CALMODULIN-DEPENDENT MODULATION OF PH SENSITIVITY OF THE AMINO-ACID-TRANSPORT SYSTEM-L IN HUMAN PLACENTAL CHORIOCARCINOMA CELLS

The JAR human placental choriocarcinoma cells express the amino acid transport system L. The activity of this system is Na+-independent and is stimulated by acidic extracellular pH. Treatment of eels with the calmodulin antagonist CGS 9343B results in a marked stimulation of the system L activity. At a CGS 9343B concentration of 50 mu M, the stimulation of activity measured at pH 7.5 is about 75-100%. This effect is not blocked by cycloheximide, actinomycin D, colchicine or cytochalasin D suggesting that the stimulation is not due to de novo synthesis of the carrier protein or recruitment of the carrier protein from an intracellular pool. The stimulatory effect of CGS 9343B is reproducible with other calmodulin antagonists. Treatment with CGS 9343B significantly modifies pH sensitivity of the system. The stimulatory effect of H+ is markedly reduced in treated cells compared to control cells. The stimulation of activity at pH 5.5 vs. pH 7.5 is 55% in control cells but only 8% in treated cells. Similarly, the stimulatory effect of CGS 9343B is reduced by H+. The stimulation of activity seen with 50 mu M CGS 9343B is 80% at pH 8.0, but only 26% at pH 5.5. In addition, H+ and CGS 9343B affect the kinetic parameters of system L in a similar manner, the stimulation in bath cases being primarily due to an increase in the maximal velocity. The apparent competitive nature between the effects of H+ and CGS 9343B is also observed with other calmodulin antagonists. These results show that the transport function and pH sensitivity of the amino acid transport system L in placental choriocarcinoma cells are modulated by calmodulin by processes which do not involve de novo synthesis nor recruitment of the carrier protein.