**3.1.3 Sodium-dependent phosphate cotransporters**

Sodium-dependent phosphate cotransporters reabsorb inorganic phosphate in a sodiumdependent manner in PTCs. These will be discussed in detail in section 4.1.

#### **3.2 Hormones regulating the expression and function of the transporters involved in sodium reabsorption in PTCs**

Many hormones are involved in the regulation of sodium reabsorption in PTCs, particularly through the function of NHE3 (Fig. 2).

#### **3.2.1 Angiotensin II**

Angiotensin II is important for increasing sodium uptake by PTCs. Intrarenal levels of angiotensin II are augmented in CKD and are also involved in the development of hypertension (Kobori et al. 2007; Navar et al. 2011). The action of angiotensin II on NHE3 has been intensively investigated, indicating biphagic dosage-dependent acute effects and chronic stimulatory effects on NHE3 (Bobulescu and Moe 2009). Classically, angiotensin II has been thought to act on its specific receptors in PTC plasma membranes by endocrine

Hormonal Actions and Interactions in Proximal Tubule Cells

**3.2.5 Aldosterone** 

therapeutic target in CKD.

**in PTCs** 

**regulated** 

**4.1.1 NaPi-IIa** 

**3.2.6 Prolactin, dopamine and renalase** 

Associated with the Development of Chronic Kidney Disease 311

Aldosterone stimulates the activity and surface expression of NHE3 in human primary PTCs, which is inhibited by the mineralocorticoid receptor blocker spironolactone and by the epidermal growth factor receptor (EGFR)-kinase inhibitor (Drumm et al. 2006). Aldosterone was found to regulate NHE3 by a nongenomic mechanism (Good, George, and Watts 2006). Aldosterone/mineralocorticoid receptor stimulation is also known to affect PTCs by inducing cellular senescence (Fan et al. 2011), suggesting that it is a potential

Prolactin is also a natiriuretic hormone that inhibits Na+, K+, ATPase activity in PTCs, depending on the renal dopamine system (Ibarra et al. 2005). Dopamine is known to both acutely and chronically inhibit the function of NHE3 by reducing its cellular expression (Bobulescu and Moe 2009). Renalase, a secreted amine oxidase that is synthesized in the kidney, metabolizes circulating catecholamines (Xu et al. 2005). Urinary renalase metabolizes urinary catecholamines, and perhaps regulates dopamine concentration in luminal fluid, and

modulate the function of NHE3 for proximal tubular sodium transport (Desir 2011).

**4.1 Transporters involved in PTC phosphate reabsorption that is hormonally** 

NaPi-IIa inactivation, inducing the increased reabsorption of phosphate (Fig. 3).

regulation of phosphate uptake in PTCs is clinically important.

mediated by NaPi-IIa in the adult kidney (Ohkido et al. 2003).

**4. Hormonal actions and interactions for regulating phosphate reabsorption** 

Phosphate uptake is an important function of PTCs which is regulated by the actions and interactions of various hormones and hormone-like substances, such as parathormone (PTH), fibroblast growth factor 23 (FGF23), klotho, glucocorticoids, vitamin D3, dopamine and so on (Figs. 2 and 3). Phosphate regulation is particularly associated with the development of cardiovascular disease and high mortality in patients with CKD, independently of the glomerular filtration rate (Kestenbaum et al. 2005). Therefore, the

At least three types of sodium-dependent phosphate transporters have been identified in the brush border membrane of PTCs: NaPi-IIa, NaPi-IIc, and sodium-dependent phosphate symporter 2 (Pit-2) (Fig. 2). Renal reabsorption of inorganic phosphate is mainly (about 70%)

NaPi-IIa is a major phosphate transporter of PTCs, and changes in renal phosphate handling are primarily attributable to altered NaPi-IIa brush border membrane expression (Hernando et al. 2005). NaPi-IIa is expressed in the S1–S3 segment of superficial and juxtamedullary nephrons (Picard et al. 2010) and mediates the electrogenic transport of inorganic phosphate coupled to three sodium ions (Murer, Forster, and Biber 2004). NaPi-IIa is regulated by several hormones and other factors, as described in section 4.2. Adequate steady-state expression of NaPi-IIa and its inactivation by endocytosis and intracellular translocation require the presence of megalin (Bachmann et al. 2004), such that megalin dysfunction is likely to impair

and local tissue paracrine mechanisms. However, more recently it was also shown to function in an autocrine or intracrine manner (Li et al. 2011).

#### **3.2.2 Insulin**

Insulin is an important hormone that regulates sodium uptake as well as glucose metabolism in PTCs. In patients with metabolic syndrome, serum levels of insulin are elevated by a mechanism of insulin resistance in the liver and skeletal muscles. Increased levels of insulin are likely to augment sodium uptake in PTCs (Baum 1987) as well as other nephron segments such as thick ascending limbs (Mandon et al. 1993) and collecting ducts (Takahashi, Ito, and Abe 1996), causing extracellular fluid expansion and hypertension. Insulin has been shown to acutely and chronically stimulate the function of NHE3 in cultured PTCs (Fuster et al. 2007), and its infusion is associated with increased blood pressure in rats (Brands et al. 1991) and humans (Gans et al. 1991).

Diabetic kidneys, however, have been demonstrated to have impaired expression of renal insulin receptors (Tiwari et al. 2007). In kidney-specific insulin receptor KO mice, basal systolic blood pressure was increased and natriuresis was reduced in response to a volume load (Tiwari et al. 2008). The greatest relative reduction of the insulin receptor in these mice was found in the thick ascending limb and collecting duct, and renal production of NO was likely to be reduced. A previously uncharacterized role of the insulin receptor in these nephron segments may thus be associated with sodium and water reabsorption via NO production.

## **3.2.3 Glucocorticoids**

Glucocorticoids, whether endogenous, as in Cushing syndrome, or exogenous, via pharmacologic provision, induce hypertension (Mangos et al. 2003). They directly stimulate NHE3 through both acute and chronic mechanisms (Bobulescu and Moe 2009). The acute effect of glucocorticoids on cell surface NHE3 is mediated by the activation of HNE3 exocytosis (Bobulescu et al. 2005) and a glucocorticoid receptor-dependent mechanism that activates serum and glucocorticoid-inducible kinase1 (SGK1) in a nongenomic manner (Wang et al. 2007). Glucocorticoids have also been found to enhance the function of insulin in stimulating NHE3 activity (Klisic et al. 2002).

## **3.2.4 Glucagon-like peptide 1**

Glucagon-like peptide 1 (GLP1) receptors are expressed in pancreatic cells and other cells such as PTCs (Schlatter et al. 2007). Administration of recombinant GLP1 to Dahl saltsensitive rats resulted in an antihypertensive effect, due mainly to its diuretic and natriuretic actions (Yu et al. 2003). GLP1 was also found to induce natriuresis in healthy subjects and insulin-resistant obese men through its action on PTCs (Gutzwiller et al. 2004). Hyperglomerular filtration in obese men was improved following this treatment, which was likely to be mediated by tubuloglomerular feedback mechanisms.

GLP1 receptor agonistic agents have been used in the treatment of hyperglycemia by acting on GLP1 receptors in pancreatic ß-cells to stimulate insulin synthesis. The GLP1 receptor agonist exendin-4 also decreases sodium reabsorption in PTCs by affecting NHE3 activity (Carraro-Lacroix, Malnic, and Girardi 2009) and reduces blood pressure in a salt-sensitive hypertension mice model (Hirata et al. 2009). Such agents are also suggested to be clinically relevant in reducing blood pressure (Vilsboll et al. 2007; Klonoff et al. 2008).
