Cytoplasmic free Ca2+ ([Ca2+]i) appears to be an important signal for DNA synthesis in early stages of lymphocyte activation. In spite of many experimental studies which employ fluorescent Ca2+ indicator dye to demonstrate an early increase of [Ca 2+]i in T-lymphocytes after stimulation with lectins 1-3, specific antigens4,5, and monoclonal antibodies to T-lymphocyte receptors6-8, the mechanism responsible for the rise of [Ca2+]i is unknown. We have used the extracellular patch clamp technique9 to investigate this mechanism. Unitary inward currents, mediated by Ca2+ or Ba2+, were recorded in the membrane of T-lymphocytes. The inward current channel was characterized by a conductance of 7 pS and extrapolated reversal potential (Erev) 110mV positive to resting potential (Vr). While gating kinetic parameters were not affected by membrane potential changes, the probability of channel opening markedly increased upon activation of the T-lymphocyte by the mitogenic lectin, phytohaemagglutinin (PHA). PHA also evoked a cadmium-sensitive, inward Ba2+ current on whole-cell clamp. We suggest that this mitogen-regulated channel introduces Ca2+ into the cytoplasm upon activation and represents a new class of voltage-independent Ca2+ channels.
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