The role of the baroreceptor reflex in the cardiovascular effects of propranolol in the conscious spontaneously hypertensive rat.

1. The role of baroreceptors in the cardiovascular mechanism of action of DL-propranolol has been studied by comparing the acute effects of subcutaneous injection of 1 and 5 mg/kg (3.3 x 10(-6) and 16.5 x 10(-6) mol/kg) of this drug in conscious baroreceptor-denervated spontaneously hypertensive (SH) rats with those in sham-operated control SH rats. 2. At 5 mg/kg (16.5 x 10(-6) mol/kg) propranolol caused a small, but significant, increase in blood pressure in sham-operated SH rats, whereas both after 1 and 5 mg/kg (3.3 x 10(-6) and 16.5 x 10(-6) mol/kg) immediate hypotension was observed in baroreceptor-denervated animals. 3. Heart rate dropped rapidly after injection of 1 or 5 mg/kg (3.3 x 10(-6) and 16.5 x 10(-6) mol/kg) propranolol both in the baroreceptor-denervated and sham-operated SH rats. Bradycardia was significantly larger in the baroreceptor-denervated animals after an injection of 5 mg/kg (16.5 x 10(-6) mol/kg). 4. It is concluded that the lack of an early hypotensive effect of propranolol in intact animals is caused by an increased baroreceptor reflex activity as a consequence of the fall in cardiac output.