Supersensitivity to isoprenaline induced in cultured neonatal rat heart cells by certain eicosatetraenoic acids

Among a number of racemic eicosatetraenoic acids (HETEs) investigated, 15-R/S HETE and 11-R/S-HETE were the only ones that enormously potentiated the chronotropic action of isoprenaline on neonatal rat heart myocytes in rocked culture. The effect of the 15-R/S-HETE was exclusively due to the S isomer.

Functional analysis of rabbit anti-peptide antibodies which mimic autoantibodies against the beta 1-adrenergic receptor in patients with idiopathic dilated cardiomyopathy

A synthetic peptide corresponding to the second extracellular loop of the beta 1-adrenergic receptor was used as an antigen for antibody production in three rabbits. Antibodies of high titers were obtained in all rabbits. Only one rabbit yielded antibodies which decreased radioligand binding on the receptor in a similar way to that described for autoantibodies in patients with dilated cardiomyopathy. These antibodies recognized the receptor protein in immunoblots. Epitope mapping indicated that the N-terminal sequence of the loop used as antigen was the target of the major antigen fraction. Incubation of antibodies with C6 glioma cell membranes or inner membranes of E. coli, which express the human beta 1-adrenergic receptor, resulted in a decrease in number of radioligand binding sites. This decrease was dependent on the concentration of antibody and of Mg++ ions. It was not affected by the GTP analog GppNHp or the beta 1 subtype-specific antagonist metoprolol. The agonist, isoproterenol, also induced a decrease but the effects of antibody and agonist were not additive. These results suggest that the antibodies induce a Mg(++)-dependent, 'active', labile conformation of the receptor, independent from coupling to the GTP regulatory protein, but similar to that induced by the agonist isoproterenol. This interpretation was corroborated by the beta 1-adrenergic receptor agonist-like effect of the antibodies on cardiomyocytes in culture.

Pluripotent mouse embryonic stem cells are able to differentiate into cardiomyocytes expressing chronotropic responses to adrenergic and cholinergic agents and Ca2+ channel blockers

A defined cultivation system was developed for the differentiation of pluripotent embryonic stem cells of the mouse into spontaneously beating cardiomyocytes, allowing investigations of chronotropic responses, as well as electrophysiological studies of different cardioactive drugs in vitro. The beta-adrenoceptor agonists (-)isoprenaline and clenbuterol, the mediators of cAMP metabolism, forskolin and isobutylmethylxanthine (IBMX), the alpha 1-adrenoceptor agonist (-)phenylephrine, and the heart glycoside digitoxin induced a positive, the muscarinic cholinoceptor agonist carbachol and L-type Ca2+ channel blockers nisoldipine, gallopamil and diltiazem induced a negative chronotropic response. In early differentiated cardiomyocytes beta 1-, alpha 1-, but not beta 2-adrenoceptors, cholinoceptors, as well as L-type Ca2+ channels participated in the chronotropic response. In terminally differentiated cardiomyocytes beta 2-adrenoceptors and digitoxin responses were also functionally expressed. The contractions of spontaneously beating cardiomyocytes were concomitant with rhythmic action potentials very similar to those described for embryonic cardiomyocytes and sinus-node cells. We conclude that cardiomyocytes differentiating from pluripotent embryonic stem cells are able to develop adrenoceptors and cholinoceptors and signal transduction pathways as well as L-type Ca2+ channels as a consequence of cell-cell interactions during embryoid body formation in vitro, independent of the development in living organisms. The cellular system described may be useful as in vitro assay for toxicological investigations of chronotropic drugs and a model system for studying commitment and cellular differentiation in vitro.

Autoantibodies to beta 2-adrenergic receptors with antiadrenergic activity from patients with allergic asthma

The serum gamma-globulin fraction from patients with allergic bronchial asthma (8/8), in contrast to the fraction from nonatopic control subjects (10/10), was found to inhibit the positive chronotropic action of the beta 2-selective adrenergic agonist, clenbuterol, on pyruvate- or lactate-treated cultured neonatal rat heart myocytes. No inhibition was exerted on the positive chronotropic response to prenalterol, which acted via beta 1-adrenergic receptors. The inhibitory effect of the asthmatic gamma-globulins was concentration dependent. It could nearly be abolished by immunoprecipitation with antihuman gamma-globulin and antihuman IgG, but not with antihuman IgM. This finding means that the inhibitory immunoglobulins of the patients with asthma were chiefly autoantibodies of the IgG isotype, capable of cross-reacting with chronotropic beta 2-adrenergic receptors on the cultured rat cardiomyocytes. These findings provide support for the notion that autoimmunity plays a role in the impairment of beta-adrenergic responsiveness in patients with allergic bronchial asthma.

Autoantibodies against the beta-adrenergic receptor in human myocarditis and dilated cardiomyopathy: beta-adrenergic agonism without desensitization

The serum of patients with myocarditis and dilated cardiomyopathy contains immunoglobulins capable of causing in vitro acceleration of beating of neonatal rat heart myocytes. This effect is stereoselectively inhibited by (-)-propranolol and is inhibited also by the beta 1-selective adrenergic antagonists, bisoprolol and metoprolol. This effect is not reversed by washing and continues unabated for at least 24 h.

Modulation of the beta-adrenergic response in cultured rat heart cells. I. Beta-adrenergic supersensitivity is induced by lactate via a phospholipase A2 and 15-lipoxygenase involving pathway

Incubation of rocker-cultured neonatal rat heart cells with 3 mM L(+)-lactate led to a sharp increase in the sensitivity of cardiomyocytes to the beta-adrenergic agonist isoprenaline, as measured by their chronotropic response. This effect was accompanied by a reduction in the arachidonic acid content of the total phospholipids. The phospholipase A2-activator melittin as well as free arachidonic acid induced this supersensitivity to the same degree. On the other hand, the L(+)-lactate-evoked supersensitivity could be blocked by the phospholipase A2 inhibitors mepacrine and n-bromophenacyl-bromide, suggesting an involvement of phospholipase A2 in the process of beta-adrenergic sensitization. The sensitizing action of arachidonic acid was blocked by the lipoxygenase inhibitors esculetin and nordihydroguaiaretic acid, but not by the cyclo-oxygenase inhibitor indomethacin. Supersensitivity was likewise evoked by 15-S-hydroxyeicosatetraenoic acid (15-S-HETE), but not by 5-S-HPETE or 5-S-HETE. These findings suggest that the phospholipase A2-15-lipoxygenase pathway plays a role in the induction of beta-adrenergic supersensitivity in the cultured cardiomyocytes and point to a new physiological role of the lipoxygenase product 15-S-HETE.

Modulation of the beta-adrenergic-response in cultured rat heart cells. II. Mammary-derived growth inhibitor (MDGI) blocks induction of beta-adrenergic supersensitivity. Dissociation from lipid-binding activity of MDGI

'Mammary-derived growth inhibitor (MDGI)' is a 14.5 kDa polypeptide with growth-inhibitory activity for various mammary epithelial cells in vitro which is highly homologous to cardiac fatty acid-binding protein (H-FABP). Here we describe a new biological activity of MDGI: Inhibition of L(+)-lactate-, arachidonic acid- and 15-S-hydroxyeicosatetraenoic acid-induced supersensitivity of neonatal rat heart cells for beta-adrenergic stimulation, concerning particularly a small population of beta 2-receptors. Synthetic peptides corresponding to the MDGI-sequence, residue 121-131 mimic the effect of MDGI. Measurements of lipid-binding to MDGI and synthetic peptides excluded the binding of arachidonic acid, 15-S-hydroxyeicosatetraenoic acid or beta-adrenergic agonists to MDGI or the peptides as the mechanism for this effect. Also, no direct interference of MDGI and the synthetic peptides with the binding of the beta-adrenergic agent CGP 12177 to its receptor on A431 cells could be detected. We suggest that MDGI and the peptides act by interference with the function of the beta 2-adrenergic receptor and that this mechanism might also be relevant for the growth-inhibitory activity of MDGI. Furthermore, the data point to a possible function of H-FABP for the modulation of beta-adrenergic sensitivity of cardiac myocytes.

Spin trapping evidence for radical generation by isolated hearts and cultured heart cells

The spin trap 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) has been applied to monitor the generation of free oxy-radicals in samples derived from isolated hearts and heart cells. .OH was trapped in the effluent of isolated hearts in the early phase of reperfusion following an ischemia time of only 10 min. Radical detection was possible even when the cardioactive DMPO was added to the effluent after draining off the heart, demonstrating that the short-lived .OH was generated by components released from the affected heart. These results support the hypothesis that radicals are of relevance for reperfusion injury. By omitting antioxidants commonly used for incubation media of cultured cells, it was possible for the first time to demonstrate the formation of .OH in the incubation solution of cardiac cells.

Adrenergic and antiadrenergic activity of cycloheximide in cultured rat heart cells

Cycloheximide at concentrations between 10(-5) to 10(-3) M increased the rate of beating of rocker-cultured neonatal rat myocardial cells through activation of alpha 1-adrenoceptors. At concentrations between 3 X 10(-10) and 10(-6) M, at which [14C]-leucine incorporation into the cultured cells was not or not greatly affected, the antibiotic inhibited surmountably the positive chronotropic action of the rather nonselective beta-adrenergic agonist isoprenaline. It also antagonized the cyclic AMP response to isoprenaline. A powerful positive chronotropic action of the beta 2-selective agonist clenbuterol was not opposed by cycloheximide. Neither did cycloheximide (10(-8) and 10(-6) M) influence the acceleration of beating by phenylephrine, dibutyryl cyclic AMP, and elevation of the extracellular calcium concentration. Displacement of the nonselective beta-adrenoceptor ligand [125I]iodopindolol from its specific binding sites on the cultured heart cells by cycloheximide was diphasic, with 15 to 35 per cent of the displacement taking place below 10(-8) M cycloheximide (IC50 = = 2 X 10(-9) M) and the remaining 65 to 85 per cent above 10(-5) M. The action of low concentrations of cycloheximide on beta-adrenoceptors remains to be delineated.