Ni2+ blocks the Ca2+ influx in human keratinocytes following a rise in extracellular Ca2+

In keratinocytes a rise in extracellular Ca2+ induces differentiation and is associated with a sustained increase in intracellular Ca2+, due to Ca2+ entry across the plasma membrane. We have investigated the mechanism of Ca2+ entry in human keratinocytes following this rise, using Fura-2-loaded cells and the cations Ni2+, Co2+, Mn2+, and La3+ and Ca2+ channel blocker verapamil. Keratinocytes were permeable to La3+, Mn2+, and Co2+; Fura-2 fluorescence was quenched by Mn2+ and Co2+. Verapamil was unable to block Ca2+ entry. Ni2+ did not enter keratinocytes, but blocked the influx of extracellular Ca2+ and Mn2+. Thapsigargin depleted Ca2+ stores, inducing a large transient intracellular rise, and the efflux of this Ca2+ was not blocked by Ni2+. We conclude that keratinocytes are permeable to a number of cations, but not Ni2+, which may be used to block the entry of the other cations during the study of cation flux into cells. The data presented are consistent with calcium entry by a nonspecific cation channel recently described on keratinocytes.