Biochemical Basis of Heart Function. VI. Influence of Cations on Norepinephrine Transport, Storage, and Synthesis in the Isolated Perfused Rat Heart

The effects of Ca2+ (0–5 mM), Mg2+ (0–16 mM), K+ (0–20 mM), and Na+ (0–145 mM) on the uptake, subcellular distribution, and release of exogenously given 3H-norepinephrine were investigated in the isolated perfused rat heart. The uptake of 14C-tyrosine, the synthesis of 14C-catecholamines, and the endogenous levels of norepinephrine were also determined under these experimental conditions. The uptake of 3H-norepinephrine was markedly decreased on perfusing the hearts with media containing low concentrations of Na+. Addition of Ca2+ to the perfusion medium containing Na+ increased the uptake of 3H-norepinephrine while Mg2+ or K+ had no effect. Increasing the concentration of Na+ from 0 to 145 mM in the perfusion medium increased the 3H-label in the granular fraction and decreased it in the soluble fraction whereas Ca2+, Mg2+, or K+ did not show any action. The spontaneous release of 3H-label was not influenced by the presence or absence of Ca2+, Mg2+, or K+ in the medium. On the other hand, the absence of Na+ was found to accelerate the spontaneous release of 3H-label and this effect was further enhanced when Ca2+ was also omitted from the perfusion medium. Increasing the concentrations of Ca2+ or Na+ in the perfusion medium decreased the rate of synthesis of 14C-catecholamincs from 14C-tyrosine whereas Mg2+ and K+ showed no effect. Absence of Na+, but not of K+, Ca2+, or Mg2+, resulted in a decrease in the endogenous level of norepinephrine. The amount of newly synthesized 14C-catecholamines in the perfusate was more on perfusing the hearts with Ca2+-free or Na+-free medium. These results are consistent with the view that Na+ is required for storage as well as uptake of norepinephrine in the adrenergic nerve terminals in the heart. Ca2+ appears necessary for the optimal uptake of norepinephrine due to Na+ and has been shown to inhibit the spontaneous release of norepinephrine due to Na+ lack. In addition, Na+ and Ca2+ reduce the rate of synthesis of catecholamines in the adrenergic nerve endings in the heart.