Contractile activity and passive stretch regulate tubulin mRNA and protein content in cardiac myocytes

Accumulation of tubulin protein and an increased array of microtubules have been associated with contractile dysfunction in cardiac myocytes after pressure overload in vivo. Experiments were performed to assess the ability of mechanical stimuli experienced by ventricular cardiac myocytes during the progression of hypertrophic and dilated pathology to increase beta-tubulin production in cultured neonatal rat cardiac myocytes. Results indicate that both contractile activity and load due to passive stretch increase beta-tubulin protein content in neonatal rat cardiac myocytes through accumulation of beta-tubulin mRNA, which occurs without increased beta-tubulin gene transcription. Western blot analysis demonstrated that contraction resulted in the accumulation of beta-tubulin in neonatal rat cardiac myocytes above increases observed in the content of total cellular protein. Northern blot analysis indicated that beta-tubulin mRNA content increased in response to both stretch and contraction. alpha-Adrenergic agonists that lead to pathophysiological growth in cardiac myocytes also stimulated an increase in beta-tubulin mRNA content. Treatment of contracting neonatal cardiac myocytes with angiotensin II (ANG II) further increased beta-tubulin mRNA content, whereas ANG II treatment in arrested neonatal cardiac myocytes failed to increase beta-tubulin mRNA. Nuclear run-on experiments indicate that contraction stimulates beta-tubulin mRNA accumulation without an increase in beta-tubulin gene transcription. These results imply that tubulin production in cultured cardiac myocytes can be regulated directly by mechanical forces. In mechanically challenged hearts, the accumulation of beta-tubulin and the development of contractile dysfunction may be directly related to the mechanical forces imposed on the myocardium during the onset and progression of cardiovascular disease.