The influence of the extracellular concentration of calcium, magnesium and sodium on the repolarizing phase of the receptor potential of the Limulus ventral nerve photoreceptor.

Lowering the extracellular calcium concentration from 10 mmol/l to 1 nmol/l causes, besides reducing membrane potential (PMP) and peak amplitude (hmax) of the light response of the Limulus ventral nerve photoreceptor (see Stieve and Bruns [1]), a prolongation of the time course of the light response. The retarded time course (characterized by latent-period tlat, time-to-peak tmax, decrease time t2 and decline quotient QHN) caused by low calcium concentration is not antagonized by either reducing the sodium concentration (from 0.5 to 0.05 mol/l) or increasing the magnesium concentration (from 5 X 10(-5) to 5 X 10(-2) mol/l) in contrast to the effects on the PMP and hmax. This effect of lowering the calcium concentration on the time course of the ReP is distinctly different from that on membrane potential and transient of the ReP described before. It is not characterized by a calcium/sodium binding competition but is probably more closely related to the bump-generating mechanism. It can be explained on the assumption that the time parameters of the ReP are primarily determined by the latency distribution of the underlying bumps which is expanded under low calcium conditions.