Lysophosphoglycerides are amphiphilic phospholipids that accumulate in ischemic myocardium and elicit electrophysiological alterations in normoxic Purkinje fibers and ventricular muscle that are analogous to alterations characteristic of ischemic tissue in vivo and that are compatible with altered sarcolemmal permeability to divalent cations. To assess directly the potential influence of lysophosphoglycerides on calcium transport, we characterized changes in the accumulation of 45Ca2+ by cultured cardiac myocytes exposed to selected concentrations of lysophosphatidyl choline (LPC). Perfusion for 10 min with 80 microM LPC augmented the amount of 45Ca2+ in myocytes compared with that in control cells (5.1 +/- 0.7 vs. 2.8 +/- 0.26 nmols Ca2+/mg protein, respectively; P less than 0.005) but did not alter total cell calcium content measured by atomic absorption spectrometry (11.6 +/- 1.0 nmols/mg protein), suggesting equivalent augmentation of bidirectional Ca2+ flux by LPC. In contrast, perfusion for 15 min with 100 microM LPC not only augmented 45Ca2+ accumulation but also increased total cellular Ca2+ content, as the quantity of 45Ca2+ accumulated reached 16.9 +/- 1.4 nmols/mg protein, a value substantially exceeding the normal total Ca2+ content (P less than 0.0025 compared with control cells). In contrast to results observed after only a 5-min exposure to 100 microM LPC, Ca2+ accumulation induced by 15 min of perfusion was not precluded by verapamil (10(-8)M), could not be reversed by perfusion without LPC, and was associated with complete cessation of beating, markedly altered morphology, and substantial depletion of cellular creatine kinase activity. Thus LPC may not only contribute to malignant ventricular dysrhythmias but also may potentiate ischemic injury by facilitating calcium ingress.

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Fibronectin, a glycoprotein that binds to collagen and modifies the adhesion properties and motility of cells in culture, is present in the interstitium of rat hearts. To localize fibronectin more precisely and to assess its relationship to the myocyte and to connective tissue elements, we employed a double antibody technique to label myocardial fibronectin with electron-dense ferritin to permit an ultrastructural analysis. Fibronectin was found to be associated with collagen, and in some cases appeared to link collagen fibers. Fibronectin was also found inserted along the surfaces of cardiac myocytes, connecting these cells to perimyocytic collagen. These ultrastructural relationships imply that fibronectin is a major component of the myocardial interstitium, and may affect myocardial compliance and control the motion of myocytes during the contraction and relaxation of the heart.

Since lysophosphatides have been implicated as arrhythmogenic metabolites, modulation of their catabolism in cardiac myocytes has been characterized. Rat cardiac myocytes and mesenchymal cells grown in culture were found to contain cytosolic lysophospholipase with specific activities of 1.3 +/- 0.1 and 0.9 +/- 0.1 nmol X mg-1 X min-1, respectively. Rat myocytic lysophospholipase had a molecular mass of approximately 20,000 daltons, estimated by gel filtration chromatography. Kinetic analysis of cytosolic myocytic lysophospholipase demonstrated a Michaelis constant of 11 microM, a pH optimum of 8.0, and competitive inhibition by L-palmitoyl carnitine (inhibitory constant of 12 microM). Although lysophospholipase-transacylase activity could not be detected in rat myocyte or mesenchymal cell cultures, rabbit myocytes isolated by perfusion of isolated hearts with collagenase contained lysophospholipase-transacylase in cytosolic extracts with a specific activity of 0.2 nmol X mg-1 X min-1. These results demonstrate the presence of lysophospholipase in cardiac myocytes and suggest that the increase in long-chain acyl carnitine, which occurs during myocardial ischemia, may contribute to accumulation of lysophosphatides within cardiac myocytes.

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