Pharmaco-kinetics of current skeletal-seeking radiopharmaceuticals.

The blood clearance of all current bone-seeking radiopharmaceuticals is biexponential during the first four hours after injection. Exponent I represents bone uptake and its clearance half-time is less than 30 min. Exponent II represents mainly urinary excretion and its clearance half-time varies from 168 to 512 min. The blood background is highest with 99mTc-labeled polyphosphate (Tc-Poly) and lowest with sodium fluoride (F-18); 99mTc-labeled diphosphonate (Tc-Dip) and 99mTc-labeled pyrophosphate (Tc-Pyro) show intermediate blood levels. The slower blood clearance of 99mTc-phosphate complexes in comparison with F-18 is due primarily to their increased protein binding. Tc-Poly blood clearance, which is slower than that of Tc-Dip and Tc-Pyro, is due primarily to its increased red cell binding and the larger size of its molecule. Bone uptake and urinary excretion of all 99mTc-labeled phosphate complexes are approximately the same: in the range of six to ten per cent in the blood, 30-33% in the urine, and 55-58 percent in the bone and other tissues. F-18 concentration in the bone is almost 1.5 times that of the 99mTc-labeled phosphate complexes. The sensitivity and resolution of lesions are identical for all 99mTc-labeled phosphate complexes and are far better than those for F-18. No toxicity is noted with the amount of phosphate present in the marketed kits, and it appears reasonable to use the minimal amount so long as efficiency is not compromised.

Kinetics of 99mTc-labeled pyrophosphate and polyphosphate in man.

Thekinetic of 99mTc-labeled pyrophosphate were compared with those of polyphosphate in ten patients in a combined study. Both agents cleared from the blood in a biexpoential fashion. The clearance half-time of Exponent I was the same for both and was shorter than the clearance half-time of Exponent ii. Urinary excretion of both agents was the same during the first hour but during the next 3 hr Tc-pyrophosphate cleared at a slightly more rapid rate, resulting in lower blood background radioactivity. Both agents were bound loosely to plasma proteins, mainly to globulin fractions. The sensitivity of lesion detection was similar for both. Excellent bone images were obtained with both agents. The images with Tc-pyrophosphatewere consistently superior owing to the low blood background and they took less time to accumulate an identical number of counts from identical regions. With the amount of 99mTc-complex used, no hyocalcemia or tetany was noted, nor was there any significant effect on 1-hr serum levels of inorganic phosphours and alkaline phosphatase. Four hours after injection, 9.5% of the dose of Tc-pyrophosphate was circulating in blood, 31.7% was excreted in urine, and the remaining 58.8% was taken up by bone and other tissues. The corresponding values with Tc-polyphosphate were 12.5% in blood, 29.0% in urine, and 58.5% in bone and other tissues. Among the soft tissues, the genitourinary system is most consistently visualized. It is concluded that both Tc-pyroposphate and Tc-polyphosphate are excellent skeletal-imaging agents and that Tc-pyrophosphate appears slightly superior to Tc-polyphosphate.

Lipogenic activation after nibbling and gorging in mice.

Lipogenic activation was studied in mice that had been restricted to a single large meal once a day rather than being allowed to eat at frequent intervals throughout the night. Mice were injected intravenously with [U-(14)C]glucose, and the flux of glucose C to total lipid fatty acids (TLFA) and to all "end products" was estimated from serial plasma glucose specific activities and measurements of incorporation of (14)C into TLFA of hepatic and extrahepatic tissues. Tracer studies were carried out in mice fasted for 1 day and at various times after the mice ate one or two small test meals or a single large test meal. Test meals consisted of a fat-free, 58% glucose diet. The flux of glucose C to TLFA increased by an order of magnitude within an hour after mice nibbled a test meal for several minutes. After ingestion of two small test meals or a single large test meal, the flux of glucose C to TLFA increased from a fasting rate of 0.5 to 35 and 87 micro g of glucose C/min/20 g body wt, respectively. Although trained meal eaters are thought to have abnormally increased lipogenesis, their lipogenic response to a single test meal was the same as that previously reported for untrained nibbling mice. Most of the newly synthesized fatty acids were found in extrahepatic tissues. Ingestion of a first test meal completely prevented the expected hyperglycemic response following ingestion of a second test meal even though the latter contained over 10 times more glucose than that in the total body glucose pool.

Specific role of glucose in rapid lipogenic activation in vivo.

Lipogenesis from glucose C was previously found to be rapidly activated as soon as mice nibbled a fat-free, glucose-rich diet. We have studied here whether such rapid activation is a specific effect of dietary glucose. The flux of endogenous glucose C to total lipid fatty acids (TLFA) in mice fasted for 1 day was compared with the minimal average flux of exogenous dietary glucose to TLFA during a 40-min period after the ingestion of various glucose-rich test meals by previously fasted mice. The fasted mice were injected intravenously with [U-(14)C]glucose, and the flux of glucose C to TLFA and to all "end products" was estimated from serial plasma glucose specific activity measurements and (14)C incorporation into TLFA 30 min after (14)C injection. Only 0.6 to 0.8 micro g of glucose C/min/20 g body wt was converted to TLFA, whereas 208 +/- 16 micro g of glucose C/min/20 g body wt was converted to all "end products" in the fasted animals. Previously fasted mice were fed [(14)C]glucose in small test meals as a neat solid, as a 30% aqueous solution, or as a fat-free, 58% glucose diet. During the next 40 min, the average flux of glucose C into TLFA increased at least 50- to 60-fold, regardless of the form in which glucose was fed; however, when glucose was fed as part of a complete fat-free diet, glucose was utilized at a much lower plasma glucose level than in mice fed either pure solid glucose or an aqueous glucose solution. Rapid activation of lipogenesis from glucose requires only glucose as a dietary constituent.

Compartmental and semicompartmental approaches for measuring glucose carbon flux to fatty acids and other products in vivo.

We have attempted to estimate the flux of glucose carbon to total body fatty acids and to other metabolic end products in Bar Harbor 129/J mice fasted 5-8 hr. Tracer [U-(14)C]glucose was injected intravenously, and the following data were obtained at various times up to 180 min: plasma glucose C specific activity, plasma glucose concentration, total body glycogen, and (14)C in total body fatty acid, total body lipid, unsaponifiable lipid, expired CO(2), and in hepatic and extrahepatic glycogen. The data were analyzed by three techniques, namely, multicompartmental, semicompartmental, and noncompartmental analyses. All three methods yielded comparable rates of glucose C conversion to total body fatty acids (2-3 micro g of glucose C/min/20 g of body weight). Although the semicompartmental approach is extremely simple (it only requires analyses of plasma glucose specific activity as a function of time and (14)C-labeled fatty acid at one point in time), it gives an apparently valid approximation for the flux of glucose C to fatty acids. Other quantitative aspects of glucose metabolism in postabsorptive mice are also considered.

Rapid activation and inactivation of fatty acid synthesis from glucose in vivo.

The flux of glucose carbon to total body fatty acids was measured in unanesthetized mice either after fasting or 50-80 min after they nibbled a small test meal containing 120 mg of glucose (fasted-refed). Flux was calculated from plasma [(14)C]glucose specific activity curves and from total body (14)C-labeled fatty acid 30 min after intravenous injection of tracer [(14)C]glucose. Mobilization of liver glycogen, changes in the body glucose pool size, and total flux of carbon through the glucose pool during periods of fasting and refeeding were defined. Liver glycogen was almost completely depleted 8 hr after food removal. Body glucose pool size fell during fasting and increased after refeeding the test meal. Irreversible disposal rate of glucose C varied directly with body glucose pool size; but flux of glucose C into fatty acids increased exponentially as body glucose concentration increased. Within an hour after nibbling a small test meal, the flux of glucose C into total body fatty acids increased 700% in mice previously starved for 24 hr. However, flux of glucose C into fatty acids in postabsorptive mice (food removed for 2 hr; livers rich in glycogen) was only about 2% of the value calculated from published studies in which the incorporation of an intubated [(14)C]glucose load into total body fatty acid was measured in mice. A possible explanation for this phenomenon is presented.

A rapid assay for lipoprotein lipase.

A rapid assay for lipoprotein lipase activity employing a (14)C-labeled substrate is described. The method is very sensitive and suitable for routine use.