A kinetic investigation of the effects of adrenaline on 45Ca2+ exchange in isolated hepatocytes at different Ca2+ concentrations, at 20 degrees C and in the presence of inhibitors of mitochondrial Ca2+ transport

Abstract

The effects of adrenaline on 45Ca2+-exchange curves for isolated hepatocytes incubated under various steady-state conditions were investigated. Kinetic analysis showed that the simplest compartment configuration consistent with each set of data was a series configuration of a three-compartment closed system comprising compartment 1 (C1), the extracellular medium, and two kinetically distinct compartments of cellular exchangeable Ca2+, C2 and C3 (C1 = C2 = C3). Subcellular fractionation of hepatocytes labelled with 45Ca2+ at 0.1 mM-Ca2+ indicated that C3 includes exchangeable Ca2+ in the mitochondria and endoplasmic reticulum. The following results were obtained from experiments conducted at 37 degrees C at five different extracellular Ca2+ concentrations. For both untreated and adrenaline-treated cells, plots of the flux from C1 to C2 as a function of the extracellular Ca2+ concentration were best described by straight lines consistent with Ca2+ influx across the plasma membrane being a diffusion process. Adrenaline increased the value of the permeability constant for Ca2+ influx by 40%. For untreated cells, plots of the flux between C2 and C3 as a function of the concentrations of Ca2+ in these compartments approached a plateau at high Ca2+ concentrations. Adrenaline caused a 3-fold increase in the concentration of Ca2+ that gives half-maximal rate of Ca2+ transport from C2 to C3. At 1.3 mM extracellular Ca2+, a decrease in incubation temperature from 37 degrees C to 20 degrees C decreased the quantity of Ca2+ in C3 and the flux and fractional transfer rates for the transport of Ca2+ between C2 and C3. At 20 degrees C adrenaline increased the quantity of Ca2+ in C3 and the fractional transfer rates for the transfer of Ca2+ from C1 to C2, and from C2 to C3. At 37 degrees C and 2.4 mM extracellular Ca2+, antimycin A plus oligomycin decreased the quantity of Ca2+ in C3 and increased the fractional transfer rate for the transport of Ca2+ from C3 to C2. In the presence of antimycin A and oligomycin, adrenaline did not increase the quantity of Ca2+ in C2 or the flux and fractional transfer rate for the transport of Ca2+ from C1 to C2, whereas these parameters were increased in the absence of the inhibitors.