Inhibition by guanosine cyclic monophosphate (cGMP) analogues of uptake of [(3)H]3',5'-cGMP without stimulation of ATPase activity in human erythrocyte inside-out vesicles

The cellular extrusion of guanosine 3',5'-cyclic monophosphate (3',5'-cGMP) is a unidirectional ATP-dependent process that is inhibited by probenecid, a non-selective transport inhibitor of organic anions. In the present study, various cGMP analogues were tested for their ability to inhibit 3',5'-cGMP efflux and stimulate the cGMP-selective ATPase in human erythrocytes. The difference in uptake of 1 microM [(3)H]3',5'-cGMP to inside-out vesicles in the presence and absence of 1 mM ATP at 37 degrees was defined as active transport. Two ATP-dependent components were detected for unlabelled 3',5'-cGMP (0.01--100 microM) with respective K(i) of 1.3 +/- 0.2 and 280 +/- 50 microM (mean +/- SEM, N = 3). The high-affinity transport was inhibited by the analogues with a typical pattern: Rp-monophosphorothioate guanosine 3',5'-cyclic monophosphate (Rp-cGMPS) > 3',5'-cGMP > 2'-O-monobutyryl guanosine 3',5'-cyclic monophosphate (O-mb-cGMP) approximately N(2)-monobutyryl guanosine 3',5'-cyclic monophosphate (N-mb-cGMP) > or = N(2),2'-O-dibutyryl guanosine 3',5'-cyclic monophosphate (Db-cGMP) approximately 8'-bromo guanosine 3',5'-cyclic monophosphate (Br-cGMP) approximately Guanosine 2',3'-cyclic monophosphate (2'3'-cGMP) > Sp-monophosphorothioate guanosine 3',5'-cyclic monophosphate (Sp-cGMPS). A concentration-dependent inhibition was found for the low-affinity transport, but no distinct order of potency was identified. Analysis according to Lineweaver--Burk of active [(3)H]3',5'-cGMP transport (0.2--2 microM) gave a K(m) value of 1.5 +/- 0.1 microM (mean +/- SEM, N = 3). The presence of 10 microM cGMP analogues did not change the ordinate intercept, but made the slopes steeper with a typical order: Rp-cGMPS > 3',5'-cGMP > N-mb-cGMP approximately O-mb-cGMP approximately db-cGMP approximately 8-Br-cGMP > 2',3'-cGMP > Sp-cGMPS. Only 3',5'-cGMP and 2',3'-cGMP were able to activate the cGMP-specific ATPase, 640 +/- 200% and 430 +/- 160% (mean +/- SEM, N = 5) above basal levels, respectively. The present data show that the binding is less selective than ATPase activation of the cellular cGMP transport system.