Increases in cytosolic calcium, but not fluid flow, affect aggrecan mRNA levels in articular chondrocytes

Fluctuations in intracellular free calcium concentration ([Ca 2+ ] i ) is thought to be one mechanism by which cells transduce mechanical signals into biological responses. Primary cultures of bovine articular chondrocytes (BAC) respond to oscillating fluid flow with a transient rise in [Ca 2+ ] i . However, specific down‐stream effects of [Ca 2+ ] i on gene expression and phenotype in BAC remain to be defined. The present work was designed to determine whether [Ca 2+ ] i mobilization regulates aggrecan mRNA levels. [Ca 2+ ] i was transiently elevated by exposing BAC to the [Ca 2+ ]‐specific ionophore, ionomycin. The results show that ionomycin increases [Ca 2+ ] i in a dose‐dependent fashion. Semi‐quantitative real time (RT)‐PCR was used to study the effects of increased [Ca 2+ ] i on steady state levels of aggrecan mRNA. Four hours after a brief exposure to 1.5 μM ionomycin, BAC displayed a nearly four‐fold decrease in aggrecan mRNA levels compared to control cells. This effect of ionomycin on aggrecan mRNA was no longer evident 6 or 10 h later. Despite previous observations that oscillating fluid flow elicits increased [Ca 2+ ] i in BAC, it did not affect aggrecan mRNA levels. Taken together, these data suggest that ionomycin‐induced [Ca 2+ ] i fluctuations regulate aggrecan mRNA levels, but that flow induced [Ca 2+ ] i fluctuations do not. © 2003 Wiley‐Liss, Inc.