The influence of the rate of perfusion on the kinetics of neuronal uptake in the rabbit isolated heart

Summary

• 1. Rabbit hearts (with monoamine oxidase and catechol-O-methyl transferase inhibited) were obtained from reserpine-pretreated animals. They were perfused at rates ranging from 1.3–11.3 ml·g⁻¹·min⁻¹ with 0.1 mM ¹⁴C-sorbitol and various concentrations of ³H-(−)noradrenaline (NA). From measurements of the arterio-venous concentration difference of ³H and ¹⁴C activity the removal of NA and sorbitol from the perfusion fluid was followed for 2–3 min at intervals of 5 s. The uptake of NA into intracellular spaces of the heart (known to be over-whelmingly into sympathetic nerve terminals) was obtained by subtracting the removal of sorbitol from that of NA. If was cumulated and plotted against time.

• 2. The progress curves of NA uptake were sigmoid in shape: following a lag period, uptake proceeded at first at a constant initial rate and from then on at gradually decreasing rates. Irrespective of the NA concentration used, the lag period became shorter and the initial rate of uptake increased whenever the rate of perfusion was increased. Furthermore, at high rates of perfusion the initial rate was maintained for a shorter time than at low ones.

• 3. At any given perfusion rate, the initial rates of NA uptake obeyed Michaelis-Menten kinetics. While changes of the rate of flow did not alter the apparent K _m (range: 2.2–2.4 μM), a rectangular hyperbolic relationship was found between V _max and the perfusion rate. The V _max was half-maximal at a rate of flow of 2.7 ml·g⁻¹·min⁻¹ and approached a maximum value of 9.0 nmoles·g⁻¹·min⁻¹.

• 4. From the lack of change in the K _m it can be concluded that the uptake sites of the perfused heart are functionally arranged in parallel. The change in V _max, on the other hand, indicate that the accessibility of the sites is limited by the rate of perfusion.