Chondrocytes Embedded in Agarose Generate Distinct Metabolic Heat Profiles Based on Media Carbon Sources.

Human chondrocytes are responsible for cartilage repair and homeostasis through metabolic production of precursors to collagen and other matrix components. This metabolism is sensitive both to the availability of media energy sources as well as the local temperature. Central carbon metabolites such as glucose and glutamine are essential not only for producing energetic compounds such as ATP and NADH, but also for assembling collagen and aggrecan from non-essential amino acid precursors. The rate at which this metabolism takes place directly relates to temperature: a moderate increase in temperature results in faster enzyme kinetics and faster metabolic processes. Furthermore, these biological processes are exothermic and will generate heat as a byproduct, further heating the local environment of the cell. Prior studies suggest that mechanical stimuli affect levels of central metabolites in three-dimensionally cultured articular chondrocytes. But these prior studies have not determined if articular chondrocytes produce measurable heat. Thus, the goal of this study is to determine if three-dimensionally encapsulated chondrocytes are capable of heat production which will improve our knowledge of chondrocyte central metabolism and further validate in vitro methods. Here we show the results of microcalorimetric measurements of heat generated by chondrocytes suspended in agarose hydrogels over a 2-day period in PBS, glucose, and glutamine media. The results show that a significant amount of heat is generated by cells (Cells Only: 3.033 ± 0.574 µJ/cell, Glucose: 2.791 ± 0.819 µJ/cell, Glutamine: 1.900 ± 0.650 µJ/cell) versus the absence of cells (No Cells: 0.374 ± 0.251 µJ/cell). This suggests that cells which have access to carbon sources in the media or as intracellular reserves will generate a significant amount of heat as they process these metabolites, produce cellular energy, and synthesize collagen precursors. The length of the microcalorimeter experiment (48 h) also suggests that the metabolism of articular chondrocytes is slower than many other cells, such as human melanoma cells, which can produce similar quantities of heat in less than an hour. These data broadly suggest that chondrocyte metabolism is sensitive to the available nutrients and has the potential to alter cartilage temperature through metabolic activity.