A calorimetric flow vessel optimised for measuring the metabolic activity of animal cells

The heat flux of animal cells growing in vitro is a direct measurement of t heir specific metabolic activity because the majority of the substrate Gibb s energy is dissipated as heat with only a small quantity being dissipated as entropy conserved in the form of biomass. Heat flux can be used as a var iable to monitor the growth of engineered cells producing heterologous prot eins in large-scale, industrial culture. Indeed, since heat flux is a funct ion of metabolism, it can be used as the control variable in fed-batch cult ure. Until now, monitoring a culture by circulating a cell suspension to a standard thermometric TAM how calorimeter has been compromised by direction of flow, slow maximum flow rate, narrow bore tubing and small volume of th e measuring vessel. A new twin flow module for the TAM calorimeter is descr ibed which is optimised for use with cell suspensions as well as for cells growing on microcarrier beads. It consists of a continuous length of stainl ess steel tubing of 1.5 mm ID which forms the transmission lines, the heat exchangers and the coiled measuring vessel nominally of 1 cm(3) in volume. The heat exchangers are of sufficient volume to permit fast pumping rates ( <200 cm(3) h(-1)) while the increased size of the vessel allows for a lower detection limit. The module is designed for downward flow through this ves sel. It also has a calibration heater (nominal 50 Ohm resister) between the fine heat exchanger and the measuring vessel. The detector is downstream o f this vessel and consists of a thermopile operating on the heat conduction principle. The flow module was chemically calibrated by the exothermic hyd rolysis of triacetin in imidazole-acetate buffer At 100 cm(3) h(-1), the ef fective thermal volume was found to 1.05 cm(3). For validation, the experiments were on the growth of CHO320 cells producin g recombinant interferon-gamma in batch culture and under the steady state conditions of continuous culture. The on-line heat flow rate was smoothed b y the moving-average technique and showed an increase proportional to cell growth during the batch phase and only small changes after setting up the c ontinuous culture. Repeated experiments gave consistent results without sig ns of erratic recordings and thus indicated that the specially designed flo w module can indeed realise the potential of heat flux to monitor metabolic activity in batch and perfusion cultures together with being a control var iable in fed-batch cultures. (C) 1999 Elsevier Science B.V. All rights rese rved.