Modulation of sympathetic activity by brain neuropeptide Y in cardiac hypertrophy

Effects of neuropeptide Y on coronary artery vasomotion in patients with microvascular angina

BACKGROUND

Patients with microvascular angina (exertional angina, positive exercise tests and normal coronary arteriograms) usually have a reduced coronary blood flow reserve. Neuropeptide Y (NPY) is a potent endogenous vasoconstrictor involved in modulation of coronary vasomotor tone and may play a role in microvascular angina.

METHODS

We compared the effects of NPY (0.2-1.0pmol/kg, intracoronary) on the vasomotor response of proximal and distal segments of the coronary arteries in 7 patients with microvascular angina, 9 with chronic stable angina, and 9 control individuals. The coronary response to the administration of ergonovine was also assessed in 9 other patients with microvascular angina. Computerized coronary artery diameter measurements were carried out before (baseline) and after the administration of the vasoactive agents.

RESULTS

Mean baseline coronary lumen diameters were similar in control, microvascular angina, and coronary artery disease patients. NPY constricted proximal coronary segments by 8±2%, 5±2% and 6±3% and distal segments by 14±2%, 11±2% and 10±2% in control, microvascular angina, and coronary artery disease patients, respectively (p=NS between groups). In patients with microvascular angina, ergonovine constricted proximal coronary segments by 7±1.5% and distal segments by 12.5±3% (p=NS vs. NPY). During NPY administration four microvascular angina patients developed chest pain, ST segment depression, and a marked lengthening of the contrast medium run off, in the absence of epicardial coronary artery spasm. Control individuals and coronary artery disease patients did not experience chest pain, ST segment shifts, or lengthening of the run off during NPY administration. Ergonovine administration caused chest pain and lengthening of the contrast run-off, in the absence of epicardial coronary artery spasm, in one microvascular angina patient.

CONCLUSIONS

Exogenous NPY causes mild epicardial coronary artery constriction which is similar in patients with non-cardiac chest pain, microvascular angina and coronary artery disease. Myocardial ischemia and marked lengthening of the contrast run off in response to NPY occurred in microvascular angina patients but not in control or coronary artery disease patients. An abnormal constrictor response to NPY at the microcirculation level could be the mechanism underlying the ischemic manifestations observed in patients with microvascular angina.

CONDENSED ABSTRACT (TABLE OF CONTENTS)

The vasomotor response of proximal and distal coronary artery segments was studied in twenty five patients: 7 microvascular angina, 9 chronic stable angina, and 9 control subjects. Computerized measurements of coronary diameters were carried out before and after the intracoronary administration of neuropeptide Y (NPY) and ergonovine. Constriction of epicardial arteries in response to NPY was mild and not significantly different in control, microvascular angina and coronary artery disease patients. Ergonovine-induced epicardial coronary artery constriction was similar to that of NPY. However, NPY caused transient myocardial ischemia in patients with microvascular angina (probably through constriction of the small intramyocardial vessels), but not in control subjects or coronary artery disease patients.

Neuropeptide Y level in paraventricular nucleus of experimental diabetic rats: correlation with sympathetic activity and body weight

Neuropeptide Y (NPY), colocalized with norepinephrine neuron, is known to modulate sympathetic activity and feeding behavior. Although experimental type 1 diabetes has increased sympathetic activity at the early part of the disease process, little effort was made so far to understand the correlation between NPY level in the hypothalamus and sympathetic activity in diabetes. Male Sprague Dawley rats were made diabetic by a single injection of streptozotocin (65 mg/kg body weight, IV). The animals were then studied after 2, 4, and 8 weeks. Control animals received only citrate vehicle. In an effort to clarify the modulatory effect of NPY at the early stage of diabetes, the paraventricular nucleus (PVN) of hypothalamus was sampled by microdialysis for NPY and norepinephrine level. While NPY level was increased immediately within 2 weeks (along with feeding behavior), norepinephrine level was increased only after 8 weeks following injection of streptozotocin. The animals lost significant weight. These results are interpreted to mean that a strong correlation exists between the feeding behavior and NPY level in PVN. Since NPY is known to inhibit sympathetic activity it is possible that NPY receptors are down-regulated following diabetes. The higher level of norepinephrine indicating higher sympathetic activity did not allow the animals to gain weight. In addition, controversy exists regarding pleiotropic activities of NPY related to the feeding behavior of these animals.

Catecholamine metabolism inhibitors and receptor blockades only partially suppress cardiac hypertrophy of juvenile visceral steatosis mice with systemic carnitine deficiency

To clarify the mechanism of cardiac hypertrophy in carnitine-deficient JVS mice, we studied the possible role of catecholamine metabolism. Cardiac hypertrophy occurs 2 weeks after birth. The turnover of norepinephrine in the ventricles of JVS mice at 2 weeks was 3 times that of control, but it was not different from control at 5 days when the heart weight was not changed. To evaluate the accelerated norepinephrine turnover, we examined the effects of catecholamine metabolism inhibitors (alpha-methyltyrosine and 6-hydroxydopamine) and catecholamine receptor blockades (propranolol, prazosin and yohimbine) on the ratio of heart weight to body weight (HW/BW) and on the augmented expression of atrial natriuretic peptide (ANP) and the down-regulated carnitine deficiency-associated gene expressed in ventricle (CDV-1). The HW/BW ratio in JVS mice treated with catecholamine metabolism inhibitors and receptor blockades was significantly lower than in JVS mice without treatment, but still higher than in controls treated with each drug and in JVS mice treated with carnitine. The HW/BW ratio of JVS mice with propranolol was not significantly different from that of JVS mice treated with catecholamine metabolism inhibitors and was significantly lower than that of JVS mice treated with prazosin and yohimbine. Northern blot analysis showed that the altered expression of ANP and CDV-1 was not corrected in the ventricles of JVS mice treated with any of the drugs except carnitine. These results suggest that the catecholamine metabolism accelerated in JVS mice ventricles at 2 weeks is not the major cause of cardiac hypertrophy, but probably promotes cardiac hypertrophy mainly through the beta-adrenergic signaling pathway. The aberrant gene expression of ANP and CDV-1 found in JVS mice seems to be independent of catecholamine metabolism, and mediated primarily by the systemic carnitine deficiency.

Neuropeptide Y modulation of sympathetic activity in myocardial infarction

OBJECTIVES

We examined the possible effect of neuropeptide Y in modulating central sympathetic activity after myocardial infarction in rats.

BACKGROUND

Previous studies have shown the coexistence of neuropeptide Y and norepinephrine in the brain and a possible functional interaction between the two. Neuropeptide Y inhibits the release of norepinephrine at the presynaptic level and can be considered to act as a neuromodulator.

METHODS

Two groups of rats were examined in this study-an experimental group, defined as those rats undergoing left coronary artery ligation, and a sham group without coronary artery ligation, serving as the control group. The animal in both groups underwent microdialysis in the paraventricular nucleus at 2, 4 and 8 weeks after operation. Microdialysis samples were collected with and without injecting neuropeptide Y in the paraventricular nucleus. The concentration of norepinephrine was determined by injecting purified microdialysate samples during high performance liquid chromatography. To explore the receptor's possible role, autoradiographic localization of neuropeptide Y receptors in the paraventricular nucleus was also carried out in the experimental and sham groups.

RESULTS

The concentration of norepinephrine measured in the samples was decreased by 50% with neuropeptide Y in 2- and 4-week old rats after infarction, but by only 20% (p < 0.05) in 8-week old rats after infraction. The diminished inhibitory effects of neuropeptide Y on norepinephrine release was associated with increased sympathetic activity, as reflected by plasma norepinephrine; 8-week old rats after infarction had almost a 100% (p < 0.05) increase in their plasma norepinephrine level compared with the sham group. Autoradiography revealed a significant decrease in density of neuropeptide Y receptors in the paraventricular nucleus in 8-week old rats after infarction (p < 0.05).

CONCLUSIONS

The data presented in this report suggest that the reduction of the inhibitory activation of neuropeptide Y on sympathetic release may contribute to elevated norepinephrine levels after myocardial infarction.

Reduction by oral propranolol treatment of left ventricular hypertrophy secondary to pressure-overload in the rat

1. Studies on cardiac myocyte cell cultures have postulated a role for alpha 1-adrenoceptors and mechanical stretch in the induction of cellular changes thought to be important in compensatory cardiac hypertrophy. However, in vivo work suggests that beta-adrenoceptors are important and the present study was designed to analyse the effect of propranolol on the cardiac hypertrophy caused by a pressure-overload in a way that takes account of the effects of propranolol on the work load itself. 2. The compensatory cardiac hypertrophy that develops in response to experimental coarctation of the aorta was studied in the rat. Pressure gradients and total cardiac work load (expressed as rate x pressure product) were assessed, and the relationship between increasing cardiac work load and the resulting left ventricular hypertrophy was established in a control group and compared with left ventricular hypertrophy in a group treated with a high dose of oral propranolol (80 mg kg-1 body weight). 3. In the rats with mean pressure gradients over the coarctation in the range of 15-31 mmHg, the animals on control diet showed a 38% increase in left ventricular weight/body weight ratio (LV ratio) and a 30% increase in heart weight/body weight ratio (heart ratio), whereas rats given high dose oral propranolol-treatment showed increases of only 13% and 9%, respectively. 4. In a second series of rats with a wider range of pressure gradients, the regression lines of LV ratio versus mean pressure gradient, and of LV ratio versus cardiac work, were different in the two groups with a slope that was only half as steep in the propranolol-treated rats as in the controls. Thus, for the same increment in cardiac work load, the degree of compensatory cardiac hypertrophy in propranolol-treated rats was half that observed in controls. 5. The reduction in compensatory cardiac hypertrophy was not associated with an increase in incidence of congestive heart failure and the propranolol-treated rats were able to sustain equally high (or higher) degrees of pressure over-load as controls did. 6. It is concluded that propranolol treatment approximately halves the compensatory cardiac hypertrophy occurring in response to a left ventricular pressure over-load by a mechanism independent of its effect on cardiac work load. This finding provides further support for the view that noradrenaline released from sympathetic nerve terminals in the heart exerts a trophic effect on cardiac myocytes, and that the sympathetic nervous system may be the final common pathway in many forms of compensatory cardiac hypertrophy. In contrast to in vitro models, this effect appears to be largely mediated via beta-adrenoceptors in the intact animal.