Calcium influx at the plasmalemma of Chara corallina

Influx of ⁴⁵Ca into internodal cells of Chara corallina has been measured, using short uptake times, and a wash in ice-cold La³⁺-containing pondwater after the labelling period to overcome the difficulty of distinguishing extracellular tracer from that in the cell. Over 5–15 min the uptake was linear with time, through the origin. The basal influx from 0.1 mM Ca²⁺ externally was 0.25–0.5 pmol·cm^-2·s^-1, but some batches of cells showed higher fluxes. The influx was markedly stimulated by depolarisation in pondwater containing 20 mM K⁺. In cells in which the control flux was less than about 0.5 pmol·cm^-2·s^-1 there was no effect of 50 M nifedipine. In cells in which the control flux was greater than about 0.5 pmol·cm^-2·s^-1 (whether by natural variability, pretreatment, or by depolarisation in 20 mM K⁺), the flux was reduced by 50 M nifedipine to a value in the range 0.25–0.59 pmol·cm^-2·s^-1. It is suggested that two types of Ca-channel are probably involved, both opening on depolarisation, but only one sensitive to nifedipine. The flux was inhibited by 10 M BAY K 8644, which in animal cells more commonly opens Ca-channels. The apparent influx measured over long uptake times was much reduced, and the kinetics indicated filling a pool of apparent size about 1.45 nmol·cm^-2 with a halftime of about 38 min, probably representing cytoplasmic stores. It is argued that in spite of the very small pool of (free+bound) cytoplasmic Ca²⁺ the measured influx is a reasonable estimate of the influx at the plasmalemma.Abbreviations 0.4K-APW6 artificial pondwater, pH 6, containing 0.4 mM KCl - 20 K-APW6 artificial pondwater, pH 6, containing 20 mM KCl - Ca_o external Ca²⁺