The effects of verapamil, a calcium antagonist, on lipolysis in isolated rat adipocytes were studied. Verapamil (100 microM) potentiated lipolysis due to dibutyryl cyclic AMP (Bt2cAMP) at submaximal concentrations, with or without extracellular Ca2+. Lipolysis due to 0.5 mM-Bt2cAMP was potentiated by verapamil in a dose-dependent manner up to 200 microM, whereas at concentrations higher than 100 microM the stimulatory effect of verapamil was progressively diminished with or without extracellular Ca2+. Verapamil showed only an inhibitory effect on lipolysis due to adrenaline (0.1-10 microM) or 3-isobutyl-1-methylxanthine (IBMX; 25-200 microM). The stimulatory effect of verapamil on lipolysis due to Bt2cAMP was not blocked by alpha-adrenergic antagonists. These results suggest (i) that verapamil has a biphasic effect on lipolysis due to Bt2cAMP and only an inhibitory effect on that due to adrenaline or IBMX, and (ii) that extracellular Ca2+ or alpha-adrenergic receptors are not involved in the action of verapamil.
The effects of dithiothreitol on basal glucose oxidation, hormone-induced lipolysis and insulin receptors in isolated rat adipocytes were studied. Dithiothreitol produced a dose-dependent stimulation of basal glucose oxidation and inhibition of adrenaline-induced lipolysis. Dithiothreitol also inhibited corticotropin-induced lipolysis, but failed to inhibit dibutyryl cyclic AMP-induced lipolysis. Dithiothreitol did not inhibit the binding of the beta-adrenergic antagonist [3H]dihydroalprenolol to adipocytes. Neither catalase (100 micrograms/ml) nor EDTA (2 mM) abolished the antilipolytic effect of dithiothreitol. Treatment of isolated adipocytes with 1 mM-dithiothreitol for 20 min at 37 degrees C also caused stimulation of basal glucose oxidation and inhibition of adrenaline-induced lipolysis. A Scatchard plot of insulin binding to control adipocytes was curvilinear. However, treatment of cells with 1 mM-dithiothreitol decreased the curvilinearity of the plot, indicating that only a low-affinity state of the insulin receptors exists in the dithiothreitol-treated adipocytes. These findings suggest that the insulin-like activities of dithiothreitol are mediated through the interaction of dithiothreitol with insulin receptors.
The relative permittivity and conductivity of aqueous solutions of oxyhaemoglobin and carboxyhaemoglobin were measured over the frequency range 150kHz–100MHz. To minimize errors of measurement the investigations were carried out with three different samples of each type of haemoglobin, independent apparatus being used in two different laboratories. The dielectric increment and relaxation time were calculated at each of several temperatures from the results. These lead to a dipole moment of 400 Debyes and an activation enthalpy of 17.6±1.4kJ·mol −1 , both of which were found to be independent of temperature to within experimental error over the range 3–35°C. The value of the dipole moment shows that the distribution of charge throughout the haemoglobin molecule is nearly symmetrical with respect to the centre of charge. The magnitude of the activation enthalpy is similar to that of the viscosity of water, in accord with the common observation that dielectric relaxation and viscosity are related phenomena. No significant differences are found between the dielectric parameters of oxyhaemoglobin and carboxyhaemoglobin. Combining the results with those obtained from X-ray diffraction of the solid a hydration value of 0.45g of water/g of protein is suggested, subject to the limitations of the model used. Finally, the results indicate the presence of a subsidiary dispersion, which could be attributed to the above quantity of bound water having a static permittivity of about 100 and a relaxation frequency in the region 100–200MHz.