Total and exchangeable calcium in lymphocytes: effects of PHA and A23187

Calcium has been suggested as an internal second messenger when lymphocytes are stimulated by mitogens to enter the cell cycle. We have assessed the effect of 2 lymphocyte stimulants, the plant lectin phytohemagglutinin (PHA) and the calcium ionophore A23187, on human lymphocyte nucleic acid synthesis, total cell calcium content, and 45Ca labeling. We have used an ultrasensitive method for the measurement of total cell calcium in the same samples used for radiolabeling. Mitogenic concentrations of A23187 (approximately .25 mumole/liter) caused an increase in both total cell calcium and 45Ca labeling. These increases were almost completely blocked by inhibitors of mitochondrial respiration, suggesting that the calcium increment after ionophore treatment was located in the mitochondria. In contrast, total cell calcium was not altered at optimal mitogenic PHA concentrations (0.1 microgram/ml and above). However, at the minimum PHA concentrations that caused stimulation (0.025 to 0.1 microgram/ml), the dose response of 45Ca uptake was very similar to that of DNA synthesis. Importantly, we could not stimulate DNA synthesis with PHA without increasing lymphocyte 45Ca labeling. Thus, an increase in total cell calcium is not essential for mitogenesis; however, an increase in 45Ca exchange is closely associated with the mitogenic effects of A23187 and PHA.