Effects of long-term administration of clonidine on plasma renin activity.
Plasma renin activity (PRA) was studied before and during long-term treatment with moderate oral doses (0.2 or 0.3 mg/d) of clonidine. Nine outpatients with essential hypertension received clonidine for 12 weeks; a significant decrease in blood pressure was evident in all patients. Except for a nonsignificant increase after 12 weeks of treatment, PRA values were not notably changed by clonidine therapy. No correlation was found between individual blood pressure changes and PRA variation during the study. The absence of a net effect on PRA in this study does not exclude more complex interactions of clonidine with the renin-angiotensin system. Nonetheless, clonidine cannot generally be classified as a "renin-inhibiting" drug. (+info
Phasic right coronary artery blood flow in conscious dogs with normal and elevated right ventricular pressures.
We studied phasic right coronary blood flow in well trained normal dogs and dogs with pulmonic stenosis. We installed electromagnetic flow transducers and pressure tubes under anesthesia to monitor right coronary blood flow, cardiac output, central aortic blood pressure, and right ventribular pressure. In normotensive dogs, systolic flow amplitude equaled early diastolic flow levels. The ratio of systolic to diastolic flow at rest was substantially greater in the right coronary bed (36+/-1.3%) than in the left circumflex bed (13+/-3.6%). Right diastolid flow runoff, including the cove late in diastole, resembled left circumflex runoff. Blood flow to the normotensive right (37+/-1.1 ml/min 100(-1) g) and the left (35+/-1.0 ml/min(-1) g) ventricular myocardium indicated equal perfusion of both cardiac walls. Throttling of systolic flow was related directly to the right ventricular systolic pressure level in the dogs with pulmonic stenosis. Retrograde systolic flow occurred in severe right ventricular hypertension. The late diastolic runoff pattern in dogs with pulmonic stenosis appeared the same as for the normotensive dogs. We obtained systolic to diastolic flow ratios of 1/3 the value of normotensive hearts in high and severe pulmonic hypertension. Electrocardiograms and studies of pathology suggested restricted blood flow to the inner layers of the right myocardium in the dogs with severe and high right ventricular hypertension. Normotensive and hypertensive peak hyperemic flow responses were similar, except for an increased magnitude of diastolic flow, with proportionately less systolic flow in hypertensive states. (+info
Reduction in baroreflex cardiovascular responses due to venous infusion in the rabbit.
We studied reflex bradycardia and depression of mean arterial blood pressure (MAP) during left aortic nerve (LAN) stimulation before and after volume infusion in the anesthetized rabbit. Step increases in mean right atrial pressure (MRAP) to 10 mm Hg did not result in a significant change in heart rate or MAP. After volume loading, responses to LAN stimulation were not as great and the degree of attenuation was propoetional to the level of increased MRAP. A change in responsiveness was observed after elevation of MRAP by only 1 mm Hg, corresponding to less than a 10% increase in average calculated blood volume. after an increase in MRAP of 10 mm Hg, peak responses were attenuated by 44% (heart rate) and 52% (MAP), and the initial slopes (rate of change) were reduced by 46% (heart rate) and 66% (MAP). Comparison of the responses after infusion with blood and dextran solutions indicated that hemodilution was an unlikely explanation for the attenuation of the reflex responses. Total arterial baroreceptor denervation (ABD) abolished the volume-related attenuation was still present following bilateral aortic nerve section or vagotomy. It thus appears that the carotid sinus responds to changes inblood volume and influences the reflex cardiovascular responses to afferent stimulation of the LAN. On the other hand, cardiopulmonary receptors subserved by vagal afferents do not appear to be involved. (+info
Ventricular pressure-volume curve indices change with end-diastolic pressure.
Many indices have been proposed to describee the diastolic pressure-volume curve mathematically and permit quantification of the elastic properties of the myocardium itself in hopes that changes in the muscle caused by disease would b.e reflected in the diastolic pressure-volume curve. To date, none of the proposed indices has been shown convincingly to discriminate one group of patients from another. While this situation in part arises from the relatively large amount of noise introduced by the technical difficulties of measuring synchronous pressures and volumes during diastole in man, ther is a more fundamental difficulty. In practice, one can measure only a short segment of the entire pressure-volume curve, and the values of all diastolic pressure-volume curve parameters investigated change significantly when one uses different segments of the same pressure-volume curve to compute them. These results were derived from relatively noise-free pressure-volume curves obtained by filling nine excised dog left ventricles at a known rate and monitoring pressure-volume curve used to compute the parameter. Merely increasing measurement fidelity will not resolve this problem, because none of these parameters accurately characterizes the entire diastolic pressure-volume curbe from a segment like that which one can reasonably expect to obtain from humans. (+info
Quantification of baroreceptor influence on arterial pressure changes seen in primary angiotension-induced hypertension in dogs.
We studied the role of the sino-aortic baroreceptors in the gradual development of hypertension induced by prolonged administration of small amounts of angiotensin II (A II) in intact dogs and dogs with denervated sino-aortic baroreceptors. Short-term 1-hour infusions of A II(1.0-100 ng/kg per min) showed that conscious denervated dogs had twice the pressor sensitivity of intact dogs. Long-term infusions of A II at 5.0 ng/kg per min (2-3 weeks) with continuous 24-hour recordings of arterial pressure showed that intact dogs required 28 hours to reach the same level of pressure attained by denervated dogs during the 1st hour of infusion. At the 28th hour the pressure in both groups was 70% of the maximum value attained by the 7th day of infusion. Both intact and denervated dogs reached nearly the same plateau level of pressure, the magnitude being directly related both the the A II infusion rate and the daily sodium intake. Cardiac output in intact dogs initially decreased after the onset of A II infusion, but by the 5th day of infusion it was 38% above control, whereas blood volume was unchanged. Heart rate returned to normal after a reduction during the 1st day of infusion in intact dogs. Plasma renin activity could not be detected after 24 hours of A II infusion in either intact or denervated dogs. The data indicate that about 35% of the hypertensive effect of A II results from its acute pressor action, and an additional 35% of the gradual increase in arterial pressure is in large measure a result of baroreceptor resetting. We conclude that the final 30% increase in pressure seems to result from increased cardiac output, the cause of which may be decreased vascular compliance. since the blood volume remains unaltered. (+info
Acute and chronic dose-response relationships for angiotensin, aldosterone, and arterial pressure at varying levels of sodium intake.
We examined the acute and chronic dose-response relationships between intravenously infused angiotensin II (A II) and the resulting changes in arterial pressure and plasma aldosterone concentration at varying levels of sodium intake. Sequential analysis of plasma aldosterone at each A II infusion rate resulted in an acute dose-related increase in plasma aldosterone which was markedly attenuated after the first 24 hours of infusion, the final level being directly related to the dose of A II and inversely related to sodium intake. A II infused at 5,15, and 23 ng/kg per min was associated with an initial increase (2nd to 8th hour) in plasma aldosterone to 2,6, and 9 times control values, respectively, in dogs receiving 40 mEq Na+/day. But, after the 1st day, aldosterone averaged only 1, 1.7, and 3 times control values for the next 2 weeks at the same rates of A II infusion. Dogs receiving 120 mEq Na+/day during A II infusion exhibited only a transient increase in plasma aldosterone during the 1st day. Sustained hypertension developed over a period of a week at all doses of A II at normal and high sodium intake, but did not occur at any dose of A II in sodium-depleted dogs. Increasing sodium intake from 40 to 120 mEq/day resulted in higher levels of hypertension, 125% compared to 140% of ocntrol values for dogs infused with A II, 5.0 ng/kg per min. We conclude that primary angiotensin-induced hypertension need not be associated with increased levels of plasma aldosterone, which appears to remain elevated only with amounts of A II greater than those required to sustain a significant degree of hypertension. (+info
The effect of cardiac contraction on collateral resistance in the canine heart.
We determined whether the coronary collateral vessels develop an increased resistance to blood flow during systole as does the cognate vascular bed. Collateral resistance was estimated by measuring retrograde flow rate from a distal branch of the left anterior descending coronary artery while the main left coronary artery was perfused at a constant pressure. Retrograde flow rate was measured before and during vagal arrest. We found that in 10 dogs the prolonged diastole experienced when the heart was stopped caused no significant change in the retrograde flow rate, which indicated that systole has little effect on the collateral resistance. However, when left ventricular end-diastolic pressure was altered by changing afterload or contractility, a direct relationship between end-diastolic pressure and collateral resistance was noted. (+info
Evaluation of the force-frequency relationship as a descriptor of the inotropic state of canine left ventricular myocardium.
The short-term force-frequency characteristics of canine left ventricular myocardium were examined in both isolated and intact preparations by briefly pertubing the frequency of contraction with early extrasystoles. The maximum rate of rise of isometric tension (Fmas) of the isolated trabeculae carneae was potentiated by the introduction of extrasystoles. The ratio of Fmas of potentiated to control beats (force-frequency ratio) was not altered significantly by a change in muscle length. However, exposure of the trabeculae to isoproterenol (10(-7)M) significantly changed the force-frequency ratio obtained in response to a constant frequency perturbation. Similar experiments were performed on chronically instrumented conscious dogs. Left ventricular minor axis diameter was measured with implanted pulse-transit ultrasonic dimension transducers, and intracavitary pressure was measured with a high fidelity micromanometer. Atrial pacing was performed so that the end-diastolic diameters of the beats preceding and following the extrasystole could be made identical. Large increases in the maximum rate of rise of pressure (Pmas) were seen in the contraction after the extrasystole. The ratio of Pmax of the potentiated beat to that of the control beat was not changed by a 9% increase in the end-diastolic diameter, produced by saline infusion. Conversely, isoproterenol significantly altered this relationship in the same manner as in the isolated muscle. Thus, either in vitro or in situ, left ventricular myocardium exhibits large functional changes in response to brief perturbations in rate. The isoproterenol and length data indicate that the force-frequency ratio reflects frequency-dependent changes in the inotropic state, independent of changes in length. (+info