How can one relate in vitro studies of caffeine (dose expressed as concentration) to dietary intake of caffeine (dose in mass)?
Caffeine rapidly diffuses throughout the body's water, both intra and extracellular.
Therefore, since the caffeine will be relatively evenly distributed, you can calculate the amount of caffeine needed to reach a given molar concentration within any particular cell by knowing the total water in a person and the molar mass of caffeine (194.19 g/mol ). A 72 kg person has about 40 liters of water in their body.
Combining those numbers, reaching 1mM requires 7.7676 grams of caffeine. Reaching 5mM requires 38.838 grams.
How are the “in vitro” results of the below charts relevant to levels that cells would experience "in vivo" after the consumption of caffeinated beverages?
Wikipedia puts the LD50 of caffeine at 150-200 mg/kg, or 10.8-14.4g for our hypothetical 72kg man. These numbers are likely very approximate as there isn't going to be a lot of well controlled clinical data on caffeine toxicity in humans, but someone reaching those concentrations stands a good chance of dying from caffeine poisoning.
For that reason I don't think these results are relevant to normal caffeine consumption. I interpreted the high concentrations used in that study as a deliberate attempt to induce cell injury, not as a means to simulate normal caffeine consumption.