Gαq inhibits cardiac L-type Ca2+ channels through phosphatidylinositol 3-kinase

Abstract

Cardiac myocyte contractility is initiated by Ca2+ entry through the voltage-dependent L-type Ca2+ channel (LTCC). To study the effect of Gαq on the cardiac LTCC, we utilized two transgenic mouse lines that selectively express inducible Gαq-estrogen receptor hormone-binding domain fusion proteins (GαqQ209L-hbER or GαqQ209L-AA-hbER) in cardiac myocytes. Both of these proteins inhibit phosphatidylinositol (PI) 3-kinase (PI3K) signaling, but GαqQ209L-AA-hbER cannot activate the canonical Gαq effector phospholipase Cβ (PLCβ). L-type Ca 2+ current (ICa,L) density measured by whole-cell patch clamping was reduced by more than 50% in myocytes from both Gαq animals as compared with wild-type cells, suggesting that inhibition of the LTCC by Gαq does not require PLCβ. To investigate the role of PI3K in this inhibitory effect, ICa,L was measured in the presence of various phosphoinositides infused through the patch pipette. Infusion of PI 3,4,5-trisphosphate (PI(3,4,5)P3) into wild-type myocytes did not affect ICa,L, but it fully restored ICa,L density in both Gαq transgenic myocytes to wild-type levels. By contrast, PI 4,5-bisphosphate (PI(4,5)P2) or PI 3,5-bisphosphate had no effect. Infusion with p110β/p85α or p110γ PI3K in the presence of PI(4,5)P2 also restored ICa,L density to wild-type levels. Last, infusion of either PTEN, a PI(3,4,5)P3 phosphatase, or the pleckstrin homology domain of Grp1, which sequesters PI(3,4,5)P3, reduced the peak ICa,L density in wild-type myocytes by ∼30%. Taken together, these results strongly suggest that the inhibitory effect of Gαq on the cardiac LTCC is mediated by inhibition of PI3K.