No inotropic action of enkephalins or enkephalin derivatives on electrically-stimulated atria isolated from lean and obese rats

Effect of dynorphin A(1?13) on cardiomyocytes in culture: modulation of the response to increased extracellular calcium, but no effect on intrinsic cardiac contractile frequency or the response to isoproterenol or increased extracellular potassium

The purpose of this study was to determine whether the endogenous opioid peptide dynorphin A(1-13) has a direct effect on the heart or acts to modulate the cardiac chronotropic response to calcium, potassium, or beta-adrenergic receptor stimulation. Spontaneously contracting myocardial cell aggregates were prepared from 7-day-old chick embryos and were maintained in culture for 72 h before study. Dynorphin A(1-13), 10(-8) to 10(-6)M, did not alter spontaneous contractile frequency. Increases in [Ca2+]o spontaneously suppressed cardiac contractile frequency, and dynorphin A(1-13) significantly (p less than 0.05) enhanced this response. Nifedipine, 10(-8) M, antagonized the effect of increased [Ca2+]o on cardiac contractile frequency, but did not block the action of dynorphin A(1-13) to accentuate the effect of increasing [Ca2+]o. Dynorphin A(1-13) did not alter the significant (p less than 0.05) increase in contractile frequency produced by beta-adrenergic receptor stimulation by isoproterenol, or the suppression in contractile frequency produced by increases in extracellular potassium ([K+]o). These data indicate that dynorphin A(1-13) does not act directly on the cardiac myocyte to alter cardiac contractile frequency or alter the response to increases in [K+]o or to isoproterenol, but that dynorphin A(1-13) does modulate the response to increases in extracellular calcium.

Kappa and delta opioid receptor stimulation affects cardiac myocyte function and Ca2+ release from an intracellular pool in myocytes and neurons

We investigated the effects of mu, delta, and kappa opioid receptor stimulation on the contractile properties and cytosolic Ca2+ (Cai) of adult rat left ventricular myocytes. Cells were field-stimulated at 1 Hz in 1.5 mM bathing Ca2+ at 23 degrees C. The mu-agonist [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (10(-5) M) had no effect on the twitch. The delta-agonists methionine enkephalin and leucine enkephalin (10(-10) to 10(-6) M) and the kappa-agonist (trans-(dl)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclo-hexyl]- benzeneacetamide)methanesulfonate hydrate (U-50,488H; 10(-7) to 2 x 10(-5) M) had a concentration-dependent negative inotropic action. The sustained decrease in twitch amplitude due to U-50,488H was preceded by a transient increase in contraction. The effects of delta- and kappa-receptor stimulation were antagonized by naloxone and (-)-N-(3-furyl-methyl)-alpha-normetazocine methanesulfonate, respectively. In myocytes loaded with the Ca2+ probe indo-1, the effects of leucine enkephalin (10(-8) M) and U-50,488H (10(-5) M) on the twitch were associated with similar directional changes in the Cai transient. Myofilament responsiveness to Ca2+ was assessed by the relation between twitch amplitude and systolic indo-1 transient. Leucine enkephalin (10(-8) M) had no effect, whereas U-50,488H (10(-5) M) increased myofilament responsiveness to Ca2+. We subsequently tested the hypothesis that delta and kappa opioid receptor stimulation may cause sarcoplasmic reticulum Ca2+ depletion. The sarcoplasmic reticulum Ca2+ content in myocytes and in a caffeine-sensitive intracellular Ca2+ store in neurons was probed in the absence of electrical stimulation via the rapid addition of a high concentration of caffeine from a patch pipette above the cell. U-50,488H and leucine enkephalin slowly increased Cai or caused Cai oscillations and eventually abolished the caffeine-triggered Cai transient. These effects occurred in both myocytes and neuroblastoma-2a cells. In cardiac myocyte suspensions U-50,488H and leucine enkephalin both caused a rapid and sustained increase in inositol 1,4,5-trisphosphate. Thus, delta and kappa but not mu opioids have a negative inotropic action due to a decreased Cai transient. The decreased twitch amplitude due to kappa-receptor stimulation is preceded by a transient increase in contractility, and it occurs despite an enhanced myofilament responsiveness to Ca2+. The effects of delta and kappa opioids appear coupled to phosphatidylinositol turnover and, at least in part, may be due to sarcoplasmic reticulum Ca2+ depletion.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of proenkephalin products on rat cardiac and vascular tissue in-vitro

The cardiovascular effects of the four proenkephalin products [Met]-enkephalin (ME), [Leu]-enkephalin (LE), [Met]-enkephalyl-arg6phe7 (MEAP) and [Met]-enkephalyl-arg6-gly7-leu8 (MEAGL) have been studied on the isolated spontaneously beating rat atria and the perfused rat mesentery in-vitro. All four peptides (at concentrations up to 10(-6)M and in the presence of peptidase inhibitors) had no direct effect on atrial rate or contractility and did not alter responses to noradrenaline or field stimulation. In addition, the peptides had no effect on the perfusion pressure of the mesentery and did not alter vasoconstrictor responses to noradrenaline. The results show that proenkephalin products are without direct or modulating effects on atrial muscle or mesenteric vasculature of the rat, and suggest that there is no endogenous opioid control in these tissues.

Proenkephalin-A derived peptides do not modulate cardiovascular effects of epinephrine on the isolated rat atrial preparations

The catecholamines and the opioid peptides are found to be co-localized in the adrenomedullary chromaffin cells. They are co-secreted from the chromaffin granules in response to various stimuli. The stress-induced released of epinephrine is known to exert its effect on the cardiovascular system resulting in the changes in heart rate and blood pressure. However, the role of the co-released proenkephalin-A derived peptides has not been extensively characterized. Previous work from several investigators suggested that the peptides modulate cardiac functions of the catecholamines. There is considerable conflicting results among these reports. Results from the isolated rat atrial preparation indicated that enkephalins attenuated the increase in atrial rate induced by norepinephrine through restriction of the calcium fluxes. Nonetheless, others reported insensitivity of the enkephalins in similar or different test systems. We further re-examined these discrepancies using the isolated rat atrial preparation to investigate the opioid peptide modulatory effect on the cardiovascular changes induced by exogenous epinephrine. Alterations in rate and force of contraction resulting from epinephrine and the peptides were both studied in parallel. The opioid peptides used in this study were [Met5]-enkephalin (ME), [Leu5]-enkephalin (LE), FMRFamide, [Met5]-enkephalyl-Arg6-Phe7 (MEAP), peptide E, and the non-selective opioid agonist, etorphine. We report here that none of the opioid peptides were effective in alleviating or attenuating the increase in heart rate and developed tension caused by epinephrine. The peptides did not affect the basal beating rate nor the force of contraction. Thus, the present results clearly demonstrate the insensitivity of the enkephalins in modulating the cardiac effects of epinephrine. They further indirectly support the prejunctional synaptic nerve endings as the potential peripheral site of action of the peptides.

Circulating [Met]enkephalin and catecholamine responses to acute hypotension and hypertension in anaesthetized greyhounds

The effects of either hypotension induced by sodium nitroprusside or hexamethonium or hypertension produced by angiotensin II or noradrenaline on the circulating levels of methionine enkephalin ([Met]enkephalin)-like immunoreactivity (MLI), adrenaline and noradrenaline in anaesthetized greyhounds were examined. Nitroprusside infusions (200 and 400 micrograms min-1) induced a fall in blood pressure accompanied by significant rises in plasma MLI and catecholamine concentrations. Concomitant administration of a high dose of naloxone did not alter the fall in blood pressure produced by nitroprusside but was associated with greater rises in circulating MLI and catecholamines when compared to nitroprusside alone, suggesting that [Met]enkephalin is not involved in the hypotensive action of nitroprusside. Intravenous hexamethonium (2.5 mg kg-1) provoked a fall in blood pressure which was not associated with any changes in plasma MLI. However, it produced a fall in plasma noradrenaline and a rise in plasma adrenaline. Thus it appears that neural mechanisms are required, at least in part, for the release of MLI. Angiotensin II (1.25 micrograms kg-1 min-1) and noradrenaline (8 micrograms kg-1 min-1) infusions produced an elevation in blood pressure without altering the circulating MLI levels. Study of the molecular forms of circulating MLI, before and during hypotension, revealed that the large molecular weight enkephalin-containing peptides with approximate molecular sizes of 18kD and 8kD were the predominant forms both in the basal and stimulated states. It is concluded that circulating [Met]enkephalin is not involved in the tonic control of blood pressure but it may modulate catecholamine release following hypotension as part of the stress response.

Enkephalins increase cyclic adenosine monophosphate content, calcium uptake, and contractile state in cultured chick embryo heart cells

Peripheral vascular effects of opioid peptides are well known, but direct myocardial effects have not been established. We studied the inotropic response of spontaneously beating cultured chick embryo ventricular cells to the enkephalin analogue [D-Ala2]-enkephalin. Amplitude of cell motion increased in a concentration-dependent manner with 0.53 microM [D-Ala2]-enkephalin producing half-maximal response. The mechanism of this positive inotropic effect was investigated by examining alterations in 45Ca influx, cyclic AMP accumulation and adenylate cyclase activity in response to [D-Ala2]-enkephalin. At maximally inotropic concentrations, the 45Ca influx rate increased 39%, adenylate cyclase was stimulated by 30%, and cyclic AMP content rose more than twofold. Thus, in contrast to neural tissue, receptors for enkephalin in cultured heart cells are coupled to adenylate cyclase in a stimulatory manner. Occupancy of these receptors produces an increase in cyclic AMP levels and exerts a positive inotropic effect via a verapamil-sensitive enhancement of Ca influx.