Objective: 

Two kidney one clip (2K1C) hypertension in rats is associated with sympathetic hyperactivity. The alpha-2 agonist, clonidine, reduces sympathetic activity. The aim of the present study was to investigate the effects of clonidine on mean arterial blood pressure (MAP), heart rate (HR), cardiac sympathetic tonus (ST), and parasympathetic tonus (PT) in conscious 2K1C hypertensive rats.

Methods: 

For this purpose, a one week after renal artery clipping or sham operation, we treated the rats with either clonidine (200 mg/kg/d, orally) or distilled water for two weeks. Then, the rats were instrumented with catheters in the femoral artery and vein to measure MAP and to infuse the drugs, respectively. After a one day of recovery, cardiac ST and PT were determined by HR response to atenolol (1 mg/kg) and atropine methyl nitrate (1 mg/kg), respectively.

Results: 

Clonidine increased MAP in both sham-operated (70 mmHg vs 79) and hypertensive rats (112 mmHg vs 134; p<0.001). Resting HR tended to decrease in clonidine-treated sham-operated rats (p=0.058). Blockade of b1 receptors decreased HR in only distilled water treated sham-operated rats (p<0.01). Clonidine did not affect cardiac ST, and PT in both sham-operated and hypertensive rats. Intrinsic HR was less in clonidine-treated sham-operated rats than distilled water-treated sham-operated rats (361 beats/min vs 402; p<0.01).

Conclusions: 

We conclude that clonidine increases MAP and does not change cardiac sympathetic and parasympathetic activity in 2K1C hypertensive rats. In addition, clonidine decreases intrinsic HR in normotensive rats. Consequently, cardiac sympathetic and parasympathetic pathways may not contribute to the increase in MAP after clonidine treatment in normotensive and hypertensive rats.