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Abstract-Microtubules have been shown to alter contraction in cardiac myocytes through changes in cellular stiffness.However, an effect on excitation-contraction coupling has not been examined. Here we analyze the effects of microtubule disruption by 1 mol/L colchicine on calcium currents (I Ca ) and [Ca 2ϩ ] i transients in rat ventricular myocytes. I Ca was studied using the whole-cell patch-clamp technique. Colchicine treatment increased I Ca density (peak values, Ϫ4.6Ϯ0.4 and Ϫ9.1Ϯ1.3 pA/pF in 11 control and 12 colchicine-treated myocytes, respectively; PϽ0.05). I Ca inactivation was well fitted by a biexponential function. The slow component of inactivation was unchanged, whereas the fast component was accelerated after colchicine treatment (at Ϫ10 mV, 11.8Ϯ1.0 versus 6.7Ϯ1.0 ms in control versus colchicine-treated cells; PϽ0.005). [Ca 2ϩ ] i transients were analyzed by fluo-3 epifluorescence simultaneously with I Ca . Peak [Ca 2ϩ ] i transients were significantly increased in cardiac myocytes treated with colchicine. The values of F/F 0 at 0 mV were 1.1Ϯ0.02 in 9 control cells and 1.4Ϯ0.1 in 11 colchicine-treated cells (PϽ0.05). ␤-Adrenergic stimulation with 1 mol/L isoproterenol increased both I Ca and [Ca 2ϩ ] i transient in control cells. However, no significant change was induced by isoproterenol on colchicine-treated cells. Colchicine and isoproterenol effects were similar and not additive. Inhibition of adenylyl cyclase by 200 mol/L 2Ј-deoxyadenosine 3Ј-monophosphate blunted the colchicine effect. We suggest that ␤-adrenergic stimulation and microtubule disruption share a common pathway to enhance The microtubule network is dynamic, composed by the self-association of ␣,␤-tubulin dimers. Thus, by polymerization and depolymerization, the cell can change the amount of microtubules at constant tubulin amount. The presence of microtubules in the cardiac myocytes is well known, but its role in physiology and pathology is thought to be purely mechanical. In this regard, it has been shown that in pressureoverload cardiac hypertrophy, there is an increase in the microtubule network, which would be responsible for the contractile dysfunction in hypertrophied cells. 1 In this elegant work, Tsutsui et al 1 studied right ventricular cardiac myocytes isolated from cats subjected to pulmonary artery constriction. Under these experimental conditions, hypertrophied cells presented an increased number of microtubules and contracted weakly. When treated with the depolymerizing agent colchicine, hypertrophied myocytes contracted normally. Tsutsui et al 1 concluded that the contractile defect of hypertrophied cells is due to an increase in stiffness and viscosity on the cell imposed by the increased microtubule network triggered by the pressure overload. 2 However, it is also possible that microtubule polymerization and depolymerization play other roles in addition to the mechanical one. In this regard, we have recently shown that heart failure after pressure-overload cardiac hypertrophy induces a dysfunction of...

Abstract-We recently showed that phosphoinositide-3-kinase-␥-deficient (PI3K␥ Ϫ/Ϫ ) mice have enhanced cardiac contractility attributable to cAMP-dependent increases in sarcoplasmic reticulum (SR) Ca 2ϩ content and release but not L-type Ca 2ϩ current (I Ca,L ), demonstrating PI3K␥ locally regulates cAMP levels in cardiomyocytes. Because phosphodiesterases (PDEs) can contribute to cAMP compartmentation, we examined whether the PDE activity was altered by PI3K␥ ablation. Selective inhibition of PDE3 or PDE4 in wild-type (WT) cardiomyocytes elevated Ca 2ϩ transients, SR Ca 2ϩ content, and phospholamban phosphorylation (PLN-PO 4 ) by similar amounts to levels observed in untreated PI3K␥