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Expression of calcium channel subunits in the normal and diseased human myocardium
Insights
Alpha1 and beta2 subunit expression of L-type Ca2+ channels is upregulated in hypertrophic obstructive cardiomyopathy (HOCM) human hearts. This suggests a coordinated upregulation of these key cardiac channel components in HOCM.
Area of Science:
- Cardiovascular Physiology
- Molecular Cardiology
- Ion Channel Biology
Background:
- L-type Ca2+ channels are crucial for cardiac function.
- Alterations in these channels are implicated in heart diseases like hypertrophic obstructive cardiomyopathy (HOCM).
Purpose of the Study:
- To investigate the protein expression levels of alpha1 and beta subunits of L-type Ca2+ channels in normal human hearts versus hypertrophied hearts from HOCM patients.
- To determine the specific beta subunit isoforms (beta2 and beta3) present and their association with the L-type Ca2+ channel complex.
Main Methods:
- Utilized 1,4-dihydropyridine (DHP) binding assays to quantify channel density.
- Employed polyclonal antibodies against beta2 and beta3 subunits for immunoprecipitation and immunoblot analysis.
- Compared protein expression in cardiac ventricles from normal individuals and HOCM patients.
Main Results:
- Maximal DHP binding (Bmax) was significantly higher in HOCM hearts (35 +/- 2 fmol/mg protein) compared to normal hearts (20 +/- 2 fmol/mg protein).
- The beta2 subunit was detected and found to be associated with the DHP receptor/Ca2+ channel complex in human, rabbit, and pig cardiac muscle.
- The beta3 subunit isoform was not detected in purified cardiac Ca2+ channels from any species studied.
- Immunoprecipitation experiments showed a partial association of the beta2 subunit with DHP-labeled Ca2+ channels in human, rabbit, and pig hearts.
Conclusions:
- Both alpha1 and beta2 subunit expression appear to be coordinately upregulated in the hypertrophied septum of HOCM patients.
- The beta2 subunit is a significant component of the cardiac L-type Ca2+ channel complex in humans, while beta3 is not detected.
- These findings highlight potential molecular adaptations in L-type Ca2+ channel composition contributing to HOCM pathophysiology.
Abstract:
We investigated the expression of alpha1 and beta subunits of the L-type Ca2+ channel on the protein level in cardiac preparations from normal human heart ventricles and from the hypertrophied septum of patients with hypertrophic obstructive cardiomyopathy (HOCM). 1,4-Dihydropyridine (DHP) binding and immunorecognition by polyclonal antibodies directed against the C-terminal amino acid sequences of the beta2 and beta3 subunits were used for detection and quantification of alpha1, beta2, and beta3 subunits. Bmax of high-affinity DHP binding was 35 +/- 2 fmol/mg protein in HOCM and 20 +/- 2 fmol/mg protein in normal human hearts (P<0.05). In rabbit hearts the anti-beta2 subunit antibody immunoprecipitated 80% of the total amount of DHP-labeled Ca2+ channels present in the assay. Under identical experimental conditions 25% of labeled Ca2+ channels were recovered in the immunoprecipitates of both normal and HOCM ventricles. A similar partial immunoprecipitation was observed in pig hearts. Immunoblot analysis demonstrated that the beta2 subunit was associated with the DHP receptor/Ca2+ channel in cardiac muscle of rabbit, pig, and human heart. In neither of these purified cardiac Ca2+ channels was the beta3 subunit isoform detected. Our results suggest that both alpha1 and beta2 subunit expression is upregulated in HOCM in a coordinate manner.