Effects of L-glutamate on the anomalous rectifier potassium current in horizontal cells of Carassius auratus retina.

The effects of externally applied L-glutamate (Glu) on K currents through the anomalous rectifier were studied in solitary horizontal cells dissociated from goldfish retinae under whole-cell voltage-clamp or cell-attached patch-clamp conditions using 'giga-seal' suction pipettes. In the whole-cell clamp experiments, hyperpolarization of the membrane below the resting potential (ca. -57 mV) induced a large voltage-dependent inward current which has been identified as the K current through the anomalous rectifier (Ianomal.). Application of Glu to the external medium reduced Ianomal.. Reduction of the inward current was not seen in preparations in which Ianomal. has been blocked by an application of Cs or Ba ions to the external medium. Single-channel currents through the anomalous rectifier were recorded under cell-attached patch-clamp conditions. The current showed an inward rectification; its amplitude increased with hyperpolarization of the patch membrane, and became below the noise level near the equilibrium potential of K ions (EK). No polarity reversal was observed even by a strong membrane depolarization. The patch membrane potential at which the current amplitude became undetectable shifted in parallel to the shift of EK. The open probability changed little with polarization of the patch membrane. When Glu (greater than 100 microM) was applied to the outside of the patch membrane, the number of available Ianomal. channels was decreased, but neither the single-channel conductance, open or closed time constants, nor the open probability changed significantly. Removal of Glu produced the opposite sequence; i.e. the number of available Ianomal. channels increased with time. It was concluded that the reduction of the Glu-induced current at hyperpolarized potentials in the whole-cell recording configuration is due to the blocking action of Glu on Ianomal..