A high PTH levels affects which of the following processes to return plasma calcium levels toward normal?
PTH acts directly on bone to increase bone resorption and mobilize Ca2+. In addition to increasing plasma Ca2+, PTH increases phosphate excretion in the urine and thereby suppresses plasma phosphate levels. PTH also increases reabsorption of Ca2+ in the distal tubules, although Ca2+ excretion in the urine is often increased in hyperparathyroidism because the increase in the load of filtered calcium overwhelms the effect on reabsorption. PTH also increases the formation of 1,25-dihydroxycholecalciferol, and this increases Ca2+ absorption from the intestine. On a longer time scale, PTH stimulates both osteoblasts and osteoclasts. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 21. Hormonal Control of Calcium & Phosphate Metabolism & the Physiology of Bone. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.
Inhibit calcium secretion from the gastrointestinal tract
Reduce the expression of plasma calcium-binding proteins
Stimulate bone resorption, leading to the release of calcium into the plasma
Stimulate the release of calcium from muscle cells