The purification and characterization of rabbit placental lactogen

Mechanism of ruminant placental lactogen action: molecular and in vivo studies

Ruminant placental lactogens (PLs) are structurally related to prolactins (PRLs) and growth hormones (GHs) and are secreted by placentae. Ruminant PLs are unusual in their capacity to bind and activate PRL and GH receptors (Rs) from other species. The present minireview summarizes several works showing that unlike in heterologous species (rat, rabbit, human), in homologous (ruminant) species, PLs act by activating PRLRs or by heterodimerizing GHRs and PRLRs, and suggests that this may be the main mechanism of PL action in vivo. Mutations impairing the ability of ovine (o)PL or bovine (b)PL to form complexes with PRLRs (but not with GHRs) do not cause loss of biological activity, because the transient existence of the homodimeric complex is still sufficient to initiate the signal transduction; however, mutants do lose their ability to activate homologous PRLRs. To explain this difference, we proposed a novel term-minimal time of homodimer persistence-which assumes that to initiate the signal transduction, a "minimal time" of homodimer existence is required for transphosphorylation of associated JAK2s. In interactions between ruminant PLs and homologous PRLRs, this minimal time is met through the interaction with homologous PRLRs, which has a shorter half-life than with heterologous PRLRs. Thus oPL or bPL are active in cells possessing both homologous and heterologous PRLRs. Mutations of PLs decrease the affinity, shortening the "time of homodimer persistence." In heterologous interactions, the minimal time is still sufficient to initiate the biological activity, whereas in homologous interactions, which in any case are weaker, further destabilization of the complex shortens its persistence below the minimal time, causing loss of biological activity.

Giant and binucleate trophoblast cells of mammals

The cellular origin, structure, and function of trophoblastic giant cells (GC) and binucleate cells (BNC) are reviewed. Mammals in which these cells have received the greatest attention include rodents, rabbits, and humans (GCs), and ruminants and equids (BNCs). In almost all cases these cells arise from the cytotrophoblast. All are large cells and contain either two diploid nuclei (BNCs), multiple nuclei (human placental bed GCs), or single nuclei with amplified DNA content (rodent and rabbit GCs). Giant and binucleate cells typically exhibit the capacity for migration or invasion, although the degree of migratory activity varies between species. While most end up within, or at the interface with, endometrial tissue, in some instances the GCs or BNCs contribute directly to the interhemal membrane of the placenta. Hormone production is a property which most GC-BNC populations have in common. Lactogen or gonadotropin has been documented in almost all cells of this type examined to date, and in some animals they are also steroidogenic (e.g., rats and sheep). In spite of some common features, both structural and functional differences remain and it is suggested that use of terms such as mononuclear giant cells, multinucleate giant cells, and binucleate cells be continued rather than assuming that these cells are all members of a single trophoblastic subtype.

Pregnancy-induced insulin resistance in liver and skeletal muscles of the conscious rabbit

Insulin sensitivity of maternal nonuterine tissues (liver and skeletal muscles) has been investigated in the conscious rabbit during late gestation (24 and 30 days). The specific effect of insulin on glucose production and utilization was evaluated with the hyperinsulinemic euglycemic clamp technique. The net balance of glucose across the hindlimb muscles was studied by means of the Fick principle in basal and insulin stimulated conditions (clamp study). Our results show that an insulin-resistant state develops between days 24 and 30 of gestation in the rabbit and involves both glucose producing (liver) and utilizing (muscles) tissues. On day 30 of gestation, muscle glucose uptake was not significantly stimulated at a plasma insulin concentration of 700 microU/ml, whereas it was stimulated by 30-40% in nonpregnant and 24 day pregnant rabbits. At similar plasma insulin concentration, endogenous glucose production was suppressed by 85% in both nonpregnant and 24 day pregnant rabbits, whereas it was decreased by only 30% in 30 day pregnant rabbits. The present data suggest that hindlimb muscles of late pregnant rabbits are able to reduce their insulin-induced glucose utilization. This could contribute to meet the glucose requirements of pregnant uterus in late gestation.