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Using the whole-cell patch-clamp configuration in rat ventricular myocytes, we recently reported that microtubule disruption increases calcium current (I(Ca)) and [Ca(2+)](i) transient and accelerates their kinetics by adenylyl cyclase activation. In the present report, we further analyzed the effects of microtubule disruption by 1 micromol/L colchicine on Ca(2+) signaling in cardiac myocytes with intact sarcolemma. In quiescent intact cells, it is possible to investigate ryanodine receptor (RyR) activity by analyzing the characteristics of spontaneous Ca(2+) sparks. Colchicine treatment decreased Ca(2+) spark amplitude (F/F(0): 1.78+/-0.01, n=983, versus 1.64+/-0.01, n=1660, recorded in control versus colchicine-treated cells; P&lt;0.0001) without modifying the sarcoplasmic reticulum Ca(2+) load and enhanced their time to peak (in ms: 6.85+/-0.09, n=1185, versus 7.33+/-0.13, n=1647; P&lt;0.0001). Microtubule disruption also induced the appearance of Ca(2+) sparks in doublets. These alterations may reflect RyR phosphorylation. To further investigate Ca(2+) signaling in cardiac myocytes with intact sarcolemma, we analyzed [Ca(2+)](i) transient evoked by field stimulation. Cells were loaded with the fluorescence Ca(2+) indicator, Fluo-3 cell permeant, and stimulated at 1 HZ: [Ca(2+)](i) transient amplitude was greater and its decay was accelerated in colchicine-treated, field-stimulated myocytes. This effect is reversible. When colchicine-treated myocytes were placed in a colchicine-free solution for 30 minutes, tubulin was repolymerized into microtubules, as shown by immunofluorescence, and the increase in [Ca(2+)](i) transient was reversed. In summary, we demonstrate that microtubule disruption by colchicine reversibly modulates Ca(2+) signaling in cardiac cells with intact sarcolemma.

Microtubule Disruption by Colchicine Reversibly Enhances Calcium Signaling in Intact Rat Cardiac Myocytes