A comparison of some cardiovascular properties of propranolol, MJ 1999 and quinidine in relation to their effects in hypertensive animals.

1. A comparison of some cardiovascular effects of propranolol, MJ 1999 and quinidine has been made in rats and dogs.2. After intravenous, subcutaneous or oral administration to rats and dogs, propranolol was found to be 2-4 times more potent than MJ 1999 in blocking the chronotropic and vasodepressor responses to intravenously administered isoprenaline.3. Propranolol and quinidine affected the e.c.g. of rats and dogs in a similar manner.4. At dose-levels causing effective blockade of beta-receptors propranolol and MJ 1999 had no hypotensive effect after short- or long-term administration to conscious hypertensive rats and dogs.5. At very high dose-levels propranolol and quinidine, but not MJ 1999, lowered blood pressure in the hypertensive rat. This effect of propranolol is probably related to one or more of the properties that propranolol and quinidine have in common rather than to a blockade of beta-receptors.6. The possible relevance of these results to the use of propranolol as a hypotensive agent in man is discussed.