Uptake kinetics and ion requirements for extraneuronal uptake of noradrenaline by arterial smooth muscle and collagen

Abstract

1 The ionic requirements for noradrenaline uptake into vascular smooth muscle cells were studied by perfusing rabbit isolated ear arteries with noradrenaline (10⁻³M) either in Krebs solution or in Krebs solution modified by altering the concentration of one or more ion. Noradrenaline uptake was measured by quantitative microphotometry. 2 Some uptake into smooth muscle continued in isotonic sucrose in the absence of all ions. Omission of Na⁺ from the Krebs solution partially inhibited uptake as did high (100 mm) K⁺. Omission of K⁺, Ca⁺⁺ or Mg⁺⁺ had no effect on uptake. Lithium was able completely to substitute for Na⁺. 3 Alteration in ion concentration did not affect the binding of noradrenaline to collagen. 4 The kinetics of uptake of noradrenaline into smooth muscle were analysed and found to be saturable with a Km of 4·9 × 10⁻⁴M. 5 It is concluded that the ionic requirements of the transport mechanism for the uptake of noradrenaline by vascular smooth muscle show a relatively low specificity.