CYTOPLASMIC CA2-DARBY CANINE KIDNEY-FOCUS (MDCK-F) CELLS( DETERMINES THE RATE OF CA2+ ENTRY INTO MARDIN)

Transformed Mardin-Darby canine kidney-focus (MDCK-F) cells exhibit sp ontaneous Ca2+ oscillations from an inositol 1,4,5-trisphosphate-sensi tive cytoplasmic Ca2+ store. In this study, Ca2+ entry from the extrac ellular space and its role in generation of oscillations were investig ated by means of Ca2+ video imaging and the Fura-2/Mn2+ quenching tech nique. Oscillations were dependent on extracellular Ca2+ concentration and were inhibited by extracellulary applied La3+, Co2+ and Ni2+. Dep olarization of the cell membrane with high K+ concentrations and the L -type Ca2+ channel blocker nifedipine had no effect on oscillations, i ndicating the lack of involvement of voltage-gated Ca2+ channels. Mn2 quenching experiments disclosed significant Ca2+ influx into MDCK-F c ells. The rate of this influx was constant between Ca2+ spikes, but ma rkedly increased during the spontaneous Ca2+ spikes. Similar transient increases in Ca2+ entry could be mimicked by agents triggering intrac ellular Ca2+ release such as bradykinin and thapsigargin. We conclude that the plasma membrane of MDCK-F cells exhibits a marked voltage-ind ependent Ca2+ permeability permitting Ca2+ entry into the cytoplasm. T he rate of Ca2+ entry which determines the frequency of oscillations i s most likely to be regulated by the cytoplasmic Ca2+ concentration.