Calcium influx at the plasmalemma of Chara corallina

Influx of (45)Ca into internodal cells of Chara corallina has been measured, using short uptake times, and a wash in ice-cold La(3+)-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 Ca(2+) 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 μM 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 μM 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 μM 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 Ca(2+) the measured influx is a reasonable estimate of the influx at the plasmalemma.