Expression profiling of Ral-depleted bladder cancer cells identifies RREB-1 as a novel transcriptional Ral effector.

Although the monomeric GTPases RalA and RalB have been shown to regulate a variety of transcription factors, little is known regarding the differences or similarities in transcriptional programs regulated by RalA compared to RalB. Further, the association of these transcriptional pathways to human carcinogenesis and progression remains unclear. Here, we studied the role of RalA and/or RalB in transcriptional regulation by combining short interfering RNA depletion of Ral with gene expression profiling via microarray in the human bladder cancer cell line, UMUC-3. A large number of genes were found to be similarly modulated in cells with RalA and RalB depletion, suggesting that RalA and RalB impinge on overlapping transcriptional signaling pathways. However, smaller sets of genes were modulated by depletion of RalA or RalB, indicating that these closely related proteins also regulate nonoverlapping transcriptional pathways. Computational analysis of upstream sequences of genes modulated by Ral depletion identified Ras-responsive element-binding protein (RREB)-1, as a putative Ral transcriptional target, which we verified experimentally. Importantly, as a group, Ral-regulated probe sets identified here were disproportionally represented among those differentially expressed as a function of human bladder transformation. Taken together, these data strongly suggest that Ral family members mediate both common and specific transcriptional programs that are associated with human cancer and identify RREB-1 as a novel transcriptional effector of Ral.

Involvement of the nitric oxide-cyclic GMP pathway in the desensitization of bradykinin responses of cultured rat sensory neurons.

Bradykinin (BK) excites a subset of dorsal root ganglion neurons by inducing an inward cation current (IBK) that strongly desensitizes and is accompanied by elevations in cGMP. We have examined the links between cGMP metabolism and IBK. The BK dose dependencies of IBK activation, desensitization, and cGMP production are comparable. Stimulation (with sodium nitroprusside [NP] or 8-bromo-cGMP [8Br-cGMP]) or inhibition (with methylene blue, hemoglobin, and nitric oxide synthase [NOS] inhibitors) of cGMP levels did not mimic or diminish IBK. However, desensitization was affected by the following agents: first, desensitization was enhanced by NP and reduced by NOS inhibitors. Second, the effects of NOS inhibitors could be overcome by 8Br-cGMP or L-arginine. Third, 8Br-cGMP modification of desensitization required receptor occupancy. We conclude that the NO-cGMP pathway affects a component of IBK desensitization at the receptor or G protein level.

Functional analysis of capsaicin receptor (vanilloid receptor subtype 1) multimerization and agonist responsiveness using a dominant negative mutation.

The recently cloned vanilloid receptor subtype 1 (VR1) is a ligand-gated channel that is activated by capsaicin, protons, and heat. We have attempted to develop a dominant negative isoform by targeting several mutations of VR1 at highly conserved amino acids or at residues of potential functional importance and expressing the mutants in Chinese hamster ovary cells. Mutation of three highly conserved amino acid residues in the putative sixth transmembrane domain disrupts activation of the VR1 receptor by both capsaicin and resiniferatoxin. The vanilloid binding site in this mutant is intact, although the affinity for [(3)H]resiniferatoxin (RTX) is diminished by nearly 40-fold. Interestingly, this mutant retains a significant but diminished response to protons, supporting the existence of multiple gating mechanisms for different stimuli. The mutant appears to function by interfering with the gating induced by vanilloids rather than the expression level or permeability of the receptor. In addition, this mutant was found to function as a strong dominant negative mutation when coexpressed with wild-type VR1, providing functional evidence that the VR1 receptor forms a multimeric complex. Analysis of both current density and [(3)H]RTX affinity in cells cotransfected with different ratios of wild-type and mutant VR1 is consistent with tetrameric stoichiometry for the native capsaicin receptor.

Reduced oral wound healing in the NOD mouse model for type 1 autoimmune diabetes and its reversal by epidermal growth factor supplementation.

Using the NOD mouse, a model for type 1 diabetes, we examined how reduced concentrations of epidermal growth factor (EGF) in the saliva, after onset of type 1 diabetes, affect oral wound healing. Diabetic NOD/LtJ mice on insulin therapy, prediabetic NOD/LtJ, and age- and sex-matched BALB/cJ mice were given a cutaneous tongue punch and allowed to undergo normal healing. With diabetes onset and a reduction in saliva-derived growth factor levels, the rate of tongue wound healing was reduced compared with nondiabetic NOD/LtJ and healthy BALB/cJ mice. Addition of exogenous EGF to the drinking water did not accelerate the rate of healing in BALB/cJ or prediabetic NOD/LtJ; however, diabetic NOD/LtJ mice exhibited accelerated wound healing similar to healthy mice. These results demonstrate that loss of growth factors from saliva is associated with profoundly reduced oral wound healing, suggesting that therapeutic treatment with topical delivery may be beneficial to patients with type 1 diabetes and oral wound complications.

Genetic drift within a protected polymorphism: enigmatic variation in color-morph frequencies in the candy-stripe spider, Enoplognatha ovata.

The candy-stripe spider, Enoplognatha ovata, exhibits a striking color polymorphism comprising three morphs. A number of lines of evidence strongly suggest that this polymorphism is maintained by natural selection: its presence in a sister species, E. latimana; the physical nature of the variation; the virtual lack of monomorphic populations; the highly consistent rank-order of morphs within populations; and the presence of large-scale clines associated with climatic variables. However, the absence of selection is equally strongly suggested by very local surveys of morph frequencies over space and time, perturbation experiments, and a variance in morph frequency between populations that is virtually independent of spatial scale. In addition, local spatial patterns in one study site (Nidderdale, Yorkshire, England) have been explained in terms of intermittent drift over half a century ago, a hypothesis supported here by the distributions of four other genetic markers (two allozyme and two visible polymorphisms). A heuristic model is suggested that reconciles these apparently contradictory messages regarding the importance of drift and selection in this system. It is proposed that when allele frequencies of the color morph redimita lie between approximately 0.05 and 0.3, the deltaq on q plot is very shallow, so that within this region, where the majority of populations lie, selection is weak and drift is the major force determining local morph frequencies. However, outside this range of frequencies, powerful selection acts to protect the polymorphism. This model may apply to polymorphisms in other species and explain why evidence of selection in natural populations is often elusive.

Some kinetic and steady-state properties of sodium channels after removal of inactivation.

To study the kinetic and steady-state properties of voltage-dependent sodium conductance activation, squid giant axons were perfused internally with either pronase or N-bromoacetamide and voltage clamped. Parameters of activation, tau m and gNa(V), and deactivation, tau Na, were measured and compared with those obtained from control axons under the assumption that gNa oc m3h of the Hodgkin-Huxley scheme. tau m(V) values obtained from the turn-on of INa agree well with control axons and previous determinations by others. tau Na(V) values derived from Na tail currents were also unchanged by pronase treatment and matched fairly well previously published values. tau m(V) obtained from 3 x tau Na(V) were much larger than tau m(V) obtained from INa turn-on at the same potentials, resulting in a discontinuous distribution. Steady-state In (gNa/gNa max - gNa) vs. voltage was not linear and had a limiting logarithmic slope of 5.3 mV/e-fold gNa. Voltage step procedures that induce a second turn-on of INa during various stages of the deactivation (Na tail current) process reveal quasiexponential activation at early stages that becomes increasingly sigmoid as deactivation progresses. For moderate depolarizations, primary and secondary activation kinetics are superimposable. These data suggest that, although m3 can describe the shape of INa turn-on, it cannot quantitatively account for the kinetics of gNa after repolarization. Kinetic schemes for gNa in which substantial deactivation occurs by a unique pathway between conducting and resting states are shown to be unlikely. It appears that the rate-limiting step in linear kinetic models of activation may be between a terminal conducting state and the adjacent nonconducting intermediate.

Interactions of monovalent cations with sodium channels in squid axon. I. Modification of physiological inactivation gating.

Inactivation of Na channels has been studied in voltage-clamped, internally perfused squid giant axons during changes in the ionic composition of the intracellular solution. Peak Na currents are reduced when tetramethylammonium ions (TMA+) are substituted for Cs ions internally. The reduction reflects a rapid, voltage-dependent block of a site in the channel by TMA+. The estimated fractional electrical distance for the site is 10% of the channel length from the internal surface. Na tail currents are slowed by TMA+ and exhibit kinetics similar to those seen during certain drug treatments. Steady state INa is simultaneously increased by TMA+, resulting in a "cross-over" of current traces with those in Cs+ and in greatly diminished inactivation at positive membrane potentials. Despite the effect on steady state inactivation, the time constants for entry into and exit from the inactivated state are not significantly different in TMA+ and Cs+. Increasing intracellular Na also reduces steady state inactivation in a dose-dependent manner. Ratios of steady state INa to peak INa vary from approximately 0.14 in Cs+- or K+-perfused axons to approximately 0.4 in TMA+- or Na+-perfused axons. These results are consistent with a scheme in which TMA+ or Na+ can interact with a binding site near the inner channel surface that may also be a binding or coordinating site for a natural inactivation particle. A simple competition between the ions and an inactivation particle is, however, not sufficient to account for the increase in steady state INa, and changes in the inactivation process itself must accompany the interaction of TMA+ and Na+ with the channel.

Interactions of monovalent cations with sodium channels in squid axon. II. Modification of pharmacological inactivation gating.

The time-, frequency-, and voltage-dependent blocking actions of several cationic drug molecules on open Na channels were investigated in voltage-clamped, internally perfused squid giant axons. The relative potencies and time courses of block by the agents (pancuronium [PC], octylguanidinium [C8G], QX-314, and 9-aminoacridine [9-AA]) were compared in different intracellular ionic solutions; specifically, the influences of internal Cs, tetramethylammonium (TMA), and Na ions on block were examined. TMA+ was found to inhibit the steady state block of open Na channels by all of the compounds. The time-dependent, inactivation-like decay of Na currents in pronase-treated axons perfused with either PC, 9-AA, or C8G was retarded by internal TMA+. The apparent dissociation constants (at zero voltage) for interaction between PC and 9-AA with their binding sites were increased when TMA+ was substituted for Cs+ in the internal solution. The steepness of the voltage dependence of 9-AA or PC block found with internal Cs+ solutions was greatly reduced by TMA+, resulting in estimates for the fractional electrical distance of the 9-AA binding site of 0.56 and 0.22 in Cs+ and TMA+, respectively. This change may reflect a shift from predominantly 9-AA block in the presence of Cs+ to predominantly TMA+ block. The depth, but not the rate, of frequency-dependent block by QX-314 and 9-AA is reduced by internal TMA+. In addition, recovery from frequency-dependent block is not altered. Elevation of internal Na produces effects on 9-AA block qualitatively similar to those seen with TMA+. The results are consistent with a scheme in which the open channel blocking drugs, TMA (and Na) ions, and the inactivation gate all compete for a site or for access to a site in the channel from the intracellular surface. In addition, TMA ions decrease the apparent blocking rates of other drugs in a manner analogous to their inhibition of the inactivation process. Multiple occupancy of Na channels and mutual exclusion of drug molecules may play a role in the complex gating behaviors seen under these conditions.

Modulation of calcium channels by norepinephrine in internally dialyzed avian sensory neurons.

Modulation of voltage-dependent Ca channels by norepinephrine (NE) was studied in chick dorsal root ganglion cells using the whole-cell configuration of the patch-clamp technique. Cells dialyzed with K+ and 2-10 mM EGTA exhibited Ca action potentials that were reversibly decreased in duration and amplitude by NE. Ca channel currents were isolated from other channel contributions by using: (a) tetrodotoxin (TTX) to block gNa, (b) internal K channel impermeant ions (Cs or Na/N-methylglucamine mixtures) as K substitutes, (c) external tetraethylammonium (TEA) to block K channels, (d) internal EGTA to reduce possible current contribution from Ca-activated channels. A marked decline (rundown) of Ca conductance was observed during continual dialysis, which obscured reversible NE effects. The addition of 2-5 mM MgATP to the intracellular solutions greatly retarded Ca channel rundown and permitted a clear assessment of modulatory drug effects. The inclusion of an intracellular creatine phosphate/creatine phosphokinase nucleotide regeneration system further stabilized Ca channels, which permitted recording of Ca currents for up to 3 h. NE reversibly decreased both steady state Ca currents and Ca tail currents in Cs/EGTA/MgATP-dialyzed cells. A possible role of several putative intracellular second messengers in NE receptor-Ca channel coupling was investigated. Cyclic AMP or cyclic GMP added to the intracellular solutions at concentrations several orders of magnitude higher than the Kd for activation of cyclic nucleotide-dependent protein kinases did not block or mask the expression of the NE-mediated decrease in gCa. Addition of internal EGTA to a final concentration of 10 mM also did not affect the expression of the NE response. These results suggest that neither cyclic AMP nor cyclic GMP nor Ca is acting as a second messenger coupling the NE receptor to the down-modulated Ca channel population.

Cation permeation through the voltage-dependent potassium channel in the squid axon. Characteristics and mechanisms.

Characteristics of cation permeation through voltage-dependent delayed rectifier K channels in squid giant axons were examined. Axial wire voltage-clamp measurements and internal perfusion were used to determine conductance and permeability properties. These K channels exhibit conductance saturation and decline with increases in symmetrical K+ concentrations to 3 M. They also produce ion- and concentration-dependent current-voltage shapes. K channel permeability ratios obtained with substitutions of internal Rb+ or NH+4 for K+ are higher than for external substitution of these ions. Furthermore, conductance and permeability ratios of NH+4 or Rb+ to K+ are functions of ion concentration. Conductance measurements also reveal the presence of an anomalous mole fraction effect for NH+4, Rb+, or Tl+ to K+. Finally, internal Cs+ blocks these K channels in a voltage-dependent manner, with relief of block by elevations in external K+ but not external NH+4 or Cs+. Energy profiles for K+, NH+4, Rb+, Tl+, and Cs+ incorporating three barriers and two ion-binding sites are fitted to the data. The profiles are asymmetric with respect to the center of the electric field, have different binding energies and electrical positions for each ion, and (for K+) exhibit concentration-dependent barrier positions.

Ionic currents in two strains of rat anterior pituitary tumor cells.

The ionic conductance mechanisms underlying action potential behavior in GH3 and GH4/C1 rat pituitary tumor cell lines were identified and characterized using a patch electrode voltage-clamp technique. Voltage-dependent sodium, calcium, and potassium currents and calcium-activated potassium currents were present in the GH3 cells. GH4/C1 cells possess much less sodium current, less voltage-dependent potassium current, and comparable amounts of calcium current. Voltage-dependent inward sodium current activated and inactivated rapidly and was blocked by tetrodotoxin. A slower-activating voltage-dependent inward calcium current was blocked by cobalt, manganese, nickel, zinc, or cadmium. Barium was substituted for calcium as the inward current carrier. Calcium tail currents decay with two exponential components. The rate constant for the slower component is voltage dependent, while the faster rate constant is independent of voltage. An analysis of tail current envelopes under conditions of controlled ionic gradients suggests that much of the apparent decline of calcium currents arises from an opposing outward current of low cationic selectivity. Voltage-dependent outward potassium current activated rapidly and inactivated slowly. A second outward current, the calcium-activated potassium current, activated slowly and did not appear to reach steady state with 185-ms voltage pulses. This slowly activating outward current is sensitive to external cobalt and cadmium and to the internal concentration of calcium. Tetraethylammonium and 4-aminopyridine block the majority of these outward currents. Our studies reveal a variety of macroscopic ionic currents that could play a role in the initiation and short-term maintenance of hormone secretion, but suggest that sodium channels probably do not make a major contribution.

Dominant-negative mutants identify a role for GIRK channels in D3 dopamine receptor-mediated regulation of spontaneous secretory activity.

The human D3 dopamine receptor can activate G-protein-coupled inward rectifier potassium channels (GIRKs), inhibit P/Q-type calcium channels, and inhibit spontaneous secretory activity in AtT-20 neuroendocrine cells (Kuzhikandathil, E.V., W. Yu, and G.S. Oxford. 1998. Mol. Cell. Neurosci. 12:390-402; Kuzhikandathil, E.V., and G.S. Oxford. 1999. J. Neurosci. 19:1698-1707). In this study, we evaluate the role of GIRKs in the D3 receptor-mediated inhibition of secretory activity in AtT-20 cells. The absence of selective blockers for GIRKs has precluded a direct test of the hypothesis that they play an important role in inhibiting secretory activity. However, the tetrameric structure of these channels provides a means of disrupting endogenous GIRK function using a dominant negative approach. To develop a dominant-negative GIRK mutant, the K(+) selectivity amino acid sequence -GYG- in the putative pore domain of the human GIRK2 channels was mutated to -AAA-, -GLG-, or -GFG-. While the mutation of -GYG- to -GFG- did not affect channel function, both the -AAA- and -GLG- GIRK2 mutants were nonfunctional. This suggests that the aromatic ring of the tyrosine residue rather than its hydroxyl group is involved in maintaining the pore architecture of human GIRK2 channels. When expressed in AtT-20 cells, the nonfunctional AAA-GIRK2 and GLG-GIRK2 acted as effective dominant-negative mutants and significantly attenuated endogenous GIRK currents. Furthermore, these dominant-negative mutants interfered with the D3 receptor-mediated inhibition of secretion in AtT-20 cells, suggesting they are centrally involved in the signaling pathway of this secretory response. These results indicate that dominant-negative GIRK mutants are effective molecular tools to examine the role of GIRK channels in vivo.

Interaction of internal anions with potassium channels of the squid giant axon.

The interaction of internal anions with the delayed rectifier potassium channel was studied in perfused squid axons. Changing the internal potassium salt from K+ glutamate- to KF produced a reversible decline of outward K currents and a marked slowing of the activation of K channels at all voltages. Fluoride ions exert a differential effect upon K channel gating kinetics whereby activation of IK during depolarizing steps is slowed dramatically, but the rate of closing after the step is not much altered. These effects develop with a slow time course (30-60 min) and are specific for K channels over Na channels. Both the amplitude and activation rate of IK were restored within seconds upon return to internal glutamate solutions. The fluoride effect is independent of the external K+ concentration and test membrane potential, and does not recover with repetitive application of depolarizing voltage steps. Of 11 different anions tested, all inorganic species induced similar decreases and slowing of IK, while K currents were maintained during extended perfusion with several organic anions. Anions do not alter the reversal potential or shape of the instantaneous current-voltage relation of open K channels. The effect of prolonged exposure to internal fluoride could be partially reversed by the addition of cationic K channel blocking agents such as TEA+, 4-AP+, and Cs+. The competitive antagonism between inorganic anions and internal cationic K channel blockers suggests that they may interact at a related site(s). These results indicate that inorganic anions modify part of the K channel gating mechanism (activation) at a locus near the inner channel surface.

Selective modification of sodium channel gating in lobster axons by 2, 4, 6-trinitrophenol: Evidence for two inactivation mechanisms.

Trinitrophernol (TNP) selectively alters the sodium conductance system of lobster giant axons as measured in current clamp and voltage clamp experiments using the double sucrose gap technique. TNP has no measurable effect on potassium currents but reversibly prolongs the time-course of sodium currents during maintained depolarizations over the full voltage range of observable currents. Action potential durations are increased also. Tm of the Hodgkin-Huxley model is not markedly altered during activation of the sodium conductance but is prolonged during removal of activation by repolarization, as observed in sodium tail experiments. The sodium inactivation versus voltage curve is shifted in the hyperpolarizing direction as is the inactivation time constant curve, measured with conditioning voltage steps. This shift speeds the kinetics of inactivation over part of the same voltage range in which sodium currents are prolonged, a contradiction incompatible with the Hodgkin-Huxley model. These results are interpreted as support for a hypothesis of two inactivation processes, one proceeding directly from the resting state and the other coupled to the active state of sodium conductance.

Removal of sodium channel inactivation in squid giant axons by n-bromoacetamide.

The group-specific protein reagents, N-bromacetamide (NBA) and N-bromosuccinimide (NBS), modify sodium channel gating when perfused inside squid axons. The normal fast inactivation of sodium channels is irreversibly destroyed by 1 mM NBA or NBS near neutral pH. NBA apparently exhibits an all-or-none destruction of the inactivation process at the single channel level in a manner similar to internal perfusion of Pronase. Despite the complete removal of inactivation by NBA, the voltage-dependent activation of sodium channels remains unaltered as determined by (a) sodium current turn-on kinetics, (b) sodium tail current kinetics, (c) voltage dependence of steady-state activation, and (d) sensitivity of sodium channels to external calcium concentration. NBA and NBS, which can cleave peptide bonds only at tryptophan, tyrosine, or histidine residues and can oxidize sulfur-containing amino acids, were directly compared with regard to effects on sodium inactivation to several other reagents exhibiting overlapping protein reactivity spectra. N-acetylimidazole, a tyrosine-specific reagent, was the only other compound examined capable of partially mimicking NBA. Our results are consistent with recent models of sodium inactivation and support the involvement of a tyrosine residue in the inactivation gating structure of the sodium channel.

Dynamics of aminopyridine block of potassium channels in squid axon membrane.

Aminopyridines (2-AP, 3-AP, and 4-AP) selectively block K channels of squid axon membranes in a manner dependent upon the membrane potential and the duration and frequency of voltage clamp pulses. They are effective when applied to either the internal or the external membrane surface. The steady-state block of K channels by aminopyridines is more complete for low depolarizations, and is gradually relieved at higher depolarizations. The K current in the presence of aminopyridines rises more slowly than in control, the change being more conspicuous in 3-AP and 4-AP than in 2-AP. Repetitive pulsing relieves the block in a manner dependent upon the duration and interval of pulses. The recovery from block during a given test pulse is enhanced by increasing the duration of a conditioning depolarizing prepulse. The time constant for this recovery is in the range of 10-20 ms in 3-AP and 4-AP, and shorter in 2-AP. Twin pulse experiments with variable pulse intervals have revealed that the time course for re-establishment of block is much slower in 3-AP and 4-AP than in 2-AP. These results suggest that 2-AP interacts with the K channel more rapidly than 3-AP and 4-AP. The more rapid interaction of 2-AP with K channels is reflected in the kinetics of K current which is faster than that observed in 3-AP or 4-AP, and in the pattern of frequency-dependent block which is different from that in 3-AP or 4-AP. The experimental observations are not satisfactorily described by alterations of Hodgkin-Huxley n-type gating units. Rather, the data are consistent with a simple binding scheme incorporating no changes in gating kinetics which conceives of aminopyridine molecules binding to closed K channels and being released from open channels in a voltage-dependent manner.

Reduced expression of metastasis suppressor RhoGDI2 is associated with decreased survival for patients with bladder cancer.

PURPOSE: RhoGDI2 was recently shown to be a metastasis suppressor gene in models of bladder cancer. We sought to further understand its importance in human cancer by determining the level of its expression and the distribution of its encoded protein in normal human tissues and cell lines and to evaluate whether its protein expression is a determinant of human bladder cancer progression. EXPERIMENTAL DESIGN: RhoGDI2 mRNA and protein expression was evaluated in cell lines and human tissues using Affymetrix and tissue microarrays, respectively. Tissue microarrays represented most human normal adult tissues and material from 51 patients that had undergone radical cystectomy for bladder cancer. In these 51 patients, the chi(2) test was used to test for associations between RhoGDI2 and stage, grade of urothelial carcinoma, histological type, and disease-specific survival status. Cox proportional hazards regression analyses were used to estimate the effect of RhoGDI2 expression level on time to development of metastatic disease and disease-specific survival time, adjusting for grade, stage, and histological type. RESULTS: In normal tissues, there was strong RhoGDI2 protein expression in WBCs, endothelial cells, and transitional epithelium. RhoGDI2 mRNA expression was inversely related to the invasive and metastatic phenotype in human bladder cancer cell lines. In patients with bladder cancer, univariate analysis indicated that reduced tumor RhoGDI2 protein expression was associated with a lower actuarial 5-year disease-free and disease-specific survival (P = 0.01). In addition, patients with tumors that had low or absent RhoGDI2 had a shorter time to disease-specific death (P

Absence of coupling between D2 dopamine receptors and calcium channels in lactotrophs from cycling female rats.

Recent evidence suggests that an important mechanism underlying the inhibition of PRL secretion by dopamine in the anterior pituitary is a direct inhibition of current through voltage-gated calcium channels. An alternative mechanism involves the activation of G protein-coupled potassium channels by D2 receptor activation, subsequent hyperpolarization of the lactotroph membrane, and an indirect inhibition of calcium influx as spontaneous electrical activity is reduced. Using patch voltage clamp methods, we have reexamined the effect of D2 receptor activation on calcium currents (ICa) in pituitary cells from normal cycling female rats and in GH4Cl pituitary tumor cells expressing cloned D2 receptors. Furthermore, we have examined secretory responses using a single cell immunoblot method. Dopamine (0.1-10 microM) failed to significantly inhibit ICa in either GH4Cl cells or normal female lactotrophs. Similarly, the D2 agonist quinpirole (20-100 microM) did not reduce ICa in lactotrophs. No responses to D2 agonists were seen when barium was substituted for calcium or when experiments were performed using the nystatin-permeabilized patch technique to avoid loss of intracellular macromolecules. Quinpirole also failed to inhibit ICa in lactotrophs isolated from lactating female rats. We have thus far been unable to observe a significant inhibition of ICa by activation of D2 receptors. PRL secretion assessed by immunoblotting methods was dramatically inhibited by quinpirole at normal (5 mM) extracellular K+. However, in elevated (50 mM) K+ that depolarizes the cells and activates calcium channels, quinpirole produced only a very modest inhibition of secretion. We conclude that direct inhibition of ICa by D2 receptor activation is not a major mechanism underlying the dopaminergic inhibition of PRL, secretion in normal female lactotrophs.

Action of zolpidem on responses to GABA in relation to mRNAs for GABA(A) receptor alpha subunits within single cells: evidence for multiple functional GABA(A) isoreceptors on individual neurons.

The relationship between zolpidem sensitivity and GABA(A) receptor alpha subunits was studied in individual dissociated neurons from rat brain. Using whole-cell recording, similar EC50 values were demonstrated for the effect of gamma-aminobutyric acid (GABA) on gated-chloride currents from substantia nigra reticulata (SNR) and lateral septal neurons. Subsequently, many neurons from both the SNR or lateral septum were found to exhibit enhanced GABA-gated chloride currents across concentrations of zolpidem ranging from 10 to 300 nM. Some neurons exhibited a greater than 20% increase in responsiveness to GABA at 30 nM of zolpidem without further increase at higher concentrations of zolpidem. Conversely, zolpidem enhancement of GABA from another group of neurons was not observed at 30 nM zolpidem, but between 100 and 300 nM the response to GABA increased greater than 20%. Finally, a third group of neurons reached both of these criteria for zolpidem enhancement of GABA. This latter spectrum of responses to GABA after varying concentrations of zolpidem was consistent with the presence of either two GABA(A) receptors or a single receptor with differing affinities for zolpidem on an individual neuron. Following determination of the sensitivity of neurons from SNR or lateral septum to zolpidem, cytoplasm was extracted from some individual cells to allow identification of cellular mRNAs for the alpha1, alpha2 and alpha3 GABA(A) receptor subunits with RT-PCR. Those neurons that responded to the 30 nM zolpidem concentration invariably expressed the alpha1-GABA(A) receptor subunit. This result is consistent with the GABA(A) alpha1-receptor subunit being an integral part of a functional high-affinity zolpidem type 1-BZD receptor complex on neurons in brain. Those neurons which showed enhancement of GABA from 100 to 300 nM zolpidem contained mRNAs for the alpha2 and/or the alpha3 receptor subunits, a finding consistent with these alpha subunits forming type 2-BZD receptors. Some individual dissociated SNR neurons were sensitive to both low and high concentrations of zolpidem and contained mRNAs for all three alpha-receptor subunits. These latter individual neurons are proposed to have at least two functional GABA(A) receptor subtypes. Thus, the present investigation emphasizes the importance of characterizing the relationship between endogenous GABA(A) receptor function and the presence of specific structural components forming GABA(A) receptor subtypes on neurons.

Gastrointestinal absorption of insulin-like growth factor in the mouse in the absence of salivary insulin-like growth factor binding protein.

Based on previous observations of the presence of both insulin-like growth factors I and II (IGF-I and IGF-II) in murine saliva (kerr et al., Biochem Pharmacol 49: 1521-1531, 1995), the saliva from BALB/c and Non-obese diabetic (NOD) mice was examined for the presence of insulin-like growth factor binding proteins (IGFBPs). Using a western-blot type ligand binding assay with 125I-labeled IGF-I, a series of binding proteins with molecular masses (M), between 25 and 45 kDa were detected in the sera, but not saliva, from both BALB/c and diabetic NOD mice. In the diabetic NOD mice, there were detectable changes in the concentrations of several of the IGFBPs relative to BALB/c mice. Using specific antibody to the binding proteins, one of these was identified as IGFBP-2. Gavage administration of [125I]IGFI indicated substantial uptake from the gastrointestinal tract and significant tissue distribution. There was an increase in serum concentrations of radiolabeled IGF-I in diabetic NOD mice over that in BALB/c mice but less recovered from most of the tissues. Intact 125I-labeled IGF-I was extracted and purified from various tissues, following gavage, and shown to retain biological activity. Thus, the uptake of biologically active IGFs from saliva would appear to take place independently of specific binding proteins.