Influence of an adrenergic beta-receptor blocking agent on the effect of various hypotensive agents in the hypertensive rat

The essential action of propranolol in hypertension

The unique action of propranolol and other beta blockers in lowering raised arterial pressure is discussed. Although the onset of the antihypertensive effect is not immediate, many trials have confirmed the efficacy of these drugs. Animal experiments have thrown little light on the mechanism of action of beta blockers in hypertension: this may be because in animals, especially the rat, peripheral beta adrenoceptor vasodilatation is relatively more important than in man. Five principal theories have been advanced to explain the antihypertensive effect. None of these, the renin, central nervous system, cardiac, baroceptor or metabolite theory, is totally satisfactory. A new theory is proposed suggesting that the essential action is to diminish sympathetic nerve output by damping sensory input to the central nervous system from a heart whose capacity to respond to exercise and stress is blunted by beta adrenoceptor blockade.

Mechanism of the hypotensive action of methyldopa in normal and immunosympathectomized rats

1. The antihypertensive action of methyldopa was investigated in the light of the prevailing false sympathetic neurotransmitter hypothesis of Day & Rand (1963). Immunosympathectomized and normal animals were used for the investigation.2. Methyldopa in a single injection significantly decreased the blood pressure in control hypertensive, immunosympathectomized normotensive, and normal rats. Guanethidine did not decrease the blood pressure of immunosympathectomized rats; it decreased the blood pressure of control and control hypertensive rats.3. The hypotensive effect of methyldopa was not antagonized by adrenoceptor and ganglion blocking agents or by the inhibition of dopamine beta-oxidase.4. High doses of methyldopa produced less hypotension than relatively low doses. This might be due to an antagonism of the hypotensive effect by methylnoradrenaline, which is formed from methyldopa.5. The antihypertensive action of methyldopa could not be correlated with any change in aortic sodium or potassium.6. It is concluded that methyldopa does not exert its hypotensive effect by producing a weakly active false sympathetic neurotransmitter.

Mechanism of the antagonism of the hypotensive action of guanethidine by propranolol

1. Propranolol antagonizes the hypotensive effect of guanethidine in renal hypertensive but not in adrenal demedullated renal hypertensive rats.2. Guanethidine does not interfere with the release of catecholamines from the adrenal gland which follows splanchnic nerve stimulation in cats anaesthetized with pentobarbitone.3. Guanethidine does not release or interfere with the release of catecholamines after the intravenous injection of physostigmine in the rat.4. Propranolol antagonizes the antihypertensive effect of guanethidine by potentiating the vasopressor effects of catecholamines released from the adrenal glands during the induction stage of ether anaesthesia.