Acute resetting of arterial baroreflexes in hypertensive rats.

Acute baroreflex resetting was studied in one-kidney, one-clip renal hypertensive (HT) and normotensive (NT) control male Wistar-Kyoto rats. Systolic blood pressure (SBP) was monitored weekly in conscious rats by the indirect tail-cuff occlusion method. At the time of experimentation (4-6 wk after placement of the clip), SBP in the HT rats (238 +/- 9 mmHg) was elevated compared with NT rats (117 +/- 5 mmHg). Baroreflex control of lumbar sympathetic nerve activity (LSNA) was assessed in alpha-chloralose (120 mg/kg)-urethan (600 mg/kg)-anesthetized rats by raising and lowering mean arterial pressure (MAP) between 50 and 200 mmHg (phenylephrine and nitroprusside infusions). The baroreflexes of the hypertensive rats were less sensitive to increments in pressure compared with normotensive rats as evidenced by a decreased slope of the baroreflex curve (-0.396 +/- 0.064 HT vs. -0.637 +/- 0.042 NT). In both groups, curves were obtained before (control) and after (reset) base-line MAP was increased by 30-55 mmHg for 15 min. The MAP for 50% maximum inhibition of LSNA (EP50) was increased in both groups of rats after exposure to the elevated pressure (NT, control = 109 +/- 5.2, reset = 120 +/- 5.0 mmHg; HT, control = 114 +/- 6.5, reset = 131 +/- 10.0 mmHg) with no significant change in slope. There was no significant difference in degree of upward resetting between the two groups (NT, 21 +/- 4%; HT, 30 +/- 7%). Thus, although hypertensive rats exhibited the depressed baroreflex function associated with chronic resetting, the ability to reset acutely was maintained.