Role of the α-Adrenergic Blocking Effect in the Acute Hypotensive Effect of β-Adrenergic Blocking Drugs with α-Blocking Activities in Conscious SHR

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Abstract
Acute hypotensive effects and the mechanisms of three .beta.-adrenergic blocking drugs with .alpha.-blocking activity were studied in comparison with those of prazosin, propranolol and hydralazine in the conscious spontaneously hypertensive rat (SHR). Prazosin lowered the blood pressure dose-dependently and inhibited the pressor response to phenylephrine. Three .beta.-adrenergic blocking drugs with .alpha.-blocking activity, labetalol (30 mg/kg), arotinolol (100 mg/kg) and nipradilol (100 mg/kg) also lowered the blood pressure to the same extent as prazosin (0.3 mg/kg), but the inhibition of the pressor response to phenylephrine produced by them was disproportionately slight. Propranolol (100 mg/kg) did not lower the blood pressure. These results suggest that the acute hypotensive effects of three .beta.-adrenergic blocking drugs with .alpha.-blocking activity were attributable only partially to the .alpha.-adrenergic blocking effect; a mechanism or mechanisms other than the .alpha.-adrenergic blocking effect must be invoked to explain the acute hypotensive effect produced by lower doses of these drugs in the conscious SHR.