Etiology of the membrane potential of rat white fat adipocytes

Abstract

The plasma membrane potential ( Vm) is key to many physiological processes; however, its ionic etiology in white fat adipocytes is poorly characterized. To address this question, we employed the perforated patch current clamp and cell-attached patch clamp methods in isolated primary white fat adipocytes and their cellular model 3T3-L1. The resting Vm of primary and 3T3-L1 adipocytes were −32.1 ± 1.2 mV ( n = 95) and −28.8 ± 1.2 mV ( n = 87), respectively. Vm was independent of cell size and fat content. Elevation of extracellular K+ to 50 mM by equimolar substitution of bath Na+ did not affect Vm, whereas substitution of bath Na+ with the membrane-impermeant cation N-methyl-d-glucamine+-hyperpolarized Vm by 16 mV, data indicative of a nonselective cation permeability. Substitution of 133 mM extracellular Cl− with gluconate-depolarized Vm by 25 mV, whereas Cl− substitution with I− caused a −9 mV hyperpolarization. Isoprenaline (10 μM), but not insulin (100 nM), significantly depolarized Vm. Single-channel ion activity was voltage independent; currents were indicative for Cl− with an inward slope conductance of 16 ± 1.3 pS ( n = 11) and a reversal potential close to the Cl− equilibrium potential, −29 ± 1.6 mV. Although the reduction of extracellular Cl− elevated the intracellular Ca2+ of adipocytes, this was not as large as that produced by elevation of extracellular K+. In conclusion, the Vm of white fat adipocytes is well described by the Goldman-Hodgkin-Katz equation with a predominant permeability to Cl−, where its biophysical and single-channel properties suggest a volume-sensitive anion channel identity. Consequently, changes in serum Cl− homeostasis or the adipocyte's permeability to this anion via drugs will affect its Vm, intracellular Ca2+, and ultimately its function and its role in metabolic control.