Voltage control of calcium influx in intact cells

This work describes quantitative measurements of membrane fluxes of Ca²⁺ in intact cells of Chara corallina Klein ex Will. em. R.D. Wood, in relation to the control of Ca²⁺ uptake by membrane voltage (PD). At the normal resting PD, ⁴⁵Ca influx was low but was stimulated up to 25-fold when the membrane was depolarised by voltage clamping to values more positive than -100 mV. Similarly, in cells in which the PD was varied by changing the concentration of K⁺ in the external solution, influx was strongly stimulated in the depolarised state. Transient depolarisation during action potentials resulted in large influxes of ⁴⁵Ca which, when averaged over the duration of the action potential, yielded influxes of up to 2000 times higher than the resting level. Thus, there appears to be a large surplus of Ca²⁺ channels over that required for adequate nutrition. The possible physiological role of voltage-dependent Ca²⁺ channels is discussed in relation to the fact that, under normal growth conditions, cells of Chara corallina exist in the hyperpolarised state where these channels would not be activated.