Compartmental analysis of 45Ca2+ efflux in perfused rat liver: effects of hormonal stimulation

The kinetics of calcium movements in the isolated perfused rat liver were examined using compartmental analysis of the efflux profiles of 45Ca2+ from 45Ca(2+)-equilibrated livers under a variety of calcium concentrations and hormonal treatments. From the 45Ca2+ efflux profiles, we determined that a three compartment model was appropriate to describe the movements of calcium in the liver on the time scale of the experiments. Hormonal treatment with the alpha-adrenergic agonist, phenylephrine, or the vasoactive peptide, vasopressin, during the efflux period lowered significantly the rate of transfer of Ca2+ between the internal compartments at all of the calcium concentrations employed. Also, phenylephrine treatment leads to increased transfer of Ca2+ into the liver from the perfusate. The temporal characteristics of the phenylephrine and vasopressin sensitive Ca2+ pools were examined by pulsing livers, loaded for variable periods of time with 45Ca2+, with the two hormones during the efflux of 45Ca2+ to measure the kinetics of Ca2+ exchange in the hormone-sensitive pools. Results from these experiments indicate that the rate of unstimulated Ca2+ efflux, k2, for the phenylephrine and vasopressin sensitive Ca2+ pools, modeled as a one compartment system, are the same, 0.074 and 0.078 min-1 for phenylephrine and vasopressin respectively, corresponding to half times for turnover of the pool(s) of 9.3 and 8.9 min, respectively.