Modulation of cardiac PIP₂ by cardioactive hormones and other physiologically relevant interventions

Phosphatidylinositol 4,5-bisphosphate (PIP₂) affects profoundly several cardiac ion channels and transporters, and studies of PIP₂-sensitive currents in excised patches suggest that PIP₂ can be synthesized and broken down within 30 s. To test when, and if, total phosphatidylinositol 4-phosphate (PIP) and PIP₂ levels actually change in intact heart, we used a new, nonradioactive HPLC method to quantify anionic phospholipids. Total PIP and PIP₂ levels (10-30 μmol/kg wet weight) do not change, or even increase, with activation of Gα_q/phospholipase C (PLC)-dependent pathways by carbachol (50 μM), phenylephrine (50 μM), and endothelin-1 (0.3 μM). Adenosine (0.2 mM) and phorbol 12-myristate 13-acetate (1μM) both cause 30% reduction of PIP₂ in ventricles, suggesting that diacylglycerol (DAG)-dependent mechanisms negatively regulate cardiac PIP₂. PIP₂, but not PIP, increases reversibly by 30% during electrical stimulation (2 Hz for 5 min) in guinea pig left atria; the increase is blocked by nickel (2 mM). Both PIP and PIP₂ increase within 3 min in hypertonic solutions, roughly in proportion to osmolarity, and similar effects occur in multiple cell lines. Inhibitors of several volume-sensitive signaling mechanisms do not affect these responses, suggesting that PIP₂ metabolism might be sensitive to membrane tension, per se.