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 cells with the
calmodulin antagonist
CGS 9343B results in a marked stimulation of the system L activity. At a
CGS 9343B concentration of 50 microM, 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 microM
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 both 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.