Abstract 83: β1 Vs β2 Adrenergic Signaling Differentially Regulates Mitochondrial Dynamics Through Alterations In Calcineurin And Drp1

Signal transduction through β1 and β2-adrenergic receptors (ARs) is considered a primary mechanism for regulating cardiovascular function and remodeling. Upon β-AR activation (i.e., physical activity, cardiac pathology) inotropy and chonotropy increase and mitochondria must quickly meet increased energy demand. This suggests that βARs and mitochondria are coupled mechanistically to rapidly respond to the functional and energetic needs of the heart. To investigate the role of β1 vs. β2-AR signaling on mitochondrial dynamics, we compared β1-/- and β2-/- to WT controls. β2-/- had increased mitochondrial fragmentation (increased number and decreased size) by electron microscopy vs. both WT and β1-/-. β2-/- showed altered regulation of mitochondrial fission: increased Drp1 translocation to the mitochondria vs. WT, whereas β1-/- had lower Drp1 translocation. These data suggest differential regulation of fission by βAR signaling, β1 activating and β2 suppressing fission. Since Ca2+-dependent calcineurin is known to activate Drp1 and [Ca2+]i is differentially regulated by β-AR signaling, we examined calcineurin as the bridge between β-AR signaling and Drp1 activation. In β2-/-, both Ca2+ transients and calcineurin activity were increased, suggesting β1-AR/Ca2+/calcinurin-mediated fission. To quantify mitochondrial fragmentation and biogenesis, mitotimer-transfected C2C12 cells were treated with the non-specific β-AR agonist isoproterenol resulting in mitochondrial fragmentation that was inhibited by the β1-antagonist CGP 12177 but not by the ß2-antagonist ICI 118551. Taken together, our data indicate that β1 and β2-AR signaling differentially regulate mitochondrial dynamics in the heart through alterations in [Ca2+]i, leading to calcineurin-induced translocation of Drp1.

Folic Acid Supplementation Modifies β-Adrenoceptor–Mediated In Vitro Lipolysis of Obese/Diabetic (+db/+db) Mice

The effects of folic acid (5.7 and 71 μg/kg, 4 weeks) consumption on the β-adrenoceptors (β-ARs)–elicited lipolysis in vitro of the abdominal adipocytes of lean/control (+ m/+ db) and obese/diabetic (+ db/+ db) mice (female) were investigated. β-AR agonists (salbutamol, a β2-AR agonist; BRL 37344 and CGP 12177, β3-AR agonists; adrenaline, a β-AR agonist)–mediated lipolysis, β2-, and β3-ARs protein expression of the adipose tissues after folic acid consumption were evaluated. Our results demonstrate that a smaller magnitude of the basal (spontaneous) and the β-AR agonists–triggered lipolysis was observed in + db/+ db mice, and folic acid supplementation (71 μg/kg) resulted in an improvement of both the baseline and the β-ARs–mediated lipolysis. In controls, a lower β2-and β3-ARs protein expression of the adipose tissues was detected in + db/+ db mice, compared to + m/+ db mice. In both strains fed with folic acid (71 μg/kg), a reduction of β2-AR protein expression was observed compared to the respective controls. In + db/+ db mice, folic acid (5.7 and 71 μg/kg) consumption caused a dose-dependent increase of β3-AR protein expression compared to controls. We demonstrate that lipolysis elicited by β-AR (β2- and β3-ARs) agonists was blunted in + db/+ db mice. Folic acid consumption has significant modulatory effects on β-ARs protein expression and lipolysis.

Chronic prenatal hypoxia sensitizes β-adrenoceptors in the embryonic heart but causes postnatal desensitization

Prenatal hypoxia in mammals causes fetal growth restriction and catecholaminergic overstimulation that, in turn, alter signaling pathways associated with adrenergic receptors. β-Adrenoceptors (β-ARs) are essential for fetal cardiac development and regulation of cardiac contractility. We studied the effects of chronic prenatal hypoxia on cardiac β-AR signaling and the incidence of alterations in the juvenile β-AR system due to the embryonic treatment. We measured functional β-AR density (Bmax) through binding with [3H]CGP-12177 and the effect of agonists on β-AR-dependent contractility (pEC50) through concentration-response curves to epinephrine. Eggs from broiler chickens were incubated in normoxia (N, 21% O2) or chronic hypoxia (H, 14% O2). Cardiac tissue from embryos and juveniles was used (15 and 19 day of embryonic development and 14 and 35 days posthatching, E19, E15, P14, and P35, respectively). Relative cardiac enlargement was found in the hypoxic groups at E15, E19, and P14, but not P35. Bmax significantly decreased in E19H. Bmax more than doubled posthatching but decreased from P14 to P35. The sensitivity to epinephrine was lower in E19N compared with E15N, but hypoxia increased the sensitivity to agonist in both E15H and E19H. Despite maintained receptor density, the P35H juvenile displayed a decreased sensitivity to β-AR agonist, something that was not seen in P14H. The postnatal decrease in β-AR sensitivity as an effect of chronic prenatal hypoxia, without a concomitant change in β-AR density, leads us to conclude that the embryonic hypoxic challenge alters the future progression of β-AR signaling and may have important implications for cardiovascular function in the adult.

NO production and eNOS phosphorylation induced by epinephrine through the activation of β-adrenoceptors

Epinephrine plays a key role in the control of vasomotor tone; however, the participation of the NO/cGMP pathway in response to β-adrenoceptor activation remains controversial. To evaluate the involvement of the endothelium in the vascular response to epinephrine, we assessed NO production, endothelial NO synthase phosphorylation, and tissue accumulation of cGMP in the perfused arterial mesenteric bed of rat. Epinephrine elicited a concentration-dependent increase in NO (EC50 of 45.7 pM), which was coupled to cGMP tissue accumulation. Both NO and cGMP production were blocked by either endothelium removal (saponin) or NO synthase inhibition ( Nω-nitro-l-arginine). Blockade of β1- and β2-adrenoceptors with 1 μM propranolol or β3-adrenoceptor with 10 nM SR 59230A displaced rightward the concentration-NO production curve evoked by epinephrine. Selective stimulation of β1-, β2-, or β3-adrenoceptors also resulted in NO and cGMP production. Propranolol (1 μM) inhibited the rise in NO induced by isoproterenol or the β2-adrenoceptor agonists salbutamol, terbutaline, or fenoterol. Likewise, 10 nM SR 59230A reduced the effects of the β3-adrenoceptor agonists BRL 37344, CGP 12177, SR 595611A, or pindolol. The NO production induced by epinephrine and BRL 37344 was associated with the activation of the phosphatidylinositol 3-kinase/Akt pathway and phosphorylation of eNOS in serine 1177. In addition, in anaesthetized rats, bolus administration of isoproterenol, salbutamol, or BRL 37344 produced NO-dependent reductions in systolic blood pressure. These findings indicate that β1-, β2-, and β3-adrenoceptors are coupled to the NO/cGMP pathway, highlighting the role of the endothelium in the vasomotor action elicited by epinephrine and related β-adrenoceptor agonists.

Inhibition of β-adrenergic receptor trafficking in adult cardiocytes by MAP4 decoration of microtubules

Decreased β-adrenergic receptor (β-AR) number occurs both in animal models of cardiac hypertrophy and failure and in patients. β-AR recycling is an important mechanism for the β-AR resensitization that maintains a normal complement of cell surface β-ARs. We have shown that 1) in severe pressure overload cardiac hypertrophy, there is extensive microtubule-associated protein 4 (MAP4) decoration of a dense microtubule network; and 2) MAP4 microtubule decoration inhibits muscarinic acetylcholine receptor recycling in neuroblastoma cells. We asked here whether MAP4 microtubule decoration inhibits β-AR recycling in adult cardiocytes. [3H]CGP-12177 was used as a β-AR ligand, and feline cardiocytes were isolated and infected with adenovirus containing MAP4 (AdMAP4) or β-galactosidase (Adβ-gal) cDNA. MAP4 decorated the microtubules extensively only in AdMAP4 cardiocytes. β-AR agonist exposure reduced cell surface β-AR number comparably in AdMAP4 and Adβ-gal cardiocytes; however, after agonist withdrawal, the cell surface β-AR number recovered to 78.4 ± 2.9% of the pretreatment value in Adβ-gal cardiocytes but only to 56.8 ± 1.4% in AdMAP4 cardiocytes ( P < 0.01). This result was confirmed in cardiocytes isolated from transgenic mice having cardiac-restricted MAP4 overexpression. In functional terms of cAMP generation, β-AR agonist responsiveness of AdMAP4 cells was 47% less than that of Adβ-gal cells. We conclude that MAP4 microtubule decoration interferes with β-AR recycling and that this may be one mechanism for β-AR downregulation in heart failure.