Novel expression and regulation of the renin-angiotensin system in metanephric organ culture

To evaluate the presence and regulation of the renin-angiotensin system (RAS) in metanephric organ culture, embryonic day 14 (E14) rat metanephroi were cultured for 6 days. mRNAs for renin and both ANG II receptors (AT(1) and AT(2)) are expressed at E14, and all three genes continue to be expressed in culture. Renin mRNA is localized to developing tubules and ureteral branches in the cultured explants. At E14, renin immunostaining is found in isolated cells scattered within the mesenchyme. As differentiation progresses, renin localizes to the ureteric epithelium, developing tubules and glomeruli. E14 metanephroi contain ANG II, and peptide production persists in culture. Renin activity is present at E14 (6.13 +/- 0.61 pg ANG I. kidney(-1). h(-1)) and in cultured explants (28.84 +/- 1. 13 pg ANG I. kidney(-1). h(-1)). Renin activity in explants is increased by ANG II treatment (70.1 +/- 6.36 vs. 40.97 +/- 1.94 pg ANG I. kidney(-1). h(-1) in control). This increase is prevented by AT(1) blockade, whereas AT(2) antagonism has no effect. These studies document an operational local RAS and a previously undescribed positive-feedback mechanism for renin generation in avascular, cultured developing metanephroi. This novel expression pattern and regulatory mechanism highlight the unique ability of developing renal cells to express an active RAS.

Extracellular cGMP inhibits transepithelial sodium transport by LLC-PK1 renal tubular cells

Both atrial natriuretic peptide (ANP) and sodium nitroprusside (SNP) inhibit tubular sodium reabsorption by generation of guanosine 3',5'-cyclic monophosphate (cGMP). To determine the role of extracellular cGMP in this response, monolayers of porcine renal tubular LLC-PK1 cells were incubated for 5 min with ANP, SNP, cGMP, or 8-bromoguanosine 3',5'-cyclic monophosphate (8-BrcGMP) (10 nM to 0.1 mM). Transepithelial sodium transport was measured as amiloride-inhibitable short-circuit current (Isc). Incubation of cell monolayers with 1 microM of ANP, cGMP, or 8-BrcGMP inhibited Isc by > 70%, as did SNP at 100 microM (P < 0.01). Adenosine 3',5'-cyclic monophosphate (0.1 mM) had no significant effect. Incubation of monolayers with 1 microM LY-83583 (an inhibitor of guanylyl cyclase), 10 microM probenecid (an organic anion transport inhibitor), or preincubation with 1 microgram/ml nocodazole (a microtubule disrupter) reduced extracellular accumulation of cGMP (P < 0.05) and abolished the SNP-mediated reduction of Isc. However, addition of these inhibitors did not affect reduction of Isc by exogenous cGMP. We conclude that SNP inhibits sodium transport by LLC-PK1 monolayers through generation of cGMP but that extrusion of cGMP out the cell is necessary for its effect.

Localization of cGMP after infusion of ANP or nitroprusside in the maturing rat

To determine the ontogeny of intrarenal distribution of guanosine 3',5'-cyclic monophosphate (cGMP) formation in response to atrial natriuretic peptide (ANP) or sodium nitroprusside (SNP), adult and neonatal Sprague-Dawley rats were anesthetized and infused for 60 s with rat ANP (5-2,500 micrograms/kg) or SNP (0.1-10.0 mg/kg). cGMP was identified by the immunoperoxidase technique using a specific antibody. In adult rats, infusion of ANP localized cGMP primarily to the glomerular podocytes, whereas stimulation by SNP increased cGMP in the mesangium only (P less than 0.01). In neonatal rats, although overall renal cGMP immunostaining was greater than in adults, specific localization to podocytes (ANP) or mesangium (SNP) resulted only with higher doses of agonists. Although basal generation of cGMP by isolated glomeruli was greater in neonatal rats, the threshold for stimulation by ANP was lower in glomeruli from adult rats. We conclude that in vivo ANP stimulates glomerular particulate guanylate cyclase primarily in the podocytes, whereas SNP stimulates soluble guanylate cyclase localized to the mesangial cells. There is a maturational increase in the sensitivity for activation of glomerular particulate and soluble guanylate cyclase.

Inhibition of ANP clearance receptors and endopeptidase 24.11 in maturing rats

Systemic clearance of atrial natriuretic peptide (ANP) decreases during postnatal development. To determine the relative contribution of ANP clearance (C) receptors and neutral endopeptidase 24.11 (NEP; EC 3.4.24.11) in regulation of plasma ANP concentration ([ANP]) during maturation, 18- to 60-day-old male Sprague-Dawley rats were anesthetized and infused with rat ANP (35 ng.kg-1.min-1). Infusion of the NEP inhibitor phosphoramidon increased [ANP] and urine guanosine 3',5'-cyclic monophosphate (cGMP) excretion in both weanling and adult rats. Infusion of C-ANP, an analogue that binds C receptors selectively, resulted in a greater rise in [ANP] in preweaned than in adult rats, suggesting a maturational decrease in function of C receptors. Despite the increase in [ANP], however, urine flow, cGMP, and sodium excretion failed to increase in preweaned compared with adult rats. Combined infusion of phosphoramidon and C-ANP resulted in a marked increase in [ANP] and cGMP excretion in weanling and adult rats. These results indicate that both C receptors and NEP modulate plasma [ANP] in the physiological range and that each pathway compensates when the other is inhibited. Age-related differences in the renal response to ANP clearance inhibitors may have important physiological implications in the regulation of sodium balance during development.