On-chip acidification rate measurements from single cardiac cells confined in sub-nanoliter volumes.

The metabolic activity of cells can be monitored by measuring the pH in the extracellular environment. Microfabrication and microfluidic technologies allow the sensor size and the extracellular volumes to be comparable to single cells. A glass substrate with thin film pH sensitive IrO( x ) electrodes was sealed to a replica-molded polydimethylsiloxane (PDMS) microfluidic network with integrated valves. The device, termed NanoPhysiometer, allows the trapping of single cardiac myocytes and the measurement of the pH in a detection volume of 0.36 nL. For wild-type (WT) single cardiac myocytes an acidification rate of 6.45 +/- 0.38 mpH/min was measured in comparison to 19.5 +/- 0.38 mpH/min for very long chain Acyl-CoA dehydrogenase (VLCAD) deficient mice in 0.8 mM of Ca(2+). VLCAD deficiency is a fatty acid oxidation disease leading to cardiomyopathy and arrhythmias. The acidification rate increased to 11.96 +/- 1.33 mpH/min for WT and to 32.0 +/- 4.64 mpH/min for VLCAD -/- in 1.8 mM of Ca(2+). The NanoPhysiometer concept can be extended to study ischemia/reperfusion injury or disorders of other biological systems to identify strategies for treatment and possible pharmacological targets.