ETA receptor-mediated Ca2+ signaling in thin descending limbs of Henle's loop: impairment in genetic hypertension.

BACKGROUND: Endothelins (ET) have diuretic and natriuretic actions via ETB receptors that are found in most renal tubular segments, although the thin limbs have not been studied. Data also suggest that dysfunction of the renal ET system may be important in the pathogenesis of hypertension. The present study was aimed at determining the presence and nature of ET receptors in the thin limbs of Henle's loop and their ability to activate a Ca2+-dependent signaling pathway, as well as whether ET-induced Ca2+ signals are altered in hypertension. METHODS: Reverse transcription-polymerase chain reaction (RT-PCR) and Fura 2 fluoreselected strains of Lyon rats with low-normal (LL), normal (LN), and high (LH) blood pressure. RESULTS: In SD rats, ET induced Ca2+ signals in DTL of long-looped nephrons, but not in DTL of short loops, or in ascending thin limbs. Ca2+ increases were abolished by BQ123, an antagonist of the ETA receptor, but not by BQ788, an antagonist of the ETB subtype. Endothelin-3 and sarafotoxin 6c, two ETB receptor agonists, were both inactive. RT-PCR showed the presence of both ETA and ETB receptor mRNA. Ca2+ signals measured scence measurements of [Ca2+]i were made to characterize ET receptors in descending thin limbs (DTL) of Sprague-Dawley rats, spontaneously hypertensive (SH) rats, and control Wistar-Kyoto (WKY) rats, and the three in DTL of WKY LL and LN rats were similar to those in Sprague-Dawley rats, but were significantly diminished (LH) or abolished (SH) in hypertensive rats. CONCLUSION: A functional ETA receptor activating a Ca2+-dependent pathway is expressed in DTL. This ETA-induced calcium signaling is impaired in two strains of genetically hypertensive rats.

Effect of luminal atrial natriuretic peptide on chloride reabsorption in mouse cortical thick ascending limb: inhibition by endothelin.

Insofar as neutral endopeptidase inhibition has afforded evidence for a tubular luminal action of atrial natriuretic peptide (ANP), the present study was undertaken to investigate a possible effect of the peptide on chloride reabsorption (JCl) in thick ascending limb (TAL). Luminal addition of ANP to in vitro microperfused cortical TAL (CTAL) significantly decreased JCl with a threshold and a maximum concentration of 10(-12) M and 10(-9) M, respectively. A similar effect of 10(-9) M ANP was observed in medullary TAL (MTAL). The effect of luminal ANP was significantly reduced by HS-142-1, a specific inhibitor of guanylyl cyclase receptor, and by H-8, a protein kinase G inhibitor, but was not affected by the protein kinase C inhibitor bisindolylmaleimide I. Unexpectedly, the effect of ANP was not additive with that of endothelin (ET), a peptide that was previously shown to decrease JCl in TAL through a calcium-independent, protein kinase C-mediated pathway. Indeed, ET-1 (10(-8) M in the lumen) significantly decreased JCl and prevented a further effect of ANP on the same tubule. Similarly, the decrease of JCl induced by simultaneous addition of ET and ANP was not higher than that obtained with each agent alone. Conversely, the inhibitory effect of ANP was enhanced in the presence of cyclic guanosine monophosphate (cGMP; 10(-6) M in the lumen). ET-1 significantly attenuated the ANP-stimulated generation of cGMP in microdissected CTAL and failed to prevent a further decrease of JCl promoted by a permeant cGMP analogue. It is concluded that luminal ANP decreased Cl reabsorption in mouse CTAL and MTAL. This effect was abrogated by ET-1 as a result of the inhibition of ANP-stimulated cGMP generation.