Many mammalian cell types exhibit Ca2l-dependent K+ channels, and activation of these channels by increasing intracellular calcium generally leads to a hyperpolarization of the plasma membrane. Their presence in B lymphocytes is as yet uncertain. Crosslinking Ig on the surface of B lymphocytes is known to increase the level of free cytoplasmic calcium ([Ca2l']). However, rather than hyperpolarization, a depolar- Ca2+1; response by omission of external Ca2" abolished the prolonged hyperpolarization. In fact, a sizable Na+-dependent depolarization was unmasked. This study demonstrates that in human B lymphocytes, Ca2l-dependent K+ channels can be activated by crosslinking of surface IgM. Moreover, it is likely that, by analogy with voltage-sensitive Ca2+ channels, Na+ can permeate through these ligand-gated Ca2l "channels" in the absence of extracellular Ca2".

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Addition of crosslinking antibody to B lymphocytes results in a rapid rise in cytoplasmic-free Ca2+ ([Ca2+]i) due to release of Ca2+ from internal stores and uptake of Ca2+ across the plasma membrane. Inositol 1,4,5-trisphosphate is believed to mediate the release of internal Ca2+ stores and has also been proposed to mediate extracellular Ca2+ entry. We have compared the properties of these two pathways for Ca2+ mobilization by dissociating the [Ca2+]i changes in ligand-activated human B cells after loading of the cells with the Ca2+ chelator BAPTA. In the present paper we show that: (a) the sustained increase in [Ca2+]i is due to increased unidirectional influx of external [Ca2+]i; (b) entry of extracellular Ca2+, but not release of internal stores, is sensitive to the transmembrane potential; and (c) entry of extracellular Ca2+, but not release of internal stores, is inhibited by increasing [Ca2+]i. These findings suggest that the permeation pathways mediating the translocation of Ca2+ across the plasma membrane and endoplasmic reticulum membrane are not identical.