Activation of a Ca2+-permeable cation channel by two different inducers of apoptosis in a human prostatic cancer cell line

We have combined patch clamp recording with simultaneous [Ca 2+ ] i measurements in single LNCaP cells (a human prostate cancer cell line), to study the activation of Ca 2+ -permeable channels by two different inducers of apoptosis, ionomycin and serum deprivation. In perforated patch recording, LNCaP cells had a membrane potential of -40 mV and a resting [Ca 2+ ] i of 90 nM. Application of ionomycin at levels that induced apoptosis in these cells (10 μM) produced a biphasic increase in [Ca 2+ ] i . The first rise in [Ca 2+ ] i was due to release of Ca 2+ from internal stores and it was associated with a membrane hyperpolarization to -77 mV. The latter was probably due to the activation of high conductance, Ca 2+ - and voltage-dependent K + channels (maxi-K). Conversely, the second rise in [Ca 2+ ] i was always preceded by and strictly associated with membrane depolarization and required external Ca 2+ . Serum deprivation, another inducer of apoptosis, unmasked a voltage-independent Ca 2+ permeability as well. A lower concentration of ionomycin (1 μM) did not induce apoptosis, and neither depolarized LNCaP cells nor produced the biphasic increase in [Ca 2+ ] i . However, the first increment in [Ca 2+ ] i due to release from internal Ca 2+ stores was evident at this concentration of ionomycin. Simultaneous recordings of [Ca 2+ ] i and ion channel activity in the cell attached configuration of patch clamp revealed a Ca 2+ -permeable, Ca 2+ -independent, non-selective cation channel of 23 pS conductance. This channel was activated only during the second increment in [Ca 2+ ] i induced by ionomycin. The absence of serum activated the 23 pS channel as well, albeit at a lower frequency than with ionomycin. Thus, the 23 pS channel can be activated by two unrelated inducers of apoptosis and it could be another Ca 2+ influx mechanism in programmed cell death of LNCaP cells.