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The Abnormal Renal Vasodilator Response to D1-Like Receptor Stimulation in Conscious SHR Can Be Normalized by AT1 Blockade

Background: We previously showed that the renal vasodilator response to a D1-like receptor agonist is blunted in conscious SHR compared with WKY rats. The mechanism of this impaired dopaminergic responsiveness in SHR is unclear. An altered balance between the renin-angiotensin-aldosterone system (RAAS) and the dopaminergic system may be involved. To determine the interaction between the RAAS and the dopaminergic system in the blunted D1-like responsiveness in SHR, we studied the renal vasodilator response to the D1-like receptor agonist fenoldopam before and after 7 days of pretreatment with the AT1-receptor antagonist (AT1-A) L158,809 in conscious SHR and WKY rats. Methods: Effective renal plasma flow (ERPF) was measured by the clearance of 131I-hippuran. Mean arterial pressure (MAP) was measured via an intraarterial catheter. Results: Without pretreatment, MAP was reduced to comparable degrees by fenoldopam in WKY (−7 ± 4%, ns) and SHR (−6 ± 1%, P < 0.05). However, ERPF was significantly more increased (P < 0.006) by fenoldopam in WKY (+26 ± 2%, P < 0.0001) than in SHR (+2 ± 2%, ns). AT1-A treatment reduced MAP and increased ERPF and glomerular filtration rate significantly in both strains. Pretreatment with AT1-A significantly potentiated the fenoldopam-induced rise in ERPF in SHR, but not in WKY, without affecting the blood pressure responses in either strain. As a result, during pretreatment with an AT1-A, the rise in ERPF by fenoldopam was similar in both strains (SHR +25 ± 2%, P < 0.0001; WKY +33 ± 2%, P < 0.0001). Conclusions: These results suggest that the RAAS accounts for the blunted renal vasodilator response to a D1-like receptor agonist in SHR. A dysbalance between the dopaminergic system and the RAAS may be involved in the abnormal renal hemodynamic regulation in SHR.