1. Solitary horizontal cells were isolated from catfish retinas and their membrane current was recorded with a whole-cell voltage clamp. Reducing the extracellular Ca2+ concentration produced a current that could be suppressed by dopamine. This Ca(2+)- and dopamine-sensitive current is hereafter termed I gamma. The voltage dependence, cytoplasmic regulation, and permeability of the I gamma channel suggest that it is half of a gap-junction channel. 2. I gamma was voltage and time dependent. In the steady state, the current-voltage relation displayed outward rectification at voltages more depolarized than 0 mV and a negative resistance region at voltages more hyperpolarized than -15 mV. The reversal potential was 3.3 +/- 1.5 mV when NaCl was the predominant extracellular salt and potassium-D-aspartate was the predominant intracellular salt. 3. The size of I gamma depended on the extracellular Ca2+ concentration. I gamma was maximal at external Ca2+ concentrations below 10 microM, half-maximal at 220 microM-Ca2+, and reduced to less than 4% of its maximum amplitude at external Ca2+ concentrations above 1 mM. Increasing the extracellular Ca2+ concentration reduced the amplitude of I gamma without changing the shape of the current-voltage relation or the kinetics of inactivation. Thus, rectification does not result from a voltage-dependent block by extracellular Ca2+. 4. Patches of cell membrane were voltage clamped in both the cell-attached and excised-patch configurations. In the cell-attached configuration, the addition of dopamine to the solution outside the patch pipette blocked the opening of channels within the membrane patch. Thus, dopamine closes I gamma channels by initiating an intracellular messenger cascade. In the excised-patch configuration, a maximum conductance of 145 pS was measured while Cs+ and tetraethylammonium+ (TEA+) were the only monovalent cations on both sides of the membrane. 5. The ability of dopamine to suppress I gamma was blocked by introducing an inhibitor of the cyclic AMP-dependent protein kinase, PKI5-24, into the cytoplasm. Thus, the action of dopamine is mediated by a pathway that includes the activation of a cyclic AMP-dependent kinase. 6. I gamma was suppressed by nitroprusside, an agent which activates guanylate cyclase and increases the intracellular cyclic GMP concentration. The effect of nitroprusside was not altered by the intracellular application of PKI5-24. Thus, nitroprusside suppresses I gamma through a pathway that does not include the activation of a cyclic AMP-dependent kinase.(ABSTRACT TRUNCATED AT 400 WORDS)