Basis for increased microtubules in pressure-hypertrophied cardiocytes

We have shown the levels of the sarcomere and the cardiocyte that a persistent increase in microtubule density accounts to a remarkable degree for the contractile dysfunction seen in pressure-overload right ventricular hypertrophy. In the present study, we have asked whether these linked phenotypic and contractile abnormalities are an immediate and direct effect of load input into the cardiocyte or instead a concomitant of hypertrophic growth in response to pressure overloading.

The feline right ventricle was pressure-overloaded by pulmonary artery banding. The quantity of microtubules was estimated from immunoblots and immunofluorescent micrographs, and their mechanical effects were assessed by measuring sarcomere motion during microtubule depolymerization. The biogenesis of microtubules was estimated from Northern and Western blot analyses of tubulin mRNAs and proteins. These measurements were made in control cats and in operated cats during and after the completion of right ventricular hypertrophy; the left ventricle from each heart served as a normally loaded same-animal control. We have shown that the alterations in microtubule density and sarcomere mechanics are not an immediate consequence of pressure overloading but instead appear in parallel with the load-induced increase in cardiac mass. Of potential mechanistic importance, both these changes and increases in tubulin poly A+ mRNA and protein coexist indefinitely after a new, higher steady state of right ventricular mass is reached.

Because we find persistent increases both in microtubules and in their biosynthetic precursors in pressure-hypertrophied myocardium, the mechanisms for this cytoskeletal abnormality must be sought through studies of the control both of microtubule stability and of tubulin synthesis.