A reader's guide to the evaluation of causation

In this article, we review the criteria used to establish a causal association. A literature search strategy will be outlined. We present a clinical scenario that was formulated to examine the evidence for nonsteroidal anti-inflammatory drugs as a possible cause of gastrointestinal ulceration and bleeding. Several of the studies identified in the literature search were critically appraised and the criteria needed to confirm a causal link were then applied. These guidelines may be employed to not only demonstrate the aetiology of a disease but have wider applicability in terms of determining whether a medication may be causing certain adverse events.     

Decline in striatal dopamine D1 and D2 receptor activation in aged F344 rats

Aging differentially affects receptor function. In the present electrophysiological study we compared neuronal responsiveness to locally applied dopamine D1 and D2 receptor agonist in the striatum of female Fischer 344 rats aged 3 and 26-27 months. In a subgroup of the old rats, the nigrostriatal dopamine bundle was destroyed unilaterally with 6-hydroxydopamine (6-OHDA) to assess receptor plasticity in response to denervation. Spontaneous firing rate of striatal neurons was higher in aged compared to young rats. Higher doses of the D1 agonist SKF 38393 or the D2 agonist quinpirole were required to elicit a 50% change in firing rate in aged compared to young rats. No difference with SKF 38393 or quinpirole was detected between 6-OHDA denervated and control (nonlesioned) striatum in aged rats. Supersensitivity to D2 agonists has been reported following 6-OHDA lesions in young rats. These observations suggest that D2 receptors in aged rat striatum might not be as plastic as in younger rats.     

Native and oxidized low density lipoproteins modulate mesangial cell apoptosis

Hyperlipidemia has been demonstrated to contribute to hypercellularity of the mesangium in experimental animal models of glomerulosclerosis. We studied whether it also has the potential to convert a hypercellular mesangium into a hypocellular one by inducing mesangial cell (MC) apoptosis. Low density lipoprotein (LDL) enhanced (P < 0.001) mouse mesangial cell (MMC) proliferation at lower concentrations (control, 10.3 +/- 0.3 vs. LDL 100 micrograms/ml, 24.2 +/- 0.3 x 10(4) cells/ml) but augmented (P < 0.001) apoptosis at higher concentrations (control, 5.6 +/- 0.5% vs. LDL, 500 micrograms/ml 26.2 +/- 3.4% apoptotic cells/field). Oxidized (OX) LDL enhanced MMC apoptosis in concentrations of 50 to 200 micrograms/dl. There was a direct relationship between MMC apoptosis and oxidation of LDL as judged by measuring thiobarbituric acid reactive species (TBARS). Since superoxide dismutase (SOD) attenuated (P < 0.001) LDL-induced MMC apoptosis, it seems to be mediated through the generation of free radicals by mesangial cells (control, 4.3 +/- 1.5%; LDL, 200 micrograms/ml, 19.4 +/- 0.5%; LDL + SOD, 8.1 +/- 1.3% apoptotic cells/field). LDL also induced a similar effect on human mesangial cells. These studies were further confirmed by DNA fragment assays and ELISA for programmed cell death. LDL treated cells also showed enhanced mRNA expression for RSG-2, a marker for active cell death. These in vitro results provide a basis for the speculation that LDL has the potential to cause an initial hypercellular and subsequent hypocellular mesangium in the course of the development of glomerulosclerosis.     

Passive mechanical behavior of human neutrophils: effects of colchicine and paclitaxel

The role of microtubules in determining the mechanical rigidity of neutrophils was assessed. Neutrophils were treated with colchicine to disrupt microtubules, or with paclitaxel to promote formation of microtubules. Paclitaxel caused an increase in the number of microtubules in the cells as assessed by immunofluorescence, but it had no effect on the presence or organization of actin filaments or on cellular mechanical properties. Colchicine at concentrations <1.0 microM caused disruption of microtubular structures, but had little effect on either F-actin or on cellular mechanical properties. Higher concentrations of colchicine disrupted microtubular structure, but also caused increased actin polymerization and increases in cell rigidity. Treatment with 10 microM colchicine increased F-actin content by 17%, the characteristic cellular viscosity by 30%, the dependence of viscosity on shear rate by 10%, and the cortical tension by 18%. At 100 microM colchicine the corresponding increases were F-actin, 25%; characteristic viscosity, 50%; dependence of viscosity on shear rate, 20%; and cortical tension, 21%. These results indicate that microtubules have little influence on the mechanical properties of neutrophils, and that increases in cellular rigidity caused by high concentrations of colchicine are due to a secondary effect that triggers actin polymerization. This study supports the conclusion that actin filaments are the primary structural determinants of neutrophil mechanical properties.     

A deoxyribonucleic acid-replication intermediate in the growing mosquito

The arrangement of 18-S rRNA and 28-S rRNA within their 40-S common precursor molecule (pre-rRNA) of Xenopus laevis was investigated by electron microscopic analysis of secondary structure of nascent pre-rRNA chains of oocytes, and by 5'-end analysis of 18-S rRNA and 28-S rRNA hybridized to the EcoRI fragment of rDNA cloned as plasmid pCD42. Secondary structure mapping of phenol-extracted RNA from nucleolar cores revealed complete pre-rRNA chains or molecules at various stages of processing and pre-rRNA molecules apparently lacking one end. In this latter group, which was regarded as representing nascent chains, more than 90% of the molecules had no 28-S rRNA REGION. This shows that the 28-S rRNA sequence is transcribed after the 18-S rRNA region and hence must be located nearer to the 3' end of the pre-rRNA molecule. For 5' end-group determination [3H]uridine-labelled 18-S rRNA and 28-S rRNA were hybridized, as fragments of about 200 nucleotides, to the plasmid pCD42 containing coding sequences for four-fifths of the 18-S rRNA sequence, the external transcribed spacer, the non-transcribed spacer and a tenth of the 28-S rRNA sequence. The RNA was recovered from the hybrids and analyzed for uridine 3',5'-bisphosphate (pUp) after alkaline hydrolysis. The pUp content of the hybridized 18-S rRNA fragments was 20-fold higher than in those of 28-S rRNA, THUS DEMONSTRATING THAT THE 5' END OF THE 18-S rRNA is located next to the external spacer region. From these results it is concluded that the 18-S rRNA is located close to the 5' end of the 40-S pre-rRNA molecule.     

Obstructive jaundice due to a chlorinated hydrocarbon in breast milk

A 6-week-old breast-fed infant had obstructive jaundice and hepatomegaly. When a dry-cleaning solvent, tetrachloroethylene, was detected in the mother's milk and blood, breast-feeding was discontinued. Rapid clinical and biochemical improvement followed. The child grew normally and had normal liver function during 2 years of follow-up.     

Injuries to the phrenic nerve resulting in diaphragmatic paralysis with special reference to stretch trauma

The effectiveness of purines and purine analogues as inducers of erythroid differentiation in cultured murine erythroleukemia cells has been investigated. These cell lines have previously been shown to differentiate in vitro in response to dimethylsulfoxide (DMSO) and a number of other polar solvents. Two purine analogues, 6-thioguanine and 6-mercaptopurine, as well as the naturally occuring purine, purine, hypoxanthine, are shown to be extremely potent inducers. 6-Thioguanine is effective at a concentration of 0.06 mM, 750 fold lower than the DMSO concentration required for equivalent induction. 6-Mercaptopurine and hypoxanthine are effective inducers at a concentration of approximately 2 mM. Accumulation of globin mRNA was monitored during induction with purine inducers and shown to be similar in amount to globin mRNA levels reached in DMSO-induced cultures. Induction of differentiation by all three compounds follows a similar time course to induction with DMSO. All three compounds are potent inducers of HGPRT (hypoxanthine-guanine phosphoribosyltransferase)-negative cell lines; hence incorporation of purines into DNA is not required for induction of differentiation. Comparison of these compounds with other purines and purine analogues suggests a high degree of specificity in their interaction with a cellular target.