A spin label study on fluidization of human red cell membrane by esters of p-hydroxybenzoic acid: structure-functional aspects on membrane glucose transport

The effects of glucose, insulin and metformin on the order parameters of isolated red cell membranes. An electron paramagnetic resonance spectroscopic study

Human red blood cell (RBC) membranes (RBC ghosts) were treated with glucose, insulin and metformin. The order parameters of RBC membranes were determined by 5- and 16-doxyl-stearic acid spin labels. Metabolic effects were excluded using an isolated system of RBC membranes. The membranes were incubated with glucose in physiological (5 mM), renal threshold (10 mM) and manifested diabetic (20 mM) concentrations for limited times. High concentrations of glucose (10, 20, 100 mM) increase the order parameters of RBC membranes significantly. Insulin by itself has a similar effect which is, however, not strictly concentration-dependent. By contrast, metformin at therapeutic concentrations (0.5 and 5.0 microM) decreases the order parameters. At 50 microM concentration the metformin effect is expressed less and recurs at 100 microM concentration. The effects are significant with 5-doxyl-stearic acid, but are not significant with the 16-doxyl derivative. When RBC membranes are co-incubated with 20 mM glucose and metformin at 0.5 and 5.0 microM concentrations the order parameters as determined by 5-doxyl-stearic acid remain normal (= control values). Higher concentrations of metformin (50 and 100 microM) cause an overshoot to very low order parameters. Insulin at 10, 100 and 200 mU/L does not influence significantly the effects of metformin. Addition of physiological amounts of bovine serum albumin does not abolish the effects of metformin. Metformin, at therapeutic concentrations (0.5 and 5.0 microM), maintains the normal fluidity at the polar interface of isolated RBC membranes by counterbalancing non-enzymatic glycosylation with 20 mM glucose in vitro.

Preservatives in the vehicle of naloxone: pharmacological effects

The pharmacological activity of the preservatives methyl and propyl hydroxybenzoate, until recently components of the vehicle of naloxone (Narcan), was investigated in vitro. This vehicle produced reversible, concentration-dependent relaxation of guinea pig trachea, not mediated via adrenergic or cholinergic receptors, prostanoid activity or phosphodiesterase inhibition. Sensitivity of the tissue to calcium-induced contraction was decreased. In single isolated rat hepatocytes, surface receptor stimulation elicits repetitive transient rises in intracellular free calcium measured with the photoprotein aequorin. The vehicle reversibly inhibited these transients. These observations suggest that the effect of hydroxybenzoates may be mediated via a perturbation of intracellular calcium-related processes.