Characterization of prostaglandin-F2 alpha-binding sites on rat hepatocyte plasma membranes

Prostaglandin (PG) F2 alpha has previously been shown to increase glucose output from perfused livers and isolated hepatocytes, where it stimulated glycogen phosphorylase via an inositol-trisphosphate-dependent signal pathway. In this study, PGF2 alpha binding sites on hepatocyte plasma membranes, that might represent the putative receptor, were characterized. Binding studies could not be performed with intact hepatocytes, because PGF2 alpha accumulated within the cells even at 4 degrees C. The intracellular accumulation was an order of magnitude higher than binding to plasma membranes. Purified hepatocyte plasma membranes had a high-affinity/low-capacity and a low-affinity/high-capacity binding site for PGF2 alpha. The respective binding constants for the high-affinity site were Kd = 3 nM and Bmax = 6 fmol/mg membrane protein, and for the low-affinity site Kd = 426 nM and Bmax = 245 fmol/mg membrane protein. Specific PGF2 alpha binding to the low-affinity site, but not to the high-affinity site, could be enhanced most potently by GTP[gamma S] followed by GDP[beta S] and GTP, but not by ATP[gamma S] or GMP. PGF2 alpha competed most potently with [3H]PGF2 alpha for specific binding to hepatocyte plasma membranes, followed by PGD2 and PGE2. Since the low-affinity PGF2 alpha-binding site had a Kd in the concentration range in which PG had previously been shown to be half-maximally active, and since this binding site showed a sensitivity to GTP, it is concluded that it might represent the receptor involved in the PGF2 alpha signal chain in hepatocytes. A biological function of the high-affinity site is currently not known.