Modulation of high affinity ATP-dependent cyclic nucleotide transporters by specific and non-specific cyclic nucleotide phosphodiesterase inhibitors

Abstract

Intracellular cyclic nucleotides are eliminated by phosphodiesterases (PDEs) and by ATP Binding cassette transporters such as ABCC4 and ABCC5. PDE5 and ABCC5 have similar affinity for cGMP whereas ABCC5 has much higher affinity for cGMP compared with cAMP. Since the substrate (cGMP) is identical for these two eliminatory processes it is conceivable that various PDE inhibitors also modulate ABCC5-transport. Cyclic GMP is also transported by ABBC4 but the affinity is much lower with a K_m 50–100 times higher than for that of ABBCC5. The present study aimed to determine K_i-values for specific or relative specific PDE5 inhibitors (vardenafil, tadalafil, zaprinast and dipyridamole) and the non-specific PDE inhibitors (IBMX, caffeine and theophylline) for ABCC5 andABCC4 transport. The transport of [³ H]-cGMP (2 &#956M) was concentration-dependently inhibited with the following K_i-values: vardenafil (0.62 &#956M), tadalafil (14.1 &#956M), zaprinast (0.68 &#956M) and dipyridamole (1.2 &#956M), IBMX (10 &#956M), caffeine (48 &#956M) and theophylline (69 &#956M). The K_i-values for the inhibition of the [³ H]-cAMP (2 &#956M) transport were: vardenafil (3.4 &#956M), tadalafil (194 &#956M), zaprinast (2.8 &#956M), dipyridamole (5.5 &#956M), IBMX (16 &#956M), caffeine (41 &#956M) and theophylline (85 &#956M). The specificity for ABCC5 we defined as ratio between K_i-values for inhibition of [³ H]-cGMP and [³ H]-cAMP transport. Tadalafil showed the highest specificity (K_i-ratio: 0.073) and caffeine the lowest (K_i-ratio: 1.2).