A Ca paradox: Electric and behavioural responses of Paramecium following changes in external ion concentration

Abstract

Summary 1. Raising [Ca2+]o depolarized the membrane but caused augmented forward swimming which is characteristic of hyperpolarizing stimulation. 2. Lowering [Ca2+]o hyperpolarized the membrane but induced backward swimming or reduction in forward swimming; these motor responses are known to follow depolarizing stimulation. 3. The apparent inconsistencies in responses (“Ca paradox”) were suspended when changes in Ca2+ concentration had been compensated by equivalent amounts of Mg2+. 4. Mechanical disturbance of the cells during solution transfer affected the early electrical and behavioural responses of Paramecium for up to 5 min. 5. External Ca2+ modulated the rates of forward swimming, but not the frequency rates of reversals. Swimming was elevated in high-Ca solutions; peak rates of reversals occurred in solutions of high Mg2+ plus low Ca2+. 6. The analysis of the data suggests that ionically-induced changes of observed membrane potentials do not always reflect changes of the transmembrane potential which controls the behaviour via modulation of voltage-sensitive membrane channels. 7. The data are discussed using a model [22] which resolves the Ca paradox by incorporation of three instantaneous effects on the membrane potential of [Ca2+]o (and other divalent cations): (1) external surface charge neutralization, (2) shifts in equilibrium potential, and (3) changes in ion conductance. 8. Three observed parameters: Ca-dependent shifts in current-voltage relation, changes in input resistance, and behavioural adaptation suggest that the transmembrane potential accommodates (i.e. returns to rest in a time-dependent manner)