Calcium channel blockers inhibit cellular uptake of thymidine, uridine and leucine: the incorporation of these molecules into DNA, RNA and protein in the presence of calcium channel blockers is not a valid measure of lymphocyte activation

Abstract An important role of transmembrane flux of calcium in lymphocyte activation has been previously demonstrated. Herein, we demonstrate that the calcium channel blockers verapamil and isradipine are able to inhibit in a concentration-dependent manner 3 H-thymidine incorporation into DNA in phytohemagglutinin (PHA)-stimulated human peripheral blood mononuclear cells (PBMC). However, verapamil and isradipine diminish PHA-stimulated thymidine incorporation into DNA to the same extent whether they are added at the beginning of culture of 4 h prior to completion of a 72-h culture. Thus, 3 H-thymidine incorporation into DNA in the presence of verapamil or isradipine is not a valid measure of mitogen-induced lymphocyte proliferation. Similarly, verapamil and isradipine also inhibit PHA-stimulated incorporation of 3 H-leucine into protein and 3 H-uridine into RNA whether the drugs are added at the beginning of culture of 4 h prior to completion of 24-h cultures. There is no intracellular accumulation of 3 H-thymidine, 3 H-leucine, or 3 H-uridine into 10% trichloroacetic acid-soluble molecules during inhibition with verapamil or isradipine, suggesting that these drugs impair the cellular uptake of these substances rather than directly inhibiting their incorporation into DNA, protein, or RNA, respectively. Since previous reports documenting the inhibitory effects of calcium channel blockers on lymphocyte proliferation have utilized 3 H-thymidine incorporation into DNA to measure proliferation, we have re-examined the antiproliferative effects of these drugs by determining their effect on PHA-stimulated cell cycle progression, employing cytofluorometric analysis of propidium iodide-stained cells. When added at the initiation of culture, both verapamil and isradipine inhibited in a concentration-dependent manner PHA-stimulated cell cycle progression. Thus, both verapamil and isradipine are capable of inhibiting PHA-induced proliferation of human lymphocytes.