Hemodynamic and metabolic effects of angiotensin II during rest and exercise in normal healthy subjects

Angiotensin II affects basal, pulsatile, and glucose-stimulated insulin secretion in humans

Angiotensin II (Ang II) modulates the tissue response to insulin (insulin sensitivity), but the effect of Ang II on the secretion of insulin has not been investigated thus far. Nineteen healthy volunteers (17 male; mean age, 26+/-1 years) were studied. In a double-blind, randomized, placebo-controlled study, seven volunteers were allocated on three occasions in random order after an overnight fast to three interventions: (1) solvent (placebo) infusion; (2) infusion of 1.0 ng Ang II x kg(-1) x min(-1) (subpressor dose); and (3) infusion of 5.0 ng Ang II x kg(-1) x min(-1) (pressor dose). Frequent blood samples (each minute) were obtained for estimation of plasma insulin concentrations over a period of 120 minutes to assess basal and pulsatile insulin secretion. In an ancillary study, plasma glucose and insulin levels were measured after an oral glucose tolerance test while solvent (placebo) or Ang II was infused in 12 fasting healthy volunteers. Plasma insulin concentrations were measured immunoenzymatically (enzyme-linked immunosorbent assay). Insulin secretion pulses were analyzed with the deconvolution technique, and the regularity of insulin secretion was analyzed with the approximate entropy technique. Plasma insulin half-life was assessed using the hyperinsulinemic euglycemic clamp method. The pressor dose of Ang II reduced total, basal, and pulsatile insulin secretion, and this effect was highly significant (P<.01). The subpressor dose tended to suppress insulin secretion. The burst frequency (number of peaks) and the regularity of insulin secretion were not affected by administration of Ang II. After the oral glucose load, the insulinemic response was significantly lower and plasma glucose concentrations were significantly higher with infusion of Ang II compared with placebo. Ang II affects both the basal (nonpulsatile) and the pulsatile component of spontaneous insulin secretion and the glucose-stimulated insulin secretion in humans. This observation is of potential interest with respect to the interaction of Ang II and insulin, eg, in the genesis of hyperinsulinemia and hypertension.

Depression of heart function after angiotensin II infusion in patients on chronic hemodialysis

The response of heart function to angiotensin II (AT II) was studied in 18 patients on regular hemodialysis. The mean age was 33 years and they had been dialyzed for 55 months in the average. AT II was infused from a large vein and systolic blood pressure was raised by 40 mmHg. Before and after the change in blood pressure, M-mode echocardiogram of left ventricle was recorded. Left ventricular enddiastolic dimension, stroke index and cardiac index were found to be normal except for 9 patients who showed cardiac index above 4.0L/min/m2. No significant change was found in these parameters after the rise of blood pressure by AT II. Control ejection fraction (EF) was slightly but nonsignificantly lower in the patients than the healthy subjects; 0.73 +/- 0.13 vs. 0.80 +/- 0.05. Though significant falls in EF were found in the patient and in the healthy group, the former showed a profound depression of EF to 0.64 +/- 0.10. This value was significantly lower than the value of the latter group; 0.76 +/- 0.04 (p less than 0.01). Since none had overt heart failure, a depression of EF after AT II can be regarded as subclinical abnormality of heart function. AT II will be useful to detect this limited reserve of heart function in patients on regular hemodialysis who may show normal function at rest.

[Hemodynamic effects of sar1-ala8-angiotensin in patients with renovascular hypertension (author's transl)]

In 7 hypertensive patients with renal artery stenosis and in 1 patient with hypertension and unilateral pyelonephritic nephrophthisi the influence of the angiotensin II antagonist, saralasin on systemic hemodynamics was studied. In the patients with normal renin infusion of saralasin produced an increase in total peripheral resistance, in patients with elevated renin a decrease in peripheral resistance was observed. In 3 patients who had extremely high renin levels while under sodium saralasin produced a dangerous drop in blood pressure concomitant with a marked decrease in cardiac output and in central venous pressure, heart rate remained unchanged or increased just slightly. The findings suggest that in patients with high plasma renin peripheral resistance, venous tone, venous retrun, and cardiac output are to a large extent controlled by circulating angiotensin II.

The angiotensin infusion test as a method of evaluating left ventricular function

Fifteen patients had left ventricular function measured by the angiotensin infusion method. Seven patients had no evidence of heart disease, and eight patients had angina pectoris and coronary arteriographic evidence of coronary disease without congestive heart failure. During angiotensin infusion, those patients without heart disease had a decrease in cardiac index (average, 0.63 L. per minute per square meter) and a decrease in heart rate (average, 12 beats per minute.) The ventricular function curve had a poor SWI response in four of the seven subjects. The patients with coronary artery disease also had a reduction in cardiac index during angiotensin (average, 0.44 L. per minute per square meter) and the heart rate was unchanged in four subjects, increased in two subjects, and decreased in two subjects. Six of the subjects had flat or descending slopes on the function curve, and in one subject there was only a very gradual ascending slope. Many of the curves of both groups looked similar so that the function curves did not differentiate between those patients with or without heart disease. The mechanism for production of bradycardia, reduction of cardiac output, and depressed function curves with angiotensin is multifactorial, but is probably due to the baroreceptor reflex response, the increase in coronary artery resistance, and possible to the direct effect of increased left ventricular afterload itself. The ventricular response to angiotensin is so variable that the angiotensin infusion method of evaluating ventricular function is not reliable.

Systemic hemodynamic and myocardial metabolic effects of isoproterenol and angiotensin after open-heart surgery

Cardiovascular dynamics and metabolism after open heart surgery were studied in 10 patients before and after infusion of angiotensin and isoproterenol. Patients with rheumatic mitral disease showed marked lowering of cardiac index with elevation of systemic vascular resistance in the control state while those with aortic disease demonstrated less depression of cardiac index and normal resistance. The group with mitral disease also showed lower coronary blood flow and much higher coronary vascular resistance than the aortic patients. Angiotensin caused further depression of cardiac index and marked increase in pressure work. Isoproterenol augmented cardiac index but did not affect pressure work significantly. Both agents increased oxygen consumption significantly by increment in coronary flow (isoproterenol) or by widening of myocardial oxygen extraction (angiotensin). Isoproterenol is favored over angiotensin in low output states after cardiac surgery as a means of meeting oxygen demands as well as the response of cardiac index.

MYOCARDIAL ACTIONS OF ANGIOTENSIN

Synthetic angiotensin a has pronounced inotropic effects on isometrically contracting, isolated papillary muscles of kittens. The response increases with drug concentration over the range from 10 -10 M to 10 -5 M. The mean maximum increase in tension development is 120%. Contractility is raised by angiotensin through an increase in the degree of activation of the contractile elements with no significant change in the duration of their active state. Low concentrations of angiotensin have a greater inotropic action than equimolar concentrations of l -norepinephrine; but the maximum inotropic effect obtainable with l -norepinephrine is almost twice that of angiotensin.

The inotropic effects of angiotensin on cat atrial muscle are slight, and the drug is inactive on frog ventricular myocardium. It has no effect on the resting length-tension relationship of mammalian atrial or ventricular muscle. The frequency of impulse formation in the SA node is not significantly changed by concentrations of angiotensin up to 10 -5 M. In marked contrast to levarterenol, angiotensin does not cause ectopic impulse formation in atrial or ventricular muscle.

It is suggested that the positive inotropic effect of angiotensin on ventricular myocardium is of importance for the pressor action of the drug in the intact circulation. The increase in myocardial contractility tends to minimize or prevent decreases in cardiac output in the face of increased resistance to cardiac ejection and thus supports the elevation of arterial pressure.