Venous and arterial responses to stimulation of beta adrenergic receptors

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Abstract
The effects of intra-arterial injections of isoproterenol on small vessels, arteries, and veins were studied in the paw and muscle of the foreleg of dog. Total blood flow to the foreleg was held constant. Blood pressures were measured in large and small arteries and veins. In some experiments venous outflow from paw and muscle was measured. The small vessel segment was the site of major dilatation. Venodilator responses were small, but they were enhanced when isoproterenol was injected in the presence of venous constriction induced by intra-arterial infusions of norepinephrine. Dichloroisoproterenol and nethalide blocked the dilator responses to isoproterenol in all segments but they did not block the dilator responses to acetylcholine or glyceryl trinitrate. Isoproterenol did not change the distribution of blood flow within the foreleg; responses in muscle and paw were not significantly different. The results indicate that stimulation of beta receptors with isoproterenol causes much more dilatation in small vessels than in veins in the foreleg and that small vessels of both paw and muscle dilate equally.