Long-Term Systemic Administration of Human Recombinant Interleukin-1β Induces a Dose-Dependent Fall in Circulating Parathyroid Hormone in Rats

Abstract

The synergism/antagonism between interleukin (IL)-1β and parathyroid hormone (PTH) has been the subject of in vitro and in vivo work, but a possible direct action of the cytokine on PTH release has not been reported. We have investigated the effect of a continuous infusion of human recombinant IL-1β (rIL-1β) on circulating PTH during a 14-day period in 7-week-old female rats. This time interval was chosen in order to exclude initial hypercalcemia and to enable data collection under steady-state conditions. Five groups of 20 animals each had miniosmotic pumps (Alzet 2002, 200 µl) implanted subcutaneously and primed to release either distilled water (controls) or 100, 500, 1,000 and 2,000 ng/24 h of rIL-1β. Blood was drawn on days 1 and 14 for PTH, corticosterone and Ca²⁺ determinations. Adequate biological activity of the infused rIL-1β was supported by elevated rectal temperature records and significant elevations of plasma corticosterone on day 14. The 100-ng dose had no effect but 500–2,000 ng rIL-1β/24 h significantly reduced plasma PTH in a dose-dependent manner down to 54% of basal value (20.4 ± 1.1 vs. 15.3 ± 1.4 pg/ml for 500 ng, p < 0.005; 20.5 ± 1.3 vs 12.3 ± 1.1 for 1,000 ng, p < 0.001, and 19.5 ± 2.0 vs. 10.6 ± 1.1 pg/ml for 2,000 ng, p < 0.0008). Despite these findings, no differences in blood Ca²⁺ could be detected between treated animals and controls. The following conclusions can be inferred from the foregoing: Systemic administration of rIL-1β to rats induced a dose-dependent fall in circulating PTH without altering calcemia, calling into question the biological relevance of the former finding. Although the recorded PTH depression may indeed not have been severe enough to cause hypocalcemia, it can be hypothesized that osteoclast activation by rIL-1β would enhance bone mineral release into the pool compensating for depressed PTH activity.