RAT-HEART SMOOTH-MUSCLE CELLS EXPRESS HIGH AND LOW-AFFINITY RECEPTORS FOR SOMATOSTATIN-14, WHICH ARE INVOLVED IN REGULATION OF CELL-PROLIFERATION

We demonstrate that rat heart coronary artery smooth muscle cells express specific binding sites (receptors) for somatostatin-14. The sigmoidal shape kinetics of the somatostatin-14 binding by the cells suggests the presence of either an allosteric binding site or binding sites with different affinities towards the ligand. Scatchard analysis reveals presence of high affinity (Kd = 0.039 x 10(-9)M) and low affinity (Kd = 0.602 x 10(-9)M) binding sites. Somatostatin-14 and Angiopeptin can displace each other from the binding sites. The concentrations of Angiopeptin required to displace 28%-48% of bound somatostatin-14 (10(-9)M) are in the range of 10(-4) - 10(-3)M. The concentrations of somatostatin-14 required to displace 8-27% of bound Angiopeptin (10(-6)M) are in the range of 10(-6) - 10(-5)M. Thus, somatostatin-14 seems to possess much higher binding affinity than Angiopeptin. Binding of somatostatin-14 and Angiopeptin to rat smooth muscle cells triggers intracellular event(s) leading to inhibition of smooth muscle cell proliferation. Exposure of smooth muscle cells to somatostatin-14 and Angiopeptin decreases amount of phosphorylated tyrosine residues. The effect of somatostatin-14 and Angiopeptin on the expression of phosphotyrosine preceedes and is most likely responsible,at least in part, for the inhibition of smooth muscle cell proliferation. This demonstrates that rat heart smooth muscle cells express physiologically active receptor(s) for somatostatin-14.