Passage
Parathyroid hormone (PTH) is an important regulator of calcium and phosphate homeostasis. Specifically, PTH functions to increase plasma calcium concentration and reduce plasma phosphate levels. PTH secretion is closely regulated by changes in plasma calcium concentration, which are detected by the human calcium-sensing receptor (hCaSR) , a G protein-coupled receptor located on chief cells of the parathyroid gland. hCaSR activation ultimately inhibits PTH release by inducing a second messenger cascade in which intracellular calcium levels rise due to the hydrolysis of the membrane phospholipid phosphatidylinositol bisphosphate (PIP₂) into inositol trisphosphate (IP₃) and diacylglycerol (DAG) by the membrane-bound enzyme phospholipase C (PLC) .Secondary hyperparathyroidism is a condition characterized by excessive PTH release in response to depressed serum calcium or elevated serum phosphate levels. The novel peptide AMG 416, an hCaSR agonist, is being investigated as a possible treatment for secondary hyperparathyroidism caused by end-stage renal disease (ESRD) . IP-1, a downstream metabolite of IP₃, is a marker of hCaSR activity. Researchers used embryonic kidney cells lacking hCaSR (HEK-293T) and HEK-293T clone cells stably transfected with hCaSR (hCaSR) to measure IP-1 levels following AMG 416 treatment (Figure 1) .
Figure 1 IP-1 concentration in embryonic kidney cells lacking hCaSR (HEK-293T) and HEK-293T clone cells transfected with hCaSR (hCaSR) following AMG 416 treatmentThe dependence of AMG 416 on extracellular calcium was investigated by incubating hCaSR cells with increasing concentrations of AMG 416 in the presence or absence of calcium treatment (Figure 2) .
Figure 2 IP-1 levels in hCaSR cells incubated with and without calcium and AMG 416 peptide (Note: IP-1 levels are normalized to the number of cells present in each culture.) In addition, PTH concentration was measured in 5/6 Nx mice, an animal model of secondary hyperparathyroidism in human ESRD. Blood samples were taken from male and female 5/6 Nx mice for 8 hours following administration of varying concentrations of AMG 416 (Figure 3) , and PTH levels were measured.
Figure 3 Serum PTH concentration following administration of AMG 416 to 5/6 Nx mice
-Which of the following conclusions is NOT consistent with the experimental findings presented in the passage?

Question

Passage
Parathyroid hormone (PTH) is an important regulator of calcium and phosphate homeostasis. Specifically, PTH functions to increase plasma calcium concentration and reduce plasma phosphate levels. PTH secretion is closely regulated by changes in plasma calcium concentration, which are detected by the human calcium-sensing receptor (hCaSR) , a G protein-coupled receptor located on chief cells of the parathyroid gland. hCaSR activation ultimately inhibits PTH release by inducing a second messenger cascade in which intracellular calcium levels rise due to the hydrolysis of the membrane phospholipid phosphatidylinositol bisphosphate (PIP₂) into inositol trisphosphate (IP₃) and diacylglycerol (DAG) by the membrane-bound enzyme phospholipase C (PLC) .Secondary hyperparathyroidism is a condition characterized by excessive PTH release in response to depressed serum calcium or elevated serum phosphate levels. The novel peptide AMG 416, an hCaSR agonist, is being investigated as a possible treatment for secondary hyperparathyroidism caused by end-stage renal disease (ESRD) . IP-1, a downstream metabolite of IP₃, is a marker of hCaSR activity. Researchers used embryonic kidney cells lacking hCaSR (HEK-293T) and HEK-293T clone cells stably transfected with hCaSR (hCaSR) to measure IP-1 levels following AMG 416 treatment (Figure 1) .

Figure 1 IP-1 concentration in embryonic kidney cells lacking hCaSR (HEK-293T) and HEK-293T clone cells transfected with hCaSR (hCaSR) following AMG 416 treatmentThe dependence of AMG 416 on extracellular calcium was investigated by incubating hCaSR cells with increasing concentrations of AMG 416 in the presence or absence of calcium treatment (Figure 2) .

Figure 2 IP-1 levels in hCaSR cells incubated with and without calcium and AMG 416 peptide (Note: IP-1 levels are normalized to the number of cells present in each culture.) In addition, PTH concentration was measured in 5/6 Nx mice, an animal model of secondary hyperparathyroidism in human ESRD. Blood samples were taken from male and female 5/6 Nx mice for 8 hours following administration of varying concentrations of AMG 416 (Figure 3) , and PTH levels were measured.

Figure 3 Serum PTH concentration following administration of AMG 416 to 5/6 Nx mice
-Which of the following conclusions is NOT consistent with the experimental findings presented in the passage?

Quizplus · Accepted Answer

11ecba2d_0fbe_cd9a_a3bf_376d2232c18c_MD0008_00 The hCaSR is a G_α protein-coupled receptor and by definition increases intracellular IP₃ when activated. The passage describes IP-1 as a metabolite of the second messenger IP₃ and as a marker of hCaSR activity. This means that IP-1 concentration is directly correlated with hCaSR activity. The only option not consistent with the passage data is the conclusion that AMG 416 increases cellular IP₃ levels through both a hCaSR-dependent and a hCaSR-independent mechanism. Figure 1 shows that AMG 416 increases IP-1 levels in cells expressing hCaSR. This increase concurs with the passage description of AMG 416 being a hCaSR agonist, a molecule that can bind a receptor and induce a biological response similar to that of ligand binding. However, Figure 1 also shows that treatment of cells lacking hCaSR (HEK-293T) results in no change in IP-1 concentration. Therefore, it is reasonable to conclude that AMG 416 mediates increased intracellular IP₃ only through binding and activation of hCaSR but not through a hCaSR-independent mechanism. In contrast, the following conclusions are consistent with the data provided in the passage: Figure 2 shows that IP-1 levels increase when hCaSR cells are incubated in the calcium-free condition (No Ca²⁺) and treated with AMG 416 peptide concentrations greater than 10^-4 M [Note: in the figure, this number is shown as -4, which the result derived from calculating log10(10^-4)]. Therefore, calcium-free conditions cause the activation of hCaSR in these cells at higher AMG 416 concentrations (Choice A). Figure 3 shows that plasma PTH levels are equally suppressed two hours after administering AMG 416 treatment at different dosages (ie, 0.3 mg/kg, 1.0 mg/kg, and 3.0 mg/kg) (Choice B). Figure 3 also shows that PTH levels remain suppressed below baseline levels (100%) for at least eight hours following AMG 416 treatment at 3.0 mg/kg (Choice D). Educational objective: An agonist enhances or duplicates the effect of a ligand binding to a receptor. Agonists are able to bind a receptor and induce a biological response that is similar to that of normal ligand binding.

Passage Parathyroid Hormone (PTH) Is an Important Regulator of Calcium and and Phosphate