Phospholipase-induced abnormalities in the sarcolemma

20 ULTRASTRUCTURAL LOCALIZATION OF LIPIDS IN HEART CELL MEMBRANES. N. J. Severs. Cardiothorocic Institute (University of London), 2 Beaumont Street, London, WlN 2DX, England. Electron microscopy has made a significant contribution to our current understanding of membrane structure and organisotion. Standard ultrastructural techniques permit some discrimination between the lipid and protein components of membranes-in particular, freeze-fracture is capable ofdetecting specialized protein domains, though membrane lipids are viewed only as on undifferentiated motrix by this method. With the recent development of freeze-fracture cytochemistry, however, attempts can now be mode to localize specific classes of lipids in membranes. These techniques depend on the action of cytochemical probes, which by binding to the target lipids, induce distinct olterations in membrane structure. Because these structural alterations are easily recognised in freeze-fracture replicas, they can be used to label the target lipids and examine their distribution in the membrane plane. Two classes of lipids have so far been studied in this way; 1) cholesterol (and related 3+hydroxysterols) ond 2) anionic phospholipids. Filipin (o polyene antibiotic) and tomatin and digitonin (saponins) are used as probes for the former, and polymyxin B (a peptide antibiotic) for the latter. Results obtained using these probes require careful interpretation but provide useful insights into the distribution of lipids in cardiac muscle cell membranes. (Supported

by British

Heart

Foundation

Grants

779

and

81/44).

PHOSPHOLIPASE-INDUCED ABNORMALITIES IN THE SARCOLEMMA, N.A. Shaikh. Department of Clin. Biochem. and Medicine, University of Toronto, Canada. In tissue bath experiments, isolated Purkinje fibres or moderator bands containing both Purkinje and muscle fibres were superfused with buffer containing various amounts of phospholipase A & C and electrophysiological characteristics (EP) were monitored. Superfusion WI4 h phospholipase A that produced lysophospholipids (LPL.) found in ischemic myocarendogenously in sarcolemma in amounts equiva f ent to those dium or more (up to 0.4mM) did not produce any changes in the EP of either fibre. Resting membrane potentials, upstroke velocity, action potential duration and amplitude remained similar to controls. At high endogenously produced LPL concentrations, (over 0.4mM), action potential deteriorated but normal EP were restored after washing (6-30 min.) with oxygenated normal buffer. At this stage of complete recovery, LPL levels in fibres remained abnormally high (0.3-0.4mM). Superfusion with phospholipase C which resulted in less than 0.1% loss of membrane phospholipids with no LPL production, completely abolish action potentials in 6-10 min., the fibres remaining unresponsive to stimulus even after prolonged washings with oxygenated normal buffer. These results suggest that LPL alone are not responsible for the electrophysiological manifestations of ischemia and that the degradation of sarcolemmal phospholipids may play an important role in membrane dysfunction.

FATTY ACID AND A.L. Shug, D.J. Memorial Veterans

CARNITINE-LINKED ABNORMALITIES Paulson. Metabolic Research Hospital and Department of

DURING Laboratory,

ISCHEMIA William

AND

CARDIOMYOPATHY. S. Middleton

Neurology, University of Wisconsin, Madison, Wisconsin, USA. During the onset of ischemia, long-chain fatty acid oxidation is slowed and the esters of CoA (LCACAE) and carnitine (LCAC) acumulate. This results in a rapid increase in the LCACAE/free carnitine (FC) ratio, which may be important because of its potential for the regulation of mitochondrial function. Myocardial deficiency in FC, either by esterfication or by leakage fran the ischemic heart, further increases the LCACAE/FC ratio. The point at which elevated LCACAE/FC ceases to be beneficial and becomes detrimental appears to correlate with the onset of the irreversible phase of ischemia. Cardiomyopathy induced by carnitine deficiency results in the disruption of normal fatty acid oxidation, tissue accumulation of lipids, increased LCACAE/FC and total CoA/total carnitine ratios, abnormal mitochondrial structure and function, and very low acetyl carnitine levels, which may adversely affect heart contraction. Carnitine treatment may protect the ischemic and cardiomyopathic heart by restoring fatty acid oxidation and returning the LCACAE/FC ratio to normal. Supported by the Veterans Administration and NIH HL17736.