that the natriuretic peptides and both the GC-linked NPR and the NPR-C, once thought to be a 'quiescent' receptor, act to modulate cell growth.