Barbara A. Barron

Opioid peptides and the heart

The heart is a complex neuroendocrine (opioids, NPY, VIP) or mechanoendocrine (ANP) organ. Opioid peptides and receptors are present in the heart and nerves such that they can easily modulate cardiac function. Cardiac opioids may have autocrine, paracrine and endocrine function. The challenge is to find the signal that turns them on and then what they do in the face of an overwhelmingly redundant system making knock-out technology hard to interpret. Determining the cardiac release of opioids in an intact system still requires a larger animal model.

Methionine-enkephalin-Arg-Phe immunoreactivity in heart tissue

Previous findings of enkephalins in cardiac tissue led us to investigate enkephalin distribution in animal models used for cardiovascular research. Canine cardiac methionine-enkephalin (ME) concentrations are low and evenly distributed between atria (4.2 +/- 0.6 fmol/mg protein, n = 30) and ventricles (4.4 +/- 0.5). In contrast, methionine-enkephalyl-arginyl-phenylalanine (MEAP) immunoreactivity (IR) is higher and preferentially concentrated in the ventricle (112 +/- 12) vs. the atria (23.2 +/- 2.6 fmol/mg protein). HPLC analysis suggests the atrial/ventricular difference is partly due to altered posttranslational processing. Nearly 90% of ventricular IR is comprised of MEAP (46%) and peptide B (40%) whereas these peptides represent less than half of the atrial content. A nonneuronal localization is indicated because the peptide distribution does not correspond to the catecholamine distribution. Canine left ventricular tissue sections were processed for immunohistochemistry with the MEAP antibody. Fluorescence was distributed throughout the myocytes and concentrated in ordered lines perpendicular to the myocyte longitudinal axis corresponding to the area of the intercalated disc. This suggests opioids may be important in communication between cardiomyocytes, and possibly the presence of a unique peptide secretory mechanism utilizing the intercalated disc. The relative peptide content in cat and pig hearts was similar to the dog; however, the distribution was different. Feline cardiac ME content was distributed 2:1 in favor of the ventricles and corresponded with a preferential ventricular norepinephrine distribution. The MEAP-IR pattern gave a ventricular/atrial ratio lower (3.5:1) in cat heart vs. dog (5:1). In contrast, pig heart ME and MEAP-IR ventricular/atrial ratios were reversed for both ME (1:10) and MEAP (1:2). HPLC of pig left ventricle showed that MEAP and peptide B represented 33% and 39% of the MEAP-IR, respectively. These species variations may correlate to the differences observed in cardiac function.

Pericardial repair depresses canine cardiac catecholamines and met-enkephalin

Decreased cardiac catecholamines were observed following incision and repair of the pericardium in sham-operated vs. unoperated control dogs. Animals were assigned to five groups: unoperated, sham-operated intact pericardia, open pericardia, sutured pericardia and complete ventricular sympathectomy. Hearts were collected four weeks after surgery. Sympathectomy decreased catecholamine content when compared to all other groups. Hearts with open/sutured pericardia contained significantly less catecholamines than controls. When the pericardium was intact or left open following incision, cardiac catecholamines were unchanged compared to unoperated controls. Since opioid peptides are colocalized with catecholamines, we measured met-enkephalin and met-enkephalin-arg-phe, proenkephalin A peptide products, in parallel samples. Similar to norepinephrine, met-enkephalin was decreased following both sympathectomy and pericardial repair. However, met-enkephalin-arg-phe, which may be more associated with the myocardium than its innervation, was not changed by any treatment. The sutured pericardium more than the stress of surgery apparently alters the tissue catecholamines and enkephalin. This may have resulted from the mechanical friction at the site of repair. Epinephrine and met-enkephalin contents in sympathectomized hearts were significantly lower than unoperated controls but were not significantly different from the intermediate values observed in the sutured group. The functional consequences of these changes on neuroendocrine status are unclear and will require further evaluation. The results also emphasize the need for careful attention to proper controls for surgical studies.

Selective parasympathetic denervation following posteroseptal ablation for either atrioventricular nodal reentrant tachycardia or accessory pathways

Baroreflex gain and coronary sinus norepinephrine and epinephrine levels were measured before and immediately after radiofrequency ablation in the posteroseptal region in 9 patients with atrioventricular nodal reentrant tachycardia or posteroseptal accessory pathways. Arterial baroreflex gain was significantly reduced after radiofrequency ablation (p = 0.046), whereas coronary sinus epinephrine and norepinephrine levels did not change significantly compared with preablation levels.

Intrinsic cardiac enkephalins & vagal control

Abstract Met-enkephalin-arg-phe (MEAP) has been identified in acid extracts of canine heart. The effects of synthetic MEAP on the vagal control of heart rate and atrial contractility were investigated in anesthetized dogs. Arterial MEAP (3 nmol/min/kg) inhibited right vagal bradycardia by two thirds. Postinfusion responsiveness to vagal stimulation returned to normal with an estimated half-time of 2–3 min. Inhibition by MEAP was reversed by the high affinity opiate antagonist, diprenorphine at pmolar concentrations. Since MEAP did not alter the negative chronotropic effect of methacholine, the effective site must reside in the efferent vagal tract proximal to nodal muscarinic receptors. Left vagal stimulation dramatically suppresses left atrial contractile activity. Infused MEAP also interrupts this negative inotropic response through a diprenorphine sensitive mechanism. Increasing MEAP infusions (0.09 – 3.00 nmol/min/kg) produced a graded suppression of vagal bradycardia with an ED50 near 0.3 nmol/min/kg. Heart rate dose responses for methionine-enkephalin were shifted to the right of MEAP and required approximately 3 X the dose to produce the same effect. The data suggest that the intrinsic cardiac enkephalin, MEAP can regulate vagal control of heart rate at physiologically achievable concentrations and may serve as a local regulator of the parasympathetic/myocardial interface.