Abstract
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 ^4 5^Ca 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 (βΌ .25 ΞΌ mole/liter) caused an increase in both total cell calcium and ^4 5^Ca 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 ΞΌg/ml and above). However, at the minimum PHA concentrations that caused stimulation (0.025 to 0.1 ΞΌg/ml), the dose response of ^4 5^Ca uptake was very similar to that of DNA sysnthesis. Importantly, we could not stimulate DNA synthesis with PHA without increasing lymphocyte ^4 5^Ca labeling. Thus, an increase in total cell calcium is not essential for mitogenesis; however, an increase in ^4 5^Ca exchange is closely associated with the mitogenic effects of A23187 and PHA.