New reversed-phase liquid chromatographic method to detect aflatoxins in food and feed with cyclodextrins as fluorescence enhancers added to the eluent.

The effect of succynil-beta-cyclodextrin (beta-CD-Su), dimethyl-beta-cyclodextrin (DIMEB) and beta-cyclodextrin (beta-CD) on the fluorescence of aflatoxins B1, B2, G1, G2 and M1 (AFB1, AFB2, AFG1, AFG2 and AFM1) was studied: beta-CD-Su promoted the largest fluorescence enhancement for AFB1 and AFM1 while DIMEB showed better results for AFG1 . On the basis of the fluorescence enhancement, a new RP-HPLC method for detecting aflatoxins B1, B2, G1, G2 and M1 was developed using cyclodextrins directly dissolved in the LC eluent. Aflatoxins B1, B2, G1 and G2 were resolved using a MICRA NPS ODS-1 column using methanol-water as mobile phase to which 6 x 10(-3) M beta-CD-Su or beta-CD were added. Chromatographic responses of AFB1 and AFG1 achieved using beta-CD dissolved in the mobile phase were enhanced, respectively, 8 and 12 times, and 10 and 15 times with beta-CD-Su. Detection limits lower than 0.3 microg/kg were achieved for all the four aflatoxins. Aflatoxin M1 was analysed using a Spherisorb S3 ODS-2 Narrow Bore column and methanol-water as mobile phase with added 2 x 10(-3) M beta-CD-Su. An area enhancement of 1.5 was detected for the toxin and the detection limit achieved under these analytical conditions was lower than 0.0005 microg/kg. Both methods were statistically validated showing a linear response for all the aflatoxins tested (R2 > or = 0.99), and applied to the analysis of spiked and naturally contaminated food samples.

Stimulatory effect of (R) alpha-methylhistamine on duodenal HCO3- secretion in anaesthetized rats.

The effect of the histamine H3 receptor agonist (R) alpha-methylhistamine on duodenal bicarbonate secretion was investigated in the anaesthetized rat. (R) alpha-methylhistamine (3-30 mumol/kg i.v.) caused a dose-dependent increase in alkaline secretion which was completely blocked by the H3 receptor antagonist clobenpropit (3 mumol/kg i.v.). This antagonist caused a slight reduction (19%) of the secretory response to PGE2 50 micrograms/kg i.v. These data indicate that the alkaline response to (R) alpha-methylhistamine is related to the activation of histamine H3 receptors and suggest that this could be an additional mechanism involved in the previously observed gastroprotective effect of this compound.

Cardiovascular effects of the novel histamine H2 receptor agonist amthamine: interaction with the adrenergic system.

The cardiovascular effects of the new histamine H2 receptor agonist amthamine were studied in the anaesthetized rat, with particular reference to a possible interaction with the adrenergic system. Amthamine (0.03-3 mumol/kg i.v.) caused vasodepressor responses which were antagonized by famotidine (3 mumol/kg i.v.). At higher doses (30-100 mumol/kg i.v.), amthamine induced a modest increase in the mean arterial pressure, which was significantly enhanced by the blockade of H2 receptors and significantly reduced by the alpha 2 adrenoceptor antagonist yohimbine (1 mumol/kg i.v.). The vasopressor response to amthamine was not modified in rats pre-treated with reserpine or 6-hydroxydopamine, and was only minimally modified in adrenalectomized animals, thus suggesting a predominant interaction with postjunctional alpha 2 adrenoceptors in the vascular muscle. The H2 receptor agonist dimaprit (0.3-100 mumol/kg i.v.) caused a reduction in arterial pressure, which was antagonized by famotidine, no pressor response being unmasked. Dimaprit (0.1-30 mumol/kg i.v.) did not modify heart rate but caused a modest bradycardia at 100 mumol/kg i.v. Amthamine (1-100 mumol/kg i.v.) induced a dose-dependent tachycardia, which was only partially (approximately 20%) reduced by famotidine and was totally blocked by propranolol (0.3 mg/kg i.v.). This effect was significantly reduced in rats pre-treated with reserpine or 6-hydroxydopamine and was further reduced by cocaine, thus suggesting a tyramine-like action of amthamine. In conclusion, these data demonstrate that the H2 receptor agonist amthamine can also interact with the adrenergic system when used at doses higher than those necessary to activate H2 receptors. Whereas the increase in blood pressure induced by amthamine seems to be mainly mediated by a direct activation of postjunctional alpha 2 adrenoceptors, the increase in heart rate is predominantly due to neuronal release of catecholamines. These effects should be considered when using amthamine in cardiovascular or other studies when high doses are employed.

Cardiovascular effects of selective agonists and antagonists of histamine H3 receptors in the anaesthetized rat.

The cardiovascular responses to a series of selective histamine H3 receptor agonists, (R) alpha-methylhistamine, imetit and immepip and selective antagonists, thioperamide, clobenpropit and clophenpropit, were studied in anaesthetized rats. At 0.003-1 mumol/kg i.v. doses, H3 agonists failed to produce any significant change in the basal blood pressure and heart rate. Larger doses of (R) alpha-methylhistamine increased the blood pressure and heart rate and higher doses of imetit caused vasodepressor responses and reduced heart rate, whereas immepip proved virtually inactive. While (R) alpha-methylhistamine-induced effects were not blocked by histamine H1-, H2- and H3-receptor antagonists, they were however reduced by idazoxan and propranolol, which indicates that the mechanisms involved are adrenergic. The effects induced by imetit are not related to histamine H3 receptors but are mediated by indirect (via 5HT3 receptors) cholinergic mechanisms, since these effects were prevented by 1 mg/kg i.v. atropine and by 0.1 mg/kg i.v. ondansetron. Similarly, the H3 antagonists per se failed to change basal cardiovascular function up to 10 mumol/kg i.v. and only at 30 mumol/kg i.v. were marked decreases observed in the blood pressure and heart rate with a significant reduction in the effects of noradrenaline. These data indicate that in anaesthetized rats, histamine H3 receptor activation or blockade has no effect on basal cardiovascular function. The effects recorded after the administration of large doses of (R) alpha-methylhistamine and imetit are clearly unrelated to histamine H3 receptors and should be taken into account when using these compounds as H3 ligands for "in vivo" experiments.

Histamine H3 receptors: an overview.

Histamine H3 receptors represent a new class of prejunctional receptors, which, first described as modulators of histamine synthesis and release in the central nervous system, were subsequently found to occur also in peripheral tissues. They may behave both as autoreceptors located in histaminergic neurons and as heteroreceptors localized in adrenergic, cholinergic, serotoninergic and NANC nerve endings. As far as the gastrointestinal tract is concerned, H3 receptors seem to negatively control gastric acid secretion induced by endogenous cholinergic stimuli as well as gastrointestinal motility, as observed in different isolated preparations electrically stimulated. So far experimental evidence suggests that these receptors are absent in the effector organs. The most selective agonist and antagonist of H3 receptors are (R) alpha-methylhistamine and thioperamide, respectively; however, a lot of similar compounds are being synthesized in a few laboratories. It is easy to foresee that in the near future the availability of such substances will help the investigators in enlarging our knowledge in this interesting field.