Cyclic nucleotide metabolism in the human erythrocyte

Erythrocytes, which show little or no guanylate or adenylate cyclase activity, take up cyclic nucleotides from blood. Studies were done by incubating human erythrocytes in isotonic medium with the dibutyryl derivatives of cAMP and cGMP and in a hypotonic medium in which the cells are partially hemolyzed and, therefore, freely permeable to cAMP and cGMP. At cAMP and cGMP concentrations of 50 microM and above, the amount of 14CO2 generated from 1-14C-glucose was decreased significantly. This effect was suppressed by 4.6 mM theophylline. Inosine and ribose, which are catabolites of cAMP and cGMP also decreased formation of 14CO2 from 1-14C-glucose. Accordingly, it is postulated that in the absence of theophylline, the activity of phosphodiesterase resulted in AMP and GMP formation. These mononucleotides enter into the purine salvage pathways to form ribose phosphate. Additionally, the production of lactate and 2,3-diphosphoglycerate (2,3-DPG) was measured in human erythrocytes after incubation with the dibutyryl derivatives of cAMP (bt2-cAMP) and cGMP (bt2-cGMP). At a concentration of 0.1 microM, bta2-cGMP inhibits lactate production at 60 min (p less than 0.01). Slight increases in 2,3-DPG were not statistically significant. Catabolism of cyclic nucleotides may prevent diffusion equilibria allowing for the erythrocyte's continuous uptake of cyclic nucleotides and providing a detoxification mechanism that could compensate for conditions in which elevations of these substances are observed.