Beta(3)-adrenergic stimulation and insulin inhibition of non-selective cation channels in white adipocytes of the rat

Etiology of the membrane potential of rat white fat adipocytes

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 Ca(2+) 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 Ca(2+), and ultimately its function and its role in metabolic control.

TRPM4

TRPM4 is a Ca(2+)-activated nonselective cation channel. The channel is activated by an increase of intracellular Ca(2+) and is regulated by several factors including temperature and Pi(4,5)P2. TRPM4 allows Na(+) entry into the cell upon activation, but is completely impermeable to Ca(2+). Unlike TRPM5, its closest relative in the transient receptor potential family, TRPM4 proteins are widely expressed in the body. Currents with properties that are reminiscent of TRPM4 have been described in a variety of tissues since the advent of the patch clamp technology, but their physiological role is only beginning to be clarified with the increasing characterization of knockout mouse models for TRPM4. Furthermore, mutations in the TRPM4 gene have been associated with cardiac conduction disorders in human patients. This review aims to overview the currently available data on the functional properties of TRPM4 and the current understanding of its physiological role in healthy and diseased tissue.

Effect of hypoxia in mice mesenteric arteries surrounded by adipose tissue

AIM

To investigate the influence of hypoxia on the vasoactive effect of peri-vascular white adipose tissue.

METHODS

Isometric tension recordings were performed on mesenteric arteries from Swiss male mice with or without adherent adipose tissue.

RESULTS

Hypoxia (bubbling with 95% N(2), 5% CO(2)) induced a biphasic response, i.e. vasoconstriction followed by vasorelaxation, in pre-contracted (noradrenaline, 10 μm) mesenteric arteries with adipose tissue in the presence of indomethacin (10 μm) and N(ω) -nitro-l-arginine (0.1 mm). Only a small vasorelaxation was observed in arteries without adipose tissue. Pre-contraction with 60 or 120 mm K(+) , incubation with tetraethylammoniumchloride (1 and 3 mm), apamin (1 μm) combined with charybdotoxin (0.1 μm) or 1-[(2-chlorophenyl) diphenylmethyl]-1H-pyrazole (TRAM-34) (10 μm) significantly impaired the hypoxic vasorelaxation. Removal of the endothelium only reduced the hypoxic vasorelaxation. Apamin (1 μm) and charybdotoxin (0.1 μm) did not influence the vasorelaxation of sodium hydrosulfide hydrate. Zinc protoporphyrin IX (10 μm), miconazole (10 μm), 8-(p-sulfophenyl)theophylline (0.1 mm), 1 H-[1, 2, 4]oxadiazolo[4,3- A]quinoxalin-1-one (10 μm), apocynin (0.3 mm), diphenyliodonium (1 μm), catalase (2500 U mL(-1)) and tempol (0.1 mm) did not influence the hypoxic vasorelaxation. In contrast to losartan (0.1 mm), indomethacin (10 μm) and SQ-29548 (10 μm) significantly reduced the hypoxic vasoconstriction.

CONCLUSIONS

Moderate hypoxia induces a biphasic vasomotor response in mice mesenteric arteries surrounded by adipose tissue. The hypoxic vasoconstriction is endothelium independent, whereas the vasodilation is endothelium dependent, soluble guanylyl cyclase independent and in part mediated by opening K(Ca) channels. Cyclooxygenase metabolites mediate the hypoxic vasoconstriction, while endothelium-derived hyperpolarizing factor plays a small role in the hypoxic vasorelaxation.

Effect of electrical stimulation on lipolysis of human white adipocytes

The goal of the present study was to investigate the effect of 30 min of electrical stimulation on the activation of lipolysis in human white adipocytes. Two stimulation protocols (S1, S2) were conducted in vitro on isolated human white adipocytes. Subcutaneous adipose tissue was obtained from female subjects undergoing abdominal adipose tissue reduction. Adipose tissue of 10 female subjects (mean age, 38.7 ± 9.1 years) and 6 female subjects (mean age, 37.2 ± 11.3 years) was obtained for S1 and S2, respectively. All subjects fasted overnight before tissue removal. The control conditions were a basal and a β-adrenergic stimulation (isoproterenol (ISO), 10(-6) mol·L(-1)) of lipolysis. For S1, the 3 electrostimulation conditions consisted of a monopolar square-wave pulse current for 30 min at intensities of 4, 8, and 20 mA, respectively. In S2, the 2 electrostimulation conditions consisted of a bipolar square-wave alternating current for 30 min at intensities of 4 and 6 mA, respectively. Fat cell lipolysis was measured by quantifying the release of glycerol from adipocytes for 3 trials in each experimental condition. For S1, 4 mA significantly increased lipolysis 1.5 times over basal values (p ≤ 0.01), 8 mA and 20 mA did not increase lipolysis significantly, and no significant difference (p > 0.05) was found between ISO and 4 mA. For S2, 4 mA (p ≤ 0.05) and 6 mA (p ≤ 0.01) significantly increased lipolysis by 1.8 and 2.3 times above basal, respectively. Our results demonstrate that both monopolar (4 mA) and bipolar (4 and 6 mA) electrical stimulations significantly activated in vitro lipolysis. Our findings suggest the existence of a new lipolytic pathway that may involve K(v) channels shown to exist in human white adipocytes.

Comparison of functional properties of the Ca2+-activated cation channels TRPM4 and TRPM5 from mice

Non-selective cation (NSC) channels activated by intracellular Ca2+ ([Ca2+]i) play an important role in Ca2+ signaling and membrane excitability in many cell types. TRPM4 and TRPM5, two Ca2+-activated cation channels of the TRP superfamily, are potential molecular correlates of NSC channels. We compared the functional properties of mouse TRPM4 and TRPM5 heterologously expressed in HEK 293 cells. Dialyzing cells with different Ca2+ concentrations revealed a difference in Ca2+ sensitivity between TRPM4 and TRPM5, with EC50 values of 20.2+/-4.0 microM and 0.70+/-0.1 microM, respectively. Similarly, TRPM5 activated at lower Ca2+ concentration than TRPM4 when [Ca2+]i was raised by UV uncaging of the Ca2+-cage DMNP-EDTA. Current amplitudes of TRPM4 and TRPM5 were not correlated to the rate of changes in [Ca2+]i. The Ca2+ sensitivity of both channels was strongly reduced in inside-out patches, resulting in approximately 10-30 times higher EC50 values than under whole-cell conditions. Currents through TRPM4 and TRPM5 deactivated at negative and activated at positive potentials with similar kinetics. Both channels were equally sensitive to block by intracellular spermine. TRPM4 displayed a 10-fold higher affinity for block by flufenamic acid. Importantly, ATP4- blocked TRPM4 with high affinity (IC50 of 0.8+/-0.1 microM), whereas TRPM5 is insensitive to ATP4- at concentrations up to 1 mM.

Visceral Periadventitial Adipose Tissue Regulates Arterial Tone of Mesenteric Arteries

Periadventitial adipose tissue produces vasoactive substances that influence vascular contraction. Earlier studies addressed this issue in aorta, a vessel that does not contribute to peripheral vascular resistance. We tested the hypothesis that periadventitial adipose tissue modulates contraction of smaller arteries more relevant to blood pressure regulation. We studied mesenteric artery rings surrounded by periadventitial adipose tissue from adult male Sprague-Dawley rats. The contractile response to serotonin, phenylephrine, and endothelin I was markedly reduced in intact vessels compared with vessels without periadventitial fat. The contractile response to U46619 or depolarizing high K + -containing solutions (60 mmol/L) was similar in vessels with and without periadventitial fat. The K + channel opener cromakalim induced relaxation of vessels precontracted by serotonin but not by U46619 or high K + -containing solutions (60 mmol/L), suggesting that K + channels are involved. The intracellular membrane potential of smooth muscle cells was more hyperpolarized in intact vessels than in vessels without periadventitial fat. Both the anticontractile effect and membrane hyperpolarization of periadventitial fat were abolished by inhibition of delayed-rectifier K + (K v ) channels with 4-aminopyridine (2 mmol/L) or 3,4-diaminopyridine (1 mmol/L). Blocking other K + channels with glibenclamide (3 μmol/L), apamin (1 μmol/L), iberiotoxin (100 nmol/L), tetraethylammonium ions (1 mmol/L), tetrapentylammonium ions (10 μmol/L), or Ba 2+ (3 μmol/L) had no effect. Longitudinal removal of half the perivascular tissue reduced the anticontractile effect of fat by almost 50%, whereas removal of the endothelium had no effect. We suggest that visceral periadventitial adipose tissue controls mesenteric arterial tone by inducing vasorelaxation via K v channel activation in vascular smooth muscle cells.

Decavanadate modulates gating of TRPM4 cation channels

We have tested the effects of decavanadate (DV), a compound known to interfere with ATP binding in ATP-dependent transport proteins, on TRPM4, a Ca(2+)-activated, voltage-dependent monovalent cation channel, whose activity is potently blocked by intracellular ATP(4-). Application of micromolar Ca(2+) concentrations to the cytoplasmic side of inside-out patches led to immediate current activation followed by rapid current decay, which can be explained by an at least 30-fold decreased apparent affinity for Ca(2+). Subsequent application of DV (10 microm) strongly affected the voltage-dependent gating of the channel, resulting in large sustained currents over the voltage range between -180 and +140 mV. The effect of DV was half-maximal at a concentration of 1.9 microm. The Ca(2+)- and voltage-dependent gating of the channel was well described by a sequential kinetic scheme in which Ca(2+) binding precedes voltage-dependent gating. The effects of DV could be explained by an action on the voltage-dependent closing step. Surprisingly, DV did not antagonize the effect of ATP(4-) on TRPM4, but caused a nearly 10-fold increase in the sensitivity of the ATP(4-) block. TRPM5, which is the most homologous channel to TRPM4, was not modulated by DV. The effect of DV was lost in a TRPM4 chimera in which the C-terminus was substituted with that of TRPM5. Deletion of a cluster in the C-terminus of TRPM4 containing positively charged amino acid residues with a high homology to part of the decavanadate binding site in SERCA pumps, completely abolished the DV effect but also accelerated desensitization. Deletion of a similar site in the N-terminus had no effects on DV responses. These results indicate that the C-terminus of TRPM4 is critically involved in mediating the DV effects. In conclusion, decavanadate modulates TRPM4, but not TRPM5, by inhibiting voltage-dependent closure of the channel.

Human adipose cells have voltage-dependent potassium currents

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.