Complementary DNAs for each of the major sodium ion transporters and channels expressed along the renal tubule have been cloned over the past 15 years. The consequence is the generation of a broad range of cDNA and antibody probes that can be used to investigate physiological mechanisms on a molecular level. An ensemble of such probes can be exploited for comprehensive analysis of integrative physiological processes, approaches which are referred to as ‘physiological genomics’ or ‘physiological proteomics’. The current studies utilize a targeted proteomic approach to allow comprehensive analysis of sodium transporter and water channel protein abundance along the renal tubule, using an ensemble of rabbit polyclonal antibodies for semiquantitative immunoblotting and immunocytochemistry of renal homogenates (Masilamani et al. 1999; Brooks et al. 2001; Knepper & Masilamani, 2001; Wang et al. 2001). The strategy is to profile the transporter abundance changes along the nephron in response to known regulators of renal sodium excretion (aldosterone, angiotensin II, noradrenaline, vasopressin, endothelins, NO, cyclo-oxygenase products, atrial natriuretic factor, etc.), and then to look for the same patterns of responses in pathophysiological models of abnormal regulation of blood pressure (Dahl salt-senstive rat, 2-kidney, 1-clip Goldblatt rat) or extracellular fluid volume (congestive heart failure, cirrhosis, nephrotic syndrome). By this approach we can generate hypotheses regarding the mediators of abnormal sodium balance in pathophysiological models of human disease.

Current studies are focusing on regulatory targets for angiotensin II. These studies are employing renal tubule Na transporter profiling in three experimental models: (1) angiotensin II receptor AT_1a knockout mice; (2) rats infused with the AT₁ receptor blocker candesartan; and (3) rats infused with angiotensin II. The studies point to several potential molecular targets for regulation: (a) NaP_i-2 (sodium phosphate cotransporter type II in proximal tubule): marked decrease in abundance in AT_1a knockouts and with AT₁ receptor blockade. (b) NBC1 (sodium bicarbonate cotransporter in proximal tubule): marked decrease in abundance with AT₁ receptor blocked; increased abundance with angiotensin II infusion. (c) α-ENaC (α subunit of epithelial sodium channel): decreased in AT_1a knockout mice and with AT_1a receptor blockade; increased with angiotensin II infusion. (d) β- and λ-ENaC: increased in AT_1a knockout mice and with AT_1a receptor blockade. There were no consistently detectable changes in the abundances of NHE3 or NKCC2 in any of these protocols. Further studies focusing on the ENaC changes in response to candesartan demonstrated that the changes can occur in the presence of mineralocortocoid receptor blockade with spironolactone and without substantial decreases in circulating aldosterone. These findings therefore suggest the possibility that regulation of ENaC subunit abundances by angiotensin II is due a direct action of the peptide on the collecting duct cells rather than a response to mineralocorticoid.