Effect of high potassium on dopamine receptor activity in bovine retina.

Abstract

In a previous study, bovine retina incubated for a short period in light showed a subsequent decrease in dopamine-sensitive adenylate cyclase activity, as compared to retinas incubated in dark. In the present study, the hypothesis that dopamine released by light caused a subsensitivity of the dopamine receptor was investigated. Bovine eyes were obtained from a slaughterhouse, and retinas were dissected in a dark room. Filter binding assays were developed to measure agonist and antagonist binding to the dopamine receptor using ($\sp3$H) dopamine and ($\sp3$H) SCH 23390, respectively, in a retinal membrane fraction. Adenylate cyclase activation was measured by the production of ($\sp{32}$P) cyclic AMP from ($\sp{32}$) ATP. In desensitization experiments, bovine retinas were incubated for fifteen minutes with 56 mM potassium, which also causes a release of dopamine in retinas were washed, and membranes were prepared. The stimulation of adenylate cyclase evoked by dopamine and radiolabelled agonist and antagonist binding were measured. In the receptor binding characterization studies, the dissociation constant and the maximum number of binding sites were obtained for ($\sp3$H) dopamine and ($\sp3$H) SCH 23390 binding. Pharmacological characterization indicated that binding was selective for D-1 dopamine receptors that are linked to stimulation of adenylate cyclase, as opposed to D-2 receptors. In the desensitization studies, retinas incubated with high potassium showed a significant decrease in apparent V$\sb{\rm max}$ in dopamine-stimulated adenylate cyclase activity in the membrane fraction from 102 $\pm$ 15 to 71 $\pm$ 11 pmol/min/mg protein. Incubation of retinas with high potassium in the presence of a dopamine antagonist prevented the desensitization. Saturation binding studies with both ($\sp3$H) dopamine and ($\sp3$H) SCH 23390 indicated that the number of dopamine receptors or receptor affinity did not change with potassium treatment. The results obtained suggested that a heterologous desensitization occurred without a change at the level of the receptor. This hypothesis was supported by a desensitization in vasoactive intestinal peptide-sensitive adenylate cyclase activity and a desensitization in dopamine-sensitive activity after IBMX or dibutyryl cAMP preincubation. Electrophysiological studies in retina suggest that the dopaminergic system in retina may modulate the response of ganglion cells to light. A rapid desensitization in enzyme activity such as seen in this study, in response to dopamine concentration, which can result from changes in ambient illumination, may be one way in which dopaminergic neurons modulate the sensitivity of ganglion cells to light.