The renal endothelin system in the Prague hypertensive rat, a new model of spontaneous hypertension

In a new model of spontaneous hypertension, namely the Prague hypertensive rat (PHR), hypertension is transferred with a kidney transplanted from the PHR to its normotensive counterpart (PNR) by an as yet unknown mechanism. One candidate may be endothelin (ET), since this potent vasoconstrictor affects vascular tone, renal haemodynamics and renal excretory function, and all members of this peptide family are located within the kidney and act in an autocrine/paracrine fashion. In the present study we investigated, in the renal tissue of PHRs and PNRs: (1) preproET-1 and preproET-3 mRNAs as well as ET-1 and ET-3 peptide distribution, (2) endothelin-converting enzyme (ECE)-1 mRNA expression, and (3) ET receptors and their characteristics in membranes of glomeruli and papillae. In addition, plasma ET concentration and urinary ET excretion were determined. Quantitative measurements by competitive reverse transcription-polymerase chain reaction revealed ET-1 mRNA levels in the renal cortex from PHRs and PNRs of 1.09±0.13 and 1.29±0.18 amol/µg of total RNA respectively, and in red medulla of 2.72±0.82 and 3.30±0.68 amol/µg respectively. In contrast, renal papilla from PHRs showed significantly lower levels of preproET-1 mRNA (1.81±0.64 amol/µg of total RNA, compared with 4.25±0.82 amol/µg in PNRs; each n = 5; P< 0.05). The ET-1 peptide concentration in papillary tissue was also significantly lower in PHRs than in PNRs (120.2±30.8 and 491.3±53.4 fmol/mg of protein respectively; n = 5; P< 0.01), whereas it was similar in cortex and medulla from PHRs and PNRs. The preproET-3 mRNA content in renal tissue was much lower than that of preproET-1 mRNA. It was significantly higher in red medulla from PHRs compared with that from PNRs (0.25±0.05 and 0.13±0.02 amol/µg of total RNA respectively; P< 0.05), but was similar in papillae of PHRs and PNRs (0.04±0.02 and 0.05±0.01 amol/µg respectively; n = 5). Cortical preproET-3 mRNA was at the lower limit of detection. Similarly, the ET-3 peptide concentration was slightly but significantly higher in the red medulla of PHRs compared with PNRs (15.4±2.0 and 8.8±0.8 fmol/mg of protein respectively; n = 5; P< 0.05), whereas no differences in ET-3 peptide concentration were found in papillae from PHRs and PNRs. ECE-1 mRNA levels were similar in the renal cortex, red medulla and papillae from PHRs and PNRs, ranging between 0.34±0.03 and 0.56±0.12 amol/µg of total RNA. Of the total ET receptors in glomerular membranes, 39% were ET_A receptors, whereas papillary membranes contained exclusively ET_B receptors. PHRs and PNRs showed similar B_max and K_d values for ET-1 in renal glomerular membranes (B_max, 6.5±1.3 and 4.9±1.2 pmol/mg of protein respectively; K_d, 0.69±0.10 and 0.56±0.10 nM respectively) and papillary membranes (B_max, 9.7±1.1 and 11.3±1.6 pmol/mg of protein respectively; K_d, 0.30±0.04 and 0.42±0.07 nM respectively). Plasma ET-1/2 concentrations (10.4±1.3 and 12.2±1.2 fmol/ml in PHRs and PNRs respectively) and urinary ET-1 excretion (3.1±0.3 and 3.0±0.2 pmol/24 h in PHRs and PNRs respectively) were similar in hypertensive and normotensive rats. In summary, although tissue levels of preproET-3 mRNA were very low in the kidney, significantly greater amounts of preproET-3 mRNA and ET-3 peptide were found in medullary tissue from PHRs compared with PNRs, a finding that awaits further investigation. In contrast, the preproET-1 mRNA content and ET-1 peptide concentration were significantly lower in papillary tissue from PHRs compared with PNRs. Decreased synthesis of ET-1, which normally antagonizes the action of [Arg⁸]vasopressin, may allow increased water (and sodium) reabsorption at the level of the inner medullary collecting duct. This intrinsic defect of the kidney in the PHR may contribute to hypertension in this model, and may transmit high blood pressure on transplantation of the ‘hypertensive’ kidney into a normotensive rat.