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

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° 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 was 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 C_3. At 1.3 mM extracellular Ca²⁺, a decrease in incubation temperature from 37°C to 20°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°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°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 incease 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.