Presence of a Ca2+-dependent K+ channel in brown adipocytes. Possible role in maintenance of alpha 1-adrenergic stimulation

Cilnidipine regulates glucose metabolism and levels of high-molecular adiponectin in diet-induced obese mice

The aim of the present study is to examine the effects of the antihypertensive drug cilnidipine on glucose metabolism and adipocytokines, including adiponectin, in diet-induced obese (DIO) mice. The effects of cilnidipine on insulin sensitivity and the levels of adiponectin in DIO mice were examined after the mice had been treated with cilnidipine dissolved in water at a dose of 0.2 g l(-1) for 14 days. As expected, treatment with cilnidipine decreased the systolic and diastolic blood pressures in DIO mice, compared with control mice (P<0.05 for each parameter). Cilnidipine treatment improved glucose and insulin sensitivity in DIO mice. In addition, cilnidipine treatment dramatically increased the level of adiponectin in white adipose tissue (P<0.05) and the circulating levels of total and high-molecular weight (HMW) adiponectin in DIO mice (P<0.01 for each parameter). Furthermore, the secretion of HMW adiponectin and the ratio of HMW adiponectin/total adiponectin were both increased after cilnidipine treatment. Finally, the secretion of adiponectin from adipocytes was increased after cilnidipine treatment. Taken together, these results indicate that cilnidipine improves insulin tolerance and adiponectin levels, especially high-molecular type adiponectin, in DIO mice.

Brown adipose tissue: function and physiological significance

The function of brown adipose tissue is to transfer energy from food into heat; physiologically, both the heat produced and the resulting decrease in metabolic efficiency can be of significance. Both the acute activity of the tissue, i.e., the heat production, and the recruitment process in the tissue (that results in a higher thermogenic capacity) are under the control of norepinephrine released from sympathetic nerves. In thermoregulatory thermogenesis, brown adipose tissue is essential for classical nonshivering thermogenesis (this phenomenon does not exist in the absence of functional brown adipose tissue), as well as for the cold acclimation-recruited norepinephrine-induced thermogenesis. Heat production from brown adipose tissue is activated whenever the organism is in need of extra heat, e.g., postnatally, during entry into a febrile state, and during arousal from hibernation, and the rate of thermogenesis is centrally controlled via a pathway initiated in the hypothalamus. Feeding as such also results in activation of brown adipose tissue; a series of diets, apparently all characterized by being low in protein, result in a leptin-dependent recruitment of the tissue; this metaboloregulatory thermogenesis is also under hypothalamic control. When the tissue is active, high amounts of lipids and glucose are combusted in the tissue. The development of brown adipose tissue with its characteristic protein, uncoupling protein-1 (UCP1), was probably determinative for the evolutionary success of mammals, as its thermogenesis enhances neonatal survival and allows for active life even in cold surroundings.

Norepinephrine-induced sustained inward current in brown fat cells: alpha(1)-mediated by nonselective cation channels

The nature of the sustained norepinephrine-induced depolarization in brown fat cells was examined by patch-clamp techniques. Norepinephrine (NE) stimulation led to a whole cell current response consisting of two phases: a first inward current, lasting for only 1 min, and a sustained inward current, lasting as long as the adrenergic stimulation was maintained. The nature of the sustained current was here investigated. It could be induced by the alpha(1)-agonist cirazoline but not by the beta(3)-agonist CGP-12177A. Reduction of extracellular Cl(-) concentration had no effect, but omission of extracellular Ca(2+) or Na(+) totally eliminated it. When unstimulated cells were studied in the cell-attached mode, some activity of approximately 30 pS nonselective cation channels was observed. NE perfusion led to a 10-fold increase in their open probability (from approximately 0.002 to approximately 0.017), which persisted as long as the perfusion was maintained. The activation was much stronger with the alpha(1)-agonist phenylephrine than with the beta(3)-agonist CGP-12177A, and with the Ca(2+) ionophore A-23187 than with the adenylyl cyclase activator forskolin. We conclude that the sustained inward current was due to activation of approximately 30 pS nonselective cation channels via alpha(1)-adrenergic receptors and that the effect may be mediated via an increase in intracellular free Ca(2+) concentration.

Inhibition of store-operated Ca2+ entry by extracellular ATP in rat brown adipocytes

1. Modulation of intracellular free Ca2+ concentration ([Ca2+]i) by extracellular ATP was investigated in cultured adult rat brown adipocytes using the fluorescent Ca2+ indicator fura-2. 2. Bath application of ATP in micromolar concentrations caused a large increase in [Ca2+]i in cells previously stimulated with noradrenaline. This ATP-induced [Ca2+]i increase exhibited a monotonic decline to near the resting levels within approximately 2 min, even in the continued presence of the agonist. 3. The magnitude and time course of the [Ca2+]i increase in response to ATP were not significantly affected by removal of extracellular Ca2+, suggesting that a mobilization of intracellular Ca2+ primarily contributes to the increase. 4. The [Ca2+]i increase in response to ATP was sensitive to inhibition by suramin, suggesting the involvement of P2 purinoceptors in the response. 5. Thapsigargin (100 nM) evoked a gradual and irreversible increase in [Ca2+]i which was entirely dependent upon extracellular Ca2+, providing functional evidence for the expression of store-operated Ca2+ entry in these brown adipocytes. 6. Extracellular ATP at a concentration of 10 microM depressed this thapsigargin (100 nM)-induced [Ca2+]i increase by 92 +/- 3 % (n = 8 cells), strongly suggesting that ATP inhibits an influx of Ca2+ across the plasma membrane through the store-operated pathway. Bath application of phorbol 12-myristate 13-acetate (PMA, 5 microM) did not affect the thapsigargin-induced [Ca2+]i increase, indicating that the inhibitory action of ATP is not mediated by activation of protein kinase C (PKC). 7. These results indicate that extracellular ATP not only mobilizes Ca2+ from the intracellular stores but also exerts a potent inhibitory effect on the store-operated Ca2+ entry process in adult rat brown adipocytes.

Differential regulation of the expression of alpha1-adrenergic receptor subtype genes in brown adipose tissue

The physiological control of the expression of the genes for the alpha1-adrenoceptor subtypes was examined in rat brown adipose tissue by analysing Northern blots of poly(A)-enriched RNA with oligonucleotide probes. In control rats, alpha1B-receptor gene expression was much lower in brown adipose tissue than in liver, but the expression of both alpha1A and alpha1D was higher than in the heart, making brown adipose tissue one of the mammalian tissues with the highest expression of these subtypes. During acute exposure to cold, alpha1B-receptor gene expression was essentially unchanged, alpha1A-receptor gene expression was increased and alpha1D-receptor gene expression was transiently decreased. Noradrenaline injection could mimic these effects of acute cold exposure, indicating that the physiologically induced up- and down-regulations were due to the interaction of noradrenaline with cells within the tissue. In chronically cold-acclimated animals, alpha1B-receptor gene expression was decreased but that of the alpha1A-receptor gene remained at a level twice that of controls. alpha1D-Receptor gene expression was also somewhat decreased. It is suggested that the enhanced expression of the alpha1A-receptor gene explains the increased alpha1-receptor density in recruited brown adipose tissue reported previously. The intricate and differential regulation of alpha1-receptor gene expression and the markedly enhanced expression of the alpha1A-receptor may imply that alpha1-receptors are important for the recruitment process or for maintenance of the recruited state in this tissue.

A voltage-dependent and a voltage-independent potassium channel in brown adipocytes of the rat

Single-channel recordings of a voltage-dependent potassium channel in brown adipocytes of the rat confirm recordings of macroscopic currents. Single-channel conductance (gamma) is 8 pS at 20 degrees C in KF solution inside vs. a modified Ringer's solution outside. With KCl solution outside, gamma is 17 pS for outward currents and 21 pS for inward currents. The majority of the channels inactivate with a time constant around 200 ms; deactivation occurs within milliseconds. The channel is blocked by tetraethylammonium (TEA) with an inhibiting constant of 1.8 mM. The type of block is fast. Selectivity sequence for monovalent cations is K+ > Rb+ >> NH4+ >> Li+ > or = Na+ approximately Cs+. Cs+ at the outside causes a voltage-dependent block of inward currents. This channel is remarkably similar to the delayed rectifier of the F-type in the node of Ranvier. Occasionally, an additional K+ channel was found. This channel is voltage-insensitive, not blocked by 10 mM TEA, and has not been recorded in brown adipocytes before. Physiological relevance of this channel could be the steady-state membrane potential.

Attenuation of Gs alpha coupling efficiency in brown-adipose-tissue plasma membranes from cold-acclimated hamsters

In order to localize site(s) of beta-adrenergic desensitization found in brown adipocytes from cold-acclimated animals, total brown-adipose-tissue homogenates (postnuclear supernatant) were obtained from control or cold-acclimated hamsters and were fractionated on discontinuous sucrose gradients. A low-density band (cytosolic proteins) and a high-density band (mitochondria) were obtained; in the middle fractions only low levels of protein were recovered. However, these fractions displayed a high level of specific [3H]ouabain binding, indicating that they represented fractions enriched in plasma membranes. The level of [3H]ouabain binding was significantly higher in plasma membranes from cold-acclimated animals, indicating an increased density of Na,K-ATPase units. The maximal activity of adenylate cyclase, as estimated with forskolin, was not changed by cold acclimation. However, the levels of cyclase activity observed after Gs-protein-mediated activation (with guanosine 5'-[gamma-thio]triphosphate, isoprenaline, both of these, or fluoride) were decreased, indicating a decreased coupling efficiency. Notably, a significant decrease was observed in the functional activity of the Gs protein, as directly measured by estimation of the ability of cholate extracts of brown-fat plasma membranes to reconstitute Gs-protein-mediated stimulation of adenylate cyclase in cyc- membranes. Further, a functionally significant decrease (to 72%) was observed in the ratio between the amount of functional Gs proteins and adenylate cyclase units. The total content of Gs alpha protein was decreased to the same extent as the coupling efficiency of the membranes, indicating that a lower content of functionally equivalent Gs alpha molecules could explain the decreased coupling. It could therefore be concluded that a decrease in Gs-protein-mediated coupling efficiency, owing to a decrease in the amount of Gs alpha, is at least one site of beta-adrenergic desensitization in cold-acclimated animals. This may, at least in part, explain that desensitization takes place despite the fact that the beta 3-adrenoceptor itself apparently lacks some of the sites known to be involved in the desensitization process in other beta-adrenergic receptors.

Adrenergically activated Ca2+ increases in brown fat cells: effects of Ca2+, K+, and K channel block

We measured intracellular calcium concentration ([Ca2+]i) during adrenergic stimulation using fura-2 ratio imaging of individual cultured neonatal rat brown fat cells. One micromolar norepinephrine (NE) increased [Ca2+]i from an average resting value of 105 nM to 555 nM in approximately 30 s. [Ca2+]i remained elevated as long as NE was present but returned to resting levels within 2-3 min after NE removal. The response was half maximal at approximately 50 nM NE and was primarily alpha-adrenergic. The sustained, but not the initial, increase in [Ca2+]i required extracellular calcium. Cells stimulated in high-K media had [Ca2+]i responses like those in 0 Ca2+, suggesting that depolarization abrogates calcium influx. Parallel perforated-patch recordings showed that the increase in [Ca2+]i activates a calcium-activated K conductance. Blocking K channels with moderate concentrations of tetraethylammonium (TEA) had only small effects on NE-induced changes in [Ca2+]i, but high concentrations of TEA significantly reduced the response. We conclude that cytoplasmic calcium is modulated by fluxes from both intracellular and extracellular sources and that K channels may not be required for normal short-term [Ca2+]i responses to hormone.

Potassium channel block does not affect metabolic responses of brown fat cells

Hormonally stimulated brown fat cells are capable of extremely high metabolic rates, making them an excellent system in which to examine the role of plasma membrane ion channels in cell metabolism. We have previously shown that brown fat cell membranes have both voltage-gated and calcium-activated potassium channels (Voltage-gated potassium channels in brown fat cells. J. Gen. Physiol. 93: 451-472, 1989; Membrane responses to norepinephrine in cultured brown fat cells. J. Gen. Physiol. 95: 523-544, 1990). Currents through both the voltage-activated potassium channels, IK,V, and the calcium-activated potassium channels, IK,Ca, can be blocked by the membrane-impermeant K channel blocker tetraethylammonium (TEA). We used microcalorimetric measurements from isolated neonatal rat brown fat cells to assess the role these potassium conductances play in the metabolic response of brown fat cells to adrenergic stimulation. Concentrations of TEA as high as 50 mM, sufficient to block approximately 95% of IK,V and 100% of IK,Ca, had no effect on norepinephrine-stimulated heat production. These results show that neither voltage-gated nor calcium-activated K channels are necessary for a maximal thermogenic response in brown fat cells and suggest that K channels are not involved in maintaining cellular homeostasis during periods of high metabolic activity.

Phorbol myristate acetate inhibition of phospholipase C activation by carbachol in slices of rat brain cortex is a delayed and indirect effect

We examined the effect of phorbol esters on phospholipase C activation in rat brain cortical slices and membranes. There was little effect of concurrent addition of phorbol 12-myristate 13-acetate (PMA) with carbachol on phosphoinositide breakdown due to carbachol over a 1-h incubation of brain slices. However, if slices were preincubated for 3 h with 1 microM PMA or 200 microM sphingosine before addition of carbachol, there was a 35-50% inhibition of phosphoinositide breakdown. There was also a marked loss of protein kinase C (PKC) activity from both cytosol and membranes after a 3-h exposure to PMA. The loss in responsiveness to the muscarinic agonists in slices was not reflected in carbachol-stimulated phospholipase C activation using isolated membranes. However, the decrease in carbachol-induced phosphoinositide breakdown seen in slices after a 3-h exposure to PMA was abolished if the extracellular K+ concentration was elevated from 5.9 to 55mM. Because elevation of the K+ level induces depolarization and increases Ca2+ entry, we examined the effect of ionomycin, a Ca2+ ionophore. Ionomycin potentiated the effects of carbachol on phosphoinositide breakdown but was unable to reverse the effects of a 3-h incubation with PMA. Because apamin, an inhibitor of Ca2(+)-dependent K+ channels, mimicked the effects of exposure to PMA for 3 h, it is possible that these channels are involved in muscarinic cholinergic regulation of phosphoinositide breakdown in rat brain slices. These results support the hypothesis that prolonged PMA treatment in rat brain cortex has no direct effect on phospholipase C activation by muscarinic cholinergic stimulation.

Alpha 1-adrenergic stimulation of Cl- efflux in isolated brown adipocytes

Unidirectional 36Cl- efflux from preloaded isolated brown adipocytes was studied. A norepinephrine-stimulated 36Cl- efflux pathway was found which approximately doubled the rate of 36Cl- efflux from the cells. The response to norepinephrine was fully inhibited by the alpha 1-adrenergic antagonist prazosin, but was unaffected by the beta-adrenergic antagonist propranolol, showing that norepinephrine stimulated the 36Cl- efflux pathway via the alpha 1-adrenoceptor. The stimulation of 36Cl- efflux could not be mimicked by the Ca2+ ionophore A23187, indicating that the effect was not mediated by elevation of the intracellular Ca2+ level. It is concluded that brown fat cells possess a specific mechanism for alpha 1-adrenergic stimulation of Cl- efflux. The possibility is discussed that this Cl- efflux pathway could be the basis for the early alpha-adrenergic depolarization seen in brown fat cells.

Membrane responses to norepinephrine in cultured brown fat cells

We used the "perforated-patch" technique (Horn, R., and A. Marty, 1988. Journal of General Physiology. 92:145-159) to examine the effects of adrenergic agonists on the membrane potentials and membrane currents in isolated cultured brown fat cells from neonatal rats. In contrast to our previous results using traditional whole-cell patch clamp, 1-23-d cultured brown fat cells clamped with the perforated patch consistently showed vigorous membrane responses to both alpha- and beta-adrenergic agonists, suggesting that cytoplasmic components essential for the thermogenic response are lost in whole-cell experiments. The membrane responses to adrenergic stimulation varied from cell to cell but were consistent for a given cell. Responses to bath-applied norepinephrine in voltage-clamped cells had three possible components: (a) a fast transient inward current, (b) a slower outward current carried by K+ that often oscillated in amplitude, and (c) a sustained inward current largely by Na+. The fast inward and outward currents were activated by alpha-adrenergic agonists while the slow inward current was mediated by beta-adrenergic agonists. Oscillating outward currents were the most frequently seen response to norepinephrine stimulation. Activation of this current, termed IK,NE, was independent of voltage and seemed to be carried by Ca2(+)-activated K channels since the current oscillated in amplitude at constant membrane potential and gradually decreased when the cells were bathed with calcium-free external solution. IK,NE had a novel pharmacology in that it could be blocked by 4-aminopyridine, tetraethylammonium, apamin, and charybdotoxin. Both IK,NE and the voltage-gated K channels also present in brown fat (Lucero, M. T., and P. A. Pappone, 1989a. Journal of General Physiology. 93:451-472) may play a role in maintaining cellular homeostasis in the face of the high metabolic activity involved in thermogenesis.

Potassium channel toxins

Many venom toxins interfere with ion channel function. Toxins, as specific, high affinity ligands, have played an important part in purifying and characterizing many ion channel proteins. Our knowledge of potassium ion channel structure is meager because until recently, no specific potassium channel toxins were known, or identified as such. This review summarizes the sudden explosion of research on potassium channel toxins that has occurred in recent years. Toxins are discussed in terms of their structure, physiological and pharmacological properties, and the characterization of toxin binding sites on different subtypes of potassium ion channels.

Permeability of the non-selective channel in brown adipocytes to small cations

The non-selective channel for monovalent cations of cultured brown adipocytes was studied concerning its permeability to alkali metal ions, NH4+, Tris+, Ca2+, and Ba2+. Experiments were done by means of the patch clamp technique using inside-out patches. With symmetrically increasing sodium concentrations the ion fluxes saturated. They are described by a dissociation constant (KNa) of 155 mmol/l and a maximum single channel conductance of 50 pS. Permeabilities were determined in relation to those for sodium yielding values of 0.80 for potassium and 1.55 for ammonium. The complete permeability sequence for ammonium and the alkali metals is: NH4+ greater than Na+ greater than Li+ greater than K+ greater than or equal to Rb+ congruent to Cs+ . Ca2+ and Ba2+ as well as the buffer ion Tris+ are not able to pass the channel measurably. It is shown that the conductance behaviour of the non-selective channel is not sufficiently described by the Goldman-Hodgkin-Katz theory. Deviations from independence are saturation with increased activity of the permeant ion and non-linear current voltage relations in symmetrical solutions. A simple two barrier model with one binding site in the center of the electric field is shown to be more appropriate.

Alpha 1- and beta-adrenergic regulation of intracellular Ca2+ levels in brown adipocytes

In order to monitor changes in cytosolic Ca2+ levels, brown-fat cells were incubated with the fluorescent Ca2+-indicator fura-2 and the fluorescence intensity ratio followed. The addition of norepinephrine led to a rapid and persistent increase in the cytosolic Ca2+ level, which was dose-dependent with a maximal effect at about 1 microM. The response was diminished in the absence of extracellular Ca2+ and was inhibited more efficiently by phentolamine and prazosin than by propranolol or yohimbine, indicating alpha 1-adrenergic mediation. Accordingly, selective alpha 1-adrenergic stimulation also increased the cytosolic Ca2+ level. However, selective beta-adrenergic stimulation, as well as the adenylate cyclase activator forskolin, were also able to increase the cytosolic Ca2+ level in these cells to a certain extent. It was concluded that the major part of the increase in cytosolic Ca2+ was mediated, as in other cell types, via alpha 1-adrenergic receptors, but that Ca2+ levels were also positively modulated by a cAMP-mediated process. These observations are discussed in relation to known alpha 1/beta synergisms in brown adipose tissue.

Voltage-gated potassium channels in brown fat cells

We studied the membrane currents of isolated cultured brown fat cells from neonatal rats using whole-cell and single-channel voltage-clamp recording. All brown fat cells that were recorded from had voltage-gated K currents as their predominant membrane current. No inward currents were seen in these experiments. The K currents of brown fat cells resemble the delayed rectifier currents of nerve and muscle cells. The channels were highly selective for K+, showing a 58-mV change in reversal potential for a 10-fold change in the external [K+]. Their selectivity was typical for K channels, with relative permeabilities of K+ greater than Rb+ greater than NH+4 much greater than Cs+, Na+. The K currents in brown adipocytes activated with a sigmoidal delay after depolarizations to membrane potentials positive to -50 mV. Activation was half maximal at a potential of -28 mV and did not require the presence of significant concentrations of internal calcium. Maximal voltage-activated K conductance averaged 20 nS in high external K+ solutions. The K currents inactivated slowly with sustained depolarization with time constants for the inactivation process on the order of hundreds of milliseconds to tens of seconds. The K channels had an average single-channel conductance of 9 pS and a channel density of approximately 1,000 channels/cell. The K current was blocked by tetraethylammonium or 4-aminopyridine with half maximal block occurring at concentrations of 1-2 mM for either blocker. K currents were unaffected by two blockers of Ca2+-activated K channels, charybdotoxin and apamin. Bath-applied norepinephrine did not affect the K currents or other membrane currents under our experimental conditions. These properties of the K channels indicate that they could produce an increase in the K+ permeability of the brown fat cell membrane during the depolarization that accompanies norepinephrine-stimulated thermogenesis, but that they do not contribute directly to the norepinephrine-induced depolarization.

Alpha 1-adrenergic inositol trisphosphate production in brown adipocytes is Na+ dependent

In order to investigate the ionic requirements for inositol trisphosphate production, brown adipocytes were prelabelled with myo-[3H]inositol and the formation of inositol trisphosphates and inositol bisphosphates as a consequence of alpha 1-adrenergic stimulation was monitored. Omission of Ca2+ from the incubation medium diminished the norepinephrine-induced increase in inositol trisphosphate levels, but it would seem that this reduction can be fully accounted for by a decreased level of the 'inactive' isomer inositol 1,3,4-trisphosphate. Omission of Na+ fully abolished the norepinephrine-induced inositol trisphosphate response. However, it was observed that the presence of Li+ in the incubation medium could fully reconstitute the ability of the cells to yield the early response of inositol trisphosphate production; Li+ could, however, not substitute for Na+ in the entire alpha 1-adrenergic cellular pathway. It was concluded that the Na+-dependent step is found in the coupling mechanism between the alpha 1-receptor and the activation of the phosphodiesterase responsible for inositol trisphosphate production. Thus, all events in the alpha 1-adrenergic pathway which are consequences of IP3 production should appear to be Na+-dependent in these cells.

Membrane potential and cation content of osteoblast-like cells (UMR 106) assessed by fluorescent dyes

A number of cellular functions have recently been associated with alterations of the membrane potential in non-excitable cells. To assess the electrophysiologic regulation of osteoblast function, a method for measuring the membrane potential (Em) of a rat osteogenic sarcoma cell line (UMR 106) by the voltage-sensitive oxonol dye di-BA-C4(3) was developed. The fluorescent signal of di-BA-C4(3) was calibrated through a null point method using the protonophore FCCP. At null point, Em is equivalent to H+ equilibrium potential, and may be calculated by the Nernst equation. Intracellular pH (pHi) changes induced by the protonophore were monitored using BCECF, a pH-sensitive fluorescent probe. In the presence of FCCP, intracellular pH was found to be linearly correlated to extracellular pH (pHo). Therefore, the value of pHi at null point was extrapolated as well. With this technique, we estimated the plasma membrane potential of the "putative" rat osteoblasts (UMR 106) as -28.3 +/- 4.0 mV (n = 10). This method corrected the 16% overestimation of Em derived from the assumption that pHi does not change during the calibration procedure, as described in previous studies employing pH null point techniques. With null point methods, using BCECF and the carboxylic ionophores nigericin and monensin, intracellular concentrations of potassium and sodium were also measured and found to be 125 +/- 0.7 mM (n = 3) and 24 +/- 5.3 mM (n = 3), respectively. Although the Em of UMR 106 cells was dependent on extracellular potassium concentration, these cells did not behave as a potassium electrode. The sodium/potassium permeability ratio, calculated by the Goldman equation, was estimated at 0.317. This high membrane permeability to sodium may contribute to the genesis of the low plasma membrane potential of UMR 106 cells.

Requirement of gene transcription and protein synthesis for cold- and norepinephrine-induced stimulation of thyroxine deiodinase in rat brown adipose tissue

The increase in propylthiouracil-insensitive 'type II' thyroxine 5'-deiodinase activity of brown adipose tissue was investigated in rats exposed to acute cold stress or single-dose norepinephrine injection. The 20-fold cold-induced increase in enzyme activity showed a 2-h lag phase and reached a maximum after only 8 h; reacclimation occurred with a 2-h time lag and a half-life of 2.2 h. 4 h after a single norepinephrine injection, the deiodinase activity was almost identical to that after a 4-h cold stress; norepinephrine could not potentiate the effect of the cold stress. Treatment with the protein synthesis inhibitor cycloheximide before exposure to cold or before norepinephrine injection totally blocked the increase in deiodinase activity, suggesting that the increase is due to de novo protein synthesis. The half-life of the enzyme in vivo was estimated to be 0.7 h. Treatment with the RNA synthesis inhibitor actinomycin D totally abolished the cold- and norepinephrine-induced increases, indicating that the increase requires mRNA synthesis. It was concluded that the dramatic cold-induced increase in thyroxine deiodinase activity in brown adipose tissue was not due to activation of preexisting enzyme but was fully due to a norepinephrine-induced increase in expression of the gene and subsequent synthesis of the protein.

A calmodulin dependent Ca2+-activated K+ channel in the adipocyte plasma membrane

Increased membrane permeability (conductance) that is specific for K+ and directly activated by Ca2+ ions, has been identified in isolated adipocyte plasma membranes using the K+ analogue, 86Rb+. Activation of these K+ conductance pathways (channels) by free Ca2+ was concentration dependent with a half-maximal effect occurring at 32 +/- 4 nM free Ca2+ (n = 7). Addition of calmodulin further enhanced the Ca2+ activating effect on 86Rb+ uptake (K+ channel activity). Ca2+-dependent 86Rb+ uptake was inhibited by tetraethylammonium ion and low pH. It is concluded that the adipocyte plasma membrane possesses K+ channels that are activated by Ca2+ and amplified by calmodulin.

Influence of Ca2+ on the plasma membrane potential and electrogenic uptake of glycine by myeloma cells. Involvement of a Ca2+-activated K+ channel

The involvement of Ca2+-activated K+ channels in the regulation of the plasma membrane potential and electrogenic uptake of glycine in SP 2/0-AG14 lymphocytes was investigated using the potentiometric indicator 3,3'-diethylthiodicarbocyanine iodide. The resting membrane potential was estimated to be -57 +/- 6 mV (n = 4), a value similar to that of normal lymphocytes. The magnitude of the membrane potential and the electrogenic uptake of glycine were dependent on the extracellular K+ concentration, [K+]o, and were significantly enhanced by exogenous calcium. The apparent Vmax of Na+-dependent glycine uptake was doubled in the presence of calcium, whereas the K0.5 was not affected. Ouabain had no influence on the membrane potential under the conditions employed. Additional criteria used to demonstrate the presence of Ca2+-activated K+ channels included the following: (1) addition of EGTA to calcium supplemented cells elicited a rapid depolarization of the membrane potential that was dependent on [K+]o; (2) the calmodulin antagonist, trifluoperazine, depolarized the membrane potential in a dose-dependent and saturable manner with an IC50 of 9.4 microM; and (3) cells treated with the Ca2+-activated K+ channel antagonist, quinine, demonstrated an elevated membrane potential and depressed electrogenic glycine uptake. Results from the present study provide evidence for Ca2+-activated K+ channels in SP 2/0-AG14 lymphocytes, and that their involvement regulates the plasma membrane potential and thereby the electrogenic uptake of Na+-dependent amino acids.