Regulation of Calmodulin‐ and Dopamine‐Stimulated Adenylate Cyclase Activities by Light in Bovine Retina

Abstract

Neural retina from most species contains 3,4-dihydroxyphenylethylamine (dopamine) receptors coupled to stimulation of adenylate cyclase activity. Release of dopamine from its neurons and subsequent occupation of dopamine receptors is increased by light. Adenylate cyclase activity in bovine retina is highly responsive to the endogneous Ca2+-binding protein, calmodulin, and that calmodulin can increase dopamine-sensitive adenylate cyclase activity in bovine retina. Both dopamine- and calmodulin-stimulated adenylate cyclase activities can be regulated by alterations in light. Bovine retinas were dissected from the eye under a low-intensity red safety light, defined as dark conditions, and incubated for 20 min in an oxygenated Krebs Henseleit buffer under either dark or light conditions. The retinas were then homogenized and adenylate cyclase activity measured in a particulate fraction washed to deplete it of endogenous Ca2+ and calmodulin. Activation of adenylate cyclase activity by calmodulin, dopamine, and the nonhydrolyzable GTP analog, guanosine-5''-(.beta.,.gamma.-imido)triphosphate (GppNHp), was significantly (60%) greater in particulate fractions from retinas that had been incubated under dark conditions as compared to those incubated under light conditions. Basal, Mn2+-, and GTP-stimulated adenylate cyclase activities were not altered by changes in lighting conditions. Calmodulin could increase the maximum stimulation of adenylate cyclase by dopamine in retinas incubated under either dark or light conditions, but the degree of its effect was greater in retinas incubated under light conditions. Activation of adenylate cyclase by calmodulin, dopamine, and GppNHp in particulate fractions from retinas incubated under light conditions was indistinguishable from the activation obtained when retinas were incubated in the dark in the presence of exogenous dopamine. An increased release of dopamine may occur in light. The decreased response of adenylate cyclase to exogenous dopamine can then be explained by a subsequent down-regulation of dopamine receptor activity. The down-regulation of dopamine receptor activity can also regulate activation of adenylate cyclase by GppNHp and calmodulin. Apparently, dopamine, calmodulin, and GppNHp are modulators of a common component of adenylate cyclase activity, and this component is regulated by light.