Buffering and CO2 dissociation of body fluids in the pupa of the silkworm moth, Hyalophora cecropia.
To assess the extent of CO2 storage and the changes in the acid-base status that occur during intermittent CO2 excretion in insects, total CO2 content and pH were measured in whole body homogenates (= tissue homogenates) of Hyalophora cecropia pupae at various levels of Pco2 at 20 degrees C. The CO2 dissociation curve, i.e. plot of total CO2 content in tissue homogenates against Pco2 was nearly linear in the Pco2 range from 15 to 50 Torr, the mean slope being 0.138 mM . Torr-1. This value, which constitutes the effective CO2 solubility, was nearly three times the physical solubility in the tissue homogenate which averaged 0.053 mM . Torr-1. Plots of bicarbonate concentration in whole body tissue water against pH yielded an average buffer value of 75 mmol . pH-1 per kg tissue water. The high buffer value results in a small pH change, about 0.04 units, when Pco2 varies between 20 and 45 Torr in the respiratory cycle. The absolute value of mean tissue pH at Pco2 = 30 Torr predicted from the buffer line, 6.57, agrees well with direct measurement in hemolymph samples.