Natriuretic Peptide Gene Expression after β-Adrenergic Stimulation in Adult Mouse Cardiac Myocytes

Long-term primary cultures of human adult atrial cardiac myocytes: cell viability, structural properties and BNP secretion in vitro

BACKGROUND

Human adult cardiomyocytes (CM) have been used in short-term cultures for in vitro studies of the adult myocardium. However, little information is available regarding human adult CMs cultured for long term (>2 weeks).

METHODS

Human adult CMs were isolated from atrial specimens of 43 patients undergoing cardiopulmonary bypass surgery. Cell viability, cytoskeletal properties, intercellular junctional mediators and responsiveness to extracellular stimuli were monitored in CM cultures for 8 weeks.

RESULTS

Absolute numbers of CMs decreased through the first 2 weeks, with substantially lower rates of cell loss thereafter. Apoptosis predominated over necrosis as the principal mode of cell death, affecting 4.1+/-1.6% of freshly dissociated cells, that declined in culture (3.6+/-1.0% week 1, 1.3+/-0.5% week 2). CMs maintained rod-shaped morphology and cross-striated expression pattern of sarcomeric proteins desmin and beta-myosin heavy chain for the first 4 weeks. Levels of desmin remained stable on first 3 weeks, but declined thereafter. CMs expressed cardiac-specific adherence molecule N-cadherin throughout the culture duration, indicating conserved contractile potential. CMs remained functional early in culture, as indicated by BNP secretion, with maximal levels on 1st week that declined gradually by week 4. Cell responsiveness to metabolic stresses (serum deprivation) was detected, inducing an early (6 h) 1.8-fold increase in levels of BNP.

CONCLUSION

Long-term cultured human adult CMs maintain morphological integrity, adult-type cytoskeletal protein expression, cell-cell communication potential and functionality for 3-4 weeks in vitro.

Mechanisms of cardiac protection from ischemia/reperfusion injury: a role for caveolae and caveolin-1

Caveolae, small invaginations in the plasma membrane, contain caveolins (Cav) that scaffold signaling molecules including the tyrosine kinase Src. We tested the hypothesis that cardiac protection involves a caveolin-dependent mechanism. We used in vitro and in vivo models of ischemia-reperfusion injury, electron microscopy (EM), transgenic mice, and biochemical assays to address this hypothesis. We found that Cav-1 mRNA and protein were expressed in mouse adult cardiac myocytes (ACM). The volatile anesthetic, isoflurane, protected ACM from hypoxia-induced cell death and increased sarcolemmal caveolae. Hearts of wild-type (WT) mice showed rapid phosphorylation of Src and Cav-1 after isoflurane and ischemic preconditioning. The Src inhibitor PP2 reduced phosphorylation of Src (Y416) and Cav-1 in the heart and abolished isoflurane-induced cardiac protection in WT mice. Infarct size (percent area at risk) was reduced by isoflurane in WT (30.5+/-4 vs. 44.2+/-3, n=7, P<0.05) but not Cav-1(-/-) mice (46.6+/-5 vs. 41.7+/-3, n=7). Cav-1(-/-) mice exposed to isoflurane showed significant alterations in Src phosphorylation and recruitment of C-terminal Src kinase, a negative regulator of Src, when compared to WT mice. The results indicate that isoflurane modifies cardiac myocyte sarcolemmal membrane structure and composition and that activation of Src and phosphorylation of Cav-1 contribute to cardiac protection. Accordingly, therapies targeted to post-translational modification of Src and Cav-1 may provide a novel approach for such protection.

Cardiac endocrine function is an essential component of the homeostatic regulation network: physiological and clinical implications

The discovery of cardiac natriuretic hormones required a profound revision of the concept of heart function. The heart should no longer be considered only as a pump but rather as a multifunctional and interactive organ that is part of a complex network and active component of the integrated systems of the body. In this review, we first consider the cross-talk between endocrine and contractile function of the heart. Then, based on the existing literature, we propose the hypothesis that cardiac endocrine function is an essential component of the integrated systems of the body and thus plays a pivotal role in fluid, electrolyte, and hemodynamic homeostasis. We highlight those studies indicating how alterations in cardiac endocrine function can better explain the pathophysiology of cardiovascular diseases and, in particular of heart failure, in which several target organs develop a resistance to the biological action of cardiac natriuretic peptides. Finally, we emphasize the concept that a complete knowledge of the cardiac endocrine function and of its relation with other neurohormonal regulatory systems of the body is crucial to correctly interpret changes in circulating natriuretic hormones, especially the brain natriuretic peptide.

G-protein-coupled Receptor Signaling Components Localize in Both Sarcolemmal and Intracellular Caveolin-3-associated Microdomains in Adult Cardiac Myocytes

This study tests the hypothesis that G-protein-coupled receptor (GPCR) signaling components involved in the regulation of adenylyl cyclase (AC) localize with caveolin (Cav), a protein marker for caveolae, in both cell-surface and intracellular membrane regions. Using sucrose density fractionation of adult cardiac myocytes, we detected Cav-3 in both buoyant membrane fractions (BF) and heavy/non-buoyant fractions (HF); beta2-adrenergic receptors (AR) in BF; and AC5/6, beta1-AR, M4-muscarinic acetylcholine receptors (mAChR), mu-opioid receptors, and Galpha(s) in both BF and HF. In contrast, M2-mAChR, Galpha(i3), and Galpha(i2) were found only in HF. Immunofluorescence microscopy showed co-localization of Cav-3 with AC5/6, Galpha(s), beta2-AR, and mu-opioid receptors in both sarcolemmal and intracellular membranes, whereas M2-mAChR were detected only intracellularly. Immunofluorescence of adult heart revealed a distribution of Cav-3 identical to that in isolated adult cardiac myocytes. Upon immunoelectron microscopy, Cav-3 co-localized with AC5/6 and Galpha(s) in sarcolemmal and intracellular vesicles, the latter closely allied with T-tubules. Cav-3 immunoprecipitates possessed components that were necessary and sufficient for GPCR agonist-promoted stimulation and inhibition of cAMP formation. The distribution of GPCR, G-proteins, and AC with Cav-3 in both sarcolemmal and intracellular T-tubule-associated regions indicates the existence of multiple Cav-3-localized cellular microdomains for signaling by hormones and drugs in the heart.

Receptor binding occurrence and plasma levels of natriuretic peptides in response to sympathectomy

In the present investigation the relationship between the sympathetic nervous system and the endocardial levels of receptor binding sites for natriuretic peptides and the plasma content of atrial natriuretic peptide were analyzed in rats. In order to destruct the cardiac sympathetic nerve terminals, chemical sympathectomy with 6-hydroxydopamine was made in parallel with intravenous measurements of blood pressure and heart frequency. By use of immunohistochemical and enzyme-linked-immunosorbent techniques the expression of tyrosine hydroxylase-positive sympathetic nerve terminals and plasma levels of pro-atrial natriuretic peptide were determined, respectively. The occurrence of receptor binding sites for natriuretic peptides was examined by in vitro receptor autoradiography. In contrast to the marked occurrence of natriuretic peptide receptor binding sites seen in the ventricular endocardium of control rats, the sympathectomized rats exhibited a decreased number of binding sites for natriuretic peptides in the endocardium of both the right and left chambers. Interestingly, this was found in parallel with a significant decrease of systolic and diastolic blood pressure and increased plasma levels of pro-atrial natriuretic peptide in the treated group of rats. These findings, together with those in previous studies, give support to an idea that one part of the blood pressure-decreasing effects, seen in patients treated with beta-adrenergic blockade, might be through a reduction of the natriuretic clearance receptor C, then giving rise to increased levels of atrial natriuretic peptide.