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

Fluctuations in intracellular free calcium concentration ([Ca2+]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 [Ca2+]i. However, specific down-stream effects of [Ca2+]i on gene expression and phenotype in BAC remain to be defined. The present work was designed to determine whether [Ca2+]i mobilization regulates aggrecan mRNA levels. [Ca2+]i was transiently elevated by exposing BAC to the [Ca2+]-specific ionophore, ionomycin. The results show that ionomycin increases [Ca2+]i in a dose-dependent fashion. Semi-quantitative real time (RT)-PCR was used to study the effects of increased [Ca2+]i on steady state levels of aggrecan mRNA. Four hours after a brief exposure to 1.5 microM 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 [Ca2+]i in BAC, it did not affect aggrecan mRNA levels. Taken together, these data suggest that ionomycin-induced [Ca2+]i fluctuations regulate aggrecan mRNA levels, but that flow induced [Ca2+]i fluctuations do not.