Modulation of Ca2+ signaling by microtubule disruption in rat ventricular myocytes and its dependence on the ruptured patch-clamp configuration

In the absence of hypertrophic proliferation of microtubules, microtubule d isruption by colchicine does not modulate contraction of adult cardiac myoc ytes. However, Gomez et al (Circ Res. 2000;86:30-36) recently reported that disruption of microtubules by colchicine in ruptured patch-clamped myocyte s increased I-Ca,I-L density and [Ca2+](i) transient amplitude and depresse d the response of these parameters to the beta -adrenoceptor agonist isopro terenol, These effects were ascribed to stimulation of adenylyl cyclase by increased intracellular free tubulin. In the present study, we show that in intact rat ventricular myocytes, 2 to 4 hours of exposure to 10 mu mol/L c olchicine had no effect on shortening or [Ca2+](i) transient amplitude or o n the amplitude of I-Ca,I-L in perforated patch-clamped cells, under basal conditions and after stimulation with 1 mu mol/L isoproterenol. However, in ruptured patch-clamped myocytes, basal I-Ca,I-L was 2-fold higher after tr eatment with colchicine compared with vehicle and, in contrast to vehicle-t reated cells, I-Ca,I-L did not increase in response to isoproterenol. Cell width decreased during ruptured patch-clamp experiments in colchicine-treat ed but not vehicle-treated myocytes. We conclude that in cells with intact sarcolemma, colchicine does not modulate Ca2+ signaling or the response to beta stimulation. However, the combination of microtubule disruption by col chicine and the ruptured patch configuration activates I-Ca,I-L and attenua tes the response to beta stimulation. We propose that these effects may be due to loss of free tubulin by intracellular dialysis or to increased sensi tivity to mechanical stimulation as a result of microtubule disruption. The se findings have important implications for cardiomyopathies associated wit h decreased free tubulin or a diminished microtubular network.