A voltage-clamp analysis of membrane currents in solitary bipolar cells dissociated from Carassius auratus.

Membrane properties of solitary bipolar cells, mechanically dissociated from the enzyme-treated goldfish retina, were studied under current- and voltage-clamp conditions with 'giga-seal' suction pipettes (pipette solution 138 mM-K). The resting potential of solitary bipolar cells was about -30 mV. They responded to depolarizing current pulses with sustained depolarization, and to hyperpolarizing current pulses with an initial hyperpolarizing transient followed by a sag to a less hyperpolarized level. The current-voltage relationship determined under voltage-clamp conditions showed strong outward and inward rectification. The membrane currents consisted of four components; Ca current (ICa), voltage- and Ca-dependent K currents (IK(V) and IK(Ca), respectively), and an inward current activated by membrane hyperpolarization (Ih). ICa was activated by membrane depolarization beyond -40 mV, was maximum at +10 mV and became smaller with further depolarization. No polarity reversal was seen. ICa was enhanced by equimolar replacement of Ca with Ba, and was blocked by 4 mM-Co. IK(Ca) was observed by membrane depolarization beyond -10 mV, was maximum at about +40 mV, and became smaller with further depolarization. This current was suppressed by 4 mM-Co, 1.6 mM-Ba, 35 mM-TEA or 30 microM-quinine. IK(V) was activated by membrane depolarization beyond -60 mV, and had slower kinetics that ICa or IK(Ca). The reversal potential of the tail current was close to the K equilibrium potential (EK), suggesting that this current is carried purely by K ions. IK(V) was inactivated slowly and nearly completely by sustained depolarization. IK(V) was blocked by 35 mM-TEA. Ih was activated by membrane hyperpolarization (less than -60 mV). The current showed a time-dependent increase. It was also dependent on the membrane potential, but not on the driving force of K ions. This current seems to be carried by a mixture of Na and K ions, since (1) in low Na solution, Ih became small in amplitude, and (2) the reversal potential of the tail current was between the Na equilibrium potential (ENa) and EK X Ih was blocked by 10 mM-Cs, but was resistant to 0.2 mM-Ba. The resting potential and voltage responses of solitary bipolar cells are discussed in reference to the characteristics of each membrane conductance isolated in the present study.