Early impairment of calcium handling and altered expression of junctin in hearts of mice overexpressing the β ₁ ⁻ adrenergic receptor

Abstract

Chronic stimulation of cardiac beta1-adrenergic receptors contributes to disease progression and mortality in patients and animal models of heart failure. To search for the mechanism of adrenergic impairment of cardiac function in vivo, we studied transgenic mice with cardiac-specific overexpression of beta1-adrenergic receptors. Transgenic mice with cardiac overexpression of beta1-adrenergic receptors showed progressive left ventricular fibrosis starting at 4 months of age. Left ventricular catheterization revealed a modest enhancement of contractility and relaxation at 2 months of age, followed by progressive dysfunction in both parameters and ultimately cardiac failure. When the effects of endogenous catecholamines were blocked by the b-receptor antagonist propranolol, maximal rate of contractility (dp/dtmax) and maximal rate of relaxation (dp/dtmin) were significantly blunted in 2-month-old beta1-receptor transgenic mice. Isolated cardiomyocytes from these animals displayed markedly altered calcium transients with significant prolongation of the intracellular calcium transient compared with nontransgenic littermates. We determined the expression of sarcoplasmic reticulum proteins involved in calcium handling by RNase protection assay and by immunoblotting. Although the expression of calsequestrin, triadin, and phospholamban was not altered, we observed a progressive decrease in junctin abundance in beta1-receptor transgenic mice (Pbeta1-adrenergic receptors.

Neurohumoral activation in heart failure: the role of adrenergic receptors

“…In marked contrast to results obtained from β 2 -subtype that needs a high level of expression to develop of heart failure, mice overexpressing β 1 -subtype, as low as five fold the endogenous expression level, present progressive hypertrophy and ventricular dysfunction, which culminate with heart failure by the age of 35 weeks (Engelhardt et al 1999). The pathways mediating the cardiac deleterious effects of β 1 -subtype overexpression seems to involve an altered calcium handling (Engelhardt et al 2001(Engelhardt et al , 2004) and increased Na + -H + exchanger (Engelhardt et al 2002). In addition, overexpression of β 1 -subtype in mice leads to upregulation of pro-apoptotic proteins, such as Bax (Bisognano et al 2000), and chronic stimulation of β 1 -subtype has been shown to increase rate of apoptosis (Communal et al 1998, Xiao 2001, Zhu et al 2001.…”

Inhibition of Na + -H + Exchange Prevents Hypertrophy, Fibrosis, and Heart Failure in β 1 -Adrenergic Receptor Transgenic Mice

“…15 Briefly, a 446-nt fragment of murine NHE1 was amplified from murine heart cDNA by PCR (forward primer, 5Ј-CTTCCTGCTGCCACCCATCA-3Ј; reverse primer 5Ј-AGACCACGCCCACAAACACC-3Ј and subcloned into a Bluescript vector. Transcription of the radioactively labeled antisense probe was carried out using T7 polymerase (Ambion) and hybridization was allowed to occur overnight.…”

Junctin is a prominent regulator of contractility in cardiomyocytes

“…Although transgenic studies showed that junctin may be involved in the regulation of SR Ca cycling [8 and 9], adapted mechanisms or even developmental changes associated with chronic expression of high junctin levels may have contributed to the observed phenotype, masking the direct effects of junctin. Intriguingly, in the hearts of mice overexpressing beta-adrenergic receptor 1, junctin was shown to decrease as early as two weeks of age, and preceding the alteration of other Ca cycling protein levels [10]. Notably, the junctin level progressively declined with age and paralleled the development of cardiac hypertrophy and heart failure in this model [10,11, and 12].…”