Phospholipase D increases cell surface Ca²⁺ binding and positive inotropy in rat heart

An increase in the content of anionic phospholipid (AP) in the sarcolemma may lead to increased Ca²⁺ binding and a concomitant inotropic response. To test this hypothesis we treated cultured neonatal rat myocardial cells with phospholipase D (PLD), an enzyme that converts membrane phospholipids to phosphatidic acid. PLD treatment resulted in an increase in total exchangeable Ca²⁺ of 36%, or 1.56 ± 0.27 mmol Ca²⁺/kg dry wt, 79 ± 3% of which remained displaceable by La³⁺. The quantity of Ca²⁺ displaced by polymyxin B, a drug that displaces Ca²⁺ preferentially from anionic phospholipid sites, increased by 85% from a predicted 0.74 ± 0.07 to 1.37 ± 0.19 mmol Ca²⁺/kg dry wt. Simultaneously the contractility of neonatal rat ventricular tissue increased by 1.7- to 2.5-fold. Spontaneous electrical activity of treated cells was not significantly altered. Thus PLD treatment, which increases the quantity of AP in the membrane, resulted in an 85% increase in Ca²⁺ bound to AP and concomitantly resulted in a significant increase in contractility. Present results, in association with published results on the effect of PLD on Na⁺-Ca²⁺ exchange, indicate that Ca²⁺ bound to anionic sarcolemmal phospholipids plays a major role in the control of transsarcolemmal Ca²⁺ flux and force development.