Targeted α _1A -Adrenergic Receptor Overexpression Induces Enhanced Cardiac Contractility but not Hypertrophy

Abstract

Abstract-Activation of the ␣ 1A -adrenergic receptor (␣ 1A -AR)/Gq pathway has been implicated as a critical trigger for the development of cardiac hypertrophy. However, direct evidence from in vivo studies is still lacking. To address this issue, transgenic mice with cardiac-targeted overexpression of the ␣ 1A -AR (4-to 170-fold) were generated, using the rodent ␣-myosin heavy chain promoter. Heterozygous animals displayed marked enhancement of cardiac contractility, evident from increases in dP/dt max (80%, PϽ0.0001), dP/dt max /LVP inst (76%, PϽ0.001), dP/dt max :dP/dt min (104%, PϽ0.0001), and fractional shortening (33%, PϽ0.05). Moreover, changes in the dP/dt max -end-diastolic volume relationship provided load-independent evidence of a primary increase in contractility. Blood pressure and heart rate were largely unchanged, and there was a small increase in (Ϫ)norepinephrine-stimulated, but not basal, phospholipase C activity. Increased contractility was directly related to the level of receptor overexpression and could be completely reversed by acute ␣ 1A -but not ␤-AR blockade. Despite the robust changes in contractility, transgenic animals displayed no morphological, histological, or echocardiographic evidence of left ventricular hypertrophy. In addition, apart from an increase in atrial natriuretic factor mRNA, expression of other hypertrophy-associated genes was unchanged. To our knowledge, these data provide the first in vivo evidence for an inotropic action of the ␣ 1A -AR.

Genetic Profiling of α1-Adrenergic Receptor Subtypes by Oligonucleotide Microarrays: Coupling to Interleukin-6 Secretion but Differences in STAT3 Phosphorylation and gp-130

“…In addition, the ␣ 1B -AR transgenic mouse model also constitutively down-regulated gp-130 protein levels in the heart (Yun et al, 2003), again confirming the fibroblast results. Confirmatory for the ␣ 1A -AR-mediated increases in STAT3, Ser727 phosphorylation is also seen in the normal rat myocyte (P. J. Gonzalez-Cabrera, J. Yun, B. R. Rorabaugh, D. F. McCune, and D. M. Perez, in preparation), which is composed of both the ␣ 1A -and ␣ 1B -AR subtypes, but heart function is thought to be mainly driven by the ␣ 1A -AR (Lin et al, 2001). ␣ 1 -ARs as well as the Angiotension AT1 receptor have been previously linked to JAK/ STAT activation (McWhinney et al, 1997;Zhong et al, 2000).…”

“…This was also suggested in Yun et al (2003), in which many genes/proteins associated with hypertrophy were down-regulated in the heart by the ␣ 1B -AR in a transgenic mouse model. Although pharmacological studies have implicated the ␣ 1A -AR subtype in mediating the growth of rat myocytes (Rokosh et al, 1996), ␣ 1A -AR transgenic mice models have discarded this subtype as the sole mediator of hypertrophy (Lin et al, 2001;Rokosh and Simpson, 2002). Other studies have shown that overexpressed ␣ 1B -AR constitutively active mutants manifest a mild hypertrophic phenotype (Milano et al, 1994;Zuscik et al, 2001), whereas no evidence exists for the ␣ 1D -AR subtype in mediating any heart function (Tanoue et al, 2002).…”

“…This issue has been partly addressed by the use of cell lines transfected with individual ␣ 1 -AR subtypes, and transgenic animal models in which ␣ 1 -AR subtypes have been either knocked out (Cavalli et al, 1997;Rokosh and Simpson, 2002;Tanoue et al, 2002) or overexpressed (Milano et al, 1994;Zuscik et al, 2000;Lin et al, 2001).…”

Involvement of chymase-mediated angiotensin II generation in blood pressure regulation

“…Proximal aortic blood pressure and dP/ dt measurements were performed as described by us previously (34), in mice anesthetized intraperitoneally with 100 mg/kg ketamine and 20 mg/kg xylazine. The right femoral vein was cannulated for infusions (0.1 μl/g body weight, in <5 minutes) of captopril (2.5 mg/ kg), losartan (80 mg/kg), or a combination of losartan (80 mg/kg) and captopril (2.5 mg/kg).…”

Inositol Polyphosphate 1-Phosphatase Is a Novel Antihypertrophic Factor

“…Aqueous phases were treated with proteinase K (2.5 g/ml), passed through Dowex-50 columns, and lyophilized. [ 3 H]InsPs were separated using anion exchange HPLC and quantified using an on-line ␤-counter as described previously (32).…”