This study will expand upon earlier infusions studies that demonstrated: 1) There is a dose-related increase in 1,25 (OH)2 vitamin D in response to PTHrP and PTH over multiple days. 2) There is a markedly attenuated vitamin D response to PTHrP compared to PTH, particularly during the second 24 hours. 3) The increase in 1,25 (OH)2 vitamin D is almost certainly responsible for the greater calcemic effect of PTH compared to PTHrP. 4) PTHrP is obviously a weaker agonist of 1,25 (OH)2 vitamin D but does not result in its suppression as is seen in Humoral Hypercalcemia of Malignancy (HHM). Thus, the suppression of 1,25 (OH)2 vitamin D seen in HHM remained unexplained. In addition to assessing the effects of an infusion of PTHrP and PTH on calcium handling and 1,25(OH)2 vitamin D, we also measured their effects on markers of bone turnover. Given the clinical observations seen in Hyperparathyroidism (HPT) and HHM, we anticipated that PTH would stimulate both bone resorption and formation, while PTHrP would stimulate bone resorption but inhibit formation. However, we observed that infusions of PTHrP and PTH resulted in an equivalent, rapid increase in bone resorption as measured by N-telopeptide (NTx) and C-telopeptide (CTx), as well as a progressive decline in bone formation. There was no difference between PTH and PTHrP. We assumed that formation would ultimately increase with additional time, as seen in HPT, and therefore examined an additional group of subjects infused with PTHrP for 96 hours. However, N-terminal propeptide of type 1 procollagen (P1NP) continued to decline even further as is seen in HHM in contrast to HPT. We have not yet studied longer infusions of PTH.

One of the reasons for doing this pilot study is to determine the optimal dosing of PTH over a week period of time. Intravenous PTH has never been infused into human beings for prolonged periods of time. The investigators question whether a prolonged continuous intravenous infusion of PTH will lead to a sustained and progressive suppression of bone formation as occurs in HHM or an increase in bone formation as occurs in HPT. They also want to assess the direct influence of long-term continuous PTH infusions on plasma 1,25 (OH)2 vitamin D regulation in healthy human volunteers. We have shown in our previous studies that doses of 8 picomoles (pmol)/kg/hr PTH given over 48 hours result in sustained mild serum hypercalcemia, with serum calcium seeming to plateau in the range of 11 - 11.5 mg/dL after 48 hours. A dose of 8 picomoles (pmol)/kg/hr has also been shown to cause desirable effects on serum 1,25(OH)2 vitamin D and markers of bone turnover, and may therefore be the "ideal" dose. However, we do not know whether serum calcium will plateau after an infusion of 48 hours with escalating doses or whether it will continue to increase over seven days.

To determine what will happen with a prolonged infusion, we plan to start with doses lower than 8 picomoles (pmol)/kg/hr, and then gradually increase the dose of PTH in successive groups of subjects. In the event of a significant adverse effect, immediate action will be taken to reverse it. Protocols will be in place to follow in the event of expected adverse events such as hypotension, nausea, and muscle cramping. Severe sudden side effects are not anticipated; however, mild easily reversible side effects are to be expected as an outcome in order to determine the optimal dose of PTH. This study has been approved by the NIH and the Data Safety Monitoring Board (DSMB).

Seventy five normal healthy men and women will be screened for an eight day in-patient admission to the General Clinical Research Center (GCRC). Thirty evaluable research participants will receive a seven day infusion of a predetermined dose of PTH. Vitals signs, blood pressure, blood and urine lab results will be monitored frequently as per the study flow sheet. The starting dose of PTH, 2 picomoles (pmol)/kg, will be given to three normal healthy subjects. The dose will be escalated in increments with successive groups of three subjects each, until early adverse effects (mild hypercalcemia, postural hypotension, tachycardia) are seen. This dose will then be used in future studies. The investigators with this study are trying to discover if a prolonged continuous intravenous infusion of PTH will lead to a sustained and progressive suppression of bone formation as occurs in HHM or an increase in bone formation as occurs in HPT.

Subject Population will consist of healthy young adults, ages 24-35 years, as in our other safety and physiologic studies. It is anticipated that we will need to screen 75 subjects in order to obtain 30 evaluable subjects.