Real-time measurements of human chondrocyte heat production during in vitro proliferation

Isothermal microcalorimeters (IMC) are highly sensitive instruments that allow the measurement of heat flow in the microwatt range. Due to their detection of minute thermal heat, IMC techniques have been used in numerous biological applications, including the study of fermentation processes, pharmaceutical development, and cell metabolism. In this work, with the ultimate goal of establishing a rapid and real-time method to predict the proliferative capacity of human articular chondrocytes (HAC), we explored to use of IMC to characterize one of the crucial steps within the process of cartilage tissue engineering, namely the in vitro expansion of HAC. We first established an IMC-based model for the real-time monitoring of heat flow generated by HAC during proliferation. Profiles of the heat and heat flow curves obtained were consistent with those previously shown for other cell types. The average heat flow per HAC was determined to be 22.0 ± 5.3 pW. We next demonstrated that HAC proliferation within the IMC-based model was similar to proliferation under standard culture conditions, verifying its relevance for simulating the typical cell culture application. HAC growth and HAC heat over time appeared correlated for cells derived from particular donors. However, based on the results from 12 independent donors, no predictive correlation could be established between the growth rate and the heat increase rate of HAC. This was likely due to variability in the biological function of HAC derived from different donors, combined with the complexity of tightly couple metabolic processes beyond proliferation. In conclusion, IMC appears to be a promising technique to characterize cell proliferation. However, studies with more reproducible cell sources (e.g., cell lines) could be used before adding the complexity associated with primary human cells.