Sympathetic component of baroreflex control of heart rate is impaired in hypertension-prone (SBH) Sabra rats.

: Baroreflex control of heart rate in response to phenylephrine was studied in conscious Sabra hypertension-prone (SBH) rats, at a prehypertensive stage, and hypertension-resistant (SBN) rats. Baroreflex sensitivity as determined from the slope of the relationship of mean arterial blood pressure and heart period was significantly lower in SBH rats (0.58 +/- 0.06 versus 1.71 +/- 0.11 ms/mmHg in SBN rats, P less than 0.01) before the development of hypertension. Sympathetic nerve blockade with guanethidine (15 mg/kg) significantly reduced the slope of the mean arterial blood pressure-heart period relationship in SBN rats to 0.45 +/- 0.05 ms/mmHg (P less than 0.01) and increased the pressor response to phenylephrine, without having any effect on these parameters in SBH rats. Atropine methyl nitrate (1 mg/kg) abolished reflex vagal bradycardia in response to phenylephrine in both groups of rats. This suggests that SBH rats are unable to withdraw the sympathetic cardiac component of the baroreflex in response to a pressor stimulus and appear to rely only on increased vagal activity to effect bradycardia.