Biochemical basis of heart function. X. Reduction in the Na+-K+-stimulated ATPase activity in failing rat heart due to hypoxia

The activities of the membrane bound enzyme Na+–K+-stimulated ATPase were studied in control and hypoxic isolated rat hearts. The control hearts were perfused with medium gassed with 95% O2 and 5% CO2, whereas hypoxia was induced by perfusing the hearts with medium gassed with 95% N2 and 5% CO2. The specific activity of the Na+–K+ ATPase was lower in the hypoxic hearts than in the control preparations. The enzyme activity of the hypoxic hearts was also lower when determined under different assay conditions obtained by changing the concentration of Na+, K+, H+, and MgATP in the incubation medium. The ouabain-induced inhibition of Na+–K+ ATPase activity in the hypoxic hearts was similar to that in the control. The depression in Na+–K+ ATPase activity due to hypoxia was reversible on perfusing the hypoxic heart with aerobic medium. The decreased Na+–K+ ATPase activity in the hypoxic heart preparations supports the concept of membrane abnormality associated with heart failure due to lack of oxygen. Since the depression in the activity of Na+–K+ ATPase in the hypoxic heart was markedly pronounced on omitting the substrate from the perfusion medium, it is likely that exogenous glucose plays an important role in maintaining the integrity of hypoxic heart membranes.

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.

Biochemical basis of heart function. V. Effect of starvation on storage, transport, and synthesis of cardiac norepinephrine in rats

Starvation of rats was observed to increase the concentration of endogenous norepinephrine in both normal and adrenalectomized animals. Refeeding the starved animals restored the cardiac level of norepinephrine toward the control value. The proportion of norepinephrine decreased in the granular fraction and increased in the soluble fraction of the heart in fasted rats. Although the plasma level of norepinephrine in starved rats was higher than the control, no appreciable difference in the abilities of the hearts from these animals to accumulate 3H-norepinephrine was observed. The uptake of 14C-tyrosine by the hearts of starved animals was comparable to that by the control rats; however, the rate of synthesis of 14C-catecholamines in the hearts of starved rats was significantly decreased. Tyramine treatment was found to lower the cardiac norepinephrine content in the starved rats to the same extent as in the control. On the other hand, exposure of the rats to cold reduced the cardiac norepinephrine to a lesser degree in the starved animals in comparison to the control. It is suggested that starvation may decrease the proportion of norepinephrine which is available for release from the storage granules and thus reduces the influence of sympathetic activity in the heart.

Biochemical basis of heart function. 3. Influence of isoproterenol on the norepinephrine stores in the rat heart

A single subcutaneous injection of isoproterenol into rats markedly reduced the concentration of norepinephrine (NE) in their hearts, but doses greater than 20 mg/kg failed to decrease NE levels below 30–40% of the control value. Restoration of lost NE began more than 48 h after injection of the isoproterenol and was not complete in 96 h. Twenty-four hours after treatment with isoproterenol the pattern of subcellular distribution of the NE in the heart was not altered from the controls, but 96 h after treatment the NE concentration had decreased in the granular fraction and increased in the soluble fraction. Isoproterenol failed to decrease NE levels in other adrenergically innervated organs such as spleen and brain. Subcutaneous doses (1 mg/kg) of NE and epinephrine also reduced the cardiac NE in the control animals but did not further reduce the NE stores in the hearts of isoproterenol-treated rats. A second injection of isoproterenol (20 mg/kg) also failed to further decrease the NE stores in the hearts of isoproterenol-treated animals. On the other hand, tyramine treatment or exposure of the rats to cold (4 °C) for 4 h lowered the cardiac endogenous NE in both the control and isoproterenol-treated animals. Pretreatment of rats with TM-10, tranylcypromine, dibenamine, and propranolol failed to prevent the isoproterenol-induced decrease in cardiac NE stores. The data reveal that isoproterenol-resistant norepinephrine stores in the heart are susceptible to the action of tyramine or cold exposure, and the isoproterenol-induced alteration in catecholamine stores may not be the result of a direct action of this agent on the adrenergic nerve terminal.