Sodium-calcium exchange in regulation of cardiac contractility. Evidence for an electrogenic, voltage-dependent mechanism

The origin and regulatory mechanisms of tonic tension (Ca current-independent component of contractility) were investigated in frog atrial muscle under voltage-clamp conditions. Tonic tension was elicited by depolarizing pulses of 160 mV (Em = +90 mV, i.e., close to E ca) and 400--600 ms long. An application of Na-free (LiCl) or Ca-free Ringer's solutions resulted in a fast (less than 120 s), almost complete abolition of tonic tension. When [Na]o was reduced (with LiCl or sucrose as the substitutes), the peak tonic tension increased transiently and then decreased below the control level. The transient changes in tonic tension were prevented by using low-Na, low-Ca solutions where the ratios [Ca]0/[Na]40 to [Ca]o/[Na]4o were kept constant (1.1 X 10(-8) mM-3 to 8.7 X 10(-13) mM-5). Na-free (LiCl) solution elicited contractures accompanied by a membrane hyperpolarization or by an outward current even when the Na-K pump was inhibited. 15 mM MnCl2 (or 3 mM LaCl3) inhibited the development of the Na-free contracture and the related part of hyperpolarization or the outward current. In conclusion, our results indicate that tonic tension is regulated by a Na-Ca exchange mechanism. Furthermore, they suggest that this exchange could be electrogenic (exchanging three or more Na ions for one Ca ion) and thus voltage dependent. The possible contribution of an electrogenic Na-Ca exchange in the maintenance of cardiac membrane potential is discussed.     

The Electrical Activity of Canine Cardiac Purkinje Fibers in Sodium-Free, Calcium-Rich Solutions

Propagated action potentials can be obtained in canine cardiac Purkinje fibers exposed to Na-free solutions containing no inorganic cation other than Ca and K. Essentially similar action potentials are obtained if Na is replaced by tetraethylammonium (TEA), tetramethylammonium (TMA), or choline. In a solution containing 128 mM TEA and 16.2 mM Ca the characteristics of these electrical responses were: maximum diastolic potential, -59 ± 3.3 mV; overshoot, 20 ± 6.8 mV; maximum upstroke velocity, 3.7 ± 2.3 V/s; conduction velocity, 0.1 m/s; and action potential duration, 360 ± 45 ms. The magnitude of the overshoot varied with log Ca_o with a slope of about 30 mV/10-fold concentration change. The upstroke velocity was an approximately linear function of Ca_o. The active response was greatly diminished or abolished by Mn and D-600 but was unaffected by tetrodotoxin. These Ca-dependent responses appeared in a region of transmembrane potential (about -50 mV) at which the rapid Na-dependent upstroke is abolished even when Na is present.     

Electrophysiological examination of transmitter release in non-quantal form in the mouse diaphragm and the activity of membrane ATP-ase.

The subsynaptic area of mouse diaphragm fibres was hyperpolarized by 1--2 mV during local curarization of the junctional zone in the presence of the reversible anticholinesteraze prostigmine (6 X 10(-6) M), or after treatment of the muscle with organophosphate cholinesterase inhibitor Soman. In a solution containing 5 mM K+ the mean hyperpolarization was 1.1 +/- 0.27 mV at mean resting potential--70 mV. After adding 2 X 10(-5) M ouabain the hyperpolarization increased to 1.5 +/- 0.25 mV. Removal of potassium ions from the bathing medium also increased curare induced hyperpolarization to 1.80 +/- 0.40 mV. Reactivation of membrane ATP-ase by addition of K+ after a period in K+-free medium reduced the hyperpolarization to zero, where measurements were performed 10--20 min after the readdition. It was concluded that spontaneous non-quantal leakage of acetylcholine occurs at the mouse neuromuscular junction, as it does in the frog (ref. Katz and Miledi 1977). Conditions which block the Na+-K+-dependent ATP-ase of nerve terminals increased the continuous leakage of ACh and activation of the pump decreased it.     

Stoichiometry and voltage dependence of the sodium pump in voltage- clamped, internally dialyzed squid giant axon

The stoichiometry and voltage dependence of the Na/K pump were studied in internally dialyzed, voltage-clamped squid giant axons by simultaneously measuring, at various membrane potentials, the changes in Na efflux (delta phi Na) and holding current (delta I) induced by dihydrodigitoxigenin (H2DTG). H2DTG stops the Na/K pump without directly affecting other current pathways: (a) it causes no delta I when the pump lacks Na, K, Mg, or ATP, and (b) ouabain causes no delta I or delta phi Na in the presence of saturating H2DTG. External K (Ko) activates Na efflux with Michaelis-Menten kinetics (Km = 0.45 +/- 0.06 mM [SEM]) in Na-free seawater (SW), but with sigmoid kinetics in approximately 400 mM Na SW (Hill coefficient = 1.53 +/- 0.08, K1/2 = 3.92 +/- 0.29 mM). H2DTG inhibits less strongly (Ki = 6.1 +/- 0.3 microM) in 1 or 10 mM K Na-free SW than in 10 mM K, 390 mM Na SW (1.8 +/- 0.2 microM). Dialysis with 5 mM each ATP, phosphoenolpyruvate, and phosphoarginine reduced Na/Na exchange to at most 2% of the H2DTG-sensitive Na efflux. H2DTG sensitive but nonpump current caused by periaxonal K accumulation upon stopping the pump, was minimized by the K channel blockers 3,4-diaminopyridine (1 mM), tetraethylammonium (approximately 200 mM), and phenylpropyltriethylammonium (20-25 mM) whose adequacy was tested by varying [K]o (0-10 mM) with H2DTG present. Two ancillary clamp circuits suppressed stray current from the axon ends. Current and flux measured from the center pool derive from the same membrane area since, over the voltage range -60 to +20 mV, tetrodotoxin-sensitive current and Na efflux into Na-free SW, under K-free conditions, were equal. The stoichiometry and voltage dependence of pump Na/K exchange were examined at near-saturating [ATP], [K]o and [Na]i in both Na-free and 390 mM Na SW. The H2DTG-sensitive F delta phi Na/delta I ratio (F is Faraday's constant) of paired measurements corrected for membrane area match, was 2.86 +/- 0.09 (n = 8) at 0 mV and 3.05 +/- 0.13 (n = 6) at -60 to -90 mV in Na-free SW, and 2.72 +/- 0.09 (n = 7) at 0 mV and 2.91 +/- 0.21 (n = 4) at -60 mV in 390 mM Na SW. Its overall mean value was 2.87 +/- 0.07 (n = 25), which was not significantly different from the 3.0 expected of a 3 Na/2 K pump.(ABSTRACT TRUNCATED AT 400 WORDS)     

Sodium and calcium interactions in vascular smooth muscle cells of the rabbit ear artery

The effects of Na-free and of K-free solutions on the membrane potential, on tension development, and on 45Ca exchange have been investigated in rabbit ear artery. The contraction induced by Na-free solutions and the tension which develops in K-free solutions after a delay of about 1 h are both submaximal. Exposure for 4 h to K-free solutions does not affect the membrane potential, whereas Na-free solutions depolarize the cells by 10-20 mV, depending on the Na-substitute. Neither the amplitude nor the rate constant of the slowly exchanging 45Ca-fraction is affected by these experimental procedures. Substituting external Na by choline or TMA induces a transient increase of the 45Ca-efflux rate which does not occur in a Ca-free efflux medium, and which can be blocked with La. K readmission to Na-enriched tissues hyperpolarizes the cells up to -100 mV and induces a relaxation, without exerting any effect on the 45Ca efflux rate. The release of Ca from intracellular stores, induced by histamine and FCCP, and its subsequent extrusion through the plasma membrane produce a transient stimulation of the 45Ca efflux, which is not affected by the reduction of the Na gradient. The transient contraction induced by histamine in Ca-free solutions is affected in a different way by different Na substitutes. The results do not fit the Na-Ca exchange hypothesis but are consistent with an effect of the Na gradient on the passive Ca influx.     

A comparative study of the effects of tetrodotoxin and the removal of external Na+ on the resting potential: Evidence of separate pathways for the resting and excitable Na currents in squid axon

To investigate whether the Na permeability of the resting membrane is determined predominantly by the excitable Na channel, we examined the effects of tetrodotoxin (TTX) and the complete removal of external Na+ on the resting potential. In the intact squid axon bathed in K-free artificial seawater, both TTX and the removal of Na+ produced small hyperpolarizations. The effect of Na removal, however, was larger than that of TTX. In the perfused squid axon, the hyperpolarization produced by the removal of external Na+ was greatly enhanced when the internal K concentration ([K+]i) was reduced. The effect of TTX, on the other hand, was not sensitive to the [K+]i or to the membrane potential. For [K+]i = 50 mM and [K+]o = 0, the average hyperpolarization produced by TTX was 1.2 mV, while the hyperpolarization produced by Na removal was approximately 21 mV. The difference between these two effects suggests that the majority of the resting Na current passes through pathways other than the excitable Na channel.     

Cesium blockade of delayed outward currents and electrically induced pacemaker activity in mammalian ventricular myocardium

The effects of Cs+, 5-25 mM, were studied in cat and guinea pig papillary muscles using voltage clamp and current clamp techniques. In solutions containing normal K+, the major effects of Cs+ were depolarization of the resting potential and reduction of the delayed outward current (ixl) between -80 and -20 mV. Both inward and outward portions of the isochronal current voltage relation (l-s clamps) were reduced by extracellular Cs+. This resulted in a substantial reduction of inward rectification and, by subtraction from the normal I-V relationship, the definition of a Cs+-sensitive component of current. Under current clamp conditions, 5-10 mM Cs+ produced a dose-dependent slowing of repetitive firing induced by depolarization. At higher concentrations (25 mM) the resting potential was depolarized and repetitive activity could not be induced by further depolarization. However, release of hyperpolarizing pulses was followed by prolonged bursts of repetitive action potentials, suggesting partial reversal of blockade or participation of another pacemaker process. The experimental results and a numerical simulation show that under readily attainable conditions, reduction in an outward pacemaker current may slow pacemaker activity.     

Properties of sodium pumps in internally perfused barnacle muscle fibers

To study the properties of the Na extrusion mechanism, giant muscle fibers from barnacle (Balanus nubilus) were internally perfused with solutions containing tracer 22Na. In fibers perfused with solutions containing adenosine 5'-triphosphate (ATP) and 30 mM Na, the Na efflux into 10 mM K seawater was approximately 25-30 pmol/cm2.s; 70% of this efflux was blocked by 50-100 microM ouabain, and approximately 30% was blocked by removal of external K. The ouabain-sensitive and K-dependent Na effluxes were abolished by depletion of internal ATP and were sigmoid-shaped functions of the internal Na concentration ([Na]i), with half-maxima at [Na]i approximately or equal to 20 mM. These sigmoid functions fit the Hill equation with Hill coefficients of approximately 3.5. Ouabain depolarized ATP-fueled fibers by 1.5-2 mV ([Na]i greater than or equal to 30 mM) but had very little effect on the membrane potential of ATP-depleted fibers; ATP depletion itself caused a 2-2.5- mV depolarization. When fueled fibers were treated with 3,4- diaminopyridine or Ba2+ (to reduce the K conductance and increase membrane resistance), application of ouabain produced a 4-5 mV depolarization. These results indicate that an electrogenic, ATP- dependent Na-K exchange pump is functional in internally perfused fibers; the internal perfusion technique provides a convenient method for performing transport studies that require good intracellular solute control.     

Effect of sodium on voltage-current relationships in rat muscles.

(1) Replacement of Tris by Na in propionate solution causes depolarizations (3-10 or more than 30 mV) in rat muscles. As a result, the resting potentials are distributed in two groups, one at about -70 mV and the other at about -40 mV. Small inward or outward currents are often sufficient for the membrane potential to switch from one level to the other. The change from the low (more positive) to the high (more negative) resting potential can also be provoked by small increases in [K] and vice versa. (2) High frequency, low-amplitude oscillations are produced by gradually repolarizing the membrane at the low resting potential level. The frequency decreases (from a high 2/sec to 5/min or less) and the amplitude increases (up to 30 mV) with further repolarization. Low amplitude oscillations are sinusoidal, high amplitude oscillations resemble pacemaker potentials in other tissues. (3) The voltage-current relationship in Na propionate solutions containing 2 mM K frequently displays pronounced hysteresis presumably covering a negative conductance region. Hysteresis is about the same in Na and Tris containing solutions at high (more than 20 mM) [K]. The results are discussed in terms of an interaction between depolarizing K inactivation and gNa activation, possibly in a channel not involved in spike production.     

Atrioventricular block in low potassium and K dependence of the nodal resting potential

A progressive conduction block leading to atrioventricular dissociation develops in perfused rabbit hearts within 20-30 min of exposure to Krebs containing 0.5 mM potassium (low K). A decrease in potassium permeability resulting in membrane depolarization (as seen in Purkinje fibers) could be responsible for the loss of excitability in nodal cells. We investigated the K dependence of the resting potential and the long-term effects of low K perfusion on the resting and action potentials of nodal cells in rabbit hearts. The resting potential of atrial, atrionodal, and nodal cells varied by 52, 41, and 34 mV per decade of change in Ko within the range of 5-50 mM K. Hyperpolarization of the resting membrane, a progressive decline in action potential amplitude, and a decrease in maximum rate of rise were observed in nodal fibers when exposed to low K. Loss of propagated activity occurred in the middle node within 20-30 min while the cells remained hyperpolarized. There was no evidence of electrogenic Na extrusion and it seems that the low nodal resting potential results from a high resting PNa/PK permeability ratio. The early decrease in rate of rise in low K probably reflects an increase in K-dependent outward currents, whereas the progressive deterioration and final loss of conducted electrical activity may result from an accumulation of internal Na and Ca overload produced by low K inhibition of the Na pump.     

Effects of marinobufagenin on electrophysiological and contractile characteristics of the rat diaphragm

Effects of Na+,K(+)-ATPase inhibitor: marinobufagenin, on contractile and electric characteristics of isolated rat diaphragm were studied for the first time. Marinobufagenin induced dose-dependent (EC50 = 0.3 +/- 0.1 nM) increase in the contraction force (positive inotropic effect). At 1-2 nM, it slowed down the fatigue induced by continuous direct stimulation (2/s) of the muscle. Marinobufagenin at the same concentrations did not affect resting membrane potential or parameters of action potentials of muscle fibers, while at 10 and 20 nM it induced hyperpolarization by approximately 2 mV. Marinobufagenin blocked dose-dependently (IC50 = 2.9 +/- 2.0 nM) hyperpolarizing effect of acetylcholine (100 nM) mediated by increase in electrogenic contribution of alpha2 isoform of the Na+,K(+)-ATPase. This result suggests a capability of marinobufagenin to inhibit this isoform of the Na+,K(+)-ATPase. Possible mechanisms of marinobufagenin effects in skeletal muscle are discussed.     

Membrane current following activity in canine cardiac Purkinje fibers

Membrane current following prolonged periods of rapid stimulation was examined in short (less than 1.5 mm) canine cardiac Purkinje fibers of radius less than 0.15 mm. The Purkinje fibers were repetitively stimulated by delivering trains of depolarizing voltage clamp pulses at rapid frequencies. The slowly decaying outward current following repetitive stimulation ("post-drive" current) is eliminated by the addition of 10(-5) M dihydro-ouabain. The post-drive current is attributed to enhanced Na/K exchange caused by Na loading during the overdrive. Depolarizing voltage clamp pulses initiated from negative (- 80 mV) or depolarized (-50 mV) holding potentials can give rise to post- drive current because of activation of tetrodotoxin-sensitive or D600- sensitive channels. The magnitude of the post-drive current depends on the frequency of voltage clamp pulses, the duration of each pulse, and the duration of the repetitive stimulation. The time constant of decay of the post-drive current depends on extracellular [K] in accordance with Michaelis-Menten kinetics. The Km is 1.2 mM bulk [K], [K]B. The mean time constant in 4 mM [K]B is 83 s. Epinephrine (10(-5) M) decreases the time constant by 20%. The time constant is increased by lowering [Ca]o between 4 and 1 mM. Lowering [Ca]o further, to 0.1 mM, eliminates post-drive current following repetitive stimulation initiated from depolarized potentials. The latter result suggests that slow inward Ca2+ current may increase [Na]i via Na/Ca exchange.     

Binding of S-adenosylhomocysteine to hydroxyindole O-methyltransferase

Mg2+-selective microelectrodes have been used to measure the intracellular free Mg2+ concentration in frog skeletal muscle fibers. Glass capillaries with a tip diameter of less than 0.4 micron were backfilled with the Mg2+ sensor, ETH 1117. In the absence of interfering ions, they gave Nernstian responses between 1 and 10 mM free Mg2+. In the presence of an ionic environment resembling the myoplasm, the microelectrode response was sub Nernstian (18-24 mV) but still useful. The electrodes were calibrated before and after muscle-fiber impalements . In quiescent fibers from sartorius muscle (Rana pipiens), with resting membrane potentials not less than -82 mV, the intracellular free Mg2+ concentration was 3.8 +/- 0.41 (S.E.) mM (n = 58) at 22 degrees C. No significant change in the intracellular free Mg2+ was observed following extensive (approx. 6 h) incubation in Mg2+-free media. Increasing the external concentration of magnesium from 4 to 20 mM (approx. 15 min) produced a slow and small enhancement (1.8 mM) of [Mg2+]i, which was fully reverted when the divalent cation was removed from the bathing solution. No change in ionic magnesium resting concentration was observed when the muscle fibers were treated either with caffeine 3 mM or with Na+-free solutions. In depolarized muscle fibers (-23 +/- 2.7 mV) treated with 100 mM K+, the myoplasmic [Mg2+] was 3.7 +/- 0.45 (S.E.) mM, n = 6, immediately after the spontaneous relaxation of the contracture. Similar determinations in muscle fibers during stimulation at low frequency (5 Hz), and after fatigue development, showed no changes in the concentration of free cytosolic Mg2+. These results point out that [Mg2+]i is not modified under these three different experimental conditions.     

三叉神经中脑核神经元膜的电学特性

用大鼠脑干脑片,给三叉神经中脑核79个神经元作了细胞内记录,测算了20个神经元膜的电学特性:静息电位-60.3±5.6mV;输入阻抗为10.5±5.4MΩ;时间常数1.3±0.5ms。电刺激可诱发动作电位,测算32个神经元的有关参数:阈电位-50—-55mV;波幅69.5±6.1mV;超射11.9±3.6mV;波宽0.8±0.2ms。TTX(0.3μmol/L)或无钠使之消失。通以长时程矩形波电流可引起200—250Hz的2—15个重复放电,但在通电停止前终止,TEA或4-AP可延长放电。膜电位-60—-55mV时在动作电位之后可看到阈下电位波动,它不受TTX的影响,无钙时消失,TEA或4-AP使波幅增大。静息电位去极化可使45个神经元中的40个发生外向整流作用,并被TEA,4-AP或无钙抑制,超极化则发生内向整流作用,Cs或无钠抑制之。灌流液中加入各种钾通道阻断药时神经元的稳态I-V曲线发生相应变化,提示I_(DR),l_A,I_(K(Ca))及I_Q可能都与静息时的膜电导有关。     

The influence of cellular amino acids and the Na+ : K+ pump on the membrane potential of the Ehrlich ascites tumor cell.

The membrane potential of the Ehrlich ascites tumor cell was shown to be influenced by its amino acid content and the activity of the Na+ :K+ pump. The membrane potential (monitored by the fluorescent dye, 3,3'-dipropylthiodicarbocyanine iodide) varied with the size of the endogenous amino acid pool and with the concentration of accumulated 2-aminoisobutyrate. When cellular amino acid content was high, the cells were hyperpolarized; as the pool declined in size, the cells were depolarized. The hyperpolarization seen with cellular amino acid required cellular Na+ but not cellular ATP. Na+ efflux was more rapid from cells containing 2-aminoisobutyrate than from cells low in internal amino acids. These observations indicate that the hyperpolarization recorded in cells with high cellular amino acid content resulted from the electrogenic co-efflux of Na+ and amino acids. Cellular ATP levels were found to decline rapidly in the presence of the dye and hence the influence of the pump was seen only if glucose was added to the cells. When the cells contained normal Na+ (approx. 30mM), the Na+ :K+ pump was shown to have little effect on the membrane potential (the addition of ouabain had little effect on the potential). When cellular Na+ was raised to 60mM, the activity of the pump changed the membrane potential from the range -25 to -30 mV to -44 to -63 mV. This hyperpolarization required external K+ and was inhibited by ouabain.     

Electrophysiological properties of isolated photoreceptors from the eye of Lima scabra

Photoreceptor cells were enzymatically dissociated from the eye of the file clam, Lima scabra. Micrographs of solitary cells reveal a villous rhabdomeric lobe, a smooth soma, and a heavily pigmented intermediate region. Membrane voltage recordings using patch electrodes show resting potentials around -60 mV. Input resistance ranges from 300 M omega to greater than 1 G omega, while membrane capacitance is of the order of 50-70 pF. In darkness, quantum bumps occur spontaneously and their frequency can be increased by dim continuous illumination in a fashion graded with light intensity. Stimulation with flashes of light produces a depolarizing photoresponse which is usually followed by a transient hyperpolarization if the stimulus is sufficiently intense. Changing the membrane potential with current-clamp causes the early phase to invert around +10 mV, while the hyperpolarizing dip disappears around -80 mV. With bright light, the biphasic response is followed by an additional depolarizing wave, often accompanied by a burst of action potentials. Both Na and Ca ions are required in the extracellular solution for normal photoexcitation: the response to flashes of moderate intensity is greatly degraded either when Na is replaced with Tris, or when Ca is substituted with Mg. By contrast, quantum bumps elicited by dim, sustained light are not affected by Ca removal, but they are markedly suppressed in a reversible way in 0 Na sea water. It was concluded that the generation of the receptor potential is primarily dependent on Na ions, whereas Ca is probably involved in a voltage-dependent process that shapes the photoresponse. Light adaptation by repetitive flashes leads to a decrease of the depolarizing phase and a concomitant enhancement of the hyperpolarizing dip, eventually resulting in a purely hyperpolarizing photoresponse. Dark adaptation restores the original biphasic shape of the photoresponse.     

Effects of triiodothyronine on resting membrane potential of primary cultured rat submandibular gland cells

The effect of triiodothyronine (T3) on the resting membrane potential was measured in primary cultured rat submandibular gland cells. The resting membrane potential was 29.5 +/- 0.71 mV. The hormone T3, at concentrations of 10(-9) M or greater, hyperpolarized the cells 5.8 mV (p less than 0.05). Hyperpolarization was complete within 24 hours. Ouabain (1 mM) depolarized the cells 5.9 mV. Cells exposed to T3 and ouabain had the same membrane potential as cells treated with ouabain alone. These data suggest that the hyperpolarization observed can be, in part, attributed to triiodothyronine-induced synthesis of (Na-K)-adenosine triphosphatase.     

Inward rectification in rat olfactory receptor neurons.

Inwardly rectifying currents in enzymically dissociated olfactory receptor neurons of rat were studied by using patch-clamp techniques. Upon hyperpolarization to membrane potentials more negative than -100 mV, small inward-current relaxations were observed. Activation was described by a single exponential with a time constant that decreased e-fold for a 21 mV hyperpolarization. The current was not reduced by the external application of 5 mM Ba2+, but was abolished by the addition of 5 mM Cs+ to the bath solution. Increasing the external K+ concentration ([K+]o) to 25 mM dramatically enhanced the current without affecting the voltage range or the kinetics of activation. In 25 mM [K+]o, tail currents reversed at -26 mV, significantly more positive than the K+ equilibrium potential of -44 mV. These characteristics are consistent with those of a mixed Na+/K+ inward rectification that has been reported in several types of neuronal, cardiac and smooth muscle cells. The current may contribute to controlling cell excitability during the response to some odorants.     

Potential difference responses to secretory K+, Na+ and HCO3- changes in secreting and resting states of frog stomach in Cl(-)-free media

The effects of changes in secretory concentrations of K+, Na+ and HCO3- on transmucosal potential difference (PD) and resistance in Cl(-)-free (SO4(2-)) solutions were compared for secreting fundus and resting fundus of Rana pipiens. In the resting fundus experiments, histamine was not present in the nutrient solution and cimetidine was primarily used to obtain acid inhibition. Increase of K+ from 4 to 80 mM, decrease of Na+ from 156 to 15.6 mM and decrease of HCO3- from 25 to 5 mM gave, 10 min after the change, in the secreting fundus delta PD values of 39.7, -11.9 and 3.2 mV, respectively. In the resting fundus, 1.5 to 2 h after the addition of cimetidine, the same changes in secretory ion concentration gave delta PD values of 12.2, -5.6 and 1.5 mV, respectively. Replacement of cimetidine with SCN and without histamine yielded a delta PD somewhat lower than that in cimetidine, namely 9 mV for a K+ change from 4 to 80 mM. Subsequent addition of histamine with SCN present gave a delta PD of about 21 mV. The change in PD was attributed to histamine increasing the secretory membrane area, leading to an increase in K+ conductance. Another possibility is that histamine increases the K+ conductance per se.     

Glucose- and concentration-dependence of noradrenalin effects on electrical activity in mouse pancreatic beta cells

The effects of a wide range of noradrenalin (NA) concentrations (10(-11)-10(-4) M) on the membrane potential and on the glucose-induced electrical activity were investigated with microelectrodes in microdissected mouse islets. In the presence of 11.1 mM glucose, the beta cells exhibited a repetitive activity. NA at more than 10(-7) M induced a rapid hyperpolarization followed by a silent depolarization, then by the appearance of a slowed pace of repetitive activity (dose-dependent effects). NA at less than 10(-7) M did not markedly affect the electrical activity; it only induced a dose-dependent increase in the degree of activity with no change of the potential levels. The glucose-dependence of these effects were then investigated. In the absence of glucose, 10(-8) and 10(-6) M NA did not affect the resting membrane potential. Non-stimulatory glucose concentrations (2.8-7.3 mM) progressively decreased the membrane potential. 10(-8) M NA did not affect it, while 10(-6) M NA induced a dose-dependent and long-lasting hyperpolarization. In the presence of stimulatory glucose concentrations (7.3-30 mM) the degree of activity increased, 10(-8) M NA induced a slight leftward shift and 10(-6) M NA a slight rightward shift of the dose-response curve.