Somatostatin binding and modulation of adenylate cyclase in ovine retina membranes.
Somatostatin (SS) receptors in membranes from ovine retinas were examined using 125I-Tyr11-SS as a ligand. Receptor binding was rapid, specific, saturable, reversible and dependent on temperature and membrane concentration. Conditions of apparent equilibrium were obtained at 25 degrees C after a 45 min incubation in the presence of about 0.25 mg membrane protein/ml. Native SS competitively inhibited the binding of 125I-Tyr11-SS in the range of 0.01-10 nM, and half-maximal inhibition was observed at 0.2 nM SS. Scatchard analysis of these data suggested the existence of a single population of SS receptors with a dissociation constant of 0.23 +/- 0.03 nM and a maximum binding capacity of 84 +/- 6 fmol/mg protein. The binding of 125I-Tyr11-SS was inhibited by various synthetic SS analogs in a dose-dependent manner whereas peptides unrelated to SS did not show practically any effect even at concentrations as high as 10(-6) M. SS receptor occupancy appears to be coupled to inhibition of adenylate cyclase activity by a guanine nucleotide-binding regulatory protein, as suggested by the facts that: (a) SS noncompetitively inhibited the stimulatory effect of vasoactive intestinal peptide (VIP) (3 x 10(-7) M) on membrane adenylate cyclase activity but it did not alter basal enzyme activity; and (b) the addition of guanosine 5'-triphosphate (GTP) (10(-5) M) decreased the specific binding of 125I-Tyr11-SS to 26.6% of the control value due to a decrease in SS receptor affinity. The present results support the hypothesis that SS may contribute to the physiological regulation of the functions of the retina.
Department of Biochemistry and Molecular Biology, Medical School, University of Alcalá, Madrid, Spain.
B
B
Colás
A M
AM
Valencia
J C
JC
Prieto
E
E
Arilla
D000262:Adenylyl Cyclases / Q000378:metabolism*; D000595:Amino Acid Sequence; D000818:Animals; D001667:Binding, Competitive; D002462:Cell Membrane / Q000187:drug effects / Q000201:enzymology; D003864:Depression, Chemical; D004789:Enzyme Activation / Q000187:drug effects; D006160:Guanosine Triphosphate / Q000494:pharmacology; D007700:Kinetics; D008969:Molecular Sequence Data; D017481:Receptors, Somatostatin / Q000187:drug effects / Q000378:metabolism*; D012160:Retina / Q000187:drug effects* / Q000201:enzymology; D012756:Sheep; D013004:Somatostatin / Q000031:analogs & derivatives / Q000378:metabolism* / Q000494:pharmacology
D017481:Receptors, Somatostatin; D013004:Somatostatin; C032367:somatostatin, Tyr(11)-; D006160:Guanosine Triphosphate; D000262:Adenylyl Cyclases