Résumé
Little is known about the subcellular distribution and the dynamics of tubulins in adult cardiac myocytes although both are modified during cardiac hypertrophy and heart failure. Using confocal microscopy, we examined post-translational modifications of tubulin in fully differentiated ventricular myocytes isolated from adult rat hearts, as well as in immortalized and dividing HL-1 cardiomyocytes. Detyrosinated Glu--tubulin was the most abundant posttranslationally modified tubulin found in ventricular myocytes, while acetylated-and 2-tubulins were found in lower amounts or absent. In contrast, dividing HL-1 cardiomyocytes exhibited high levels of tyrosinated or acetylated -tubulins. A mild nocodazole treatment (0.1 µM, 1 hour) disrupted microtubules in HL-1 myocytes, but not in adult ventricular myocytes. A stronger treatment (10 µM, 2 hours) was required to disassemble tubulins in adult myocytes. Glu--tubulin containing microtubules were more resistant to nocodazole treatment in HL-1 cardiomyocytes than in ventricular myocytes. Endogenous activation of the cAMP pathway with the forskolin analog L858051 (20 µM) or the -adrenergic agonist isoprenaline (10 µM) disrupted the most labile microtubules in HL-1 cardiomyocytes. In contrast, isoprenaline (10 µM), cholera toxin (200 ng/ml, a G s-protein activator), L858051 (20 µM) or forskolin (10 µM) had no effect on the microtubule network in ventricular myocytes. In addition, intracellular Ca 2+ accumulation induced either by thapsigargin (2 µM) or caffeine (10 mM) did not modify microtubule stability in ventricular myocytes. Our data demonstrate the unique stability of the microtubule network in adult cardiac myocytes. We speculate that microtubule stability is required to support cellular integrity during cardiac contraction.