Characterization of prostaglandin‐F _2α‐binding sites on rat hepatocyte plasma membranes

Abstract

Prostaglandin (PG) F2. has previously been shown to increase glucose output from perfused livers and isolated hepatocytes, where it stimulated glycogen phosphorylase via an inositol-trisphosphatedependent signal pathway. In this study, PGF2. binding sites on hepatocyte plasma membranes, that might represent the putative receptor, were characterized.Binding studies could not be performed with intact hepatocytes, because PGFza accumulated within the cells even at 4°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/highcapacity binding%ite for PGF2,. The respective binding constants for the high-affinity site were Kd = 3 nM and B, , , = 6 fmol/mg membrane protein, and for the low-affinity site Kd = 426 nM and B,,, = 245 fmol/mg membrane protein.Specific PGFza binding to the low-affinity site, but not to the high-affinity site, could be enhanced most potently by GTP[yS] followed by GDPUS] and GTP, but not by ATP [yS] or GMP.PGFza competed most potently with [3H]PGF2a for specific binding to hepatocyte plasma membranes, followed by PGD2 and PGE2.Since the low-affinity PGFza-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. signal chain in hepatocytes. A biological function of the high-affinity site is currently not known.Eicosanoids play an important role in the signal pathway between non-parenchymal and parenchymal liver cells [I -31.They are involved in the regulation of liver metabolism and hemodynamics by sympathetic hepatic nerves [4, 51, cell-wall particles from yeast (zymosan) [6], endotoxins [7], heat-aggregated immunoglobulins [S], peptides of the activated complement system [9 -111, platelet-activating factor [I21 and extracellular nucleotides and nucleosides [13,14]. Prostaglandins (PG) F2., Ez and D2 stimulated glucose and lactate output and increased resistance in perfused rat liver [I 5 -171. PGF2,, and PGE2 enhanced glucose output from isolated hepatocytes by an al-adrenergic-like activation of glycogen phosphorylase via an inositol-trisphosphate-dependent signal pathway [18, 191. Besides this glycogenolytic activity, PG have an antiglycogenolytic, a,-adrenergic-like effect, i.e. they attenuate the glucagon-stimulated elevation of CAMP and glycogen phosphorylase activity in hepatocytes [20 -231.

Ca2+-mediated activation of ERK in hepatocytes by norepinephrine and prostaglandin F2α: role of calmodulin and src kinases

“…We then examined the role of Ca 2+ in activation of ERK1/2 induced by stimulation of α 1 -adrenoceptors (with norepinephrine in the presence of timolol) and prostaglandin receptors (using PGF 2α ) [21,25,26]. The hepatocytes were pretreated with BAPTA-AM, which is activated intracellularly to bind Ca 2+ , EGTA, which binds extracellular Ca 2+ and eventually may deplete intracellular Ca 2+ [27,28], or gadolinium, a competitive inhibitor of Ca 2+ influx [29-31].…”

Kupffer cell‐derived prostaglandin E2 is involved in regulation of lipid synthesis in rat liver tissue

“…Although, the concentrations of PGE 2 in the vena cava are in the nanomolar range, the local concentration inside the liver tissue, close to hepatocytes, is expected to reach about 4-10 mM. 26,27 Moreover it is important to note that prostaglandins are rapidly metabolized by hepatocytes: during one liver passage, more than 90% of exogenously added PG is degraded. On the one hand, a similar higher rate of PL and TAG synthesis from palmitate is observed in PCLS obtained from both glycine and GdCl 3 treated rats.…”

Glycogenolytic and antiglycogenolytic prostaglandin E2 actions in rat hepatocytes are mediated via different signalling pathways

“…They are degraded mainly by the parenchymal cells, i.e. hepatocytes [2-61, which also have been shown to possess binding sites for prostaglandins [2,[7][8][9][10]. The role of prostaglandins in the regulation of liver metabolism has been discussed controversially in previous studies.…”