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

Abstract

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 fluorescence 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 selected 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 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.