Tissue Radioactivity Measurements Research Articles

Imaging zinc trafficking in vivo by positron emission tomography with zinc-62.

Non-invasive imaging techniques to dynamically map whole-body trafficking of essential metals in vivo in health and diseases are needed. Despite 62Zn having appropriate physical properties for positron emission tomography (PET) imaging (half-life, 9.3 h; positron emission, 8.2%), its complex decay via 62Cu (half-life, 10 min; positron emission, 97%) has limited its use. We aimed to develop a method to extract 62Zn from a 62Zn/62Cu generator, and to investigate its use for in vivo imaging of zinc trafficking despite its complex decay. 62Zn prepared by proton irradiation of natural copper foil was used to construct a conventional 62Zn/62Cu generator. 62Zn was eluted using trisodium citrate and used for biological experiments, compared with 64Cu in similar buffer. PET/CT imaging and ex vivo tissue radioactivity measurements were performed following intravenous injection in healthy mice. [62Zn]Zn-citrate was readily eluted from the generator with citrate buffer. PET imaging with the eluate demonstrated biodistribution similar to previous observations with the shorter-lived 63Zn (half-life 38.5 min), with significant differences compared to [64Cu]Cu-citrate, notably in pancreas (>10-fold higher at 1 h post-injection). Between 4 and 24 h, 62Zn retention in liver, pancreas, and kidney declined over time, while brain uptake increased. Like 64Cu, 62Zn showed hepatobiliary excretion from liver to intestines, unaffected by fasting. Although it offers limited reliability of scanning before 1 h post-injection, 62Zn-PET allows investigation of zinc trafficking in vivo for >24 h and hence provides a useful new tool to investigate diseases where zinc homeostasis is disrupted in preclinical models and humans.

Feasibility of experimental BT4C glioma models for somatostatin receptor 2-targeted therapies

Somatostatin receptor subtype 2 (sstr2) is regarded as a potential target in malignant gliomas for new therapeutic approaches. Therefore, visualizing and quantifying tumor sstr2 expression in vivo would be highly relevant for the future development of sstr2-targeted therapies. The purpose of this study was to evaluate sstr2 status in experimental BT4C malignant gliomas.Methods. Rat BT4C malignant glioma cells were injected into BDIX rat brain or subcutaneously into nude mice. Tumor uptake of [68Ga]DOTA-(Tyr3)-Octreotide ([68Ga]DOTATOC), a somatostatin analog binding to sstr2, was studied by positron emission tomography/computed tomography (PET/CT). Additionally, subcutaneous tumor-bearing mice underwent PET imaging with 5-deoxy-5-[18F]fluororibose-NOC ([18F]FDR-NOC), a novel glycosylated peptide tracer also targeting sstr2. Ex vivo tissue radioactivity measurements, autoradiography and immunohistochemistry were performed to study sstr2 expression.Results. Increased tumor uptake of [68Ga]DOTATOC was detected at autoradiography with mean tumor-to-brain ratio of 68 ± 30 and tumor-to-muscle ratio of 9.2 ± 3.8 for rat glioma. High tumor-to-muscle ratios were also observed in subcutaneous tumor-bearing mice after injection with [68Ga]DOTATOC and [18F]FDR-NOC with both autoradiography (6.7 ± 1.5 and 4.3 ± 0.8, respectively) and tissue radioactivity measurements (6.5 ± 0.8 and 4.8 ± 0.6, respectively). Furthermore, sstr2 immunohistochemistry showed positive staining in both tumor models. However, surprisingly low tumor signal compromised PET imaging. Mean SUVmax for rat gliomas was 0.64 ± 0.28 from 30 to 60 min after [68Ga]DOTATOC injection. The majority of subcutaneous tumors were not visualized by [68Ga]DOTATOC or [18F]FDR-NOC PET.Conclusions. Experimental BT4C gliomas show high expression of sstr2. Weak signal in PET imaging, however, suggests only limited benefit of [68Ga]DOTATOC or [18F]FDR-NOC PET/CT in this tumor model for in vivo imaging of sstr2 status.

The Involvement of Organic Anion Transporting Polypeptide in the Hepatic Uptake of Telmisartan in Rats: PET Studies with [11C]Telmisartan

Telmisartan, a selective angiotensin II receptor antagonist, is primarily excreted via hepatobiliary transport. The predominant contribution of organic anion transporting polypeptide (OATP) 1B3 in its hepatic uptake of telmisartan has been demonstrated by in vitro transport studies. In the present study, a quantitative positron emission tomography (PET) methodology was developed for in vivo kinetic assessment of hepatobiliary transport of telmisartan. Serial abdominal PET scans were performed in rats following intravenous administration of [(11)C]telmisartan as a radiotracer. PET scans revealed that [(11)C]telmisartan was localized primarily in the liver and some of the radioactivity moved to the intestine, which corresponds to biliary excretion. Radiometabolite analysis by radiometric HPLC showed that [(11)C]telmisartan was converted to its acylglucuronide, which was mainly detected in bile, but little in plasma and liver. Integration plot analysis revealed that [(11)C]telmisartan was taken up into the liver as rapidly as the hepatic blood flow rate, and the radiometabolite was subsequently excreted into the bile. When rifampicin, a typical Oatp inhibitor, was coadministered with [(11)C]telmisartan in rats, hepatic uptake clearance of [(11)C]telmisartan was significantly decreased, whereas biliary efflux clearance was not changed. Coinjection with unlabeled telmisartan (4 and 10 mg/kg) also decreased hepatic uptake clearance of [(11)C]telmisartan. On the other hand, PET imaging analysis revealed a significant increase of biliary efflux when telmisartan dose was increased to more than 4 mg/kg. These results suggested that the hepatic uptake of [(11)C]telmisartan mainly consists of a saturable process mediated by Oatps in rats, according to noninvasive real-time measurement of tissue radioactivity with the use of PET. The present study with rats is expected to provide the feasibility of PET imaging study to quantitatively estimate OATP1B3 function in humans.

Renoprotective effect of Erdosteine in rats against gentamicin nephrotoxicity: a comparison of 99mTc-DMSA uptake with biochemical studies

Erdosteine is a mucolytic agent having antioxidant properties through its active metabolites in acute injuries induced by pharmacological drugs. This study was designed to investigate the renoprotective potential of Erdosteine against gentamicin (GM)-induced renal dysfunction by using Technetium-99 m dimercaptosuccinic acid (Tc-99 m DMSA) uptake and scintigraphy in rats. For this purpose, male Wistar rats were randomly allotted into one of the four experimental groups: Control, Erdosteine, GM, and GM + Erdosteine groups. GM and GM + Erdosteine groups received 100 mg/kg GM intramuscularly for 6 days. In addition, Erdosteine and GM + Erdosteine groups received 50 mg/kg Erdosteine orally for 6 days. Renal function tests were assessed by serum blood urea nitrogen (BUN), creatinine levels, as well as scintigraphic and tissue radioactivity measurements with Tc-99 m DMSA. Renal oxidative damage was determined by renal malondialdehyde (MDA) levels, by antioxidant enzyme activities; superoxide dismutase (SOD) and catalase (CAT) and activities of oxidant enzymes; xanthine oxidase (XO) and myeloperoxidase (MPO). GM administration resulted in marked renal lipid peroxidation, increased XO and MPO activities and decreased antioxidant enzyme activities. GM + Erdosteine group significantly had lower MDA levels, higher SOD and CAT activities and lower XO and MPO activities, when compared to GM. Also GM + Erdosteine had lower levels of serum BUN, creatinine and higher renal tissue Tc-99 m DMSA uptake and radioactivity with respect to GM. In conclusion, our results supported a protective role of Erdosteine in nephrotoxicity associated with GM treatment.

Comparison of 18F-FDG and 68Ga PET imaging in the assessment of experimental osteomyelitis due to Staphylococcus aureus

Although positron emission tomography (PET) using 2-[18F]fluoro-2-deoxy-D-glucose (18F-FDG) is a promising imaging modality for bone infections, the technique may still give false-positive results due to unspecific uptake in healing bone. This experimental study compared 18F-FDG and 68Ga in PET imaging of osteomyelitis and normal bone healing. A diffuse osteomyelitis model of the tibia was applied in the rat (n=50). Two weeks after operation, PET imaging with 18F-FDG and 68Ga was performed, followed by peripheral quantitative computed tomography (pQCT) and radiography. Osteomyelitis was verified by quantitative bacteriology. In addition to in vivo imaging, ex vivo measurements of tissue radioactivity were performed to verify uptake of the tracers. Compared with controls with normal bone healing, the osteomyelitic tibias showed increased SUV ratios (i.e. radioactivity ratios between the operated and non-operated sides) for both 18F-FDG (1.74+/-0.37) and 68Ga (1.62+/-0.28) (P<0.001). Ex vivo measurements also showed increased tracer accumulation in the infected bone (P=0.003 for 18F-FDG and P<0.001 for 68Ga). The intensity of 68Ga uptake reflected pathological changes of osteomyelitic bones measured by pQCT. The uptake of 18F-FDG, however, did not show as close a correlation with the anatomical changes. The healing bones without infection exhibited slightly elevated uptake of 18F-FDG (SUV ratio 1.16+/-0.06), but 68Ga did not accumulate in the healing bone, as judged on the basis of both in vivo imaging (SUV ratio 1.02+/-0.05) and ex vivo measurements (SUV 0.92+/-0.21) (P=0.003 and P=0.022 compared with 18F-FDG uptake, respectively). This study suggests the feasibility of 68Ga PET imaging of bone infections. However, further studies are needed to clarify the value of 68Ga PET for clinical purposes.

Molecular imaging of lung glucose uptake after endotoxin in mice

Positron emission tomographic imaging after administration of the glucose analog fluorine-18 fluorodeoxyglucose ([18F]FDG) may be useful to study neutrophilic inflammation of the lungs. In this study, we sought to determine the specificity of the increase in lung [18F]FDG uptake after intraperitoneal endotoxin (Etx) for neutrophil influx into mouse lungs and to determine the regulation of glucose uptake after Etx by Toll-like receptors (TLRs) and TNF-alpha. Lung tissue radioactivity measurements by imaging were validated against counts in a gamma well counter. Glucose uptake was quantified as the [18F]FDG tissue-to-blood radioactivity ratio (TBR) after validating this measure against the "gold standard" measure of glucose uptake, the "net influx rate constant." TBR measurements were made in a control group (no intervention), a group administered Etx, and a group administered Etx plus an additional agent (e.g., vinblastine) or Etx administered to a mutant mouse strain. The glucose uptake measurements were compared with measurements of myeloperoxidase. Increases in TBR after Etx were significantly but not completely eliminated by neutrophil depletion with vinblastine. Increases in TBR after Etx were consistent with signaling via either TLR-4 or TLR-2 (the latter probably secondary to peptidoglycan contaminants in Etx preparation) and were decreased by drug inhibition of TLR-4 but not by inhibition of TNF-alpha. Thus molecular imaging can be used to noninvasively monitor biological effects of Etx on lungs in mice, and changes in lung glucose uptake can be used to monitor effects of anti-inflammatory agents. Such imaging capacity provides a powerful new paradigm for translational "mouse-to-human" pulmonary research.

N,N-Bis(2-mercaptoethyl)methylamine: A New Coligand for Tc-99m Labeling of Hydrazinonicotinamide Peptides

Hydrazinonicotinamide (HYNIC) forms stable coordination complexes with Tc-99m when reacted with Tc(V)oxo species such as Tc-mannitol or other Tc-polyhydric complexes. However, radio-HPLC of [Tc-For-MLFK-HYNIC] labeled via Tc-polyhydric ligands demonstrated multiple radiochemical species each with unique biodistribution patterns. This is likely due to the fact that Tc can bind to the hydrazino moiety, as well as polyhydric ligands, in a variety of coordination geometries. Tridentate ligands, such as bis(mercaptoethyl)methylamine (NS2), may constrain the possible coordination geometries and improve overall stability. To investigate this, we synthesized NS2, converted the [Tc-mannitol-For-MLFK-HYNIC] to the corresponding NS2-containing complex [Tc-NS2-For-MLFK-HYNIC], and compared its infection imaging and biodistribution properties with [Tc-mannitol-For-MLFK-HYNIC]. Conversion to the NS2 complex was confirmed by HPLC which showed a single unique hydrophobic species with retention time greater than the [Tc-mannitol-For-MLFK-HYNIC] complex. Imaging experiments with both preparations were performed in rabbits with E. coli infections in the left thigh. Tissue radioactivity measurements demonstrated that compared to Tc-mannitol-peptide, accumulation of Tc-NS2-peptide was lower in blood, heart, and normal muscle and higher in spleen, infected muscle, and pus (p < 0.01). These results indicate that the Tc-NS2-peptide complex is chemically more homogeneous and exhibits improved infection localization and biodistribution properties. In an effort to model the interactions of the metal-HYNIC core with NS2 and related ligand types, the reactions of [ReCl3(NNC5H4NH)(NHNC5H4N)] and [99TcCl3(NNC5H4NH)(NHNC5H4N)], effective structural analogues for the [M(NNC5H4NH(x))2] core, with NS2, C5H3N-2,6-(CH2SH)2, O(CH2CH2SH)2, and S(CH2CH2SH)2 were investigated and the compounds [M[CH3N(CH2CH2S)2](NNC5H4N)(NHNC5H4N] (M = 99Tc (5a), Re (5b)), [Re[C5H3N-2,6-(CH2S)2](NNC5H4N)(NHNC5H4N)].CH2Cl2.0.5MeOH (7), [Re[SCH2CH2)2O] (NNC5H4N)(NHNC5H4N)] (8), and [Re[(SCH2CH2)2S](NNC5H4NH)(NHNC5H4N)]Cl (9) were isolated. Similarly, the reaction of [ReCl3(NNC5H4NH)(NHNC5H4N)] with the bidentate ligands pyridine-2-methanethiol and 3-(trimethlysilyl)pyridine-2-thiol led to the isolation of [ReCl(C5H4N-2-CH2S) (NNC5H4N)(NHNC5H4N)] (10) and [Re(2-SC5H3N-3-SiMe3)2 (NNC5H4N)(NHNC5H4N)] (11), respectively, while reaction with N-methylimidazole-2-thiol yielded the binuclear complex [Re(OH)Cl(SC3H2N2CH3)2(NNC5H4N)2 (NHNC5H4N)2] (12). The analogous metal-(HYNIC-OH) precursor, [ReCl3[NNC5H3NH(CO2R)] [NHNC5H3N(CO2R)]] (R = H, 13a; R = CH3, 13b) has been prepared and coupled to lysine to provide [RCl3[NNC5H3NH(CONHCH2CH2CH2CH2CH(NH2)CO2H)] [NHNC5H3NH(CONHCH2CH2CH2CH2CH(NH2)CO2H)]].2HCl (14.2HCl), while the reaction of the methyl ester 13b with 2-mercaptopyridine yields [Re(2-SC5H4N)2[NNC5H3N(CO2Me)][NHNC5H3N(CO2Me)]] (15). While the chemical studies confirm the robustness of the M-HYNIC core (M = Tc, Re) and its persistence in ligand substitution reactions at adjacent coordination sites of the metal, the isolation of oligomeric structures and the insolubility of the peptide conjugates of 13, 14, and 15 underscore the difficulty of characterizing these materials on the macroscopic scale, an observation relevant to the persistent concerns with reagent purity and identity on the tracer level.

6-Mercaptomethylpyridine-3-carboxylic acid (MEMNIC): a new reagent for peptide labeling with Tc-99m

As part of our ongoing research into the development of peptide based radiopharmaceuticals, we have explored 2-mercaptopyridines and 2-mercapto-pyrimidines (2MPs) as coligands to control the radiochemical speciation of 99mTc-labeled hydrazinonicotinamide (HYNIC) derivatized chemotactic peptides. In an attempt to develop an aminothiol ligand with greater stability, we synthesized 6-mercaptomethylpyridine–3-carboxylic acid (MEMNIC), assuming a five-member chelate ring complex would be more stable than the four-membered ring possible with 2MPs. Initial experiments suggested that 99mTc–MEMNIC was stable in vivo and led to our investigation of it as a bifunctional chelator for 99mTc labeling of peptides. The utility of MEMNIC was tested in a rabbit model of infection. The N-epsilon MEMNIC derivative of For–MLFK was prepared using the N-hydroxysuccinimidyl ester of MEMNIC. For–MLFK–MEMNIC was labeled via 99mTc–mannitol and its imaging and biodistribution properties compared to Tc–For–MLFK–HYNIC and Tc–For–MLFCys (labeled via the free thiol of cysteine). Sites of infection were produced in New Zealand white rabbits ( n=6/peptide) by injection of a suspension of Escherichia coli in a posterior thigh. Twenty-four hours after infection, the rabbits were injected intravenously with 0.5 mCi of HPLC purified 99mTc-peptide and imaged at 2.5 and 18 h p.i. The animals were sacrificed at 18 h and tissue activity determined. At 2.5 and 18 h, the organ distribution of Tc–For–MLFK–HYNIC and Tc–For–MLFK–MEMNIC were qualitatively similar with the exception of higher renal accumulation of the latter, whereas Tc–For–MLFKCys showed rapid and prolonged accumulation in bowel. ROI analysis gave target/background ratios of 3.91±0.32 and 11.89±2.21 for Tc–For–MLFK–HYNIC, 5.09±0.66 and 9.88±1.12 for Tc–For–MLFK–MEMNIC and 2.59±0.16 and 1.70±0.37 for Tc–For–MLFCys, at 2.5 and 18 h. Tissue radioactivity measurements (18 h) demonstrated that compared to Tc–For–MLFK–HYNIC, the accumulation of Tc–For–MLFK–MEMNIC was less in all organs except in kidney which was greater. Direct labeling of the thiol group of cysteine led to lower accumulation in all organs compared to Tc–For–MLFK–MEMNIC, except in GI tract which was fivefold higher. While, Tc–For–MLFK–MEMNIC had half the accumulation of Tc–For–MLFK–HYNIC in infected muscle and pus, the lower accumulation in most organs and normal muscle resulted in good infection localization. These results indicate that MEMNIC can be successfully used to label peptides with 99mTc while exhibiting good in vivo stability. The chemistry of MEMNIC with the {M(V)O} 3+ core characteristic of technetiun and its Group V congener rhenium was modeled by investigating the reactions of 2-mercaptomethylpyridine with appropriate Re(V)-oxo precursors in the presence of a variety of coligands. With diolate type tridentate donors of the class {X(CH 2CH 2O) 2} 2− (X=–NR, O, S), the ‘3+2’ compounds [ReO{η 3-(OCH 2CH 2) 2S}{η 2-SCH 2C 5H 4N}] ( 2) and [ReO{η 3-(OCH 2CH 2) 2NCH 3}{η 2-SCH 2C 5H 4N] ( 3) were isolated. In contrast, with the more sterically demanding dirmercapto class of tridentate ligands {S(CH 2CH 2S) 2} 2−, the ‘3+1’ species [ReO{η 3-(SCH 2CH 2) 2S}{η 1-(SCH 2C 5H 4N)}] ( 1) was isolated.

Fate of 6-deoxy-6-[123I]iodo-D-glucose in normal and diabetic rats.

The D-glucose analog 6-deoxy-6-¿123Iĭodo-D-glucose (6-DIG) was recently proposed as a potential tracer for the in vivo characterization of D-glucose transport in distinct cell types. In this study, the validity of such a proposal was investigated in both control and streptozotocin-induced diabetic rats. 6-DIG was injected intravenously in either control or diabetic rats. The fate of 6-DIG was assessed by scintigraphy of the injected animals, blood and urine sampling, and measurement of tissue radioactivity at the time of sacrifice, 140 min after 6-DIG injection. The half-life for 6-DIG in plasma and its accumulation in kidney and urinary bladder indicated that it was mainly eliminated from the body by glomerular filtration. The urinary elimination of 6-DIG was accelerated, however, in the polyuric diabetic rats. Bile formation also apparently contributed to the clearance of 6-DIG. Its uptake by liver, heart and muscles yielded values lower than blood concentration. The usefulness of 6-DIG as a tracer for D-glucose transport in selected organs in the perspective of clinical application, e.g. by single photon emission computed tomography, requires further investigations.

Pharmacokinetics of 18F-labeled trovafloxacin in normal and Escherichia coli-infected rats and rabbits studied with positron emission tomography.

OBJECTIVE: To measure tissue pharmacokinetics of trovafloxacin (CP 99,219) in normal and infected animals by both direct tissue radioactivity measurements and positron emission tomography (PET). METHODS: Concentrations of [18F]trovafloxacin were measured in normal and infected rats (n=6/group), at 10, 30, 60, and 120 min after injection, by radioactivity measurements. In normal rabbits (n=4) and rabbits with Escherichia coli thigh infection (n=4), tissue concentrations of drug were measured over 2 h with PET. After acquiring the final images, the rabbits were killed and tissue concentrations measured with PET were compared to the results of direct tissue radioactivity measurements. RESULTS: In both species, there was rapid distribution of [18F] trovafloxacin in most peripheral organs. Peak concentrations of more than five times the MIC90 of most Enterobacteriaceae and anaerobes (>100-fold for most organisms) were achieved in all tissues and remained above this level for >2 h. Particularly high peak concentrations were achieved in the kidney (>75 micro g/g), liver (>100 micro g/g), blood (>40 micro g/g), and lung (>10 micro g/g). Even though the concentration of trovafloxacin in infected muscle was reduced (p<0.01), the peak concentration was still >4 micro g/g and tissue levels remained above 2 micro g/g for more than 2 h. Due to the lower concentrations that were achieved in the brain (peak approximately 5 micro g/g), it is expected that trovafloxacin will have limited central nervous system toxicity. CONCLUSION: PET with [18F]trovafloxacin is a useful technique for non-invasive measurements of tissue pharmacokinetics.

Peroperative radioimmunodetection of ovarian carcinoma using a hand-held gamma detection probe.

Radioimmunoscintigraphy (RIS) can be used in the preoperative localisation of ovarian carcinoma to demonstrate uptake of radiolabelled monoclonal antibodies into neoplastic tissue. The tissue uptake of radiotracer was evaluated at laparotomy in 16 patients with suspected ovarian cancer who had preoperative RIS using technetium-99m-labelled monoclonal antibodies SM3 and H17E2. A gamma detection probe (gamma DP) was used to measure uptake in possible tumour deposits at operation and also the uptake in tissues resected for histology. The percentage uptake of the initial injected dose of radiotracer was also measured in resected tissues. Activity was found to be significantly higher in malignant than in non-neoplastic tissue by all three methods of evaluation. The gamma DP used peroperatively yielded a 82% sensitivity with a 72% specificity for an uptake ratio of 1.5:1. When tissue was examined immediately after resection, for a 100% specificity the sensitivity was 64%. In vitro measurements of monoclonal antibody uptake by tissue similarly gave a 65% sensitivity with a 100% specificity. Peroperative and immediate post-operative measurements of tissue radioactivity can be performed quickly and conveniently, and in some cases may be of benefit in the localisation of tumour at laparotomy and in providing extra information when tissue is examined by frozen section.

Distribution of β1‐ and β2‐adrenoceptors in mouse trachea and lung: a quantitative autoradiographic study

1. Binding and quantitative autoradiography were used to detect [125I]-iodocyanopindolol (I-CYP) associated with beta 1- and beta 2-adrenoceptors in mouse tracheal epithelium and airway smooth muscle as well as in lung parenchymal tissue. 2. Specific I-CYP binding to slide-mounted tissue sections of both trachea and parenchyma was of high affinity (KD = 49.0 pM, n = 3, trachea; KD = 118.9 pM, n = 3, parenchyma) and saturable, involving single populations of non-interacting binding sites (Hill coefficient nH = 1.00 +/- 0.02, trachea; nH = 0.99 +/- 0.03, parenchyma). 3. Direct measurement of tissue radioactivity also showed that specific I-CYP binding was competitively inhibited in the presence of the beta-adrenoceptor antagonists (-)-propranolol (non-selective), CGP 20712A (beta 1-selective) and ICI 118,551 (beta 2-selective). Analysis of the competition binding curves for the two selective antagonists revealed mixed populations of beta 1- and beta 2-adrenoceptors in the approximate proportions 33% and 67% respectively in mouse trachea and 28% and 72% respectively in mouse lung parenchyma. 4. Densities of autoradiographic grains derived from specific I-CYP binding to alveolar wall tissue and to tracheal epithelium and airway smooth muscle were quantified by a computer-assisted image analysis system, which allowed the construction of competition binding curves in the presence of the selective beta-adrenoceptor antagonists CGP 20712A and ICI 118,551. Analysis of these data demonstrated that in alveolar wall, beta 1- and beta 2-adrenoceptors co-existed in the proportions 18% and 82%, respectively. 5. Quantitative autoradiographic analyses also showed that beta 1- and beta 2-adrenoceptors were differentially distributed in tracheal epithelium and airway smooth muscle. The beta 2-adrenoceptor subtype accounted for 71% of all beta-adrenoceptors in epithelium. Conversely, beta l-adrenoceptors which mediate relaxant responses of mouse trachea to beta,-adrenoceptor agonists (Henry & Goldie, 1990), accounted for 69% of all beta-adrenoceptors in the airway smooth muscle.

Proliferative activity of myocardial capillary wall cells in dipyridamole--treated rats.

The hearts of rats treated with dipyridamole were studied by light microscope autoradiography and measurement of tissue radioactivity after intravenous injection of 3H-thymidine. Total tissue radioactivity was significantly increased in the hearts of the dipyridamole-treated animals. Only capillary wall cells were labelled in the autoradiograms. The observations indicate a proliferation of capillary wall cells, suggesting a neoformation of myocardial capillaries during dipyridamole treatment.

The proliferative activity of the myocardial tissue in various forms of experimental cardiac hypertrophy.

Various forms of experimental cardiac hypertrophy were investigated by light microscope autoradiography and measurements of tissue radioactivity after in vivo injections of 3H‐thymidine in rats. In the autaradiograms an attempt was made to distinguish between labelled vessel wall cells, connective tissue cells and heart muscle cells. A highly significant increase in the nuclear incorporation of 3H‐thymidine was recorded in both ventricle walls of rats with cardiac hypertrophy induced by swimming exercise. The increased incorporation was mainly confined to the nuclei of cells in the walls of the blood capillaries. A considerably smaller increase in the incorporation into capillary wall cells was recorded in the left ventricle wall of rats with cardiac hypertrophy secondary to prolonged arterial hypertension and aortic stenosis of 1–2 weeks1 duration. No increased incorporation of 3‐thymidine into the myocardial tissue was found in rats if their swimming exercise was followed by a period of rest, in rats with short‐term hypertension and in rats with aortic stenosis of 2 months' duration. The observations suggest a significant neoformation of myocardial blood vessels during swimming exercise, whereas any neoformation of myocardial blood vessels in arterial hypertension and aortic stenosis appears to be of minor degree.

Tritium‐labelled Araldite as an internal standard for quantitative autoradiography using the electron microscope

SUMMARYA method is described for making quantitative measurements of radioactivity from electron‐microscope autoradiographs. Autoradiographs are made from sections of tissue which contain a seam of tritium‐labelled Araldite of known radioactivity. The latter serves as an internal standard and an estimate of the tissue activity is obtained by comparing the tissue grain density with that over the standard. The method can avoid many of the problems which are commonly encountered in making quantitative measurements of tissue radioactivity.