Cross-talk between cardiac kappa-opioid and beta-adrenergic receptors in developing hypertensive rats

Biased, Bitopic, Opioid–Adrenergic Tethered Compounds May Improve Specificity, Lower Dosage and Enhance Agonist or Antagonist Function with Reduced Risk of Tolerance and Addiction

This paper proposes the design of combination opioid–adrenergic tethered compounds to enhance efficacy and specificity, lower dosage, increase duration of activity, decrease side effects, and reduce risk of developing tolerance and/or addiction. Combinations of adrenergic and opioid drugs are sometimes used to improve analgesia, decrease opioid doses required to achieve analgesia, and to prolong the duration of analgesia. Recent mechanistic research suggests that these enhanced functions result from an allosteric adrenergic binding site on opioid receptors and, conversely, an allosteric opioid binding site on adrenergic receptors. Dual occupancy of the receptors maintains the receptors in their high affinity, most active states; drops the concentration of ligand required for full activity; and prevents downregulation and internalization of the receptors, thus inhibiting tolerance to the drugs. Activation of both opioid and adrenergic receptors also enhances heterodimerization of the receptors, additionally improving each drug’s efficacy. Tethering adrenergic drugs to opioids could produce new drug candidates with highly desirable features. Constraints—such as the locations of the opioid binding sites on adrenergic receptors and adrenergic binding sites on opioid receptors, length of tethers that must govern the design of such novel compounds, and types of tethers—are described and examples of possible structures provided.

Involvement of the p38 MAPK-pHsp27 pathway in vascular hyporeactivity induced by obstructive jaundice in rats

Patients with obstructive jaundice are prone to develop cardiovascular complications during surgery. However, the underlying mechanisms remain largely unknown. The present study was aimed to investigate the role of p38 MAPK-pHsp27 pathway in vascular hyporesponsiveness induced by obstructive jaundice. Firstly, an experimental rat obstructive jaundice model was established by bile duct ligation (BDL). We found that the thoracic aorta rings isolated from BDL rats showed decreased response to norepinephrine and acetylcholine, while continuous intraperitoneal injection with SB203580, a selective P38 MAPK inhibitor, could significantly prevented BDL-induced hyporeactivity. Also, the immunohistochemistry and Western blot assays revealed that the up-regulation of pHsp27 and F-actin in thoracic aorta rings from BDL rats and bilirubin-treated vascular smooth muscle cells (VSMCs) were also inhibited by SB203580. Moreover, we identified that bilirubin could induced decreased cell proliferation of VSMCs by using CCK8 assay and which was also prevented by SB203580. All these data demonstrated that p38 MAPK-pHsp27 mediates vascular hyporesponsiveness in rats with obstructive jaundice by modulating the expression level of pHsp27 and F-actin, and that inhibition of p38 MAPK signaling could remodel the vascular activity.

Opioid Facilitation of β-Adrenergic Blockade: A New Pharmacological Condition?

Recently, propranolol was suggested to prevent hyperlactatemia in a child with hypovolemic shock through β-adrenergic blockade. Though it is a known inhibitor of glycolysis, propranolol, outside this observation, has never been reported to fully protect against lactate overproduction. On the other hand, literature evidence exists for a cross-talk between β-adrenergic receptors (protein targets of propranolol) and δ-opioid receptor. In this literature context, it is hypothesized here that anti-diarrheic racecadotril (a pro-drug of thiorphan, an inhibitor of enkephalinases), which, in the cited observation, was co-administered with propranolol, might have facilitated the β-blocker-driven inhibition of glycolysis and resulting lactate production. The opioid-facilitated β-adrenergic blockade would be essentially additivity or even synergism putatively existing between antagonism of β-adrenergic receptors and agonism of δ-opioid receptor in lowering cellular cAMP and dependent functions.

Analgesia and central side-effects: two separate dimensions of morphine response

AIMS

To present a statistical model for defining interindividual variation in response to morphine and to use this model in a preliminary hypothesis-generating multivariate genetic association study.

METHODS

Two hundred and sixty-four cancer patients taking oral morphine were included in a prospective observational study. Pain and morphine side-effect scores were examined using principal components analysis. The resulting principal components were used in an exploratory genetic association study of single nucleotide polymorphisms across the genes coding for the three opioid receptors, OPRM1, OPRK1 and OPRD1. Associations in multivariate models, including potential clinical confounders, were explored.

RESULTS

Two principal components corresponding to residual pain and central side-effects were identified. These components accounted for 42 and 18% of the variability in morphine response, respectively, were independent of each other and only mildly correlated. The genetic and clinical factors associated with these components were markedly different. Multivariate regression modelling, including clinical and genetic factors, accounted for only 12% of variability in residual pain on morphine and 3% of variability in central side-effects.

CONCLUSIONS

Although replication is required, this data-driven analysis suggests that pain and central side-effects on morphine may be two separate dimensions of morphine response. Larger study samples are necessary to investigate potential genetic and clinical associations comprehensively.

Cardiovascular abnormalities in obstructive cholestasis: the possible mechanisms

Cholestatic liver disease is associated with widespread derangements in the cardiovascular system, such as bradycardia, hypotension, QT prolongation and peripheral vasodilation; it is also associated with increased susceptibility to postoperative renal failure and haemorrhagic shock. A number of cellular signalling pathways have been shown to contribute to these abnormalities. In this article, we briefly review recent in vivo and in vitro findings in the field in an attempt to highlight the areas of agreement and areas of controversy. In this review, we will summarize pathogenic mechanisms underlying cardiac and vascular abnormalities in obstructive cholestasis. It seems that cardiovascular dysfunction is likely because of bile acids as one of the predominant factors. Other important factors which might play roles in these abnormalities are increased nitric oxide, endogenous opioids and endocannabinoids. These three factors interact with each other to exert vasodilation and impaired cardiovascular responses to sympathetic stimulation.

Involvement of endogenous opioid peptides and nitric oxide in the blunted chronotropic and inotropic responses to beta-adrenergic stimulation in cirrhotic rats

It is well known that chronotropic and inotropic responses to beta-adrenergic stimulation are impaired in cirrhosis, but the exact reason is not clear. Considering the inhibitory effect of endogenous opioid peptides and nitric oxide (NO) on beta-adrenergic pathway, we examined their roles in hyporesponsiveness of isolated atria and papillary muscles to isoproterenol stimulation in cirrhotic rats. Cirrhosis was induced by chronic bile duct ligation. Four weeks after ligation or sham operation, the responses of the isolated atria and papillary muscles to isoproterenol stimulation were evaluated in the absence and presence of naltrexone HCl (10(-6) m), N(omega)-nitro-L-arginine methyl ester (L-NAME, 10(-4) m), and naltrexone plus L-NAME in the organ bath. Considering the role of inducible NOS (iNOS) in hemodynamic abnormalities of cirrhotic rats, the chronotropic and inotropic responses of cirrhotic rats to isoproterenol stimulation were also assessed in the presence of aminoguanidine (a selective inhibitor of iNOS, 3 x 10(-4) m). Sham operation had no significant effect on basal atrial beating rate, contractile force, and maximal time derivatives for the development and the dissipation of papillary muscle tension. The basal atrial beating rate of cirrhotic rats did not show any significant difference compared with the sham-operated ones; however, the basal contractile parameters were significantly decreased in cirrhosis. Although the maximum effects of isoproterenol on chronotropic and inotropic responses were significantly reduced in cirrhotic rats, there was no difference in half-maximal effective concentrations of isoproterenol in these concentration-response curves. The basal abnormalities and the attenuated chronotropic and inotropic responses to isoproterenol were completely corrected by the administration of naltrexone, L-NAME and aminoguanidine. Concurrent administration of naltrexone and L-NAME also restored to normal the basal abnormalities and the blunted responses to isoproterenol in cirrhotic rats, and did not show any antagonistic effect. Based on these findings, both the endogenous opioid peptides and NO may be involved in the attenuated chronotropic and inotropic responses to beta-adrenergic stimulation in cirrhosis. It seems that the iNOS activity results in NO-induced hyporesponsiveness to beta-adrenergic stimulation in cirrhosis.

Increased PKA activity and its influence on isoprenaline-stimulated L-type Ca2+ channels in the heart from ovariectomized rats

We previously showed that oestrogen confers cardioprotection by downregulating the cardiac beta1-adrenoceptor (beta1-AR). The present study examined the effect of oestrogen on the post beta1-AR signalling cascade, with particular emphasis on the activity of protein kinase A (PKA) and its influence on the L-type Ca2+ channel. Three groups of adult female Sprague-Dawley rats were used: sham-operated controls, bilaterally ovariectomized (Ovx) rats, and Ovx rats with oestrogen replacement (Ovx + E2), which restored the oestrogen concentration to normal. The electrically induced intracellular Ca2+ transient (E[Ca2+]i), 45Ca(2+)-uptake through cardiac L-type Ca2+ channels (Ca2+ channels), heart rate and force of contraction in response to beta-AR stimulation with 10 nM isoprenaline (Iso) in hearts from Ovx rats were significantly greater than those of control and Ovx + E2 rats. The basal and Iso-induced PKA activities were also higher in hearts from Ovx rats. KT5720, a selective PKA inhibitor, completely inhibited its potentiating effect on basal Ca2+ channel activity in the Ovx rat heart. On the other hand, expression of G proteins (G(alpha)s and G(alpha)i1-3)), basal and forskolin-stimulated cAMP accumulation, and responsiveness of PKA to cAMP, were not altered by Ovx. Interestingly, the PKA inhibitor at the same concentration significantly reduced the increases in PKA activity and Ca2+ channel activity upon beta-AR stimulation in all three groups of rats and the inhibitions were significantly greater in the Ovx rat than in the other two groups of rats. This study provides the first evidence that, in addition to downregulation of beta1-AR shown previously, suppression of PKA activity, which is partly responsible for the suppressed Ca2+ channel activity, also determines the E[Ca2+]i and cardiac contractility following beta-AR stimulation in the female rat.

Estrogen reduces cardiac injury and expression of beta1-adrenoceptor upon ischemic insult in the rat heart

To test the hypothesis that estrogen confers cardioprotection by suppressing the expression of beta-adrenoceptor (beta-AR), we first correlated the infarct size in response to ischemic insult and beta-AR stimulation with the expression of beta(1)-AR in sham, ovariectomized (Ovx) and estrogen replaced (Ovx + E(2)) rats. When beta-AR is being activated during ischemia, the infarct size was significantly greater in Ovx than in the sham and Ovx + E(2) rats. There is a negative correlation between the infarct size and the expression level of beta(1)-AR as revealed by Western blotting and supported by binding analysis. Incubation of ventricular myocytes from Ovx rats with estrogen at 10(-9) M for 24 and 48 h, but not 12 h, significantly reduced lactate dehydrogenase release when the myocytes are subjected to simulated ischemia. The cardioprotective effect of 24 h estrogen incubation was accompanied by a reduction in the protein expression level of beta(1)-AR, which is estrogen receptor-dependent, whereas the lack of protection of 12-h estrogen incubation was not accompanied by any alterations in the expression level of beta(1)-AR. Together, the result from present study suggested that it is most likely that the cardioprotective effect of long-term estrogen replacement is due to suppressing the enhanced expression of cardiac beta(1)-AR in the Ovx rats, which in turn reduces cardiac injury when beta-AR is activated by sympathetic hyperactivity during ischemia. Therefore, suppression of the enhanced expression of cardiac beta(1)-AR in Ovx rats represents a novel cardioprotective mechanism of estrogen replacement therapy.

G-protein coupled receptor oligomerization in neuroendocrine pathways

Protein-protein interactions are fundamental processes for many biological systems including those involving the superfamily of G-protein coupled receptors (GPCRs). A growing body of biochemical and functional evidence supports the existence of GPCR-GPCR homo- and hetero-oligomers. In particular, hetero-oligomers can display pharmacological and functional properties distinct from those of the homodimer or oligomer thus adding another level of complexity to how GPCRs are activated, signal and traffick in the cell. Dimerization may also play a role in influencing the activity of agonists and antagonists. We are only beginning to unravel how and why such complexes are formed, the functional implications of which will have an enormous impact on GPCR biology. Future research that studies GPCRs as dimeric or oligomeric complexes will enhance not only our understanding of GPCRs in cellular function but will also be critical for novel drug design and improved treatment of the vast array of GPCR-related conditions.

Receptor crosstalk. Implications for cardiovascular function, disease and therapy

There are at least three well-defined signalling cascades engaged directly in the physiological regulation of cardiac circulatory function: the beta1-adrenoceptors that control the cardiac contractile apparatus, the renin-angiotensin-aldosterone system involved in regulating blood pressure and the natriuretic peptides contributing at least to the factors determining circulating volume. Apart from these pathways, other cardiac receptor systems, particularly the alpha1-adrenoceptors, adenosine, endothelin and opioid receptors, whose physiological role may not be immediately evident, are also important with respect to regulating cardiovascular function especially in disease. These and the majority of other cardiovascular receptors identified to date belong to the guanine nucleotide binding (G) protein-coupled receptor families that mediate signalling by coupling primarily to three G proteins, the stimulatory (Gs), inhibitory (Gi) and Gq/11 proteins to stimulate the adenylate cyclases and phospholipases, activating a small but diverse subset of effectors and ion channels. These receptor pathways are engaged in crosstalk utilizing second messengers and protein kinases as checkpoints and hubs for diverting, converging, sieving and directing the G protein-mediated messages resulting in different signalling products. Besides, the heart itself is endowed with the means to harmonize these signalling mechanisms and to fend off potentially fatal consequences of functional loss of the essential signalling pathways via compensatory reserve pathways, or by inducing some adaptive mechanisms to be turned on, if and when required. This receptor crosstalk constitutes the underlying basis for sustaining a coherently functional circulatory entity comprising mechanisms controlling the contractile apparatus, blood pressure and circulating volume, both in normal physiology and in disease.

Modulation of sympathetic actions on the heart by opioid receptor stimulation

The sympathetic nervous system, the most important extrinsic regulatory mechanism of the heart, is inhibited postsynaptically and presynaptically by opioid peptides produced in the heart via their respective receptors. The cardiac actions of beta-adrenergic receptor (beta-AR) stimulation are attenuated by activation of the opioid receptor (OR) with OR agonist at ineffective concentrations, implying cross-talk between the OR and beta-AR. This cross-talk results from inhibition of the Gs protein and adenylyl cyclase of the beta-AR pathway by the pertussis toxin-sensitive G protein of the opioid pathway. Alterations in cross-talk between these two receptors occur in pathological situations to meet bodily needs. In myocardial ischemia, when the sympathetic activity is increased, the inhibition of beta-AR stimulation by kappa-opioid stimulation is also enhanced, thus reducing the workload, oxygen consumption and cardiac injury. Whereas cardiac responsiveness to sympathetic discharges is also reduced after chronic hypoxia, the cross-talk between kappa-OR and beta-AR is reduced to prevent undue suppression of the sympathetic influence on the heart. On the other hand, impairment of the cross-talk may result in abnormality. A lack or a significant reduction in the inhibition of beta-AR stimulation by kappa-OR stimulation may lead to an excessive increase in cardiac activities, which contribute to the maintenance of high arterial blood pressure in spontaneously hypertensive rats. Other than opioid peptides, female sex hormone and adenosine also inhibit the sympathetic actions on the heart. In addition, sympathetic action is also inhibited presynaptically by kappa-opioid peptides via their receptor.

Cardiac opioids

Opioid peptides have long been considered as neuropeptides or neurotransmitters. The more recent discovery of these same peptides in non-neuronal tissue suggests that the peptides may have autocrine, paracrine, or endocrine functions as well. The opioid peptides, enkephalins, dynorphins, and endorphins, have been found in isolated cardiac myocytes and heart tissue. This review will cover the recent literature on opioid peptides in respect to cardiac distribution, biochemistry, and function.