Cardiac function in a rat model of chronic aortic stiffness.

Cardiac function was investigated in conscious normotensive rats in which increased aortic stiffness was produced as a result of vascular calcium overload after treatment with vitamin D3 plus nicotine (VDN rats, n = 16; controls, n = 17). Baseline stroke volume, cardiac output, and cardiac response to a venous volume overload were unchanged after 1 mo of exposure to increased aortic stiffness, as were baseline venous return and total vascular capacitance. The latter was estimated from the change in mean circulatory filling pressure after modification of circulatory volume. Cardiovascular reflexes were modified in VDN rats. Bradycardia evoked by an increase in arterial PCO2 (PaCO2) or hypotensive hemorrhage was more pronounced. The PaCO2-induced bradycardia was accompanied by a fall in cardiac output in VDN rats but not in controls. In VDN rats, the attenuation of sympathetic reflexes may explain the slower recovery of blood pressure after hypotensive hemorrhage. In conclusion, a chronic increase in aortic stiffness does not compromise cardiac performance, but cardiovascular reflexes are impaired in VDN rats. Whether this is because of the increase in aortic stiffness or the effect of VDN treatment on the baroreceptors or other components of the reflex arc remains to be elucidated.

Time-related normalization of maximal coronary flow in isolated perfused hearts of rats with myocardial infarction.

BACKGROUND: In the present study, we investigated the time dependency and regional differences of the vascular adaptation of the myocardium after myocardial infarction (MI) in rats. METHODS AND RESULTS: MI was induced by total occlusion of the left anterior descending coronary artery. Time-dependent adaptation of the coronary vasculature was determined by histological staining of endothelial cells and measurement of basal and maximal coronary flow at days 0, 4, 7, 21, 35, and 90 after surgery in isolated retrogradely perfused hearts of sham-operated and infarcted rats. Cardiac function was determined during anterograde perfusion. In a separate group of experiments, regional myocardial flow was measured with radiolabeled microspheres in sham-operated and infarcted hearts to determine local differences in adaptation. Basal coronary flow was completely normalized within 7 days, whereas maximal coronary flow was not normalized until 35 days after MI. Normal growth, as observed in sham-operated hearts, resulted in a parallel increase in coronary flow and tissue mass from day 7 to 35 after surgery. In contrast, the increase in coronary flow was lower than the hypertrophic response in the right ventricles and septa of infarcted hearts, whereas a parallel increase in tissue mass and coronary flow was observed in the left ventricles of these hearts. These functional data were supported by structural data that showed the presence of numerous and dilated vessels, especially in the border zone of the infarcted and noninfarcted tissue. CONCLUSIONS: These observations demonstrate that vessel growth, predominantly in the region adjacent to the infarcted zone, results in complete normalization of coronary vasodilatory capacity within 35 days after MI.

DNA synthesis in the non-infarcted cardiac interstitium after left coronary artery ligation in the rat: effects of captopril.

Effects of Captopril. Journal of Molecular and Cellular Cardiology (1991) 23, 1245-1253. This study was undertaken to investigate the alterations in interstitial DNA synthesis and collagen content in the non-infarcted left and right ventricle after induction of a myocardial infarction (MI) in the rat. MI was induced by ligation of the left anterior descending coronary artery. All animals received 5-Bromo-2'-deoxyUridine (BrdU), via a subcutaneous osmotic minipump, one day before sacrifice, to quantitate DNA synthesis. A transient rise in BrdU incorporation was found in both ventricles. Peak levels were found at day 7 and 14 after infarct induction. BrdU incorporation had returned to control levels 3 weeks after infarct induction. By using anti BRDU--anti-laminin immuno- double staining DNA synthesis was localized mainly in the cardiac interstitium. Concomitantly, a sustained increase in collagen content, measured as the Sirius red positive area on cross sections, was found from day 7 after infarct induction. No changes were found in sham animals. In the second part of the study the effects of the angiotensin I converting enzyme inhibitor captopril and the arteriolar vasodilator hydralazine on MI induced interstitial DNA synthesis and collagen content were investigated. Captopril reduced both the increase in DNA synthesis and collagen content. Hydralazine did not affect interstitial DNA synthesis, but reduced the MI induced collagen content. Both drugs had no effects in sham animals. We conclude that induction of a myocardial infarction stimulates interstitial DNA synthesis and increases the collagen content in the non-infarcted areas of the heart. Interstitial DNA synthesis is dependent on the angiotensin I converting enzyme in a direct manner independent from afterload changes.

Two weeks of intermittent dobutamine therapy restores cardiac performance and inotropic responsiveness in conscious rats with heart failure.

Dobutamine is frequently used for acute therapy in heart failure. In the present study, the hemodynamic effects of long-term intermittent dobutamine therapy were investigated in conscious rats with heart failure. Rats with healed myocardial infarctions received two i.p. injections of dobutamine per day for 2 weeks. Hemodynamic measurements were performed 90-180 min after the last injection. Two weeks of intermittent dobutamine significantly restored all hemodynamic changes induced by infarction. The maximal cardiac output during volume loading was depressed due to infarction and dose-dependently restored by 2 weeks of intermittent dobutamine. An increased stroke volume accounted for this improvement since the heart rate was not altered. In order to investigate changes in adrenergic responsiveness, the effects of acute dobutamine in nontreated and 2 weeks of dobutamine-treated infarcted rats were compared to those in control rats. Whereas chronotropic responses to acute dobutamine were comparable for all experimental groups, the inotropic response was reduced in nontreated infarcted rats but dose-dependently restored after 2 weeks of intermittent dobutamine therapy. From the data, we conclude that 2 weeks of intermittent dobutamine therapy in conscious rats with healed myocardial infarcts improved cardiac performance and restored the inotropic response to acute dobutamine administration. Data indicate that dobutamine has a long-term effect on cardiac function, which differs from the acute inotropic effect.

Delayed but not immediate captopril therapy improves cardiac function in conscious rats, following myocardial infarction.

After myocardial infarction, the renin-angiotensin system is found to be activated. While this response may be beneficial in acute failure, it could be detrimental in chronic stages. Therefore effects of captopril therapy were investigated during early and later phases after myocardial infarction in conscious rats, chronically instrumented for hemodynamic measurements. Hemodynamics were measured at baseline and after stimulating the heart by a volume load (cardiac function curve). Myocardial infarction decreased baseline cardiac output and impaired cardiac function, without effects on baseline mean arterial pressure, central venous pressure and heart rate. Captopril given 3 to 5 weeks after infarction improved cardiac function in a dose-dependent manner by increasing stroke volume, whereas stroke work was not affected. In contrast, captopril given from 1 to 21 days after infarction did not lead to improved cardiac function; instead, tachycardia together with a decreased stroke volume suggested deterioration, rather than improvement, of cardiac function. These data indicate that captopril therapy in chronically infarcted conscious rats improved cardiac function when treatment was started after completion of the healing process, but that early treatment not only failed to improve ventricular function, but may have a deleterious effect of the heart.

Beneficial hemodynamic effects of two weeks' milrinone treatment in conscious rats with heart failure.

Milrinone is a phosphodiesterase inhibitor which combines vasodilating effects with inotropic effects. Hemodynamic improvement after acute administration and increased survival with chronic milrinone therapy in rats with heart failure have been reported before, and suggest long-term hemodynamic improvement. However, no detailed hemodynamic studies are available on prolonged milrinone therapy in rats with heart failure. Therefore, the hemodynamic effects of 2 weeks' milrinone therapy were now investigated in conscious rats with heart failure due to myocardial infarction. The effects were compared to hemodynamic changes after acute administration. Acute milrinone increased the baseline cardiac output in infarcted rats by increasing heart rate rather than stroke volume. However, the maximal cardiac output achieved when the heart was stimulated through a volume load was improved due to increased stroke volume as well as increased heart rate. The increase in maximally stimulated cardiac output after acute milrinone was found to be related to infarct size. Two weeks' milrinone therapy in chronically infarcted rats dose dependently restored the hemodynamic changes which were caused by infarction. In contrast to acute administration, two weeks' milrinone restored cardiac function without an increase in heart rate. The effects were achieved at a rate of administration which presumably has no acute inotropic effects. The data indicate that acute milrinone in infarcted rats has vasodilating effects. Positive inotropic effects, possibly masked by concomitant venodilatation at baseline conditions, became overt after stimulation by volume loading. Long-term milrinone dose dependently restored cardiac function in infarcted rats without effects on heart rate or mean arterial pressure, suggesting that different mechanisms may be involved.

Acute hemodynamic effects of coronary artery ligation in conscious rats.

Because of the growing interest in the use of coronary artery ligation (CAL) in rats as a model for studies on heart failure, we have investigated the acute hemodynamic changes following CAL in conscious rats. Animals were equipped for measurement of cardiac output (CO), arterial pressure (MAP), and central venous pressure (CVP). These parameters were measured before CAL, immediately after, and 24 h after. Furthermore, peak CO, obtained by rapid infusion of 12 ml Ringer's solution (in 1 min) was measured 2 days before and 1 day after CAL. CAL resulted in immediate reduction of CO, because of reduced stroke volume (SV). CO as well as SV were inversely correlated with infarct size as determined 24 h after CAL. Heart rate (HR) and MAP did not change. Twenty-four hours later, CO was still reduced. MAP was now reduced, possibly as a result from resetting of nervous reflex mechanisms. Before CAL, peak CO and SV were similar in CAL and sham animals. At 24 h after CAL, these parameters were greatly reduced in CAL rats. Peak values were strongly correlated to infarct size. Results indicate that CAL in rats leads to hemodynamic changes similar to the ones observed following myocardial infarction in man. Cardiac function is related to infarct size and is altered both at rest and during maximal stimulation.

Role of sensory renal nerves in the development of spontaneous hypertension in rats.

In order to study the role of afferent renal nerves in the development of spontaneous hypertension, 3-4 weeks old uninephrectomized spontaneously hypertensive rats were deafferented selectively by a unilateral dorsal rhizotomy. Control rats were sham-operated. Until 10 weeks of age, systolic tail cuff blood pressures were identical in both groups. Although from 12 weeks on systolic blood pressure was slightly (10%) but significantly lower in deafferented rats, mean arterial pressures from an indwelling catheter were identical in deafferented and control rats. We therefore conclude that a selective destruction of afferent renal nerves does not delay or prevent the development of spontaneous hypertension in rats.

Failure of atriopeptin II to cause arterial vasodilation in the conscious rat.

The cardiovascular actions of the synthetic natriuretic peptide, atriopeptin II, were examined in conscious unrestrained spontaneously hypertensive rats and normotensive Wistar-Kyoto rats. The animals were chronically instrumented with miniaturized pulsed Doppler flow probes to allow measurement of regional blood flow, or with an electromagnetic flow probe on the ascending aorta to facilitate the measurement of cardiac output in the conscious rat. Intravenous infusion of increasing doses of atriopeptin II (0.25-4 micrograms/kg per min) caused a dose-dependent fall in mean arterial pressure in the hypertensive and normotensive rats. Blood flow in the renal, mesenteric, and hindquarters vascular beds was markedly decreased during the infusion of atriopeptin II, and regional vascular resistance was significantly increased in both groups of rats. Heart rate was significantly elevated (47 +/- 14 beats/min) in the spontaneously hypertensive rats during atriopeptin II infusion, but no change in heart rate was observed in the Wistar rats. In the hypertensive rats, atriopeptin II caused a marked dose-dependent decrease in cardiac output (maximal decrease = -39 +/- 4%) and stroke volume (maximal decrease = -48 +/- 4%). Central venous pressure and left atrial pressure were also significantly reduced during atriopeptin II infusion. Total peripheral resistance was increased over the infusion protocol by 26 +/- 3%. These data suggest that atriopeptin II infusion markedly attenuated cardiac output in the conscious spontaneously hypertensive rats. Total and regional vascular resistances were increased, possibly through reflex compensatory mechanisms, to maintain arterial pressure in the face of decreased cardiac output.(ABSTRACT TRUNCATED AT 250 WORDS)