Neurogenic control of peripheral resistance in renal hypertension

The increased peripheral resistance responsible for hypertension associated with unilateral renal ischemia and contralateral nephrectomy, is abolished after destruction by pithing of the central nervous system of dogs and rats, or after inactivation of the inhibitory carotid sinus reflex in dogs. The tissue catecholamine content of renal hypertensive rats was in the same range as in the normotensive animals. These results suggest that the increased peripheral resistance in hypertension is neurogenically mediated and that the increased resistance is due to an increased response of the resistance vessels to normal neurogenic stimulation.

Effect of guanethidine, hemicholinium and mebutamate on the hypertensive response to eserine and catechol amines

Guanethidine, hemicholinium and mebutamate were used to study the site and mechanism of the hypertensive response to eserine in the rat. Guanethidine was found to block very effectively the hypertensive effect of eserine and to produce at the same time a very strong potentiation of the response to catechol amines. Hemicholinium, after a certain latent period, also blocked the effect of eserine, at the same time leaving the response to adrenaline and noradrenaline intact. Mebutamate was also found to block the effect of eserine. The results of the present experiments suggest that eserine produces a central adrenergic activation in the rat.

SYSTEMIC BLOOD PRESSURE RESPONSE TO CHANGES IN RIGHT VENTRICULAR FUNCTION

The relationship between right ventricular and aortic pressure was studied in open-chest anesthetized dogs. A simultaneous increase in both pressures was observed in animals with an intact circulation and animals with the systemic circulation perfused at a constant rate and stroke volume by an extracorporeal pump. With gradual partial constriction of the main pulmonary artery, the increase in right ventricular systolic and aortic pressures occurred without consistent change in other measured parameters. Bilateral cervical vagotomy abolished the systemic vascular response.

These observations are consistent with a pressor reflex which is mediated over vagal afferent fibers from the right ventricle. It is suggested that in animals under varying physiologic stimuli an increase in right ventricular pressure is associated with a reflex increase in systemic vascular resistance. In abnormal circumstances with increased resistance to right ventricular emptying, the increased total peripheral vascular resistance would be important in maintaining systemic blood pressure and blood flow.

ENHANCEMENT OF FIBRINOLYSIS IN THE DOG BY INJECTION OF VASOACTIVE DRUGS

Intravenous injection of epinephrine into the dog produces an increase in fibrinolytic activity of plasma euglobulins. This increase appears to be unrelated to the following epinephrine-induced effects: splenic contraction, stimulation of the lymph circulation, increase in blood glucose, lactic acid, or free fatty acids. A specific involvement of either alpha or beta adrenotropic receptors is ruled out since norepinephrine and isopropylnorepinephrine are equally effective in enhancing fibrinolysis. Intravenous injection of each of a number of vasoactive drugs with different modes of action, and with either hypertensive or hypotensive properties, induces an increase in fibrinolytic activity. Hence, it appears that there is a direct relationship between appearance of increased fibrinolytic activity and vasoactive changes. For all the drugs tested except methoxamine, the maximal fibrinolytic effect is present 5 min after the injection and in general the fibrinolytic activity returns to the preinjection level within 30 min. Repeated injections of a powerful inducer of increased fibrinolytic activity, such as histamine, results in a progressive decrease of the fibrinolytic response.

DRUG ACTION ON PULMONARY CIRCULATION OF UNANESTHETIZED DOGS

A preparation is described which overcomes most of the difficulties previously encountered in the assessment of vasomotor responses of the pulmonary vessels to pharmacologic agents. The use of electromagnetic flow transducers on the main and left pulmonary arteries allows for the observation of immediate local effects following unilateral drug administration before general systemic effects occur. The opposite lung serves as a control, since it is subjected to the same changes of left artrial pressure, pulmonary arterial pressure, and pulmonary blood flow as the test lung. Our studies have demonstrated a direct pulmonary vasoconstrictor response to 5-hydroxytryptamine and acetylcholine in both the conscious and anesthetized dog. Epinephrine was also shown to have a direct pulmonary vasoconstrictor activity, but this could be elicited in only three of the eight dogs studied. Isoproterenol, methoxamine, aminophylline, phenylephrine, γ-amino butyric acid and pentobarbital had no apparent direct pulmonary vasomotor effect.

SUPRASPINAL INHIBITION OF A CUTANEOUS VASCULAR REFLEX IN THE CAT

In cats anesthetized with chloralose-urethane mixture, stimulation of an afferent nerve evoked a vasoconstrictor reflex (VCR) and a galvanic skin response (GSR) in the pads of the feet. Stimulation of the ventromedial medullary reticular substance at the level of the obex abolished the VCR and the GSR. VCR could also be reduced by occlusion during prolonged stimulation of another spinal or visceral afferent pathway. Medulla stimulation was effective without itself causing a sympathetic discharge to the paw, showing that inhibition rather than occlusion was operative. Anterior cerebellar stimulation also inhibited the VCR. Carotid sinus nerve stimulation did not abolish the VCR. It is concluded that the effective mechanism includes a bulbospinal inhibitory path projecting on a spinal vasoconstrictor reflex arc. This arrangement is similar to the descending pathways inhibiting other spinal reflexes but the VCR-inhibitory path can be activated independently of them.

MYOCARDIAL REACTIVE HYPEREMIA IN THE UNANESTHETIZED DOG

Studies of myocardial reactive hyperemia in dogs 5–24 days after the implantation of electromagnetic flowmeters on coronary artery branches reveal that the duration of occlusion and the control flow rate are independent determinants of the volume of reactive hyperemia flow. Flow debt is almost always overpaid, the average figure being 500 ± 200%. Phasic coronary flow curves show an increase in both systolic and diastolic flow during reactive hyperemia, the increase in systolic flow being due in part to decreased myocardial contractility. Myocardial reactive hyperemia is thought to be due to the accumulation of vasodilator metabolites during the period of coronary occlusion.

EFFECTS OF SMALL ORAL DOSES OF RESERPINE ON VASCULAR RESPONSES TO TYRAMINE AND NOREPINEPHRINE IN MAN

Experiments were done to see if reserpine, in small oral doses, alters the responses to tyramine and norepinephrine in man. Each of seven normotensive subjects was studied on three occasions. The first and third sessions served as pre- and post-treatment control sessions; the second session was held after oral administration of reserpine (0.25 to 1 mg. per day) for 2 weeks. Blood pressure was measured with a sphygmomanometer, and forearm blood flow was measured with a water plethysmograph. Reserpine reduced resting blood pressure and heart rate but forearm blood flow did not change. The pressor, forearm vasoconstrictor, and bradycrotic actions of intravenous tyramine were suppressed by reserpine, but the pressor, vasoconstrictor, and bradycrotic actions of three intravenous doses of norepinephrine were not augmented significantly. The results indicate that oral administration of small doses of reserpine may cause depletion of endogenous catecholamines in man as suggested by the suppressed response to tyramine. The decreased response to tyramine was not accompanied by hypersensitivity to exogenous norepinephrine. In equipressor doses norepinephrine produced greater forearm vasoconstriction and more reflex bradycardia than did tyramine.

VENOUS AND ARTERIAL RESPONSES TO NOREPINEPHRINE IN DOGS TREATED WITH RESERPINE

Experiments were done to see if the effects of norepinephrine upon veins, arteries, and small vessels are augmented in dogs after treatment with reserpine. With intra-arterial injection of increasing doses of norepinephrine into the perfused foreleg there were progressive increments in total, arterial, small vessel, and venous resistances. In animals treated with reserpine the increments in total and small vessel resistances were greatly augmented while increments in segmental arterial resistance were increased only slightly. The venous responses in these treated dogs were not augmented. Similar effects on segmental resistances were noted with infusion of norepinephrine into a systemic vein. The vasoconstrictor effect of tyramine was reduced in treated dogs and was restored after intra-arterial and intravenous norepinephrine. The results indicate that veins do not participate in the increased vascular responsiveness to norepinephrine which occurs in forelegs of dogs after treatment with reserpine.

CENTRAL CARDIOVASCULAR ACTIONS OF GAMMA-AMINOBUTYRIC ACID

The vasomotor and cardiac effects of gamma-aminobutyric acid (GABA) administered by different routes were studied in cats and dogs. The cat was resistant to the action of GABA administered intravenously or into a vertebral artery. Intrathecal injection of GABA into the cat depressed the vasomotor reponse to spinal compression. In dogs, intravenous, intrathecal or intraventricular injection, or topical application of GABA to the floor of 4th ventricle consistently produced hypotension and depressed the reflex and direct excitability of the vasomotor neurones, located at the supraspinal and spinal levels. Bradycardia observed after intravenous and intraventricular injections of GABA into dogs was abolished by stellate ganglionectomy but not by vagotomy. It has been attributed to depression of the central sympathetic neurones.