The disposition of cells whose mechanism of death is related to the age of the cell cannot be appropriately represented by pharmacodynamic (PD) models where the elimination rate is related to the number of cells. In cells with age-related death their disposition is determined by their lifespan. Thus in these cells PD models of cellular response must incorporate a lifespan component. Previous cellular lifespan PD models assumed that the lifespan of cells is predetermined and does not vary over time. However, in many instances these assumptions are inappropriate and thus extensions to the existing models are needed. An important application of these time variant PD models is determining the erythropoiesis rate, since the lifespan of reticulocytes and mature erythrocytes are known to change over time under specific conditions. The objectives us this work were to develop a general time variant lifespan-based PD model of cellular response and to use the model to determine the dynamic changes over time in both the erythrocyte lifespan and erythropoiesis rate under a variety of complex conditions. An initial time variant cellular lifespan model was formulated assuming no variability in lifespans and used to determine the dynamic changes in both the reticulocyte lifespan and erythropoiesis rate in sheep. Subsequently, the time variant model was extended to account for a distribution of cellular lifespans, which resulted in better capturing the physiology of sheep erythrocyte maturation. The model was then further extended to account for the effect of changes in the environment on cell lifespans and used to determine the effect of chemotherapy administration on sheep erythrocytes. In order to conduct studies on erythropoiesis in premature very low birth weight (VLBW) infants the ability to accurately measure erythrocytes and hemoglobin from clinically collected excess blood was validated. Then an in depth analysis of the relationship between erythropoietin, erythrocytes, and hemoglobin was conducted in a clinical study of premature VLBW infants that accounted for the dynamic hematological conditions experienced by these subjects. This analysis indicated that a nearly 4-fold increase in erythropoiesis could be achieved with only a modest increase in plasma erythropoietin concentrations.

In 2D cell cultures uptake/release of various metabolic analytes such as glucose, lactate or metabolic by-products like hydrogen peroxide from/to the extracellular environment results in concentration gradients. The magnitude, direction, and time scales of these gradients carries information that is essential for internal cellular processes and/or for communication with neighboring cells. This PhD research work focusses on the design, fabrication and characterization of electrochemical microelectrode arrays (MEAs) optimized to be positioned in commonly used 2D cell culture setups. Importantly, by simultaneously measuring accurate concentration transients and associated gradients/uxes near the cell surface (surface concentration) the capability of the device to quantify kinetic rates and distinguish mechanisms involved in various cellular processes is demonstrated. An in-situ transient calibration technique suitable for amperometric MEAs is developed and the technique is validated by quantitatively measuring dynamic concentration profiles with varying spatial (100-800 µm) and time (s to hrs.) scales set up from an electrically controlled diffusion reaction system. With the proposed MEA design and technique three physiological applications are demonstrated. Firstly, the position able 1D MEA was employed real time to quantitatively measure the hydrogen peroxide scavenging rates from astrocyte vs glioblastoma cell cultures. With the ability to extract to dynamic surface concentration and fluxes, the cell lines were shown to have hydrogen peroxide uptake rates dependent on local surface concentrations. Moreover, the cancerous glioblastoma cells demonstrated an upregulated linear peroxide scavenging mechanism as compared to astrocytes. For the next phase, spatial scales of 1D MEA device along the size and functionalization scheme of the electrodes in the MEA was further modified to selectively sense glucose and lactate to enable extracellular metabolic profiling of cancer vs normal cell lines. Secondly, measurement of glucose concentration profiles demonstrated an increased glucose uptake rate in glioblastoma as compared to astrocytes. Additionally, sigmoidal (allosteric) vs Michaelis - Menten glucose uptake kinetics was observed in glioblastoma vs astrocytes. Moreover, the presence of a glucose sensing mechanism was observed in glioblastoma cells due to the dependence of the glucose uptake rate on initial exposed concentration rather than surface concentration. Finally, simultaneous multi-analyte (glucose and lactate) gradient measurements were performed on genetically modified mouse pancreatic cancer cell lines. While glucose uptake rate was shown to increase with increasing extracellular glucose concentration for one of the cell lines, the lactate release rate was observed to be independent of the initial extracellular glucose dose.