Studies of carnitine metabolism in relation to intestinal absorption.

We studied the postabsorptive fate of L-[3H]carnitine after intraluminal injection into the proximal intestine of anesthetized rats. Carnitine absorption was characterized by slow appearance in the circulation with blood levels still rising 2 h after administration. Absorption via the portal vein was followed by hepatic extraction and appearance in bile with reabsorption of a fraction, thus establishing an enterohepatic circulation. About half of the [3H]carnitine in blood obtained 4 h after administration was free, with the rest largely acetylcarnitine. In contrast the increase in blood carnitine content after intraluminal administration of unlabeled carnitine was almost exclusively limited to the esterified fraction. We hypothesize that release of esterified endogenous or stored carnitine from some other site accounted for the increase in esterified carnitine. The liver may be that site: although about 50% of hepatic [3H]carnitine was in ester form after administration of labeled carnitine, the increase after unlabeled carnitine was primarily in the free fraction, suggesting that a large amount of esterified carnitine had been released. Thus the liver appears to be an important storage and excretory site for exogenous as well as endogenous carnitine, which may be released with an appropriate signal from the intestine.