Disparate effects of methyldopa and clonidine on cardiac mass and haemodynamics in rats.

1. Wistar-Kyoto and spontaneously hypertensive rats were given either methyldopa (400 mg day-1 kg-1) or clonidine (0.1 or 0.3 mg day-1 kg-1) for 3 weeks commencing at 20 weeks of age. 2. Both drugs significantly decreased mean arterial pressure in spontaneously hypertensive but not Wistar-Kyoto rats. Heart rate was significantly increased in spontaneously hypertensive rats by methyldopa, whereas clonidine significantly decreased heart rate. The higher dose of clonidine also decreased heart rate in Wistar-Kyoto rats. Both cardiac output and total peripheral resistance decreased slightly, but not significantly, with both agents. 3. Methyldopa, but not the lower equipotent depressor dose of clonidine, reduced left ventricular hypertrophy in spontaneously hypertensive rats. However, the higher dose of clonidine also significantly decreased the heart to body weight ratio despite an increased total peripheral resistance presumably due to the alpha-adrenergic agonist effect. 4. Minimal changes in organ blood flows were noted with both drugs. 5. These results suggest that neither systemic haemodynamics nor central inhibition of adrenergic drive are primary factors responsible for the regression of hypertrophy.