VENOUS AND ARTERIAL RESPONSES TO NOREPINEPHRINE IN DOGS TREATED WITH RESERPINE

SENSITIZATION OF ARTERIES, VEINS, AND SMALL VESSELS TO NOREPINEPHRINE AFTER COCAINE

The effects of cocaine on the responses of arteries, veins, and small vessels to norepinephrine were studied. Intra-arterial norepinephrine was injected or infused into perfused forelegs of dogs. Angiotensin and serotonin were used as agonists. The effects of postganglionic sympathetic stimulation and of intra-arterial tyramine were tested also.

Cocaine in doses of 0.5, 5.0, and 10 mg/kg IV sensitized the arterial and venous segments to norepinephrine; the responses of small vessels were not augmented significantly. The effects of angiotensin and serotonin were not increased. The vasoconstrictor effect of weak and strong submaximal postganglionic sympathetic nerve stimulations was reduced and that of intra-arterial tyramine was suppressed after cocaine.

The results allow us to conclude that vascular supersensitivity to norepinephrine after cocaine is not a uniform phenomenon but it is restricted to certain vascular segments. The reason for the sensitization of arteries and veins but not small vessels is unknown. The observations are discussed in light of the recent hypothesis proposed by Furchgott et al on the mechanism of action of cocaine.

A scanning electron microscopic study of the contraction of vascular wall cells in dog dental pulp

Ultrastructural changes in vascular wall cells in dog dental pulp after removal of connective tissue components were studied by scanning electron microscopy following administration of norepinephrine (0.2 mg/kg). Contracted smooth-muscle cells were frequently seen in arterioles of all sizes. The surfaces of these cells were highly irregular with numerous evaginations and invaginations, varying considerably in configuration and size. Many evaginations ran longitudinally or obliquely to the long axis of the vessel. Some meandering evaginations were also observed as, rarely, were small spherical or bulbous projections. Spidery smooth-muscle cells frequently seen in the tunica media of terminal arterioles and thought to be primitive smooth-muscle cells exhibited fewer irregularities than the typical spindle-shaped smooth-muscle cells beneath them. Pericytes in the larger post-capillary venules (20 microns or larger in diameter) often showed evaginations and invaginations, mainly running parallel to the vessel axis. On the other hand, no surface irregularities could be seen in pericytes of either the smaller post-capillary venules (less than 20 microns in diameter) or the capillaries, although occasional evaginations running parallel to the vessel axis were noted on the outer surface of the endothelium.

Responses of saphenous and mesenteric veins to administration of dopamine

Others have observed that dopamine (3,4-dihydroxyphenylethylamine) constricts resistance vessels in skin, but dilates these vessels in the mesentery. We studied the effects of dopamine on cutaneous and mesenteric veins of dogs to see if this agent also produced qualitatively different effects on the tone of capacitance vessels (veins) in these vascular beds. The lateral saphenous or the left colic vein was perfused at constant flow with blood from a femoral artery. Pressures at the tip of the perfusion cannula and at the tip of a catheter 15 cm downstream were recorded continuously. Increases in the pressure gradient between these two points indicated venoconstriction; decreases indicated venodilatation. Dopamine and norepinephrine injected into the perfusion tubing caused constriction of both veins. The constriction was antagonized by blockade of alpha receptors. A dilator action of dopamine was not seen, even after alpha receptor blockade or in the presence of increased venous tone produced by serotonin, norepinephrine, or nerve stimulation. Reserpine and cocaine did not alter responses to dopamine in the saphenous vein; this suggests that the venoconstrictor action of dopamine results mainly from a direct effect on alpha receptors and that uptake into sympathetic nerve endings may not be important in regulating the amount of dopamine available to receptors in the saphenous vein.

Autonomic reflexes and vascular reactivity in experimental scurvy in man

Ascorbic acid is a required cofactor in the conversion of dopamine to norepinephrine in vitro, and the deficiency of this vitamin in guinea pigs is associated with degeneration of autonomic ganglion cells and with cardiac supersensitivity to norepinephrine. Because of these findings, we tested the hypothesis that ascorbic acid deficiency in man alters autonomic cardiovascular reflexes and vasomotor responses to adrenergic stimuli. We studied five normal volunteers who had been deprived of ascorbic acid for a period of 3 months; they had developed symptoms and signs of scurvy and their plasma levels of ascorbic acid averaged 0.178 +/-SE 0.07 mg/100 ml. We repeated the studies after giving the subjects vitamin C for a period of 4 months; they had become asymptomatic and their plasma ascorbic acid had increased to an average of 1.68 +/-0.151 mg/100 ml. Blood flow to the left forearm (plethysmograph), arterial and central venous pressures, and heart rate were measured before and after exposure of the lower half of the body to subatmospheric levels of pressure and before and after intravenous and intra-arterial (left brachial artery) infusions of norepinephrine and tyramine. Average values of blood flow (7.9 +/-1.4 ml/min per 100 ml), arterial pressure (91.2 +/-4.6 mm Hg), heart rate (68 +/-4.4 beats/min), central venous pressure (6.1 +/-1.1 mm Hg), and plasma catecholamines (0.68 +/-0.20 mug/liter) obtained during ascorbic acid deficiency were not altered significantly after correction of the deficiency. Vasoconstrictor responses to intra-arterial norepinephrine and tyramine were augmented after vitamin repletion. During ascorbic acid deficiency, four subjects had reduced responsiveness of resistance vessels of the forearm to lower body negative pressure as compared to the responsiveness observed after vitamin repletion. Reflex tachycardia during lower body negative pressure and reflex bradycardia during the pressor responses to intravenous tyramine and norepinephrine were similar during the two studies. The results suggest that the decreased vascular responsiveness to intra-arterial norepinephrine and tyramine and to lower body negative pressure during ascorbic acid deficiency is caused by a defect in the ability of resistance vessels to constrict in response to adrenergic stimuli. Ascorbic acid deficiency in man does not interrupt autonomic reflexes and does not appear to cause significant depletion of endogenous norepinephrine.