K.-I. Kariya

Cloning of the rat alpha 1C-adrenergic receptor from cardiac myocytes. alpha 1C, alpha 1B, and alpha 1D mRNAs are present in cardiac myocytes but not in cardiac fibroblasts.

alpha 1-Adrenergic receptor (AR) activation in cardiac muscle has several different physiological effects that might be mediated through different alpha 1-AR subtypes. Two alpha 1-AR subtypes have been cloned from the rat, the alpha 1B and the alpha 1D; both are present in adult rat heart. A third subtype, the alpha 1C, cloned from the cow and human, was reported to be absent in the rat. However, we recently found alpha 1C mRNA in adult rat heart by using a partial alpha 1C cDNA. Thus, all three cloned alpha 1-AR subtypes are present in the heart, but it is unknown whether each is expressed in cardiac myocytes or in cardiac fibroblasts. In the present study, the full-length rat alpha 1C-AR was cloned from cultured neonatal cardiac myocytes. alpha 1C mRNA transcripts of 3, 9.5, and 11 kb were present in adult rat heart by Northern blot analysis. alpha 1B-, alpha 1C-, and alpha 1D-subtype mRNAs were each present in isolated adult and neonatal cardiac myocytes by RNase protection assay. In addition, cultured neonatal cardiac myocytes expressed the three alpha 1-AR subtype mRNAs. In contrast, none of the alpha 1-AR mRNAs was detected in cultured neonatal cardiac fibroblasts. In addition, alpha 1-ARs were absent in fibroblasts by [3H]prazosin binding and norepinephrine-stimulated [3H]inositol phosphate production. The absence of alpha 1-ARs in cardiac fibroblasts differs from beta-adrenergic and angiotensin II receptors, which are present in both cardiac fibroblasts and cardiac myocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroid hormone improves function and Ca2+ handling in pressure overload hypertrophy. Association with increased sarcoplasmic reticulum Ca2+-ATPase and alpha-myosin heavy chain in rat hearts.

We asked whether thyroid hormone (T4) would improve heart function in left ventricular hypertrophy (LVH) induced by pressure overload (aortic banding). After banding for 10-22 wk, rats were treated with T4 or saline for 10-14 d. Isovolumic LV pressure and cytosolic [Ca2+] (indo-1) were assessed in perfused hearts. Sarcoplasmic reticulum Ca2+-ATPase (SERCA), phospholamban, and alpha- and beta-myosin heavy chain (MHC) proteins were assayed in homogenates of myocytes isolated from the same hearts. Of 14 banded hearts treated with saline, 8 had compensated LVH with normal function (LVHcomp), whereas 6 had abnormal contraction, relaxation, and calcium handling (LVHdecomp). In contrast, banded animals treated with T4 had no myocardial dysfunction; these hearts had increased contractility, and faster relaxation and cytosolic [Ca2+] decline compared with LVHcomp and LVHdecomp. Myocytes from banded hearts treated with T4 were hypertrophied but had increased concentrations of alpha-MHC and SERCA proteins, similar to physiological hypertrophy induced by exercise. Thus thyroid hormone improves LV function and calcium handling in pressure overload hypertrophy, and these beneficial effects are related to changes in myocyte gene expression. Induction of physiological hypertrophy by thyroid hormone-like signaling might be a therapeutic strategy for treating cardiac dysfunction in pathological hypertrophy and heart failure.

Sympathetic activity : modulator of myocardial hypertrophy

The mechanisms regulating myocardial hypertrophy are largely unknown. Furthermore, the hypertrophic phenotype can be associated with either normal or abnormal function. To study the molecular mechanisms involved in myocardial hypertrophy, we have established a cell culture system in which stimulation of the alpha 1-adrenergic receptor leads to the development of myocardial cell hypertrophy. In addition to producing a generalized twofold increase in both cell size, total protein, and total RNA, activation of the alpha 1-receptor produces specific alterations in gene expression that are reflected by changes at both the mRNA and protein levels. In particular, alpha 1 stimulation leads to an increase in the expression of the c-myc oncogene as well as a selective increase in skeletal alpha-actin and beta-myosin heavy-chain isogene expression, isoforms normally found only in fetal/neonatal hearts. Similar changes in gene expression are seen in pressure-load hypertrophy in vivo. Skeletal alpha-actin gene expression is induced preferentially to that of the cardiac actin isogene resulting from a specific preferential increase in gene transcription. Work with subtype-specific inhibitors indicates that it is a particular alpha 1-receptor subtype that is responsible for the development of hypertrophy in culture. The finding that alpha 1 stimulation leads to an increase in protein kinase C activity is suggestive of a potential second messenger involving the phosphorylation of a transcriptional factor or factors.

Sympathetic modulation of the cardiac myocyte phenotype: studies with a cell-culture model of myocardial hypertrophy

Myocardial hypertrophy is the common endpoint of many cardiovascular stimuli such as hypertension, myocardial infarction, valvular disease, and congestive failure. Catecholamines have long been implicated in the pathogenesis of myocardial hypertrophy, however, it is very difficult to sort out catecholamine mechanisms in vivo. We have developed a cell-culture model which excludes hemodynamic effects and allows the assignment of receptor specificity to catecholamine effects. Utilizing this system, we have shown that stimulation of the alpha 1 adrenergic receptor leads to the development of myocardial hypertrophy and results in the selective up-regulation of the fetal/neonatal mRNAs encoding skeletal alpha-actin and beta-MHC, a pattern similar to that seen with hypertrophy in-vivo. Utilizing a co-transfection assay, we have also obtained data that suggest that the beta-PKC isozyme is in a pathway regulating transcription of the beta-MHC isogene. Beta adrenergic stimulation of the cultured cardiac myocytes also results in a modest degree of hypertrophy, however, this effect may be dependent upon myocyte contractile activity and may involve, at least in part, the non-muscle cells present in the culture system.