© 2001 American Heart Association, Inc.
Letters to the Editor |
A Unifying Mechanism for the Role of Microtubules in the Regulation of [Ca2+]i and Contraction in the Cardiac Myocyte
School of Biomedical Sciences, University of Leeds, Leeds, UK, s.c.calaghan@leeds.ac.uk
Nutrition, Croissance, Cancer EA2103, University of Tours, Tours, France
To the Editor:
There is evidence to show that in the absence of prior microtubule proliferation, disruption of the microtubule cytoskeleton by colchicine does not significantly modulate cardiac contractility1–8 or the intracellular Ca2+ ([Ca2+]i) transient.8 These negative observations would not be interesting by themselves, unless it had also been shown that in those models of pressure-overload hypertrophy that result in proliferation of microtubules, colchicine normalizes depressed contractility, and that chemical proliferation of the microtubules by taxol in normal myocytes decreases both contraction1,7,9 and the [Ca2+]i transient.9
Recent debate in Circulation Research was sparked by the work of Gómez et al.10 Surprisingly, this group reported that colchicine increased the global [Ca2+]i transient and ICa,L in the normal cardiocyte.10,11 Unless colchicine decreases myofilament Ca2+ sensitivity to the exact degree required to offset changes in [Ca2+]i, colchicine would be expected to increase contractility in the normal myocyte. Gómez et al10 suggested that the increase in [Ca2+]i they saw was due to stimulation of adenylyl cyclase via increased intracellular free tubulin, although cAMP, the product of adenylyl cyclase stimulation, does not increase in response to colchicine in normal or hypertrophied cardiac muscle.8 In a study in which we measured contraction and [Ca2+]i, as well as ICa,L,12 we were unable to reproduce the results of Gómez et al10 in the intact cardiac cell.
In their response, Kerfant et al11 were critical of our study,12 but unfortunately did not take the opportunity to discuss the apparent discrepancies that exist between their work and data accumulated over 8 years from several laboratories.1–8,12,13 For example, they are the only group to show the potential for a significant positive inotropic effect of colchicine in intact adult myocytes in the absence of prior microtubule proliferation.10,11 Their data imply that in models of hypertrophy in which free tubulin levels are increased, contractility would likewise be increased, whereas routinely it is depressed.1,2,8 In addition, the microtubule disruptor vincristine, which binds released tubulin as paracrystals, also increases contraction in hypertrophied tissue despite the fact that on Western blot analysis free tubulin heterodimers are virtually absent from these cardiocytes (G. Cooper, MD, written communication, July 2001).
Kerfant et al11 misinterpret our dataset12 and the action of insulin and colchicine upon K+ channels. Colchicine must be added before insulin to prevent its effects on channel function.14 Furthermore, insulin is absent in many other studies that have shown no significant effect of colchicine on contractility in the absence of hypertrophy.1–7
Kerfant et al11 state that, as microtubule disruption increases contraction in cells in which the microtubules are proliferated, there must be an effect, albeit smaller, in normal cells. This argument is based on a false premise that all microtubules are the same. In most cells, microtubules exist as a large dynamic population and a small subset of drug- and cold-stable microtubules.15 It has been shown recently16 that it is the stable subset of microtubules that modulates the beating rate of neonatal cardiac cells. Proliferated microtubules differ in appearance to microtubules present in the normal cardiac cell.1,9 Taxol-proliferated microtubules are resistant to the actions of colchicine,9 and proliferated microtubules in hypertrophy differ in terms of tubulin isoform and the ratio of microtubule-associated protein to tubulin.13 Alteration in microtubule structure, which, in turn, is reflected in microtubule function, is one explanation for the general finding to date that disruption of proliferated, but not normal, microtubules can modulate cardiac contractility.
References
1.
Tsutsui H, Ishihara K, Cooper G. Cytoskeletal role in the contractile dysfunction of hypertrophied myocardium. Science. . 1993; 260: 682–687.
2.
Tsutsui H, Tagawa H, Kent RL, McCollam PL, Ishihara K, Nagatsu M, Cooper G. Role of microtubules in contractile dysfunction of hypertrophied cardiocytes. Circulation. . 1994; 90: 533–555.
3. Collins JF, Pawloski-Dahm C, Davis MG, Ball N, Dorn GW II, Walsh RA. The role of the cytoskeleton in left ventricular pressure overload hypertrophy and failure. J Mol Cell Cardiol. . 1996; 28: 1435–1443.[Medline] [Order article via Infotrieve]
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7. Yamamoto S, Tsutsui H, Takahashi M, Ishibashi Y, Tagawa H, Imanaka-Yoshida K, Saeki Y, Takeshita A. Role of microtubules in the viscoelastic properties of isolated cardiac muscle. J Mol Cell Cardiol. . 1998; 30: 1841–1853.[Medline] [Order article via Infotrieve]
8.
Zile MR, Koide M, Sato H, Ishiguro TY, Conrad CH, Buckley JM, Morgan JP, Cooper G IV. Role of microtubules in the contractile dysfunction of hypertrophied myocardium. J Am Coll Cardiol. . 1999; 33: 250–260.
9.
Howarth FC, Calaghan SC, Boyett MR, White E. Effect of the microtubule polymerising agent taxol on contraction, Ca2+ transient and L-type Ca2+ current in rat ventricular myocytes. J Physiol. . 1999; 516: 409–419.
10.
Gómez AM, Kerfant BG, Vassort G. Microtubule disruption modulates Ca2+ signaling in rat cardiac myocytes. Circ Res. . 2000; 86: 30–36.
11. Kerfant BG, Vassort G, Gómez AM. Microtubule disruption by colchicine reversibly enhances calcium signaling in intact rat cardiac myocytes. Circ Res. . 2001; 88: e59–e65.
12. Calaghan SC, Le Guennec J-Y, White E. Modulation of Ca2+ signaling by microtubule disruption in rat ventricular myocytes and its dependence on the ruptured patch-clamp configuration. Circ Res. . 2001; 88: e32–e37.
13. Cooper G. Cardiocyte cytoskeleton in hypertrophied myocardium. Heart Failure Rev. . 2000; 5: 187-201.[Medline] [Order article via Infotrieve]
14.
Shimoni Y, Ratner JB. Type 1 diabetes leads to cytoskeleton changes that are reflected in insulin action on rat cardiac K+ currents. Am J Physiol. . 2001; 281: E575–E585.
15.
Schulze E, Kirschner M. Dynamic and stable populations of microtubules in cells. J Cell Biol. . 1987; 104: 277–288.
16.
Webster DR, Patrick DL. Beating rate of isolated neonatal cardiomyocytes is regulated by the stable microtubule subset. Am J Physiol. . 2000; 278: H1653–H1661.
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