Effects of neuropeptide Y on cell length and membrane currents in isolated guinea pig ventricular myocytes

Cardiac sympathetic activation circumvents high-dose beta blocker therapy in part through release of neuropeptide Y

The sympathetic nervous system plays an important role in the occurrence of ventricular tachycardia (VT). Many patients, however, experience VT despite maximal doses of beta blocker therapy, possibly due to the effects of sympathetic cotransmitters such as neuropeptide Y (NPY). The purpose of this study was to determine, in a porcine model, whether propranolol at doses higher than clinically recommended could block ventricular electrophysiological effects of sympathoexcitation via stellate ganglia stimulation, and if any residual effects are mediated by NPY. Greater release of cardiac NPY was observed at higher sympathetic stimulation frequencies (10 and 20 vs. 4 Hz). Despite treatment with even higher doses of propranolol (1.0 mg/kg), electrophysiological effects of sympathetic stimulation remained, with residual shortening of activation recovery interval (ARI), a surrogate of action potential duration (APD). Adjuvant treatment with the NPY Y1 receptor antagonist BIBO 3304, however, reduced these electrophysiological effects while augmenting inotropy. These data demonstrate that high-dose beta blocker therapy is insufficient to block electrophysiological effects of sympathoexcitation, and a portion of these electrical effects in vivo are mediated by NPY. Y1 receptor blockade may represent a promising adjuvant therapy to beta-adrenergic receptor blockade.

Roles of nuclear NPY and NPY receptors in the regulation of the endocardial endothelium and heart functionThis paper is one of a selection of papers published in this Special issue, entitled Second Messengers and Phosphoproteins—12th International Conference

It is now well accepted that the heart is a multifunctional organ in which endothelial cells, and more particularly endocardial endothelial cells (EECs), seem to play an important role in regulating and maintaining cardiac excitation–contraction coupling. Even if major differences exist between vascular endothelial cells (VECs) and EECs, all endothelial cells including EECs release a variety of auto- and paracrine factors such as nitric oxide, endothelin-1, angiotensin II, and neuropeptide Y. All these factors were reported to affect cardiomyocyte contractile performance and rhythmicity. In this review, findings on the morphology of EECs, differences between EECs and other types of endothelial cells, interactions between EECs and the adjacent cardiomyocytes, and effects of NPY on the heart will be presented. We will also show evidence on the presence and localization of NPY and the Y1receptor in the endocardial endothelium and discuss their role in the regulation of cytosolic and nuclear free calcium.

Regional vascular and cardiac responses to systemic neuropeptide-Y in normal and diabetic rats

Diabetes is associated with altered autonomic activity, and both the peripheral and central nervous content of NPY is altered in diabetes suggesting that part of the cardiovascular dysfunction of diabetes may be associated with altered responses to NPY. We evaluated the mean arterial pressure (MAP), heart rate (HR) and regional blood flow (BF) in response to NPY in normal and diabetic rats. Male Wistar rats were made diabetic using streptozotocin and maintained without insulin treatment for 28-30 days. Normal and diabetic animals were anesthetized, the femoral artery cannulated, and flow probes placed around the iliac, renal and superior mesenteric arteries. Dose dependent responses to NPY infusion were determined for MAP, HR and BF. Conductance was calculated from BF/MAP. In normal rats NPY increased the MAP in a dose-dependent fashion but did not alter the HR. NPY decreased the BF in the iliac artery in a dose-dependent fashion while the renal BF was decreased only at the highest dose, and the superior mesenteric BF was not affected. The conductances in all vascular beds were decreased dose dependently. NPY in diabetic rats also increased MAP and did not affect HR. In diabetic rats NPY also decreased BF in iliac and renal arteries but contrastingly increased BF in the superior mesenteric artery. Again conductances in all three vascular beds were decreased by NPY. When comparing diabetic response to NPY to normals we noted that the MAP response was less in the diabetic, but the HR and regional BF were not different. The vascular conductances in response to NPY were attenuated in the diabetic vessels especially the iliac and superior mesenteric. We conclude that systemic NPY increases MAP as a result of decreased vascular conductance and this vasopressive effect of NPY is diminished in diabetics.

Effects of neuropeptide Y on L-type calcium current in guinea-pig ventricular myocytes

1. Neuropeptide Y (NPY) reduces cell shortening at high concentrations in guinea-pig ventricular myocytes. We have studied the effects of the peptide on calcium current in cardiac myocytes. 2. We have recorded L-type calcium current in guinea-pig ventricular myocytes under conditions in which the effects of other overlapping currents have been minimised by using Na(+)-free, K(+)-free external solution and patch-clamp electrodes containing Cs+. 3. Peak inward calcium current is reduced by NPY at concentrations in excess of 1 nM, and maximal inhibition (31%) was found at and above concentrations of 100 nM. The IC50 value for NPY inhibition of peak calcium current was 1.72 nM. 4. NPY had no effect on the voltage-dependence of calcium current amplitude, on the time course of current inactivation, or on the voltage-dependence of the steady-state gating variables. 5. NPY did not reduce the calcium current in the presence of 8-Br-cyclic AMP, and it was also without effect when GTP-gamma-S or GDP-beta-S were included in the patch pipette. 6. We conclude that in guinea-pig ventricular myocytes NPY acts at low concentration to reduce L-type calcium current, via a G-protein-mediated pathway and reduction in intracellular cyclic AMP.

Changes in cell length consequent on depolarization in single left ventricular myocytes from guinea-pigs with pressure-overload left ventricular hypertrophy

Cell length was measured in single guinea-pig left ventricular myocytes by using a high-resolution photodiode array. Step depolarizations from a holding potential of -45 mV were applied using a switch-clamp technique with 2 M KCl microelectrodes, which were devoid of Ca2+ buffering. Comparison was made between myocytes from sham-operated guinea-pigs and guinea-pigs with mild pressure-overload left ventricular hypertrophy induced by infra-renal aortic constriction. The relation between cell shortening and membrane voltage was bell shaped, and a phasic component of shortening was evident at the range of potentials over which the L-type calcium current was activated. Mean cell shortening was increased in the hypertrophy group, and was maximal at +15 mV in both groups (control, 7.6 +/- 0.9 microns, n = 11, hypertrophy 11.0 +/- 1.2 microns, n = 20, p < 0.05). The latency to the onset of contraction was significantly shorter in the hypertrophy myocytes at -25 mV and at potentials positive to +50 mV. The relation between time-to-peak shortening and voltage showed a trend to shorter times in the hypertrophy group. At very positive potentials a slow component of contraction was identified which was relatively larger in the hypertrophy myocytes. This finding is consistent with increased calcium entry via sarcolemmal sodium-calcium exchange in the myocytes from the hypertrophy group.

Neuropeptide Y potentiates calcium-channel currents in single vascular smooth muscle cells

The effect of neuropeptide Y (NPY) on Ca(2+)-channel currents in isolated vascular smooth muscle cells was studied with the perforated-patch recording technique. Using Ba2+ (10 mM) as the charge carrier, inward currents sensitive to Cd2+ and nifedipine were potentiated by NPY in a concentration-dependent manner. The threshold concentration for the potentiating effect of NPY was 50 nM and reached a maximum at 150 nM. NPY shifted the steady-state activation curve to less positive membrane potentials by about 6 mV so that the potentiating effect was most prominent near the activation threshold of the current. It had no effect on steady-state inactivation of the current. These results suggest that NPY may potentiate vasoconstriction by promoting calcium entry through L-type voltage-dependent Ca(2+)-channels.