Transport of cyclic AMP and synthetic analogs in the perfused rat liver

The purpose of the present work was to investigate the transport of cyclic AMP (cAMP) and analogs in the rat liver. The experimental system was the isolated once-through perfused liver. Transport was measured by employing the multiple-indicator dilution technique. The single-pass recovery of tracer [(32)P]cAMP was equal to 94.4 +/- 1. 4%; no significant extracellular transformation of cAMP occurred during a single passage. The unidirectional influx rates of dibutyryl-cAMP were a saturable function of its concentration, with K(m) = 72.75 +/- 9.24 microM and V(max) = 0.464 +/- 0.026 micromol min(-1) (mL cellular space)(-1). The unidirectional influx rates of cAMP were much lower than those of dibutyryl-cAMP and were a linear function of the concentration (up to 100 microM). The transfer coefficient for influx (k(in)) was equal to 0.860 +/- 0.058 mL min(-1) (mL extracellular space)(-1). cAMP inhibited the influx of dibutyryl-cAMP; the IC(50) was 0.83 mM. The following series of increasing unidirectional influx rates was found: cAMP < monobutyryl-cAMP approximately 2-aza-epsilon-cAMP < rp-cAMPS approximately sp-cAMPS < 8-Br-cAMP approximately dibutyryl-cGMP approximately 8-Cl-cAMP < O-dibutyryl-cAMP. There was no precise correlation between the rates of influx of the various cyclic nucleotides and their lipophilicity. It was concluded that the penetration of cAMP and its analogs into the liver cells was a facilitated process. Lipophilicity was not the only factor determining the rate of transport. The transformation of dibutyryl-cAMP was limited by both transport and activity of the intracellular enzymic systems. The intracellular transformation of exogenous cAMP, however, was limited by the transport process.