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.