EXPORT OF GUANOSINE 3',5'-CYCLIC-MONOPHOSPHATE (CGMP) FROM HUMAN ERYTHROCYTES CHARACTERIZED BY INSIDE-OUT MEMBRANE-VESICLES

The present study was undertaken to characterize the export of cGMP fr om human erythrocytes at 37 degrees C. Inside-out membrane vesicles we re exposed to cGMP and [H-3]-cGMP in the presence and absence of 2 mmo l l(-1) ATP. In the absence of ATP, an equilibrium was reached within 15 min for the lowest tested concentration (0.65 mu mol l(-1)), and th e amount of cGMP in the vesicles was linearly correlated to the cGMP c oncentrations in the incubate. These observations suggest that the ATP -independent process represents passive diffusion or nonsaturated bind ing to membrane components. In the presence of ATP, cGMP accumulated l inearly during the test period (up to 120 min) and the transport into the inside-out vesicles was dependent on both low- and high-K-m transp ort. The kinetic parameters for the low-K-m process were determined af ter 5 and 120 min, the K-m values being 4.6 (SD 1.9) and 4.7 (SD 1.1) mu mol l(-1) (n=3), respectively. The corresponding V-max values were 400 (SD 50) and 440 (SD 70) fmol mg(-1) min(-1). The high-K-m process was characterized by K-m = 170 (SD 50) mu mol l(-1) and V-max = 1610 ( SD 280) fmol mg(-1) min(-1) (n=5). The present data demonstrate an ATP -requiring saturable transport system for cGMP in human erythrocytes.