Case Study 58 Flashcards
- PTH-rp, secreted by certain malignant tumors, is chemically homologous with PTH that is secreted by the parathyroid glands. PTH-rp has all of the biologic actions of PTH on bone and kidney. Given this information, why was Mr. Kessler hypercalcemic (increased serum Ca2+) and hypophosphatemia (decrease serum phosphate)? Why was his alkaline phosphatase level elevated?
a. Mr. Kessler was hypercalcemic and had decreased serum phosphate levels because the PTH-rp was acting on osteoblasts which then would stimulate osteoclasts to begin bone resorption. The broken down calcium and phosphate were released into the extracellular fluid. The PTH-rp inhibited Na+-phosphate cotransport and renal phosphate reabsorption. This caused high amounts of calcium in the extracellular fluid, and low amounts of phosphate in the extracellular fluid. His alkaline phosphatase levels were elevated because it is associated with high osteoblastic activity during high bone turnover.
- Why was Mr. Kessler’s serum PTH level decreased?
a. His PTH levels were decreased because it is regulated by the amount of Ca2+ in his blood. Since there waslots of Ca2+ in his blood, there was less PTH being released.
- After the 4-hour water deprivation test, Mr. Kessler’s serum osmolarity was 305 mOsm/L (normal 290 mOsm/L) and his urine osmolarity was 90 mOsm/L. Administration of an ADH analogue (dDAVP) by nasal spray did not alter his serum or urine osmolarity. The physician concluded that Mr. Kessler has nephrogenic diabetes insipidus. Why? What might be the cause of this condition?
a. Mr. Kessler’s serum osmolarity was high, but his urine osmolarity was low. This signaled to the physician that something was wrong. The doctor believed it was due to ADH deficiency and suggested he had diabetes insipidus. As a test he gave Mr. Kessler an ADH analogue (dDAVP) to observe if his urine osmolarity would increase. It did not, so he was officially diagnosed with diabetes insipidus. His nephrogenic diabetes insipidus was caused by hypercalcemia. The calcium deposits in the inner medulla of the kidney and inhibits ADH-dependent adenylyl cyclase and prevents ADH (antidiuretic hormone) from increasing water permeability in the collecting ducts.
- Why did Mr. Kessler have polyuria (increased urine production) and polydipsia (increased thirst)?
a. Polyuria occurred because the collecting ducts in his kidneys were resistant to ADH and were impermeable to water. Water that was not reabsorbed was excreted. Polydipsia occurred because increased water excretion made his body fluid more concentrated, therefor making him more thirsty due to osmoreceptors in his hypothalamus signaling.
- How was pamidronate expected to keep Mr. Kessler’s serum Ca2+ in normal range?
a. Pamidronate is expected to keep his serum Ca2+ in normal range because it would prevent Ca2+ from being released into the serum due to osteoclast activity.
