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 (J. Biol. Chem. 259: 16-19, 1984), indicate that Ca2+ bound to anionic sarcolemmal phospholipids plays a major role in the control of transsarcolemmal Ca2+ flux and force development.