Resetting of exaggerated tubuloglomerular feedback activity in acutely volume-expanded young SHR

One purpose of the present study was to evaluate the ability of 7-wk-old spontaneously hypertensive rats (SHR) to reset tubuloglomerular feedback (TGF) activity in response to acute volume expansion (VE). Second, we evaluated the contribution of ANG II, via its action on AT1 receptors, to TGF control of glomerular function during VE. TGF was assessed by micropuncture methods and proximal tubular stop-flow pressure (SFP) determinations in SHR, Wistar-Kyoto rats (WKY), and Sprague-Dawley rats (SD). During euvolemia SHR exhibited enhanced TGF activity. In the same animals acute VE was achieved by infusion of saline (5 ml. h-1. 100 g body wt-1). VE led to resetting of TGF in all three strains. Maximal SFP responses, elicited by a 30-40 nl/min loop of Henle perfusion rate, decreased from 19 to 12 mmHg in SHR and, on average, from 11 to 5 mmHg in WKY and SD (P < 0.001). Tubular flow rate producing a half-maximal response (turning point) shifted to higher flow rates during VE, from 12 to 14 nl/min in SHR and from 15 to 19 nl/min in WKY. Administration of the AT1 receptor blocker candesartan (0.05 mg/kg iv) during sustained VE decreased TGF-mediated reductions in SFP in SHR and slightly increased the turning point in WKY. Nevertheless, other parameters of TGF activity were unaffected by AT1 receptor blockade. In conclusion, young SHR possess the ability to reset TGF activity in response to VE to a degree similar to compensatory adjustments in WKY. However, TGF remains enhanced in SHR during VE. ANG II and its action on AT1 receptors are in part responsible for the exaggerated SFP responses in young SHR during VE.