Quantitation of cimetidine and cimetidine sulfoxide in serum by solid-phase extraction and solvent-recycled liquid chromatography

A high performance liquid chromatographic method for the simultaneous determination of cimetidine and its major metabolite, cimetidine sulfoxide, was developed. These compounds and the internal standard, ornidazole, were extracted from 0.5 mL of serum using a solid phase Bond Elut C18 analytical column with detection at 229 nm. Absolute recoveries were 94 to 103%, 93 to 104%, and 95 to 105% for cimetidine, cimetidine sulfoxide, and ornidazole, respectively. The minimum detection limit for cimetidine was 0.1 mg/L and for cimetidine sulfoxide was 0.05 mg/L when the concentrating step was used. Cimetidine and cimetidine sulfoxide demonstrated linearity up to 10 mg/L and 7.5 mg/L respectively, with the between-run precision of less than a 5% coefficient of variation for both compounds. Interferences from other drugs tested or endogenous substances in serum were not detected. The mobile phase was recycled to maintain better long term column stability and to minimize solvent cost. The instability of the drugs in solution was circumvented with a reduced-pressure drying process that produced working standards possessing longterm stability. The problem of drug interconversion observed during sample storage and with concentrating steps was controlled also. In addition, a resolution test mixture was chromatographed daily to control chromatographic quality.

Insulin inhibits the glucocorticoid-mediated increase in hepatocyte EGF binding

Hydrocortisone and dexamethasone produced a time-dependent increase [125I]epidermal growth factor [( 125I]EGF) binding in primary cultures of isolated rat hepatocytes. Maximally effective doses of glucocorticoids resulted in a 70-100% increase in binding. The effect was similar when hepatocytes were maintained on collagen-coated plates or directly on culture dishes. The glucocorticoid-mediated increase in [125I]EGF binding could be detected after 4 h exposure to glucocorticoid and was substantial by 8 h. The major effect of glucocorticoid appeared to be to increase the number of EGF receptors. While insulin (100 nM) had no effect on basal [125I]EGF binding, it significantly inhibited the increase produced by the glucocorticoid. Since the inhibitory effect of insulin was only observed when insulin was added with the inducing glucocorticoid, insulin appears to inhibit an early hydrocortisone-mediated event.

Dimethyl sulfoxide decreases specific EGF binding

Dimethyl sulfoxide (DMSO) stimulates tyrosine phosphorylation of the hepatic EGF receptor in isolated membrane preparations. To determine whether DMSO affects EGF binding, primary cultures of rat hepatocytes were incubated with 1-10% DMSO for 30 min prior to the addition of 125I-EGF. DMSO (1-2%) reduced specific 125I-EGF binding; the effect was maximal (a 40-60% reduction) at 5-7.5% DMSO and was reversed by removing the DMSO. Scatchard analysis showed that the reduction in binding was due to a change in receptor affinity. The decrease in binding was not seen when other, slightly less polar, solvents (eg, acetone and ethanol) were tested. DMSO also reduced 125I-EGF binding to purified rat liver plasma membranes. This reduction was seen in the absence of added ATP and in membranes that had been pretreated with TLCK, a tyrosine kinase inhibitor. Thus, completion of the receptor autophosphorylation reaction was not necessary to effect the change. The data are consistent with a DMSO-induced alteration of receptor conformation that reversibly reduces receptor affinity.