Prolactin Directly Stimulates the Liverin Vivoto Secrete a Factor (Synlactin) which Acts Synergistically with the Hormone*

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Abstract
The liver of rats, mice and pigeons secretes a PRL-synergizing activity (synlactin) in vitro. Whether PRL and/or GH [growth hormone] could stimulate the secretion of synlactin by the pigeon liver or kidney was investigated. Young birds received twice daily injections of PBS [phosphate buffer solution], ovine (o) PRL, or oGH for 5 days. On the 6th day, their livers and kidneys were removed, and slices of these organs were incubated in medium 199 for 4 h. The medium samples were filtered and diluted, then tested for PRL-like activity and synlactin activity in the local pigeon crop-sac bioassay. The latter test involved adding a dose of 1.0 .mu.g PRL to the medium samples. None of the liver or kidney medium samples had PRL-like activity when tested alone. Only the liver incubation medium from the PRL-injected pigeons contained significant amounts of synlactin activity. The next experiment was designed to determine whether PRL stimulation of hepatic secretion of synlactin involved a direct action of the hormone on the liver in vivo. A catheter attached to a coil of tubing and an osmotic minipump was inserted into an intestinal vein of pigeons, and the pump and coil were left in the abdomen. By this means, solvent, or GH or PRL in solvent, was pulse infused (4 pulses of 2 h each per day) into the intestinal venous drainage. The hormones were delivered directly to the liver via the hepatic portal vein. During the last 3 of the 7 days of infusion, the pigeons received 2 daily injections of PBS or oPRL over the contralateral lobes of the crop-sac. Intrahepatic infusion of PRL, but not GH, caused a marked augmentation of the response of the crop to local injections of PRL. Pulse infusion of the same dose of oPRL into the external jugular vein of pigeons did not have this effect; hence, it appears to be mediated by the liver. Apparently, one of the actions of PRL on the liver is to stimulate the secretion of a factor (synlactin) which acts synergistically with the hormone to promote growth of its target organs.