Surface structures of the heart cell and their role in contractile control

Studies which focus on calcium (Ca) at the cellular surface indicate: Ca in the vascular and interstitial spaces is in rapid equilibrium with that bound within the sarcolemmal-glycocalyx complex of the cell. At [Ca]o of 1.0-1.5 mM 80-85% of this Ca is bound to phospholipid of the sarcolemmal bilayer and amounts to 500-700 mumoles/kg wet wt. tissue. Phospholipids of net anionic charge (phosphatidylserine, phosphatidylinositol) or zwitterionic structure (phosphatidylethanolamine) are likely to play a predominant role in the binding. Augmentation of net anionic charge (formation of phosphatidate by phospholipase D, addition of dodecylsulfate) of the sarcolemma augments Ca binding and is positively inotropic. Conversely, insertion of a cationic amphiphile (Polymyxin B, dodecyltrimethylamine) displaces Ca and is negatively inotropic. Augmentation of anionic Ca binding sites induces a significant increase of Na-Ca exchange in sarcolemmal vesicles which may contribute to the associated positive inotropy found in the intact myocardium.