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BACKGROUND: Parathyroid hormone-related protein (PTHrP) is a ubiquitous and highly conserved vasoactive peptide whose role and regulation in normal physiology remain an enigma. Recently, we demonstrated that low-dose endotoxin (LPS) induces intrasplenic, but not systemic, levels of PTHrP; and that tumor necrosis factor, a pro-inflammatory cytokine, is the major mediator of this effect. We have therefore hypothesized that, with higher, lethal doses of endotoxin, PTHrP could be induced in multiple tissues to such a degree that it could contribute to the lethality of septic shock. MATERIALS AND METHODS: Northern blot analysis was used to measure PTHrP mRNA levels in vital organs of rats after administration of a near lethal dose (5 mg/250 g) of LPS (or vehicle alone). Plasma levels of PTHrP were also measured by immunoradiometric assay. The ability of the immunoglobulin fraction of two different PTHrP(1-34) antisera to protect from LPS-induced lethality was also studied in mice using survival analysis. RESULTS: In response to a near-lethal dose of endotoxin, PTHrP mRNA levels increased acutely in every vital organ examined (spleen, lung, heart, kidney, and liver). Circulating levels of PTHrP also increased, peaking 2 hr after administration of high-dose endotoxin. Passive immunization of mice with anti-PTHrP(1-34) antibody 6 hr prior to administration of a lethal dose of LPS protected mice from endotoxin-induced death (p < 0.00005). CONCLUSIONS: These results suggest that PTHrP belongs to the cascade of pro-inflammatory cytokines induced during lethal endotoxemia that is responsible for the toxic effects of LPS.