Permeability characteristics of erythrocyte membrane to okadaic acid and calyculin A.

The rates of transport of the protein phosphatase inhibitors okadaic acid and calyculin A through rabbit erythrocyte membranes have been estimated by measuring protein phosphatase type 2A (PP2A) activity in lysates. High concentrations of okadaic acid (100 nM) cause rapid (t 1/2 approximately 10 min) inhibition of PP2A. However, the t 1/2 for okadaic acid influx is much longer because the concentration is much higher than the concentration inhibiting 50% of the maximal response (IC50). The estimated t 1/2 is over 1 h at 37 degrees C and over 4 h at 25 degrees C. The effect of low extracellular pH indicates that the undissociated acid is the permeant species. It takes hours to reverse the effect of okadaic acid, because the efflux must proceed through several half times before the concentration is below the IC50 for PP2A. The permeation of calyculin A in contrast to okadaic acid is too fast to measure at 25 degrees C. Our results indicate that okadaic acid entry into erythrocytes is slower than is generally believed; it is crucial to consider concentration, temperature, pH, and time of exposure to okadaic acid to interpret the effects of this agent on intact cells.