Human somatotropin binding to rabbit kidney microsomal fraction

Insulin in the acute renal adaptation to dietary phosphate restriction in the rat

Dietary phosphate restriction produces a rapid increase in tubular reabsorption of phosphate. To evaluate whether insulin is important in the acute renal adaptation following a low phosphate meal, four groups of conscious rats were studied by renal clearance methods, following a single meal by gavage. Group A received a normal (0.8%) phosphate meal, followed by saline infusion; Group B, a low (0.03%) phosphate meal, followed by saline infusion; Group C, a low phosphate meal, followed by infusion of somatostatin to suppress endogenous insulin secretion; and Group D, a low phosphate meal, followed by infusion of somatostatin plus insulin. Baseline plasma phosphate, insulin, glomerular filtration rate, and fractional excretion of phosphate were similar in all four groups. Following a low phosphate meal in Groups B, C, and D, there was a decrease in plasma phosphate, as compared with Group A. Whereas fractional excretion of phosphate decreased when plasma phosphate fell in Group B, administration of somatostatin (Group C) prevented the drop in fractional excretion of phosphate, despite a lower plasma phosphate. The addition of exogenous insulin (Group D) restored the antiphosphaturic effect of the low phosphate meal. These results suggest that insulin contributes to the acute decrease in phosphate excretion following a low phosphate meal.

Formation of complexes between 125I-labelled human or bovine somatotropins and binding proteins in vivo in rat liver and kidney

At 5 min after intravenous injection, both 125I-labelled human somatotropin and 125I-labelled bovine somatotropin were concentrated in rat liver and kidney. When the labelled hormones were administered along with an excess of the corresponding unlabelled hormone, a significant decrease of the uptake was observed in the liver, but not in the kidney. Study of the subcellular distribution of radioiodinated somatotropins in liver revealed that most of the radioactivity was specifically concentrated in the microsomal fraction. In contrast, the kidney fraction that accounted for most of the radioactivity was the 100 000 g supernatant. After solubilization, with 1% (w/v) Triton X-100, of the microsomal fractions obtained from both organs, the radioactive material was analysed by gel filtration on Sepharose CL-6B. By using this approach, it was demonstrated that both 125I-labelled human somatotropin and 125I-labelled bovine somatotropin bind in vivo to proteins present in liver. A small proportion of 125I-labelled human somatotropin was also shown to form complexes with proteins present in kidney. The present results demonstrate that the liver uptake is mainly due to binding of somatotropins to specific proteins, in contrast with the kidney, in which binding to specific sites contributes minimally to the overall uptake.