Voltage Control of Calcium Influx in Intact Cells

Abstract

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.