NUCLEOSIDE INFLUX AND EFFLUX IN GUINEA-PIG VENTRICULAR MYOCYTES - INHIBITION BY ANALOGS OF LIDOFLAZINE

Adenosine influx and formycin B influx and efflux were characterized i n guinea-pig ventricular myocytes at 22 degrees. Transport by both mod es was saturable and inhibited by nitrobenzylthioinosine (NBMPR), indi cating the presence of an equilibrative NBMPR-sensitive nucleoside tra nsporter in the cardiomyocytes. The kinetic constants for influx and e fflux of formycin B, a non-metabolized nucleoside, were similar, sugge sting that the nucleoside transporter exhibits symmetrical kinetics (a pparent K-m 490 +/- 160 and 700 +/- 140 mu M; V-max, 6.5 +/- 1.7 and 3 .5 +/- 0.3 nmol/10(6) cells per min for influx and efflux, respectivel y). No evidence was found of either NBMPR-insensitive equilibrative nu cleoside transport or sodium-dependent concentrative nucleoside transp ort. Inhibition of adenosine influx (apparent K(m)100 +/- 33 mu M), by lidoflazine and the analogues mioflazine, soluflazine and R73-335, ga ve average K-i values of 730, 100, 64 and 2.9 nM, respectively. These compounds also inhibited formycin B efflux with a similar potency to t hat of adenosine influx. NBMPR-sensitive nucleoside transport was asso ciated with high affinity binding of NBMPR (apparent K-d similar to 1 nM; 9.6 x 10(5) sites/cell). Specific binding of NBMPR was also inhibi ted by lidoflazine and its analogues. Mioflazine and soluflazine were 20-30-fold more potent at inhibiting NBMPR-sensitive nucleoside influx in guinea-pig erythrocytes than ventricular myocytes, indicating that the potency of some of the compounds studied is tissue dependent.