KINETICS AND CALCIUM-SPECIFICITY OF POLYAMINE UPTAKE IN CARROT PROTOPLASTS

The kinetics of putrescine and spermidine uptake and the influence of calcium on the kinetic parameters of the transport process were invest igated in protoplasts isolated from carrot phloem parenchyma. Spermidi ne uptake dependence on external concentration was biphasic, both in t he absence and in the presence of 1 mM CaCl2. In the first case, satur ation was reached at 0.1 to 0.25 mM and the K-m value was 43 mu M. Whe n calcium was added, the K-m and V-max increased. A similar pattern wa s found with regard to putrescine uptake. Moreover, in order to clarif y the mode of action of calcium on polyamine uptake, lanthanides (lant hanum and gadolinium) were utilised as Ca+2-channel antagonists. When protoplasts were preincubated with these lanthanides, the stimulatory effect exerted by Ca+2 on polyamine uptake was almost totally abolishe d. On the other hand, if lanthanum was supplied instead of calcium, it gave rise to a small enhancement of polyamine transport. These result s induce us to suggest that calcium acts on polyamine uptake both by b inding to external sites on the plasmalemma and by penetrating into th e cell.