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.
Colás
B
B
Department of Biochemistry and Molecular Biology, Medical School, University of Alcalá, Madrid, Spain.
Valencia
A M
AM
Prieto
J C
JC
Arilla
E
E
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