Effects of the protein phosphatase inhibitors okadaic acid and calyculin A on metabolically inhibited and ischaemic isolated myocytes.

Isolated adult rat myocytes were subjected to 180 min of metabolic inhibition or incubated in ischaemic pellets, in the presence and absence of 10 microM okadaic acid (OA) or calyculin A (CL-A). Contracture and viability was determined by light microscopic analysis of trypan blue-stained preparations and ATP levels by HPLC. Osmotic fragility was assessed by brief hypotonic swelling of cells in 170 or 85 mOsm media prior to determination of viability. Neither drug significantly affected the relatively rapid rates of contracture of myocytes during metabolic inhibition, and both afforded significant protection from development of trypan blue permeability and osmotic fragility. Both OA and CL-A significantly accelerated the rates of contracture and ATP depletion of myocytes during ischaemic incubations. Despite an enhanced rate of ATP depletion, which would be expected to accelerate development of injury, neither drug accelerated development of loss of viability or development of osmotic fragility as measured by 170 mOsm swelling. Mathematical compensation for different rates of ATP depletion confirmed that a protective effect of the drugs, during ischaemic incubation, was masked by their enhancement of the rate of injury, following swelling at 170 mOsm. When the effects of CL-A on ischaemic cells were examined at 85 mOsm, a more stringent test for osmotic fragility, protection was found without compensation for differing rates of ATP depletion. A dose/response curve for CL-A showed some effect at 100 nM and a nearly full effect during metabolic inhibition at 1 microM concentrations. It is concluded that protein phosphatase inhibitors reduce the rates of development of osmotic fragility of metabolically inhibited cells and reduces the rate of injury relative to the rate of ATP depletion of ischaemic cardiomyocytes. Phosphorylation mechanisms may be important to development of irreversible myocardial cell injury.