-Receptor-mediated inhibition of amiloride-sensitive sodium absorption was observed in primary and immortalized murine renal collecting duct cell (mCT12) monolayers. The addition of epidermal growth factor (EGF) to the basolateral bathing solution of polarized monolayers reduced amiloride-sensitive short-circuit current (I sc) by 15-25%, whereas the addition of ATP to the apical bathing solution decreased Isc by 40 -60%. Direct activation of PKC with phorbol 12-myristate 13-acetate (PMA) and mobilization of intracellular calcium with 2,5-di-tert-butyl-hydroquinone (DBHQ) reduced amiloride-sensitive I sc in mCT12 monolayers by 46 Ϯ 4% (n ϭ 8) and 22 Ϯ 2% (n ϭ 8), respectively. Exposure of mCT12 cells to EGF, ATP, PMA, and DBHQ caused an increase in phosphorylation of p42/p44 (extracellular signal-regulated kinase; ERK1/2). Pretreatment of mCT12 monolayers with an ERK kinase inhibitor (PD-98059; 30 M) prevented phosphorylation of p42/p44 and significantly reduced EGF, ATP, and PMA-induced inhibition of amiloride-sensitive I sc. In contrast, pretreatment of monolayers with a PKC inhibitor (bisindolylmaleimide I; GF109203x; 1 M) almost completely blocked the PMA-induced decrease in I sc, but did not alter the EGF-or ATP-induced inhibition of Isc. The DBHQ-mediated decrease in I sc was due to inhibition of basolateral Na ϩ -K ϩ -ATPase, but EGF-, ATP-, and PMA-induced inhibition was most likely due to reduced apical sodium entry (epithelial Na ϩ channel activity). The results of these studies demonstrate that acute inhibition of amiloride-sensitive sodium transport by extracelluar ATP and EGF involves ERK1/2 activation and suggests a role for MAP kinase signaling as a negative regulator of electrogenic sodium absorption in epithelia.mitogen-activated protein kinase; epithelial ion transport; epithelial sodium channel AMILORIDE-SENSITIVE Na ϩ transport, mediated by the epithelial Na ϩ channel (ENaC), is an important pathway for Na ϩ absorption by the colon and for Na ϩ retention by the distal nephron (16). A similar mechanism for Na ϩ transport is in the salivary duct, urinary bladder, sweat duct, and airway epithelium, where the process is responsible for the clearance of fetal lung liquid at birth (9, 28) and for control of the composition and/or depth of airway surface liquid in the postnatal lung (4). Defective regulation of ENaC and resultant Na ϩ hyperabsorption by the airway surface epithelium is thought to be an important component of the pathophysiology of cystic fibrosis (5). ENaC loss-of-function mutations (e.g., pseudohypoaldosteronism type I) are known to cause hyponatremia, hyperkalemia, and salt wasting, whereas gain-of-function mutations (e.g., salt-sensitive hypertension, Liddle's syndrome) lead to an increase in ENaC activity and excess Na ϩ reabsorption (20, 31).Corticosteroids, insulin, and vasopressin cause acute and/or chronic stimulation of sodium transport in epithelial cells (45) by several mechanisms, including alterations in ENaC open probability (23), increased synthesis of channel subunits...

Amiloride-sensitive sodium entry, via the epithelial sodium channel (ENaC), is the rate-limiting step for Na+ absorption in kidney collecting ducts, and epidermal growth factor (EGF) inhibits Na+ transport and ENaC expression. A pathognomonic feature of polycystic kidney disease (PKD) is EGF receptor mislocalization to the apical plasma membrane and EGF/EGF receptor axis overactivity. Immunohistochemical and biochemical analysis revealed mislocalization of EGF receptor and excessive activation of the p42/44 extracellular signal-regulated protein kinase pathway (ERK1/2) in kidneys from cystic mice compared with noncystic littermates. Primary monolayer cultures of noncystic and cystic murine collecting duct principal cells were used to identify aberrant EGF-dependent ERK1/2 activation and regulation of Na+ transport associated with autosomal recessive PKD. Addition of EGF to the basolateral bathing solution of noncystic or cystic monolayers led to p42/44 phosphorylation and inhibition of Na+ transport (30-35%), whereas apical EGF was effective only in monolayers derived from cystic mice. p42/44 Phosphorylation and inhibition of Na+ transport were prevented by prior treatment of the cells with an ERK kinase inhibitor. Chronic treatment (24 h) of noncystic and cystic monolayers with basolateral EGF elicited sustained inhibition of Na+ absorption (50-55%) and a reduction in steady-state ENaC mRNA levels (50-75%). In contrast, addition of EGF to the apical bathing solution (24 h) had no effect in noncystic monolayers but led to inhibition of Na+ transport (50-60%) and decreased ENaC expression (45-60%) in cystic cells. Pretreatment of the monolayers with an ERK kinase inhibitor abolished the chronic effects of EGF on Na+ transport. The results of these studies reveal that the mislocalized apical EGF receptors are functionally coupled to the ERK pathway and that abnormal EGF-dependent regulation of ENaC function and expression may contribute to PKD pathophysiology.

Chloride channels are expressed along the entire mammalian nephron. They participate in transepithelial chloride transport, cell volume regulation and acidification of intracellular vesicles. Some chloride channels are constitutively active and others are regulated by either second messengers such as cAMP or Ca(++) or secondary to changes in membrane potential. The molecular identities of a number of chloride channels within the kidney are still unknown. Abnormalities in chloride channel expression and function in the kidney can cause a range of disorders such as autosomal recessive Dent's disease, Bartter's syndrome, renal tubular acidosis and diabetes insipidus. The purpose of this review is to give an overview of the chloride channels in the kidney and to focus on the function of renal chloride channels as revealed by diseases associated with channel dysfunction.