<prompt> A high PTH levels affects which of the following processes to return plasma calcium levels toward normal?Inhibit calcium secretion from the gastrointestinal tractReduce the expression of plasma calcium-binding proteinsStimulate bone resorption, leading to the release of calcium into the plasmaStimulate the release of calcium from muscle cells  <response>: 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, Ganongs Review of Medical Physiology, 24e. 