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

The effects of Ca 2+ (0–5 mM), Mg 2+ (0–16 mM), K + (0–20 mM), and Na + (0–145 mM) on the uptake, subcellular distribution, and release of exogenously given 3 H-norepinephrine were investigated in the isolated perfused rat heart. The uptake of 14 C-tyrosine, the synthesis of 14 C-catecholamines, and the endogenous levels of norepinephrine were also determined under these experimental conditions. The uptake of 3 H-norepinephrine was markedly decreased on perfusing the hearts with media containing low concentrations of Na + . Addition of Ca 2+ to the perfusion medium containing Na + increased the uptake of 3 H-norepinephrine while Mg 2+ or K + had no effect. Increasing the concentration of Na + from 0 to 145 mM in the perfusion medium increased the 3 H-label in the granular fraction and decreased it in the soluble fraction whereas Ca 2+ , Mg 2+ , or K + did not show any action. The spontaneous release of 3 H-label was not influenced by the presence or absence of Ca 2+ , Mg 2+ , or K + in the medium. On the other hand, the absence of Na + was found to accelerate the spontaneous release of 3 H-label and this effect was further enhanced when Ca 2+ was also omitted from the perfusion medium. Increasing the concentrations of Ca 2+ or Na + in the perfusion medium decreased the rate of synthesis of 14 C-catecholamincs from 14 C-tyrosine whereas Mg 2+ and K + showed no effect. Absence of Na + , but not of K + , Ca 2+ , or Mg 2+ , resulted in a decrease in the endogenous level of norepinephrine. The amount of newly synthesized 14 C-catecholamines in the perfusate was more on perfusing the hearts with Ca 2+ -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. Ca 2+ 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 Ca 2+ reduce the rate of synthesis of catecholamines in the adrenergic nerve endings in the heart.