Cardiovascular reactivity in acute and chronic renal hypertensive dogs

Pathogenesis of paradoxical hypertension after coarctation resection

The pathogenesis of paradoxical hypertension after resection of coarctation of the aorta was investigated by comparing the course of seven children undergoing repair of coarctation with five acyanotic children undergoing elective cardiovascular surgery. During the first 24 hours after surgery, all coarctation patients demonstrated a rise in systolic blood pressure (35 +/- 15.5 mm Hg; P less than 0.001), a significant depression in cold pressor test response, and only a slight elevation in plasma renin activity. In the next 24-72 hours, coarctation patients developed a rise in diastolic blood pressure (26.8 +/- 10.6 mm Hg; P less than 0.001), plasma renin activity (22.9 +/- 10.2/ml/hr; P less than 0.001) and fluid retention. By contrast, control patients had no significant postoperative changes. Abdominal pain occurred in five coarctation patients during the period of maximal plasma renin activity. The data suggest that the sympathetic nervous system may be responsible for the initial phase of hypertension after coarctation resection and that the renin angiotension system plays a major role in the second phase of hypertension and in the pathogenesis of mesenteric arteritis.

Sound-withdrawal hypertension in rats--a new form of experimental hypertension

Females of an inbred strain of Wistar rats, initially 160–180 g, develop hypertension when transferred from the stock (normal rat) room (NRR) to a sound-proofed semi-anechoic room (SPR). No hypertension develops when sounds in the NRR are relayed unchanged into the SPR. The hypertension reaches maximum at 4 weeks then decreases slightly to stabilize at a level approximately 30 mm Hg above normal before, and 20 mm Hg above normal after, pentolinium blockade. Histological changes are demonstrable in small arteries after 12 weeks in the SPR. Augmented pressor responses to fixed doses of noradrenaline and of tyramine, but not of angiotensin II and of vasopressin, and a general increase in organ/body wt ratios were evident in weeks 3–5 after entry to the SPR, but had reversed by week 12. These findings contrast with those made on salt-DOCA hypertensives.

Experimental renal hypertension in dogs. Forelimb hemodynamics

Blood flow arid segmental intravascular pressures were measured in the pump-perfused forelimb vascular beds of 25 dogs with chronic perinephritic hypertension and 26 normotensive control dogs (either unilaterally nephrectomized or sham-operated) under pentobarbital anesthesia. Measurements were made under resting conditions (pump-perfusion pressure set equal to aortic pressure) and during response to: (1) stepwise changes in pump blood flow over the range 21-330 ml/min; (2) intrabrachial arterial injection of supramaximal doses of methacholine chloride; and (3) limb denervation. Total limb vascular resistance and segmental pressure gradients were calculated. In hypertensive dogs, as compared to normotensive dogs, resting blood flow was equal ( P >0.3) and resting limb total vascular resistance was increased ( P <0.005). Small vessel pressure gradient was increased ( P <.005) and venous pressure gradient was decreased ( P <0.005). Limb vascular resistance following methacholine injections and residual resistance following neurectomy were lower ( P <0.05) in normotensive than in hypertensive dogs. The development of hypertension was accompanied by the upward displacement of limb flow-resistance curves without discernible change in shape of the curves. These data suggest that in chronic renal hypertension in dogs: (1) the limb vascular bed participates in the increased peripheral vascular resistance; (2) the increase in limb vascular resistance is confined to the small vessel segment with no evidence found for limb venoconstriction; (3) a portion of the elevated limb vascular resistance is not attributable to neural stimuli; and (4) structural changes in vessels may account for part of the increase in limb resistance.

Cardiovascular reactivity in the experimental hypertensive rat

1. In pithed preparations, deoxycorticosterone (DOCA)/NaCl-induced hypertensive rats showed increased cardiovascular reactivity to injected noradrenaline, DMPP, tyramine, angiotensin and to sympathetic nerve stimulation.2. Pithed, chronic renal hypertensive rats showed similar hyperreactivity to noradrenaline, DMPP and tyramine but responses to sympathetic nerve stimulation were within normal limits.3. In the isolated, Krebs perfused mesentery preparation, vessels obtained from both DOCA/NaCl and renal hypertensive rats showed hyperreactivity to injected noradrenaline. Responses to periarterial nerve stimulation were also markedly increased in preparations from DOCA/NaCl hypertensive rats, while preparations from renal hypertensive rats showed this effect only at higher rates of stimulation (12 and 25 Hz).

The effect of reserpine on the pressor responses to angiotensin in the conscious cat

1. Blood pressure recordings have been made in conscious cats in an attempt to reveal a possible indirect component to the angiotensin pressor response.2. Reserpine (50 to 250 mug/kg per day) caused a maximal reduction of about 50% in the pressor response to angiotensin whilst virtually abolishing the responses to tyramine and McN-A-343. Responses to noradrenaline were only slightly and transiently reduced.3. Syrosingopine (0.5 mg/kg) and reserpine (250 mug/kg) reduced the responses to angiotensin, McN-A-343 and tyramine to much the same extent, but tetrabenazine only reduced the responses to all these agents in a dose (25 mg/kg) which probably had effects on the catecholamine stores of smooth muscle.4. The reduction in the responses to angiotensin, tyramine and McN-A-343 by reserpine was partly reversed by tranylcypromine. Noradrenaline and (+/-)-dopa infusions were ineffective by themselves, but increased the effects of tranylcypromine in restoring the responses to angiotensin, tyramine and McN-A-343 after reserpine.5. Infusion of alpha-methyldopa markedly increased the responses to angiotensin, tyramine and McN-A-343 after these had been reduced by reserpine.6. The results suggest that the pressor response to angiotensin in the conscious cat is partly mediated by release of noradrenaline from peripheral neuronal stores.

Increased cardiovascular reactivity caused by depletion of endogenous renin in dogs

Increase in cardiovascular reactivity following bilateral nephrectomy has previously been found to be independent of the presence of the central nervous system, mechanical exclusion of the renal vascular bed, or loss of renal excretory function. It was considered possible that increased responsiveness results from removal of endogenous stores of renin. In nephrectomized dogs treated with crude kidney extracts, or infused continuously with synthetic angiotensin for 48 hours, enhancement of response to angiotensin and renin was inhibited while that to norepinephrine was not; response to tyramine was augmented as after infusion of angiotensin into intact dogs. Treatment with liver extracts alone did not inhibit response to renin or angiotensin; neither did continuous infusion of norepinephrine. When hog renin was added to liver extracts, the combination had the same inhibitory effect as kidney extracts. The data are in accord with the concept that depletion of endogenous renin contributes to the enhanced pressor responsiveness that occurs following bilateral nephrectomy.

A hemodynamic comparison of essential and renovascular hypertension. Cardiac output and total peripheral resistance: supine and tilted patients

Systemic hemodynamic measurements were obtained in 15 normal subjects and 30 untreated hypertensive patients (15 with essential hypertension and 15 with fibrosing stenosis of renal arteries). The essential hypertensives were selected from a larger group solely on the basis of matching their mean arterial pressures and ages with those of the patients with renal arterial disease in matched pairs; cardiac index was consistently higher in patients with renovascular hypertension than in those with essential hypertension. The higher cardiac output was maintained in the group with renal arterial disease even when reduced by tilting the subjects to a 50° head-up position. Total peripheral resistance was increased in both hypertensive groups. The low cardiac indices and mean rates of left ventricular ejection found in essential hypertension may also implicate the heart in this form of hypertension. Hence, increased total peripheral resistance, long thought to be the hemodynamic hallmark of hypertension, may not be the sole systemic abnormality; cardiac output may be increased in renal arterial disease and even reduced in essential hypertension.

Effect of angiotensin on the pressor response to tyramine in normotensive subjects and hypertensive patients

Arterial blood pressure was recorded directly in 11 normotensive subjects, in 11 patients with essential hypertension and in 10 patients with renovascular hypertension. The pressor response to intravenous injections of norepinephrine, tyramine, angiotensin and ephedrine was compared in these three groups. An infusion of angiotensin was then given and the pressor response to injections of norepinephrine, tyramine, and ephedrine was again measured; in some cases, a comparison of the effects of the cold pressor test was made.

In normotensive subjects, angiotensin infusion caused a significant increase in the responses to tyramine and ephedrine but not to norepinephrine; this occurred with subpressor doses and was unrelated to the level of blood pressure. In patients with renovascular hypertension, the mean pressor responses to injections of tyramine and ephedrine and the ratio of response to tyramine to that to norepinephrine were significantly increased, and the infusion of angiotensin failed to cause any further increase in these responses. In contrast, in patients with essential hypertension, the average response to injections of tyramine was not significantly increased, but during angiotensin infusion the response to tyramine increased as in normotensive subjects. It is proposed that increased response to tyramine, and lack of the potentiating effect of angiotensin demonstrated in renovascular hypertension, may be caused by increased formation of endogenous angiotensin due to renin secretion in this disease.