Effects of phenothiazine derivatives on the microclimate-pH in the rat jejunum

Concentration-dependent Exsorption of Quinidine in the Rat Intestine

During intravenous infusion, the luminal concentration of quinidine was higher than the plasma concentration. The intestinal clearance (CLi) of the drug was measured by dividing the rate of appearance of the drug in the intestinal luminal perfusate by the plasma concentration. The CLi of quinidine was therefore much higher than the rate of luminal perfusion. Over the infusion dose range of 0·1–2 mg h−1, the CLi of quinidine decreased with increasing plasma concentration of quinidine. Adding quinidine into the luminal perfusate had little effect on the CLi of quinidine. Co-administration of quinidine with other agents intravenously did not alter the CLi of salicylic acid and urea, while the same treatment decreased the CLi of theophylline and 5-disopyramide. In-vitro experiments on brush-border membrane vesicles showed that quinidine decreased the rate of Na+ uptake and H+ efflux. The inhibition was significant at quinidine concentrations above 20 μm. Quinidine was a more potent inhibitor than amiloride. At quinidine infusion rates less than 2 mg h−1, quinidine concentration in plasma or in the luminal perfusate was at the lower limit of the inhibitory concentration. Microclimate pH at the intestinal surface was also measured. At mid-jejunum, the microclimate pH increased 0·3 pH units by infusing 2 mg h−1 of quinidine, while the microclimate pH at most other measuring sites was not significantly altered by quinidine infusion. It was concluded that quinidine is exsorbed from blood into the intestinal lumen by a carrier-mediated pathway in addition to the passive diffusion. At high plasma concentration, quinidine exsorption becomes saturated. Quinidine inhibited the intestinal exsorption of theophylline and S-disopyramide possibly by competition on the carrier.

Effect of perfusate pH on the influx of 5-5'-dimethyl-oxazolidine-2,4-dione and dissociation of epidermal growth factor from the cell-surface receptor: the existence of the proton diffusion barrier in the Disse space

The influx clearance (PSinf.MID) of the weak acid 5,5'-dimethyl-oxazolidine-2,4-dione (DMO) was determined by the multiple indicator dilution method with the isolated perfused rat liver under various perfusate pH conditions, ranging from 6.4 to 7.6. Although the pH partition theory predicted an increase in influx clearance of ten times in proportion to the change in the unionized fraction of DMO, there was no measurable change in this value. The effect of medium pH on the steady-state cell/medium concentration ratio (C/M) ratio of DMO was also investigated using isolated hepatocytes. The C/M ratio increased while medium pH decreased, but this change was less marked than predicted by the pH partition theory. Finally the pH dependency of the dissociation rate constant (koff) of epidermal growth factor from its receptor was also investigated using both isolated rat hepatocytes and the perfused rat liver. When the extracellular pH was changed from 6.4 to 5.6, the koff value of isolated hepatocytes increased 44 times, while that of the perfused rat liver increased only 9 times. Therefore, the effect of changing the extracellular pH on pH-dependent dissociation of epidermal growth factor from its cell-surface receptor was less in the perfused liver than in isolated hepatocytes. These findings, in addition to the well-known existence of the Na(+)-H+ exchanger on the sinusoidal membrane and the possible existence of the unstirred water layer in the Disse space, seem to suggest the existence of the proton diffusion barrier in the rat liver, which remains stronger in the perfused liver than in isolated hepatocytes.

Thioridazine induces lipid peroxidation in myelin of rat brain

In this study, the oxidative effect of the commonly used phenothiazine, thioridazine, on brain tissue has been investigated. Thioridazine (0.1 and 0.5%) supplemented in pellet diet (w/w), produced a significant increase (P < 0.001) in levels of myelin lipid peroxide, after 3 weeks of treatment. Besides myelin, there was a 2-fold increase in the mitochondrial lipid peroxides, as a result of treatment with thioridazine. However, these elevated levels of lipid peroxides returned to normal after withdrawal of thioridazine for 2 weeks. Myelin-associated enzyme activities of Na+,K(+)-ATPase and 5'-nucleotidase became inhibited by 20-25%, but CNPase activity was unaffected. Studies of in vitro lipid peroxidation on purified myelin from untreated rats suggested that extensive lipid peroxidation of myelin in thioridazine-treated rats could underlie inhibition of the myelin enzymes. Morphological studies revealed little or no structural alterations in myelin, produced by thioridazine. These studies suggest that thioridazine induces a reversible lipid peroxidation in myelin, that could result in functional alterations of the myelin-associated enzymes, during use of this drug.