Cold enhanced secretion of a pinocytosis-regulating factor in Amoeba proteus

Low temperature inhibited preferentially pinocytosis induced by Na+ and Na+-like inducers, a category of inducers which are particularly dependent on the concentration of Ca++ that means susceptible to blockade by EGTA, La+++, or high Ca++. Coldness inhibited Na+-induced pinocytosis more effectively in cells from a calcium deficient medium than in normal cells and the inhibition was abolished by Ca++. In normally fed cells, a period of coldness prevented the subsequent induction of pinocytosis by Na+. This block was of the same type as that observed when the concentration of Ca++ was increased in the medium of normal cells kept at room temperature. In contrast, starved cells responded to coldness by an increased capacity for pinocytosis. These effects lasted for several hours after the cells had been rewarmed and were mediated by a cell-derived soluble factor, PRF, pinocytosis regulating factor. Solubility characteristics indicated that PRF is a lipid. Ca++, low pH, and low temperature stimulated accumulation of the factor in the medium. Preincubation with PRF or addition of PRF to the inducer stimulated pinocytosis in starved or Ca++-deficient cells and reduced the sensitivity to Na+-like inducers in normal cells. The amount of PRF secreted from cold cells was sufficient to cause the Ca++-like blockade of pinocytosis in the cold, the diminished pinocytotic activity after a cold period and the activating effect of coldness on starved cells. PRF appears to be an important physiological regulator of the capacity for pinocytosis in the amoeba.