Lability of the prolonged depolarizing afterpotential in Balanus photoreceptors

A large cell to cell variability of the prolonged depolarizing afterpotential (PDA) decay time constant (τ) has been measured in Balanus eberneus lateral ocelli. While 25% of the cells had PDAs of long duration, τ > 10 min, 45% of the cells tested showed either weak (τ < 60 s) PDA or none at all. The variability was not reflected in the late receptor potential. All the cells showed normal light-coincident responses. The variability was not due to some alteration of the thermal stability of the pigment states, since after monochromatic adaptation the amplitude of the early receptor potential remained unchanged for at least 30 min. In addition, in some cells that initially showed PDAs of long duration, the decay time was either shortened or abolished after exposure to anoxia. Again, the late receptor potential and the stability of the pigments remained unaffected. These results indicate that the mechanisms which give rise to the PDA are not always tightly coupled to the direct chain of events that lead to the light-coincident response.