Intravenously injected [1-14C]arachidonic acid targets phospholipids, and [1-14C]palmitic acid targets neutral lipids in hearts of awake rats

The differential uptake and targeting of intravenously infused [1-¹⁴C]palmitic ([1-¹⁴C] 16∶0) and [1-¹⁴C]arachidonic ([1-¹⁴C]20∶4n−6) acids into heart lipid pools were determined in awake adult male rats. The fatty acid tracers were infused (170 μCi/kg) through the femoral vein at a constant rate of 0.4 mL/min over 5 min. At 10 min postinfusion, the rats were killed using pentobarbital. The hearts were rapidly removed, washed free of exogenous blood, and frozen in dry ice. Arterial blood was withdrawn over the course of the experiment to determine plasma radiotracer levels. Lipids were extracted from heart tissue using a two-phase system, and total radioactivity was measured in the nonvolatile aqueous and organic fractions. Both fatty acid tracers had similar plasma curves, but were differentially distributed into heart lipid compartments. The extent of [1-¹⁴C]20∶4n−6 esterification into heart phospholipids, primarily choline glycerophospholipids, was elevated 3.5-fold compared to [1-¹⁴C]16∶0. The unilateral incorporation coefficient, k ^*, which represents tissue radioactivity divided by the integrated plasma radioactivity for heart phospholipid, was sevenfold greater for [1-¹⁴C]20∶4n−6 than for [1-¹⁴C]16∶0. In contrast, [1-¹⁴C]16∶0 was esterified mainly into heart neutral lipids, primarily triacylglycerols (TG), and was also found in the nonvolatile aqueous compartment. Thus, in rat heart, [1-¹⁴C]20∶4n−6 was primarily targeted for esterification into phospholipids, while [1-¹⁴C]16∶0 was targeted for esterification into TG or metabolized into nonvolatile aqueous components.