A study of microtubules in hypertrophied cardiomyocytes

Microtubules, components of the cytoskeleton, play an important role in the maintenance of cellular shape, in intracellular transport of organelles and membrane vesicles, and in signal transduction in the cell. Studies on the role of microtubules in hypertrophied myocardial cells have been carried out using an acute pressure overloading model in which stenosis was produced in the aorta. Clinically, however cardiac hypertrophy is most frequently caused by a gradual increase in blood pressure due to essential hypertension. We evaluated the role of microtubulas by observing their serial changes in cells from spontaneously hypertensive rats (SHR), which are used as a model of essential hypertension. Blood pressure, body weight, left ventricular weight, and the cell cross-sectional area were measured in SHR and Wistar-Kyoto (WKY) rats aged 4, 6, 10, and 16 weeks (5 rats each). Microtubules in cardiocytes were observed using confocal laser scanning microscopy by the immunofluorescence method against beta-tubulin. Microtubules in cardiocytes run primarily in a longitudinal direction, thinly through the intermyofibrillar spaces and densely around the nuclei. The numbers of microtubules were measured separately in the perinuclear region and the nonperinuclear region, and their total was calculated. The cross-sectional area of a whole cell and nucleus was measured at the level of the nucleus, and microtubule density was calculated by the number of nonperinuclear microtubules in the cytosolic area. Five myocardial cells were randomly selected in each rat, and the mean density (/micron2) was observed. For comparison, similar analysis was done in an acute pressure loading model. This study showed that: 1) In the acute pressure overload model, hypertrophy was immediately observed, and microtubules were increased in 16% of cardiocytes. In SHR, an increase in blood pressure and hypertrophy of hearts were observed at the age of 6 weeks and after. The density of microtubules at the age of 4, 6, 10, and 16 weeks was 0.69 +/- 0.03 (/micron2), 0.73 +/- 0.05 (/micron2), 0.64 +/- 0.02 (/micron2), respectively, in WKY rats, and 0.54 +/- 0.03 (/micron2), 0.54 +/- 0.02 (/micron2), and 0.51 +/- 0.02 (/micron2), respectively, in the SHR, showing a relatively stability in the latter. This suggested that a gradual increase of blood pressure could not be a stimulus that changes the density and distribution of microtubules. 2) The immunofluorescence method against beta-tubulin using confocal laser scanning microscopy seems to be good method for quantitative analysis of microtubules in cardiocytes.