The whole-cell patch-clamp method was used to study the membrane electrical properties of human adipocyte cells obtained by differentiating from precursors of human abdominal and mammary tissues. All differentiated cells exhibited outward currents with sigmoidal activation kinetics. The outward currents showed activation thresholds between -20 to -30 mV and slow inactivation. The ionic channels underlying the macroscopic current were highly selective for K(+). Their selectivity was for typical K(+) channels with relative permeabilities of K(+)>NH4+>Cs(+)>Na(+). No evidence of any other type of voltage-gated channel was found. The potassium currents ( I(KV)) were blocked reversibly by tetraethylammonium and barium. The IC(50) value and Hill coefficient of tetraethylammonium inhibition of I(KV) were 0.56 m M and 1.17 respectively. These results demonstrate that human adipose cells have voltage-dependent potassium currents.

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We studied the potassium currents in white adipocytes obtained by culturing preadipocytes from rat epididymal tissue, both with insulin (WA i ) and without insulin (WA o ), in order to test the role of insulin in the development of voltage-gated potassium channels (K v ) during adipogenesis. Occasionally, very small potassium currents (I K,V ) were present in preadipocytes; however these currents were measured in all differentiated cells (adipocytes). WA i exhibited greater macroscopic potassium currents than WA o with no apparent differences in kinetics or voltage dependence. The current density (pA/µm 2 ) calculated in WA i was higher than in WA o . Currents were blocked by millimolar concentrations of tetrethylamonium (TEA). The effect of insulin on adipogenesis, both with and without TEA, was analysed. Four days without insulin and three days with insulin were necessary to increase the total number of cells in culture by 2·5-fold. Insulin increased the number of differentiated cells by 73·5%. Cell proliferation and differentiation were inhibited by TEA. Proliferation was affected only by high concentration of TEA. Inhibition of differentiation was dose dependent, with the concentration necessary for half-block similar to the IC 50 values to block potassium channels. These results suggest that insulin increases the density of K v and that these channels may be necessary for the normal growth of white adipocytes in culture.

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We studied the effect of the external addition of noradrenaline (NA) on the electrical activity of white adipocytes from rat epididymal tissue. NA modified the active voltage response recorded by the current clamp technique, and decreased the macroscopic membrane potassium currents measured by a whole-cell configuration patch-clamp technique in cells obtained by culturing differentiating preadipocytes. To analyse if cAMP is implicated in the effect of this hormone on potassium conductances, the changes in electrical activity caused by an increase in the intracellular concentration of cAMP were studied using intracellular recording. The addition of forskolin, or N,N-dibutyryl cAMP plus 3-isobutyl-1-methylxanthine to the bath reduced active changes in membrane voltage responses to hyperpolarizing and deolarizing pulses. A similar effect was observed when vasoactive intestinal peptide was added to the superfusion chamber. These results suggest that NA could modulate K + conductances in white adipocytes, mediated by cAMP.