Isoproterenol-stimulated taurine influx in the perfused rat heart

Taurine influx in the perfused rat heart was characterized and the effect of isoproterenol on this process determined. Hearts were perfused by the Langendorff technique with [3H]-taurine in a non-recirculating system. The rate of taurine influx was constant for at least 20 min and the process was saturable. A Km of 45 micron indicated that taurine influx is mediated by a high affinity transport system. Competition between taurine and beta-alanine, but not alpha-amino acids, for influx indicated that the transport sites are specific for beta-amino acids. Isoproterenol (4 X 10(-7) M) stimulated the rate of taurine influx, but propranolol (1 X 10(-8) M) blocked this stimulation. The enhancement of influx by isoproterenol was specific for beta-amino acids in that alpha-amino acid influx was not affected. Dibutyryl cyclic AMP (1 X 10(-3) M) and theophylline (1 X 10(-3) M) also stimulated taurine influx, whereas alterations in heart rate had no effect on the rate of taurine influx. The results are suggestive of a beta-adrenergically activated, cyclic AMP-mediated mechanism controlling isoproterenol-stimulated taurine influx.