Catherine Ela

Expression of Opioid Receptors During Heart Ontogeny in Normotensive and Hypertensive Rats

BACKGROUND The opioidergic systems are involved in modulating nociceptive stimuli. In addition, the recent results suggest that endogenous and exogenous opioids could play a role in the modulation of blood pressure and cardiac functions. However, little is known regarding the expression and role of opioid-binding sites in the heart. The decreased sensitivity to noxious stimuli in hypertensive rats raises the possibility of different developmental pattern expression of opioid-binding sites in normotensive versus hypertensive rats. METHODS AND RESULTS Opioid receptor expression in hearts from hypertensive and normotensive rats was studied during heart development by binding assays. From P1 until P90, the development of the heart in the two rat strains was accompanied by a gradual increase in the density of kappa-opioid receptors. Hearts from hypertensive rats expressed significantly higher levels of kappa receptors compared with those of normotensive rats. At ages older than P7, mu-opioid receptors could not be detected in hearts of both strains, whereas delta-opioid-binding sites gradually increased until reaching adult levels. Seven-day-old cardiomyocyte cultures of both rat strains expressed similar densities of delta or kappa receptors to those observed in hearts from 7-day-old neonates. The mu-binding sites were not detected in cardiomyocytes cultures. Similar to the in vivo state, cultured myocytes from hypertensive rats had significantly higher levels of kappa-binding sites (1.5 fold) compared with those of normotensive rats. The kappa sites are pertussis toxin sensitive, and the state of coupling of the receptor to G protein is similar for the two rat strains. CONCLUSION The role of opioid-binding sites in the heart is not completely clear. Hypertensive rats are known to be less sensitive to noxious stimuli compared with normotensive rats. It is controversial whether the site if application of noxious stimuli plays an important role in the sensitivity to pain in hypertensive rats. We suggest that the opioidergic system could play a role in the modulation of blood pressure in addition to its known effect on nociception.

Inotropic action of σ receptor ligands in isolated cardiac myocytes from adult rats

High affinity binding sites for sigma receptor ligands were found in membranes of cardiac myocytes from adult rats. The sigma receptor ligand (+)-3-hydroxyphenyl-N-(1-propyl)piperidine ((+)-3-PPP) binds with a Kd of 17.9 +/- 4.0 nM and a Bmax of 275 +/- 32.1 fmol/mg protein. Competition experiments of (+)-pentazocine with [3H]1,3-di-O-tolylguanidine ([3H]DTG) binding yielded a Ki of 6.1 +/- 1.3 nM. The majority of the sites (> 80%) were of the sigma 1 subtype. Exposure of isolated cardiomyocytes from adult rats to (+)-3-PPP (10 nM-1.0 microM) caused a marked concentration-dependent increase in the amplitude of systolic cell contraction, reaching 149% of control level, with an apparent ED50 value of 4.5 nM. The increase in the contraction amplitude was markedly inhibited by pretreatment with verapamil or thapsigargin. An increase in the amplitude of [Ca2+]i transients, similar to that in the amplitude of cell contraction, was observed in indo-1-loaded cardiomyocytes exposed to 0.1 microM (+)-3-PPP. Exposure to 10 nM of haloperidol or (+)-pentazocine induced an increase in the amplitude of contraction, reaching 188% and 138% (respectively) of control level. A lower concentration of haloperidol or (+)-pentazocine (1 nM) did not induce an increase in the contraction amplitude but rather reduced the amplitude to 70-80% of control.

Highly selective σ receptor ligands elevate inositol 1,4,5-trisphosphate production in rat cardiac myocytes

Exposure of cardiac myocytes from adult rat ventricles to the highly selective, high affinity sigma receptor ligands BD-737 (0.1-100 nM) and BD-1047 (0.01-10 nM) caused potentiation of electrically-evoked amplitudes of contraction and Ca2q transients, while exposure to 100 nM BD-1047 caused attenuation of these amplitudes. In addition, BD-737 (1-100 nM) and BD-1047 (10-100 nM) caused an increase in the incidence of spontaneous twitches. These effects were inhibited when the incubation with BD-737 was done in the presence of the phospholipase C inhibitor, neomycin, or after pre-incubation with thapsigargin or caffeine which deplete the sarcoplasmic reticulum Ca2q stores. Inositol 1,4,5-trisphosphate (IP3) production in cardiac myocytes was determined by the IP3 binding protein assay. Both substances caused an increase in the intracellular concentration of IP3. BD-737 caused a rapid transient increase to 3.2-fold in 1 min and stabilization at 2.1-fold of control thereafter. BD-1047 caused a gradual increase reaching 4.4-fold after 5 min. The results suggest that the effects of these sigma receptor ligands on contractility and spontaneous contractions are mediated by activation of phospholipase C and elevation of intracellular IP3 level.

Apparent desensitization of a σ receptor sub-population in neonatal rat cardiac myocytes by pre-treatment with σ receptor ligands

Abstract σ Receptor ligands induce marked effects on contractility in cardiac myocytes from neonatal and adult rats (Ela et al., 1994, J. Pharmacol. Exp. Ther. 269, 1300–1309; Novakova et al., 1995, Eur. J. Pharmacol. 286, 19–30). Augmentation or attenuation of the contractile amplitude was observed under different experimental conditions. Preincubation of neonatal cardiomyocytes with a σ receptor ligand ((+)-(3-hydroxyphenyl)-N-(1-propyl)-piperidine ((+)-3PPP), (+)-pentazocine, or haloperidol) changed the response to re-application of the ligand after cell wash. The inhibitory effect was abolished, while the stimulatory effect became much more pronounced. We suggest that the effects of σ receptor ligands are mediated via two receptor subtypes, one stimulatory and the other inhibitory, and only the inhibitory subtype is subject to desensitization.