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(Circulation Research. 2001;89:e31.)
© 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

Sarah Calaghan, Ed White, Jean-Yves Le Guennec

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 contractility^1–8 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 contraction^1,7,9 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 I_Ca,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 I_Ca,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

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