Pulsating fluid flow stimulates prostaglandin release and inducible prostaglandin G/H synthase mRNA expression in primary mouse bone cells

Bone tissue responds to mechanical stress with adaptive changes in mass and structure. Mechanical stress produces now of fluid in the osteocyte lacunar-canalicular network, which is likely the physiological signal for bone ceh adaptive responses. We examined the effects of Ih pulsating fluid now (PFF; 0.7 +/- 0.02 Pa, 5 Hz) on prostaglandin (PG) E(2), PGI(2), and PGF(2 alpha) production and on the expression of the constitutive and inducible prostaglandin G/H synthases, PGHS-1, and PGHS-2, the major enzymes in the conversion of arachidonic acid to prostaglandins, using mouse calvarial bone cell cultures. PFF treatment stimulated the release of all three prostaglandins under 2% serum conditions, but with a different time course and to a different extent. PGF(2 alpha) was rapidly increased 5-10 minutes after the onset of PFF. PGE(2) release increased somewhat more slowly (significant after 10 minutes), but continued throughout 60 minutes of treatment. The response of PGI(2) was the slowest, and only significant after 30 and 60 minutes of treatment. In addition, PFF induced the expression of PGHS-2 but not PGHS-1. One hour of PFF treatment increased PGHS-2 mRNA expression about 2-fold relative to the induction by 2% fresh serum given at the start of PFF. When the addition of fresh serum was reduced to 0.1%, the induction of PGHS-2 was 8- to 9-fold in PFF-treated cells relative to controls. This up-regulation continued for at least 1 h after PFF removal. PFF also markedly increased PODS activity, measured as the conversion of arachidonic acid into PGE(2). One hour after PFF removal, the production of all three prostaglandins was still enhanced. These results suggest that prostaglandins are important early mediators of the response of bone cells to mechanical stress. Prostaglandin up-regulation is associated with an induction of PGHS-2 enzyme mRNA, which may subsequently provide a means for amplifying the cellular response to mechanical stress.