Modulation of the inwardly rectifying K+ channel in isolated human atrial myocytes by α1-adrenergic stimulation

We have examined the ₁-adrenergic modulation of the inwardly-rectifying K⁺ channel (I _K1) in isolated human atrial myocytes using the patch clamp technique. ₁-Adrenergic agonist methoxamine produced action potential prolongation and a depolarization of the resting membrane potential. Under whole-cell voltage clamp conditions, bath application of methoxamine can inhibit macroscopic I _K1. The methoxamine-induced inhibition was reversible and concentration dependent, with the concentration for half-maximal inhibition being 18 m. The methoxamine-induced inhibition of I _K1 was prevented by bath application of ₁-adrenergic blocker prazosin. The current was similarly inhibited by phorbol ester (PMA), an activator of protein kinase C (PKC). In contrast, methoxamine failed to inhibit the current in the presence of a specific PKC inhibitor H-9, suggesting that PKC is involved in the methoxamine-induced inhibition of I _K1. In single channel recording from cell attached patches, bath-applied methoxamine could suppress I _K1 channels by decreasing the frequency and duration of bursting without affecting unitary amplitude. Direct application of purified PKC to excised inside-out patches inhibited channel activity similar to methoxamine in cell-attached patches. The PKC selective inhibitor, PKC19-36, prevented the PKC-induced inhibition of the channel. We conclude that human atrial I _K1 can be inhibited by ₁-adrenergic stimulation via PKC-dependent pathways.     

Depressing action of nitroglycerin on voltage-activated calcium current in isolated smooth muscle cells of the guinea pig gut

The effect of nitroglycerin (NG) on inward voltage-activated calcium current (I _Ca) was studied in isolated smooth muscle cells (SMC) of the guinea pigtaenia coli with the voltage clamp technique in an intracellular dialysis mode. Addition of NG (10^–7 to 10^–4 M) to the extracellular solution reduced theI _Ca amplitude and increased theI _Ca inactivation rate. The maximum inhibition ofI _Ca (on the average, by 41.7 ± 4.8%,n=13) was produced by 10^–4 M NG; the inhibition was dose-dependent. No shift of theI _Ca current-voltage curve under the NG influence was observed. Application of dibutyril-cGMP (2·10^–4 M), a membrane-penetrating analog of cyclic 3,5-guanosine monophosphate (cGMP), likewise decreased theI _Ca amplitude and increased its inactivation rate. The results obtained suggest that the NG inhibitory effect onI _Ca is related to a cGMP-dependent modulation of the voltage-activated calcium channels of L-type in the SMC membrane in the guinea pigtaenia coli.Neirofiziologiya/Neurophysiology, Vol. 26, No. 3, pp. 218–222, May–June, 1994.     

Effects of anti-peptide antibodies against the second extracellular loop of human M2 muscarinic acetylcholine receptors on transmembrane potentials and currents in guinea pig ventricular myocytes

The effects of anti-peptide antibodies against the second extracellular loop of human M2 muscarinic receptor on transmembrane potentials and currents in guinea pig single ventricular cells were analyzed using whole-cell patch clamp technique. These effects were compared with those of the muscarinic receptor agonists carbachol and acetylcholine. The antibodies shortened the action potential duration in a dose-dependent manner. By using a ramp or step rectangular pulse protocol, it was found that the antibodies increased the outward K⁺ current and decreased the inward basal I Ca significantly. The reversal potential of both carbachol-and antibody-induced extra currents were close to –80 mV, being in proximity to the calculated E_k of –90 mV. A -adrenergic receptor agonist, isoprenaline, prolonged the action potential and increased the overshoot which could be inhibited by both antibody and carbachol. Isoprenaline increased inward I_ca and outward I_k simultaneously. Both antibody and carbachol could significantly reduce the isoprenaline-stimulated I_Ca but not the isoprenaline-stimulated I_k. The antibody- or carbachol-induced outward K⁺ current and the depressant effects of antibody and carbachol on isoprenaline-stimulated I_ca were partially antagonized by atropine. These results suggest that the anti-M2 muscarinic receptor antibodies display a stimulatory activity similar to muscarinic receptor agonist on the receptor-mediated electrophysiological events.     

Apparent loss of calcium-activated potassium current in internally perfused snail neurons is due to accumulation of free intracellular calcium

Summary Internal perfusion ofHelix neurons with a solution containing potassium aspartate, MgCl₂, ATP, and HEPES causes the calcium-activated potassium current (I _K(Ca)) evoked by depolarizing voltage steps to decrease with time. When internal free Ca⁺⁺ is strongly buffered to 10^–7 m by including 0.5mm EGTA and 0.225mm CaCl₂ in the internal solution,I _K(Ca) remains constant for up to 3 hours of perfusion. In cells whereI _K(Ca) is small at the start of perfusion, perfusion with the strongly buffered 10^–7 m free Ca⁺⁺ solution produces increases inI _K(Ca) which ultimately saturate. In cells perfused with solutions buffered to 10^–6 m free Ca⁺⁺,I _K(Ca) is low and does not change with perfusion. These results lead us to conclude thatI _K(Ca) is stable in perfusedHelix neurons and that the apparent loss ofI _K(Ca) seen initially with perfusion is due to accumulation of cytoplasmic calcium. Since the calcium current (I _Ca) provides the Ca⁺⁺ which activatesI _K(Ca) during a depolarizing pulse,I _Ca is also stable in perfused cells when free intracellular Ca⁺⁺ is buffered.Perfusion with 1 m calmodulin (CaM) produces no effect onI _K(Ca) with either 10^–7 or 10^–6 m free internal calcium. Inhibiting endogenous CaM by including 50 m trifluoperazine (TFP) in both the bath and the internal perfusion solution also produces no effect onI _K(Ca) with 10^–7 m free internal calciu. It is concluded that CaM plays no role inI _K(Ca) activation.     

Basolateral K channel activated by carbachol in the epithelial cell line T84

Cholinergic stimulation of chloride secretion involves the activation of a basolateral membrane potassium conductance, which maintains the electrical gradient favoring apical Cl efflux and allows K to recycle at the basolateral membrane. We have used transepithelial short-circuit current (I _SC), fluorescence imaging, and patch clamp studies to identify and characterize the K channel that mediates this response in T₈₄ cells. Carbachol had little effect on I _SC when added alone but produced large, transient currents if added to monolayers prestimulated with cAMP. cAMP also enhanced the subsequent I _SC response to calcium ionophores. Carbachol (100 m) transiently elevated intracellular free calcium ([Ca²⁺] _i ) by 3-fold in confluent cells cultured on glass coverslips with a time course resembling the I _sc response of confluent monolayers that had been grown on porous supports. In parallel patch clamp experiments, carbachol activated an inwardly rectifying potassium channel on the basolateral aspect of polarized monolayers which had been dissected from porous culture supports. The same channel was transiently activated on the surface of subconfluent monolayers during stimulation by carbachol. Activation was more prolonged when cells were exposed to calcium ionophores. The conductance of the inward rectifier in cell-attached patches was 55 pS near the resting membrane potential (–54 mV) with pipette solution containing 150 mm KCl (37°C). This rectification persisted when patches were bathed in symmetrical 150 mm KCl solutions. The selectivity sequence was 1 K > 0.88 Rb > 0.18 Na Cs based on permeability ratios under bi-ionic conditions. The channel exhibited fast block by external sodium ions, was weakly inhibited by external TEA, was relatively insensitive to charybdotoxin, kaliotoxin, 4-aminopyridine and quinidine, and was unaffected by external 10 mm barium. It is referred to as the K_BIC channel based on its most distinctive properties (Ba-insensitive, inwardly rectifying, Ca-activated). Like single K_BIC channels, the carbachol-stimulated I _SC was relatively insensitive to several blockers on the basolateral side and was unaffected by barium. These comparisons between the properties of the macroscopic current and single channels suggest that the K_BIC channel mediates basolateral membrane K conductance in T₈₄ cell monolayers during stimulation by cholinergic secretagogues.We thank Dr. Marcel Crest (Laboratoire de Neurobiologie, CNRS, Marseille) for providing a sample of kaliotoxin. This work was supported by the Canadian Cystic Fibrosis Foundation and the Respiratory Health Network of Centres of Excellence. J.W.H. is a Chercheur-Boursier of the Fonds de la recherche en santé du Québec.     

Regulation of an inwardly rectifying K channel in the T84 epithelial cell line by calcium,nucleotides and kinases

Agonists that elevate calcium in T₈₄ cells stimulate chloride secretion by activating K_BIC, an inwardly rectifying K channel in the basolateral membrane. We have studied the regulation of this channel by calcium, nucleotides and phosphorylation using patch clamp and short-circuit current (I _SC) techniques. Open probability (P ₀) was independent of voltage but declined spontaneously with time after excision. Rundown was slower if patches were excised into a bath solution containing ATP (10 m–5 mm), ATP (0.1 mm) + protein kinase A (PKA; 180 nm), or isobutylmethylxanthine (IBMX; 1 mm). Analysis of event durations suggested that the channel has at least two open and two closed states, and that rundown under control conditions is mainly due to prolongation of the long closed time. Channel activity was restimulated after rundown by exposure to ATP, the poorly hydrolyzable ATP analogue AMP-PNP, or ADP. Activity was further enhanced when PKA was added in the presence of MgATP, but only if free calcium concentration was elevated (400 nm). Nucleotide stimulation and inward rectification were both observed in nominally Mg-free solutions. cAMP modulation of basolateral potassium conductance in situ was confirmed by measuring currents generated by a transepithelial K gradient after permeabilization of the apical membrane using -toxin. Finally, protein kinase C (PKC) inhibited single K_BIC channels when it was added directly to excised patches. These results suggest that nonhydrolytic binding of nucleotides and phosphorylation by PKA and PKC modulate the responsiveness of the inwardly rectifying K channel to Ca-mediated secretagogues.This work was supported by the Canadian Cystic Fibrosis Foundation and the Medical Research Council of Canada. J.W.H. is a Chercheur-Boursier of the Fonds de la recherche en santé du Québec.     

Bifurcation Analysis of Genetically Engineered Pacemaking in Mammalian Heart

Genetically engineered pacemaking in ventricular cells has been achieved by down-regulation of the time independent inward rectifying current (I _K1), or insertion of the hyperpolarisation-activated funny current (I _f). We analyse the membrane system (i.e. ionic concentrations clamped) of an epicardial Luo-Rudy dynamic cell model using continuation algorithms with the maximum conductance () of I _K1 and I _f as bifurcation parameters. Pacemaker activity can be induced either via Hopf or homoclinic bifurcations. As _K1 is decreased by ≈74%, autorhythmicity emerged via a homoclinic bifurcation, i.e., the periodicity first appear with infinitely large periods. In contrast, the insertion of _f induced periodicity via a subcritical Hopf bifurcation at _f≈ 0.25 mSμF⁻¹. Stable autorhythmic action potentials occurred at _f > 0.329 mSμF⁻¹.     

Voltage dependence of current through the Na,K-exchange pump ofRana oocytes

Summary We have studied current (I _Str) through the Na, K pump in amphibian oocytes under conditions designed to minimize parallel undesired currents. Specifically,I _Str was measured as the strophanthidin-sensitive current in the presence of Ba^2–, Cd²⁺ and gluconate (in place of external Cl^–). In addition,I _Str was studied only after the difference currents from successive applications and washouts of strophanthidin (Str) were reproducible. The dose-response relationship to Str in four oocytes displayed a meanK _0.5 of 0.4 m, with 2–5 m producing 84–93% pump' block. From baseline data with 12 Na⁺-preloaded oocytes, voltage clamped in the range [–170, +50 mV] with and without 2–5 m Str, the averageI _Str depended directly onV _m up to a plateau at 0 mV with interpolated zero current at –165 mV. In three oocytes, lowering the external [Na⁺] markedly decreased the voltage sensitivity ofI _p , while producing only a small change in the maximal outwardI _Str. In contrast, decreasing the external [K⁺] from 25 to 2.5mm reducedI _Str at 0 mV without substantially affecting its voltage dependence. At K⁺ concentrations of 1mm, both the absolute value ofI _Str at 0 mV and the slope conductance were reduced. In eight oocytes, the activation of the averagedI _Str by [K⁺] _o over the voltage interval [–30, +30 mV] was well fit by the Hill equation, with K=1.7±0.4mm andnH (the minimum number of K⁺ binding sites) =1.7±0.4. The results unequivocally establish that the cardiotonic-sensitive current ofRana oocytes displays only a positive slope conductance for [K⁺] _o >1mm. There is therefore no need to postulate more than one voltage-sensitive step in the cycling of the Na, K pump under physiologic conditions. The effects of varying external Na⁺ and K⁺ are consistent with results obtained in other tissues and may reflect an ion-well effect.     

Inhibitory effect of phorbol ester on carbachol-induced signal transduction in cultured canine tracheal smooth muscle cells

Regulation of the increases in inositol 1,4,5-trisphosphate (IP₃) production and intracellular Ca²⁺ concentration ([Ca²⁺]_i) by activation of protein kinase C (PKC) was investigated in cultured canine tracheal smooth muscle cells (TSMCs). Stimulation of TSMCs by carbachol led to IP₃ formation and caused an initial transient peak of [Ca²⁺]_i followed by a sustained elevation in a concentration-dependent manner. Pretreatment of TSMCs with phorbol 12-myristate 13-acetate (PMA, 1 µM) for 30 min blocked the carbachol-induced IP₃ formation and Ca²⁺ mobilization. Following preincubation, carbachol-induced Ca²⁺ mobilization recovered within 24 h. The concentrations of PMA that gave half-maximal inhibition of carbachol-induced IP₃ formation and increase in [Ca²⁺]_i were 7 and 4 nM, respectively. Prior treatment of TSMCs with staurosporine (1 µM), a PKC inhibitor, inhibited the ability of PMA to attenuate carbachol-induced responses. Inactive phorbol ester, 4-phorbol 12,13-didecanoate at 1 µM, did not inhibit these responses to carbachol. The K_d and B_max of the muscarinic receptor for [³H]N-methylscopolamine binding were not significantly changed by PMA treatment. PMA also decreased PKC activity in the cytosol of TSMCs, while increasing it transiently in the membranes within 30 min. Thereafter, the membrane-associated PKC activity decreased and persisted for at least 24 h of PMA treatment. Taken together, these results suggest that activation of PKC may inhibit phosphoinositide hydrolysis and consequently attenuate the [Ca²⁺]_i increase or inhibit both responses independently. The inhibition by PMA of carbachol-induced responses was inversely correlated with membranous PKC activity.     

PKC-epsilon regulates basolateral endocytosis in human T84 intestinal epithelia: role of F-actin and MARCKS

Protein kinase C (PKC) and the actin cytoskeleton are criticaleffectors of membrane trafficking in mammalian cells. In polarized epithelia, the role of these factors in endocytic events at either theapical or basolateral membrane is poorly defined. In the present study,phorbol 12-myristate 13-acetate (PMA) and other activators of PKCselectively enhanced basolateral but not apical fluid-phase endocytosisin human T84 intestinal epithelia. Stimulation of basolateralendocytosis was blocked by the conventional and novel PKC inhibitorGö-6850, but not the conventional PKC inhibitor Gö-6976,and correlated with translocation of the novel PKC isoform PKC-. PMAtreatment induced remodeling of basolateral F-actin. The actindisassembler cytochalasin D stimulated basolateral endocytosis andenhanced stimulation of endocytosis by PMA, whereas PMA-stimulated endocytosis was blocked by the F-actin stabilizers phalloidin andjasplakinolide. PMA induced membrane-to-cytosol redistribution of theF-actin cross-linking protein myristoylated alanine-rich C kinasesubstrate (MARCKS). Cytochalasin D also induced MARCKS translocationand enhanced PMA-stimulated translocation of MARCKS. A myristoylatedpeptide corresponding to the phosphorylation site domain of MARCKSinhibited both MARCKS translocation and PMA stimulation of endocytosis.MARCKS translocation was inhibited by Gö-6850 but notGö-6976. The results suggest that a novel PKC isoform, likelyPKC-, stimulates basolateral endocytosis in model epithelia by amechanism that involves F-actin and MARCKS.

     

Different photoreceptors within the same retina express unique combinations of potassium channels

Single electrode current and voltage clamp recordings in Calliphora, and whole-cell voltage clamp recordings in Drosophila were used to characterise the voltage-gated K channels in both major classes of photoreceptors, R7/8 (long visual fibres, LVFs) and R1-6 (short visual fibres, SVFs). R7/8 were identified by their unique spectral properties, ca. 3–4 fold higher input resistances and 3–4 fold lower cell capacitance. In Calliphora SVFs possess both fast and slow activating delayed rectifier potassium conductances. Drosophila SVFs possess a slowly inactivating delayed rectifier (I_Ks), a very rapidly inactivating A channel encoded by the Shaker gene (I_A), and, in a minority of cells, a third K conductance with intermediate kinetics (I_Kf). In both specs the LVFs lack the slowest component, but exhibit the faster K conductance(s) with properties indistinguishable from those in the SVFs. These findings add to established evidence demonstrating the significant role played by potassium channels in tuning the photoreceptor membrane. The results also suggest that R1-6 photoreceptors and R7/8 form inputs to visual subsystems tuned to different temporal frequencies.Abbreviations LVF long visual fibre - SVF short visual fibre - R1-6 retinular cells 1 to 6 inclusive - R7/8 retinular cell 7 and 8 - I _A rapidly inactivating A type potassium conductance; channel coded by Shaker gene - I _Kf rapidly activating, slowly inactivating delayed rectifier-like potassium conductance - I _Ks slowly activating, slowly inactivating delayed rectifier-like potassium conductance - I _KDs slowly activating delayed rectifier potassium conductance - I _KDf rapidly activating delayed rectifier potassium conductance     

Disintegration of yeast Saccharomyces cerevisiae in the vertical perl mill with a bell-shaped impeller

Investigation of disintegration of yeast Saccharomyces cerevisiae in the laboratory batch perl mill with a bell-shaped impeller was carried out. The number of non-damaged cells, changing in time was determined using hemocytometer (Thom's chamber).To describe kinetics of the disintegration process the differential equation was applied: where N _p the number of non-damaged cells in the sample, [number of cells/ml] t time, [s] m,k constants.The effect of three operating parameters: rotation frequency of the impeller shaft n, filling of the mill with disintegrating elements (ballotini) S _k and the initial concentration of yeast cells in the suspension C ₀ on the process of disintegration was analyzed.For S _k =0.5, m=1 and dependence of constant k on the rotation frequency of the impeller and suspension concentration were obtained. For S _k =0.6 and 0.7 the values of m were higher than 1. The effect of rotation frequency of the impeller and filling of the mill, with ballotini on constant k and exponent m was determined.List of Symbols a, b constants - a ₁, b ₁, c ₁, d ₁ constants - C ₀ initial concentration of suspension g/ml - C concentration of cell suspension g/ml - k constant disintegration rate 1/s; N ₀ ^1-m /s - m variable in the equation - N ₀ initial number of cells no. of cells/ml - N _p number of non-damaged cells no. of cells/ml - r process rate g/ml·s - X(t) disintegration degree % - , variables in the equation - z variable in the equation - S _k degree of filling the mill with disintegrating elements     

Control of cardiac performance by Ca-turnover

A quantitative model of Ca-turnover in cardiac cells that incorporates negative feedback modulation of sarcolemmal calcium transport (via Ca channels and Na/Ca exchange) has been designed. The Na/Ca exchange current was expressed as I_NaCa = I _NaCar + _Naca . The component I _NaCar reflects slow changes of Ca²⁺ and Na⁺ concentrations and depends on the Na/K pump. I _NaCa is the fast component related to the Ca²⁺ transient. The single input to the model is an arbitrary sequence of intervals between excitations; outputs are sequences of calcium amounts transferred among the compartments during individual intervals. The model operates with a combination of discrete variables (amounts of Ca transferred during contraction, relaxation and rest) and continuous variables — slow changes in ionic concentrations. Since the model is not formalistic but respects the nature of the underlying elements of the system, it enables us to simulate the known effects of cardiotropic drugs or to predict their unknown mechanisms by visualizing the changes in individual Ca compartments. By altering the parameters, the model also simulates the known species and tissue differences in rate-dependent phenomena.     

Cloned T cells recognize Trichinella spiralis antigen in association with an Ek βEk α restriction element

A method is described for the production of T-cell lines and clones specific for solubilized Trichinella spiralis antigens. hese T cells are antigen-specific and do not respond to challenge with a third party antigen (lysozyme). The proliferation responses of the cloned T cells are specifically inhibited by anti-I-E but not by anti I-A subregion monoclonal reagents. The inhibition patterns obtained are consistent with cis-gene complementation in B10.K cells involving the E^k -chain and the E^k -chain of the I-E molecule. Inhibition is obtained with an E^k -specific monoclonal antibody (H9-14.8) but not with an A^k -specific monoclonal antibody (10-2.16). Inhibition was also observed with Ia.7-specific (H40-242) or Ia.22-specific (17-3-3) monoclonal antibodies. The inhibition patterns were confirmed by antigen presentation experiments using recombinant inbred mice. Only B 10.K (E^k E^k spleen cells and not B 10.A(5R) (E^b E^k ) or B10.S(9R) (E^s E^k ) spleen cells could effectively present T. spiralis antigens. The role of hybrid Ia molecules in the immune response to T. spiralis is discussed.     

Characteristics of L-Type Ca2+ Channels in the Membrane of Mesenteric Artery Myocytes from Genetically Hypertensive Rats

Using the standard voltage-clamp technique in the whole-cell mode, we studied the characteristics of barium currents (I _Ba; Ba²⁺ concentration in the external solution was 5 mM) carried through L-type Ca²⁺ channels in the membrane of myocytes of the resistive mesenteric artery from normotensive and genetically hypertensive rats (NR and GHR, respectively). To perforate the membrane, we used amphotericin B. The arbitrary density of I _Ba through the plasma membrane of GHR myocytes significantly exceeded this parameter in the NR group. For both animal groups, activation curves plotted as the dependence of the membrane conductance (G _Ba) on the membrane potential were not significantly different: the membrane potential for half activation (V _0.5) of I _Ba in the NR myocytes was equal to 1.0 ± 0.3 mV with slope factor k = 6.3 ± 0.4 mV, whereas in the GHR myocytes V _0.5 = -1.6 ± 0.2 mV and k = 6.2 ± 0.5 mV. The stationary inactivation curves for I _Ba differed significantly: in the NR myocytes, V _0.5 = -24.2 ± 0.4 mV and k = 8.3 ± 0.2 mV, whereas in the GHR myocytes such parameters were, respectively, -21.4 ± 0.4 and 8.7 ± 0.3 mV. The pattern of intersection of stationary activation and stationary inactivation curves for I _Ba was indicative of the existence of a window current, i.e., the non-inactivating component of I _Ba within the -40 to ±20 mV range; the phenomenon was clearly pronounced in the GHR myocytes. Differences in the arbitrary density of integral I _Ba and window current were observed. These differences can cause an increased tone of the blood vessels in hypertensive animals.     

Kinetics of protein aggregation. Quantitative estimation of the chaperone-like activity in test-systems based on suppression of protein aggregation

The experimental data on the kinetics of irreversible aggregation of proteins caused by exposure to elevated temperatures or the action of denaturing agents (guanidine hydrochloride, urea) have been analyzed. It was shown that the terminal phase of aggregation followed, as a rule, first order kinetics. For the kinetic curves registered by an increase in the apparent absorbance (A) in time (t) the methods of estimation of the corresponding kinetic parameters A _lim and k _I (A _lim is the limiting value of A at t and k _I is the rate constant of the first order) have been proposed. Cases are revealed when the reaction rate constant k _I calculated from the kinetic curve of aggregation of the enzymes coincides with the rate constant for enzyme inactivation. Such a situation is interpreted as a case when the rate of aggregation is limited by the stage of denaturation of the enzyme. A conclusion has been made that, in order to establish the mechanism of protein aggregation, the kinetic investigations of aggregation should be carried out over a wide range of protein concentrations. The refolding experiments after denaturation of proteins by guanidine hydrochloride or urea have been also analyzed. It was shown that aggregation accompanying refolding follows first order kinetics at the final phase of the process. The model of protein refolding explaining such a kinetic regularity has been proposed. When aggregation of protein substrate follows first order kinetics, parameters A _lim and k _I may be used for the quantitative characterization of the chaperone-like activity in the test-systems based on suppression of protein aggregation.     

Non-receptivity for kappa phage of kappa-lysogenic Serratia and reactions to superinfection of receptive cells with a mutant prophage

Summary l_I-lysogenic cells of Serratia marcescens as opposed to l_I ⁺-lysogenic cells, are receptive to further infection. After superinfection with wild type or several clear plaque mutants killing and lytic response were observed to a varying extent. From some of the surviving cells doubly lysogenic colonies with a l_I ⁺ and a l_I prophage could originate, l_I ⁺ being dominant and the cells therefore non-receptive. By this property, combined with a special color reaction, the colonies could be easily screened. Evidence is presented that the l_I ⁺ gene product probably does not interfere with the formation of new phage receptors but that under its influence receptors are masked.Two mutants resembling int^- mutants are described which only rarely give double lysogenization and prophage substitution following superinfection. The phage-coded function normally achieving these reactions is likely to work constitutively after superinfection. Transduction experiments performed with one of the two mutants pointed at integration of prophage into the host DNA near to the trp gene.Abbreviations moi multiplicity of infection - OD optical density - NG N-methyl-N-nitro-N-nitrosoguanidine - NB nutrient broth - BS buffered saline - EMB eosmemethylene blue - leu leucine - met methionine - pro proline - trp tryptophan     

Inward-rectifier K<Superscript><Emphasis Type="Bold">+</Emphasis></Superscript> Current in Guinea-pig Ventricular Myocytes Exposed to Hyperosmotic Solutions

Superfusion of heart cells with hyperosmotic solution causes cell shrinkage and inhibition of membrane ionic currents, including delayed-rectifer K⁺ currents. To determine whether osmotic shrinkage also inhibits inwardly-rectifying K⁺ current (I_K1), guinea-pig ventricular myocytes in the perforated-patch or ruptured-patch configuration were superfused with a Tyrodes solution whose osmolarity (T) relative to isosmotic (1T) solution was increased to 1.3–2.2T by addition of sucrose. Hyperosmotic superfusate caused a rapid shrinkage that was accompanied by a negative shift in the reversal potential of Ba²⁺-sensitive I_K1, an increase in the amplitude of outward I_K1, and a steepening of the slope of the inward I_K1-voltage (V) relation. The magnitude of these effects increased with external osmolarity. To evaluate the underlying changes in chord conductance (G_K1) and rectification, G_K1-V data were fitted with Boltzmann functions to determine maximal G_K1 (G_K1max) and voltage at one-half G_K1max (V_0.5). Superfusion with hyperosmotic sucrose solutions led to significant increases in G_K1max (e.g., 28±2% with 1.8T), and significant negative shifts in V_0.5 (e.g., –6.7±0.6 mV with 1.8T). Data from myocytes investigated under hyperosmotic conditions that do not induce shrinkage indicate that G_K1max and V_0.5 were insensitive to hyperosmotic stress per se but sensitive to elevation of intracellular K⁺. We conclude that the effects of hyperosmotic sucrose solutions on I_K1 are related to shrinkage-induced concentrating of intracellular K⁺.     

Molecular characterization of meiotic recombination within the major histocompatibility complex of the mouse: Mapping of crossover sites within theI region

The molecular analysis of crossing-over within the mouse major histocompatibility complex provides a useful approach for the study of the structural characteristics of meiotic recombination. In this study five intra-I-region recombinants, each derived fromI ^k/I ^b heterozygotes, were characterized for restriction-fragment length polymorphisms (RFLPs) characteristic of theI region of the two parental strains. Southern blot analysis of intra-I recombinant strains A.TBR2, A.TBR3, A.TBR5, A.TBR13, and A.TBR17 using sixI-region DNA probes revealed that the point of crossing-over in all five recombinants occurred within a 6.2-kbKpnI-EcoRI segment located within theE gene. The segments of DNA containing the crossover point from each of the recombinant chromosomes were cloned by screening partial genomic libraries constructed in gt7 bacteriophage. Construction of partial restriction maps of the cloned segments from the parental and recombinant chromosomes permitted the boundaries of the area containing the crossover site to be narrowed to a 4.0-kb segment located almost entirely within an intron of theE gene. The recognition that the points of crossing-over in all five recombinants studied are clustered in a relatively small area of theI region provides further evidence for a hot spot of recombination associated with theE _ß gene.This work was supported by Grants AI14424 and AI20317 from the National Institutes of Health. J. Kobori was supported by a postdoctoral fellowship from the Arthritis Foundation. E. Zimmerer was supported by a postdoctoral fellowship from the Charles and Johanna Busch Fund of the Bureau of Biological Research. D. Spinella was supported by a predoctoral fellowship from the Charles and Johanna Busch Fund.     

Modulation of neuronal calcium channels by arachidonic acid and related substances

Low-voltage-activated (1-v-a) and high-voltage-activated (h-v-a) Ca²⁺ currents I _Ca were recorded in whole-cell voltage clamped NG108-15 neuroblastoma x glioma hybrid cells. We studied the effects of arachidonic acid (AA), oleic acid, myristic acid and of the positively charged compounds tetradecyltrimethyl-ammonium (C₁₄TMA) and sphingosine. At pulse potentials >–20 mV, AA (25-100 m) decreased 1-v-a and h-v-a I _Ca equally. The decrease developed slowly and became continually stronger with increasing time of application. It was accompanied by a small negative shift and a slight flattening of the activation and inactivation curves of the 1-v-a I _Ca. The shift of the activation curve manifested itself in a small increase of 1-v-a I _Ca at pulse potentials <–30 mV. The effects were only partly reversible. The AA effect was not prevented by 50 m 5, 8, 11, 14-eicosatetraynoic acid, an inhibitor of the AA metabolism, and not mimicked by 0.1–1 m phorbol 12, 13-dibutyrate, an activator of protein kinase C. Probably, AA directly affects the channel protein or its lipid environment. Oleic and myristic acid acted similarly to AA but were much less effective. The positively charged compounds C₁₄TMA and sphingosine had a different effect: They shifted the activation curve of 1-v-a I _Ca in the positive direction and suppressed 1-v-a more than h-v-a I _Ca; their effect reached a steady-state within 5–10 min and was readily reversible. C₁₄TMA blocked 1-v-a I _Ca with an IC₅₀ of 4.2 m while sphingosine was less potent.