Inhibition of sarcolemmal Na+-K+-ATPase by palmitoyl carnitine: potentiation by propranolol.

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Native sarcolemma (SL) from adult canine cardiac myocytes (Na+-K+-ATPase activity 74.2 +/- 3.0 mumol X mg-1 X h-1) was preincubated (10 min, 37 degrees C, pH 7.2) with 1) 20-600 microM palmitoyl carnitine, 2) 250 nM-2.5 mM propranolol, or 3) 20-600 microM palmitoyl carnitine plus propranolol at various concentrations (0.0, 0.025, 0.25, 0.5, 1.0, and 2.5 mM); after preincubation, Na+-K+-ATPase activity was assayed. Palmitoyl carnitine alone (series 1) had no effect on ATPase activity over the range of 20-400 microM but was inhibitory (30%) at 600 microM. Propranolol alone (series 2) did not alter ATPase activity at any concentration. When SL membranes were exposed to both palmitoyl carnitine and propranolol (series 3), a dose-dependent inhibition of ATPase activity was observed. The inhibitory effect was not reversed by 3.0% bovine serum albumin. Propranolol concentrations greater than 0.025 mM significantly inhibited the activity of SL exposed to palmitoyl carnitine (above 150 microM). Palmitoyl carnitine and propranolol do not have to be added simultaneously to produce combined inhibition. Activity was inhibited 50% when SL were pretreated with 100 microM palmitoyl carnitine followed by addition of 2.5 mM propranolol no inhibition occurred if preincubation conditions were reversed. Thus exposure of SL to propranolol and reported physiological levels of palmitoyl carnitine leads to irreversible inhibition of the Na+-K+-ATPase, which may be due to the combined membrane-perturbant actions of these amphipathic agents.