Baroreflex and beta-adrenoceptor function are diminished in rat cardiac hypertrophy due to volume overload

Left ventricular response to sustained volume overload from chronic aortic valve regurgitation in rats

OBJECTIVES

Aortic regurgitation (AR) induces left ventricular (LV) eccentric hypertrophy in response to chronic volume overload. Patients suffering from this disease often remain asymptomatic for decades before progressive LV dysfunction develops silently. Because of this slow evolution, large clinical trials with long-term follow-up on subjects with chronic AR are hard to perform. To overcome this problem, animal models have been developed in the past but results were very heterogeneous.

METHODS

Helped by echocardiography, we refined a known technique to induce homogeneous degrees of severe AR in Wistar-Kyoto rats. The effects on LV function without treatment and with nifedipine (25 mg/kg daily) (a drug currently recommended in humans with chronic AR) were evaluated by echocardiography.

RESULTS

Over 6 months, nontreated animals developed progressive LV dilatation and eccentric hypertrophy, characteristic of chronic LV volume overload. The animals also developed progressive LV systolic dysfunction, mimicking closely the evolution of the disease in humans. Abnormal filling parameters were also detected in the majority of animals. Systolic and diastolic abnormalities were prevented but only partially in the group treated with nifedipine.

CONCLUSION

This model can be used to study chronic AR and LV dysfunction associated with the disease. Nifedipine seems to protect the LV against chronic volume overload but only partially. Treatment strategies currently used in humans deserve further investigation.

Spontaneous baroreflex sensitivity: toward an ideal index of cardiovascular risk in hypertension?

OBJECTIVE

Estimating the risk entailed by classical risk factors like blood pressure (BP) or serum cholesterol may be difficult because of their variability and the often unknown duration of exposure. Having variables integrating the impact of those classical risk factors on the cardiovascular system would probably aid the prediction of cardiovascular events. The present study aimed at determining whether cardiac baroreflex sensitivity (BRS), correlates with several risk factors and thus is a good candidate for being such an integrative variable. As a comparison, left ventricular mass (LVM), pulse wave velocity (PWV), and creatinine were also tested for association with risk factors.

DESIGN

A total of 302 subjects referred for hypertension, were considered. They had a 24-h BP recording and a determination of BRS by two different methods (sequence and alpha coefficient), in two different positions (lying and standing). They were also tested for the presence of left ventricular hypertrophy (LVH) (by echocardiography and electrocardiogram) and had a PWV measurement. Biological testing included serum lipids, blood glucose, creatinine, proteinuria and urinary excretion of microalbumin.

RESULTS

There was a strong correlation between the two methods of BRS measurement in each position (P < 0.001). BRS determined by the sequence method in the lying position was correlated significantly and independently with age, 24-h systolic BP, heart rate, and serum cholesterol with P values < 0.001, < 0.001, < 0.01, and < 0.05, respectively. In an univariate analysis, BRS was also correlated with echocardiographic LVM index (r = -0.21, P < 0.05) and PWV (r = -0.27, P < 0.001), which possibly reflects its dependence on both vascular and cardiac damages.

CONCLUSION

The present study supports the hypothesis that BRS could encompass the impact over time of several risk factors on the cardiovascular system. Thus, it may constitute a valuable parameter in assessing more precisely the risk of cardiovascular events.

[Torasemide (LUPRAC): a review of its pharmacological and clinical profile]

Loop diuretics potently excrete water and electrolytes and therefore have been widely prescribed for the treatment of various kinds of edema for a long time. The potent diuretic action of loop diuretics, however, often causes hypokalemia, and therefore potassium sparing diuretics have also been supplied as a concomitant drug. Torasemide (LUPRAC), a novel diuretics, shows not only an effective loop diuretic action but also a potassium sparing action due to its anti-aldosteronergic effect. Torasemide also has a high bioavailability and is only slightly influenced by meals in humans. In addition, its pharmacodynamic features contribute to its stable diuretic action without any individual differences. In animal experiments, torasemide showed about a tenfold more potent diuretic action in comparison with furosemide, an authentic loop diuretic. On the one hand, the increase in the urinary potassium excretion by torasemide was relatively slight compared to the increase in urinary sodium excretion and, as a result, the urinary sodium to potassium (Na+/K+) ratio increased. The diuretic profile of torasemide was equal to that of the concomitant use of furosemide and an anti-aldosteronergic drug, spironolactone. Torasemide showed a significant efficacy and safety in comparison with furosemide in the patients with edema in both domestic and foreign clinical studies. Moreover, torasemide also showed a decreased rate of cardiac death in comparison to furosemide in patients with chronic heart failure in a large-scale clinical study (TORIC Study). The difference in cardiac death between these two diuretics has been suggested to depend on the anti-aldosteronergic effect of torasemide. In Japan, no new loop diuretics have been developed in over 10 years. Torasemide is therefore expected to be useful as an effective diuretic for diseases with edema.

Baroreceptor sensitivity during the compensatory phase of left ventricular overloading

This study focused on the role of the baroreceptor-mediated control during the compensatory process after acute left ventricular overloading induced by aortic regurgitation (AR). Baroreceptor-heart rate sensitivity was measured using a phenylephrine-induced increase in blood pressure according to the steady state method before, 1 day, 1 week and 4 weeks after production of AR in 7 rabbits, and compared with 6 other rabbits that underwent a sham operation. Blood pressure was monitored noninvasively using Finapres in the unanesthetized state. Four weeks after the procedure, the left ventricular diameters of both end-diastole and end-systole were larger in the rabbits with AR than in the sham-operated rabbits. There was no difference in the left ventricular end-diastolic pressure or cardiac output. Left ventricular weight was higher in the rabbits with AR than in the sham-operated rabbits. Myocardial beta-adrenergic receptor density and norepinephrine content were comparable between the two groups. Baroreceptor-heart rate sensitivity significantly decreased 1 week after production of AR, and this alteration in sensitivity was partially restored 4 weeks after production of AR. These findings suggested that the altered baroreceptor-heart rate sensitivity was reversible, relating to the compensatory process after acute left ventricular overloading, and that these changes had some role in its pathophysiology.

Effect of chronic volume overload on baroreflex control of heart rate and sympathetic nerve activity

Baroreceptor-heart rate reflex sensitivity is decreased in congestive heart failure. The reflex control of heart rate and sympathetic nerve activity in rats with chronic volume overload, an established model for moderate heart failure, is still unknown. Therefore, we investigated the regulation of humoral and neuronal sympathetic activity and the baroreflex control of heart rate and sympathetic nerve activity in conscious, unrestrained rats with aortocaval shunt. Rats with aortocaval shunts had larger hearts (388 +/- 11 vs. 277 +/- 4 mg/100 g body wt), elevated central venous pressures (14 +/- 4 vs. 4 +/- 3 mmHg), and higher atrial natriuretic peptide plasma levels (87 +/- 16 vs. 25 +/- 3 pmol/l) than controls but had similar systemic blood pressure and heart rate values. Plasma epinephrine (0.63 +/- 0.16 vs. 0.21 +/- 0.08 pmol/l, P < 0.05) and norepinephrine concentrations (0.27 +/- 0.03 vs. 0.16 +/- 0.02 pmol/l, P < 0.05) were elevated in shunted rats compared with controls. Nitroprusside-induced hypotension led to a significantly greater increase in efferent splanchnic sympathetic nerve activity in shunted rats than in controls (0.9 +/- 0.1 vs. 2.6 +/- 0.6 microV, P < 0.05), whereas the heart rate responses were not different between the groups. These results indicate that the regulation of the autonomic nervous system is altered in chronically volume-overloaded rats. The arterial baroreflex control of efferent splanchnic sympathetic nerve activity was dissociated from the control of heart rate. Therefore, analysis of the activation of sympathetic nervous system assessed by direct measurements of efferent sympathetic nerve activity appears to be more sensitive for the detection of altered autonomic nervous system function than the analysis of baroreflex control of heart rate.