L ysophosphatidylcholine (LPC) accumulates in ischemic tissue, and exogenous LPC (20-100 microM) induces electrophysiologic alterations in vitro. However, to determine whether compartmentalization is critical, intracellular pressure microinjection of LPC was performed with simultaneous recording of the transmembrane action potential. Intracellular LPC in concentrations as high as 500 microM (n = 18), calculated based on calibration of injectate volume and cellular volume, did not induce electrophysiologic alterations. The concentrations and efflux of phospholipids and lysophospholipids were assessed in lymph obtained from the supracardiac lymph vessel in anesthetized dogs to assess the extent of extracellular accumulation. Prior to ischemia, phosphatidylcholine (PC) was the major phospholipid in lymph (79 +/- 2%) with substantial quantities of sphingomyelin (11 +/- 2%) and LPC (6 +/- 1%). With ischemia, the concentration of LPC increased by 18%, and net efflux of LPC increased by 24% (P less than 0.01) with no net efflux of PC or other assayed phospholipids. The calculated concentration of LPC increased from 84 to 197 microM in lymph within the ischemic region, a concentration sufficient to induce electrophysiologic derangements.