The phosphoinositide 3-kinase inhibitor LY294002 enhances cardiac myocyte contractility via a direct inhibition of I_k,slow currents

Objective: Phosphoinositide 3-kinase (PI3K) is a key component in regulating myocardial growth, survival and contractility. LY294002 and wortmannin are two PI3K inhibitors used widely to establish the role of PI3K. The goal of this study was to examine the effects of acute application of LY294002 and wortmannin on cardiac myocyte contractility and underlying machanisms. Methods: Patch-clamp, indo-1 epifluorescence and video-edge detection techniques were used to measure outward K⁺ currents, action potentials (AP), Ca²⁺ transients and shortening of myocytes isolated from mouse left ventricular free wall. Results: In field-stimulated myocytes, LY294002 (10 μmol/l) increased Ca²⁺ transient amplitude by 23%, and cell shortening amplitude by 60% in the absence or presence of wortmannin, while wortmannin alone had no effect. LY294002 (but not wortmannin) prolonged AP duration by specifically inhibiting slowly inactivating K⁺ currents (i.e., the 4-aminopyrydine-sensitive I_k,slow1 and the tetraethylammonium-sensitive I_k,slow2), leading to an increase in sarcoplasmic reticular Ca²⁺ levels. It appeared that the AP prolongation was responsible for elevated contractility since AP-clamp of myocytes with prolonged APs (recorded in LY294002-treated myocytes) induced a 29% increase in cell shortening compared with control APs, while LY294002 application did not increase contractility in voltage-clamp studies using either step or AP depolarizations. Conclusions: The putative PI3K inhibitor LY294002 increases Ca²⁺ release and myocyte contractility via direct inhibition of cardiac I_k,slow and AP prolongation, thus limiting the usefulness of this agent in the analyses of the role of PI3K in heart function.