Impaired beta-adrenergic response and decreased L-type calcium current of hypertrophied left ventricular myocytes in postinfarction heart failure

Abstract

Infarct-induced heart failure is usually associated with cardiac hypertrophy and decreased ß-adrenergic responsiveness. However, conflicting results have been reported concerning the density of L-type calcium current (I Ca(L) ), and the mechanisms underlying the decreased ß-adrenergic inotropic response. We determined I Ca(L) density, cytoplasmic calcium ([Ca 2+ ] i ) transients, and the effects of ß-adrenergic stimulation (isoproterenol) in a model of postinfarction heart failure in rats. Left ventricular myocytes were obtained by enzymatic digestion 8-10 weeks after infarction. Electrophysiological recordings were obtained using the patch-clamp technique. [Ca 2+ ] i transients were investigated via fura-2 fluorescence. ß-Adrenergic receptor density was determined by [ 3 H]-dihydroalprenolol binding to left ventricle homogenates. Postinfarction myocytes showed a significant 25% reduction in mean I Ca(L) density (5.7 ± 0.28 vs 7.6 ± 0.32 pA/pF) and a 19% reduction in mean peak [Ca 2+ ] i transients (0.13 ± 0.007 vs 0.16 ± 0.009) compared to sham myocytes. The isoproterenol-stimulated increase in I Ca(L) was significantly smaller in postinfarction myocytes (E max : 63.6 ± 4.3 vs 123.3 ± 0.9% in sham myocytes), but EC 50 was not altered. The isoproterenol-stimulated peak amplitude of [Ca 2+ ] i transients was also blunted in postinfarction myocytes. Adenylate cyclase activation through forskolin produced similar I Ca(L) increases in both groups. ß-Adrenergic receptor density was significantly reduced in homogenates from infarcted hearts (B max : 93.89 ± 20.22 vs 271.5 ± 31.43 fmol/mg protein in sham myocytes), while Kd values were similar. We conclude that postinfarction myocytes from large infarcts display reduced I Ca(L) density and peak [Ca 2+ ] i transients. The response to ß-adrenergic stimulation was also reduced and was probably related to ß-adrenergic receptor down-regulation and not to changes in adenylate cyclase activity.

Abnormal calcium “Sparks” in cardiomyocytes of post-myocardial infarction heart

“…Our findings and Aggarwal's results showed that in the early stage of heart remodeling (5-10 weeks after infarction), I CaL was significantly decreased but in the later stage, according to Gómez's result [27] , I CaL returned to normal level. Although the underlying mechanism for this phenomenon is not clear, the result implies that the heart ionic remodeling is different between early and end stage.…”

“…So far, the findings regarding I CaL in post-MI hypertrophic cardiomyocytes were conflicting. Gómez et al found that in ischemic cardiomyocytes, I CaL increased 1 week after MI development and restored to normal level 3 weeks till 6 months after myocardial infarction [19,27] . Saraiva et al found that the current was significantly decreased in cardiomyocytes 8-10 weeks after ligation [28] .…”

Computational analysis of the regulation of Ca2+dynamics in rat ventricular myocytes

“…Instead, simple phenomenological models of β-adrenergic-mediated LCC and PLB phosphorylation which are able to approximate both the steadystate and dynamic isoprenaline dose responses of relevant functional data are applied. The steady-state dose response data used to parameterize the LCC phosphorylation component come from Saraiva et al [36] and are shown along with model fits in figure 2. The data are fit to a Hill equation of form…”

“…The black line in the diagram indicates a dependency that was required in order to fit the data of Dibb et al [7], whereas the gray lines indicate dependencies that are known to exist but found to be unnecessary to fit the data of Dibb et al [7] (solid lines are dependencies that were actually tested, and dotted lines indicate dependencies that were not considered in the model). Data (circles) are from Saraiva et al [36] and represent the percentage increase in LCC current from baseline following a dose of isoprenaline. It is assumed here that I LCC increase is linearly related to LCC phosphorylation fraction.…”

Cardiac Hypertrophy Diminished the Effects of Isoproterenol on Delayed Rectifier Potassium Current in Rat Heart

“…This dissociation was probably due to the differences of severity of hypertrophy. In the models of heart failure, the reaction of I Ca to isoproterenol is generally decreased [29,36]; thus its reaction might tend to be decreased when hypertrophy became severe and heart failure is prominent. Based on the fact that both I K and I Ca were enhanced by isoproterenol via the same β-stimulation cascade [22], it was difficult to explain the mechanism of the results in our study regarding why only I Ca was enhanced by isoproterenol when the phosphodiesterase activity increased in hypertrophic hearts.…”