Pharmacokinetic and Pharmacodynamic Comparison of Sildenafil-Bosentan and Sildenafil-Ambrisentan Combination Therapies for Pulmonary Hypertension

To elucidate whether the pharmacokinetics (PK) and pharmacodynamics (PD) of sildenafil are influenced differently when it is coadministered with bosentan (S+B) or with ambrisentan (S+A), we evaluated the PK and PD profiles of sildenafil before and after 4–5 weeks of S+A or S+B treatment in patients with pulmonary arterial hypertension. The area under the plasma concentration–time curve of sildenafil was significantly higher in S+A treatment than in S+B treatment (165.8 ng•h/mL vs. 396.8 ng•h/mL, P = 0.018) and the oral clearance of sildenafil was significantly lower after S+A treatment than after S+B treatment (120.6 L/h/kg vs. 50.4 L/h/kg, P = 0.018). In the PD study, incremental shuttle walking distance was superior during treatment with S+A than during treatment with S+B (S+B; 280 m vs. S+A; 340 m, P = 0.042). There were no concerns about safety with either combination therapy regime.

Immunological abnormalities in splenic marginal zone cell lymphoma

The clinical features of patients with splenic marginal zone cell lymphoma (SMZCL) have rarely been reported. In the present study, immunological abnormalities, particularly hematological abnormalities, observed in SMZCL were described. Autoimmune hemolytic anemia, immune thrombocytopenia, and appearance of lupus anticoagulant were observed in 2 of 3 patients with SMZCL. Other abnormal data including monoclonal gammopathy and cold agglutinin were also observed in 2 of the 3 patients. Immunological abnormalities may be characteristic complications in patients with SMZCL and must be followed carefully, since they may be a reliable marker of this type of lymphoma activity.

[Syphilitic thoracic aortic aneurysm with destruction of vertebral body, producing numbness of lower extremities and paraplegia]

Numbness and paraplegia are uncommon complaints in patient with thoracic aortic aneurysm (TAA). The patient was a 64-year-old man. He suffered numbness and gait disturbance (paraplegia). The blood examination showed no positive findings except a Wassermann was positive. Roentgen examination of the chest showed two abnormal shadows like tumors. The CT and MRI revealed destruction of the vertebral bodies and TAAs adjacent to the spinal cord. After the graft replacement was performed, numbness and paraplegia disappeared. This suggests that in our patient the TAAs destruct the vertebral body and produce pressure on the spinal cord, causing numbness and paraplegia. We experienced a rare case of the syphilitic TAA producing bone destruction, numbness and paraplegia.

Stretch-activated anion currents of rabbit cardiac myocytes

1. Stretch-activated anion currents were studied in sino-atrial and atrial cells using the whole-cell patch clamp technique. With continuous application of positive pressure (5-15 cmH2O) through the patch clamp electrode, the cell was inflated and the membrane conductance was increased. 2. Voltage clamp steps revealed that the stretch-activated currents had time-independent characteristics. The increase in membrane conductance was reversible on subsequent application of negative pressure to the electrode. 3. The reversal potential of the stretch-activated currents was shifted by 60 mV for a 10-fold change in intracellular Cl- concentration, while it was unaffected by replacement of Na+ in the extracellular solution by N-methyl-D-glucamine. Cell superfusion with Cl(-)-deficient solution (10 mM Cl-) reduced the amplitude of outward current. These findings indicate that the stretch-activated conductance is Cl- selective. 4. The sequence of anion permeability through the stretch-activated conductance was determined to be I-(1.7) > NO3-(1.5) > Br-(1.2) > Cl-(1.0) > and F-(0.6). SCN- appeared to be more permeant than I-. 5. The stretch-activated conductance was reduced by the Cl- channel blockers, 4,4'-dinitrostilbene-2,2'-disulphonic acid disodium salt, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid or anthracene-9-carboxylate (9-AC). Administration of furosemide or bumetanide had no effect. 6. The stretch-activated Cl- current was recorded even though intracellular Ca2+ ions were chelated by including 10 mM EGTA in the pipette solution. Neither the specific peptide inhibitor of cyclic AMP-dependent protein kinase (50 microM), nor the non-selective blocker of protein kinases, H-7 (20 microM), was effective in reducing the stretch-activated Cl- current, suggesting that the stretch-activated Cl- current is a novel type of cardiac Cl- current, which shows a different modulatory mechanism from that of other cardiac Cl- currents.

Background current in sino-atrial node cells of the rabbit heart

1. The Ca2+ current, K+ current, hyperpolarization-activated current, Na(+)-K+ pump current and the Na(+)-Ca2+ exchange current were all blocked by appropriate blockers and the remaining time-independent currents were investigated in single pacemaker cells of the rabbit sino-atrial node using the whole-cell patch clamp technique. 2. Exchanging the bathing solution from Tris-hydroxymethyl-aminomethane hydrochloride (Tris) Na+ free to 150 mM-Na+ induced an inward current and the slope conductance of the current-voltage relationship increased from 0.45 +/- 0.18 to 0.87 +/- 0.33 nS (n = 71) at -50 mV. The remaining conductance in Tris Na(+)-free solution was essentially the same when Tris was substituted with tetraethylammonium (TEA) or N-methyl-D-glucamine (NMG). The current density of the Na(+)-dependent inward current obtained by subtracting the current in Tris Na(+)-free from that in 150 mM-Na+ solution was 0.73 +/- 0.21 pA/pF (n = 71) at -50 mV. We called this current the Na(+)-dependent background current. 3. The membrane conductance was reduced by lowering the temperature of the external solution from 36 to 23 degrees C. In Tris Na(+)-free solution, the temperature-sensitive component was outward at all potentials, whereas it showed a reversal potential at around -20 mV in 150 mM-Na+ solution. This reversal potential was interpreted as a sum of the Cs+ efflux and Na+ influx, by comparing the Na(+)-dependent inward currents obtained at 36 degrees C and those at 23 degrees C. 4. Divalent cations (2 mM-Ni2+, 1 mM-Ba2+ or 2 mM-Ca2+) reduced only the outward current in the Tris Na(+)-free solution, while in the 150 mM-Na+ solution, they reduced both the inward and outward components of the current which had a reversal potential of around -10 mV. 5. Amiloride depressed the membrane conductance in 150 mM-Na+, Cs+ or Rb+ external solution, though only at negative membrane potentials, which suggests amiloride has a voltage-dependent effect on the background current. 6. Removal of Cl- from the external solution or the addition of a Cl- channel blocker (4,4'-dinitrostilbene-2,2'-disulphonic acid disodium salt, DNDS) failed to affect the membrane conductance. 7. When the monovalent cation-dependent inward current was measured by subtracting the current in the Tris solution from those recorded in the various monovalent cation solutions, the current amplitude decreased in the order: Rb+ greater than K+ greater than Cs+ greater than Na+ greater than Li+, which suggests a poor cation selectivity of this current system.(ABSTRACT TRUNCATED AT 400 WORDS)

Automated microanalysis of creatinine by coupled enzyme reactions

Hydrogen peroxide was generated from creatinine by the sequential enzyme reactions of creatinine amidohydrolase, creatine amidinohydrolase and sarcosine oxidase. Hydrogen peroxide was, in turn, used stoichiometrically for the condensation of 4-aminoantipyrine and N-ethyl-N-(2-hydroxy-3-sulfopropyl)-m-toluidine catalyzed by horse-radish peroxidase, resulting in the formation of quinone dye with maximum absorbance at 546 nm. The optimized assay conditions for the enzymatic determination of creatinine in a HITACHI 7250 autoanalyzer was established. This system, which requires less than 5 microliters of sample, was found to be the most economical for laboratories equipped with autoanalyzers.

Regulation of spontaneous opening of muscarinic K+ channels in rabbit atrium

1. Intracellular mechanism(s) for controlling the opening of muscarinic K+ channels in the absence of an applied muscarinic agonist were studied in rabbit atrium by applying the patch clamp technique to isolated single myocytes. 2. In the cell-attached patch configuration, currents due to the activity of both the muscarinic K+ channel and the inward rectifying K+ channel were recorded. However, while the inward rectifying K+ channel currents were observed in only ten patches of 211 examined, spontaneous opening (i.e. in the absence of a muscarinic agonist) of the muscarinic K+ channel currents was observed in all patches examined in these atrial cells. 3. The single-channel currents due to spontaneous opening of muscarinic K+ channels were identified on the basis of their very similar conductance and gating properties to the unitary events which have been recorded when 0.5 microM-acetylcholine is included in the pipette and 10 microM-GTP is present in the internal side of the patch membrane. 4. Although the spontaneous opening of the muscarinic K+ channels disappeared soon after excision of the patch membrane, this type of channel activity reappeared following application of ATP and MgCl2 to the internal side of the torn-off patch, as expected from previous publications. 5. The K+ channel activity induced by the ATP and Mg2+ (measured as the product of the number of channels, N, times the probability of opening, Po) was strongly dependent upon concentration of free Mg2+; it was half-maximal at 2.2 x 10(-4) M [Mg2+]i. However, after the muscarinic K+ channels had been activated by 100 microM-guanosine 5'-O-3-thiotriphosphate (GTP gamma S) together with ATP and Mg2+, an increase in the Mg2+ concentration from 5.5 x 10(-5) to 2 x 10(-3) M failed to enhance this channel activity. 6. Pertussis toxin, which is known to uncouple muscarinic receptors from associated G-proteins (G(i) or G(o)), failed to inhibit the ATP- and Mg(2+)-induced activation of this K+ channel in the absence agonists. 7. In experiments made to test whether the Mg(2+)-ATP requirement results from an obligatory phosphorylation reaction, ATP was replaced with adenylyl-imidodiphosphate (AMP-PNP), an analogue of ATP which is resistant to hydrolysis. This K+ channel activity was not present when ATP was replaced with AMP-PNP.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of the muscarinic receptor-activated K+ current by N-ethylmaleimide in rabbit heart

The effects of N-ethylmaleimide (NEM), a sulfhydryl alkylating agent, on the ACh-activated K+ current were examined in single cells from rabbit hearts using whole-cell and single channel patch clamp techniques. Bath application of NEM (50 microM) or the muscarinic antagonist, atropine (1 microM) completely inhibited the ACh-activated K+ current in whole-cell recordings. In cell-attached patch conditions, the inhibitory effect of NEM was still observed; in contrast, atropine was ineffective when the agents were bath applied, indicating that the site of action of NEM is different from that of atropine. Inside-out patch recordings confirmed that GTP was required on the internal side of the membrane for activation of this K+ channel when ACh was present in the pipette. NEM abolished this GTP-activated K+ channel activity. GTP gamma S, a non-hydrolysable GTP analogue, was able to activate this K+ channel in the absence of a muscarinic agonist, an effect thought to be due to the direct activation of GTP-binding proteins. Pretreatment with NEM almost completely prevented this effect of GTP gamma S. In contrast, after the activation of the K+ channel by GTP gamma S had reached a steady-state, NEM failed to show a significant inhibitory effect. These results demonstrate that NEM prevents the activation of muscarinic receptor-regulated K+ channel and suggest an involvement of alkylation of the GTP-binding proteins which are coupled to this type of K+ channel.

Mechanism of acetylcholine-induced inhibition of Ca current in bullfrog atrial myocytes

The mechanism of the anti-beta-adrenergic action of acetylcholine (ACh) on Ca current, ICa, was examined using the tight-seal, whole-cell voltage clamp technique in single atrial myocytes from the bullfrog. Both isoproterenol (ISO) and forskolin increased ICa dose dependently. After ICa had been enhanced maximally by ISO (10(-6) M), subsequent application of forskolin (50 microM) did not further increase ICa, suggesting that ISO and forskolin increase ICa via a common biochemical pathway, possibly by stimulation of adenylate cyclase. ACh (10(-5) M) completely inhibited the effect of low doses of forskolin (2 x 10(-6) M), as well as ISO, but it failed to block the effects of high doses of forskolin (greater than 5 x 10(-5) M). Intracellular application of cyclic AMP (cAMP) also increased ICa. ACh (10(-5) M) failed to inhibit this cAMP effect, indicating that the inhibitory action of ACh occurs at a site proximal to the production of cAMP. ACh (10(-5) M) also activated an inwardly rectifying K+ current IK(ACh). Intracellular application of a nonhydrolyzable GTP analogue, GTP gamma S (5 X 10(-4) M), activated IK(ACh) within several minutes; subsequent application of ACh (10(-5) M) did not increase IK(ACh) further. These results demonstrate that a GTP-binding protein coupled to these K+ channels can be activated maximally by GTP gamma S even in the absence of ACh. Intracellular application of GTP gamma S also strongly inhibited the effect of ISO on ICa in the absence of ACh. Pertussis toxin (IAP) completely prevented both the inhibitory effect of ACh on ICa and the ACh-induced activation of IK(ACh). GTP gamma S (50 microM-1 mM) alone did not increase ICa significantly; however, when ISO was applied first, GTP gamma S (5 x 10(-4) M) gradually inhibited the ISO effect on ICa. These results indicate that ACh antagonizes the effect of ISO on ICa via a GTP-binding protein (Gi and/or Go). This effect may be mediated through a direct inhibition by the alpha-subunit of Gi which is coupled to the adenylate cyclase.

N-ethylmaleimide uncouples muscarinic receptors from acetylcholine-sensitive potassium channels in bullfrog atrium

The effect of N-ethylmaleimide (NEM), a sulphydryl alkylating agent, on the acetylcholine-activated K+ current, IK(ACh), has been studied in single cells from bullfrog atrium using a tight-seal, whole-cell voltage clamp technique. Addition of NEM (5 x 10(-5) M) produced a time-dependent complete block of IK(ACh). Dialysis of guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S, 5-10 x 10(-4) M), a nonhydrolyzable GTP analogue, into the myoplasm from the recording pipette gradually activated IK(ACh) even in the absence of acetylcholine. This effect is thought to be due to a GTP gamma S-induced dissociation of GTP-binding proteins (Gi and/or Go) into subunits that can directly activate these K+ channels. When NEM (5 x 10(-5) M) was applied after the GTP gamma S effect had fully developed, it failed to inhibit the GTP gamma S-induced K+ current, indicating that the NEM effect is unlikely to be on the dissociated subunits of the GTP-binding protein(s) or on the K+ channels. In contrast, pretreatment with NEM before GTP gamma S application markedly reduced the muscarinic K+ current, suggesting that NEM can block this K+ current by inhibition of the dissociation of the GTP-binding proteins into functional subunits. In NEM-treated cells the stimulatory effect of isoproterenol on ICa was present, but the inhibitory action of ACh on ICa was completely abolished. These results demonstrated that NEM can preferentially inhibit muscarinic receptor-effector interactions, probably by alkylating the GTP-binding proteins that are essential for these responses.

Calcitonin gene-related peptide regulates calcium current in heart muscle

The influx of Ca2+ due to the transmembrane calcium current, ICa, has a fundamental role in cardiac pacemaker activity, in the action potential plateau and in excitation-contraction coupling. Both sympathetic and parasympathetic neurotransmitters can modulate ICa. Recent studies indicate that in both the cardiovascular and the central nervous systems, nerve varicosities exist that contain a novel non-adrenergic, non-cholinergic peptide--calcitonin gene-related peptide (CGRP). Although CGRP is known to exert strong positive inotropic and chronotropic effects, as well as to cause vasodilation, very little is known about the ionic mechanisms of these effects. Here we report that CGRP dramatically increases ICa in single heart cells. Although this CGRP-induced increase in ICa resembles the effect of beta-adrenergic agonists, our results demonstrate some significant differences between the effects of CGRP and these agonists: (1) the increase due to CGRP cannot be blocked by beta-adrenergic antagonists; (2) the CGRP-induced effect is transient; and, (3) CGRP can inhibit isoproterenol-stimulated ICa. Our results provide the first electrophysiological evidence that CGRP can significantly modulate ICa in the heart, and suggest a new additional mechanism for the neurogenic control of cardiac function.

Modulation by intracellular Ca2+ of the hyperpolarization-activated inward current in rabbit single sino-atrial node cells

1. The sensitivity to internal Ca2+ of the hyperpolarization-activated inward current (Ih or If) in rabbit single sino-atrial node cells was investigated by the whole-cell voltage-clamp method. 2. When the patch pipette contained an internal solution of pCa 10, the amplitude of If decreased by 74.8 +/- 3.3% in 10 min (n = 7) after rupture of the patch membrane. When the pipette contained an internal solution of pCa 7, If increased by 43.7 +/- 8.7% within 10 min (n = 5). 3. Increase of If by the higher Ca2+ internal solution was confirmed in the same cell using the cell dialysis method. Both If and its tail current were increased at every membrane potential. The amplitude of If increased most markedly between pCa 8 and 7. 4. The reversal potential and kinetics of If were unaffected by the internal Ca2+ concentration. Increase of If by the high internal Ca2+ concentration was sensitively blocked by Cs+. These findings confirm that the increased current is indeed If and not a newly activated If-like current due to elevation of internal Ca2+. 5. The activation curve of If shifted approximately 13 mV in a positive direction by elevating Ca2+ from pCa 10 to 7 (n = 21), indicating that the voltage dependence of If was modulated by internal Ca2+. 6. beta-Agonists also modulated If, but the underlying mechanisms of their effects on If differed from those of the internal Ca2+. The former affected the If kinetics rather than its amplitude, whereas the latter acted on the If conductance rather than on its kinetics. 7. The increase in If by the internal Ca2+ was unaffected by protein kinase inhibitor or calmodulin inhibitor, suggesting that the internal Ca2+ directly modulates If. 8. When the patch pipette contained pCa 7 internal solution, the maximum diastolic potential shifted towards a positive potential but the heart rate remained almost constant.

Dual effects of intracellular magnesium on muscarinic potassium channel current in single guinea-pig atrial cells

1. The effects of internal Mg2+ ions on the muscarinic acetylcholine (ACh) receptor-mediated K+ currents were investigated in single atrial cells of guinea-pigs, using the whole-cell and inside-out modes of the patch-clamp technique. 2. During cell dialysis in the whole-cell-clamp condition, the depletion of internal Mg2+ increased outward muscarinic K+ currents but decreased inward currents, thereby reducing the inwardly rectifying property of the channels. 3. When inside-out patches were prepared, channel availability was abolished and was reactivated by internal application of guanosine 5'-triphosphate (GTP) or its non-hydrolysable analogue, 5'-guanylyl imidodiphosphate (GppNHp), in the presence of Mg2+. GppNHp led to a recovery of the channels also in the nominal absence of Mg2+ (0[Mg2+]i). 4. The activation of single-channel currents by intracellular GTP and Mg2+ was dose-dependent. Both concentration-response curves were fitted by saturation kinetics with Hill coefficients of 1, and the half-maximum doses were 24 +/- 8 microM for GTP and 67 +/- 14 microM for Mg2+. The effects of Mg2+ on activation of K+ currents were additive with those of GTP, suggesting the presence of two independent binding sites for GTP and Mg2+. 5. The single-channel conductance became virtually ohmic when measured at nominally zero [Mg2+]i while GppNHp was used to recover the channel activity. Micromolar [Mg2+]i reduced the unitary amplitude of single open-channel currents in a dose- and voltage-dependent manner, showing half-blocking doses of 293 microM at +40 mV and 115 microM at +60 mV. 6. Voltage-dependent kinetics of Mg2+ block were described using equations based on Eyring rate theory (Woodbury, 1971; Hille, 1984), where the coefficient for voltage dependence (delta) was 0.63. 7. Intracellular Mg2+, at a physiological concentration, has a dual action on the muscarinic K+ channel: first Mg2+ activates the channel in the presence of GTP through GTP-binding proteins (G proteins), and secondly it blocks outward currents through the channel, thereby causing the inwardly rectifying property.

Contribution of two types of calcium currents to the pacemaker potentials of rabbit sino-atrial node cells

1. Two types of calcium currents, the transient type and long-lasting type, were examined by both whole-cell and cell-attached patch-clamp modes in single isolated sino-atrial node cells of the rabbit. 2. In the whole-cell clamp mode, in response to a depolarizing pulse to -40 mV from a holding potential of -80 mV, a transient type calcium current with an amplitude of 2.1 +/- 0.7 pA/pF (mean +/- S.D.; n = 15) was recorded. The threshold potential was approximately -50 mV. 3. Nickel (40 microM) and tetramethrin (0.1 microM) blocked the transient type calcium current without appreciable effects on the long-lasting type. Nifedipine and D600 blocked the long-lasting type, but did not affect the transient type. Cadmium (20 microM) and cobalt (2 mM) inhibited both types of calcium currents equally. 4. Both types of calcium currents showed an increased amplitude with increasing extracellular calcium concentration. The values of the Michaelis constant, Km, were 0.95 mM for the transient type and 3.92 mM for the long-lasting type, indicating that these types represent two different classes of channels. 5. In the cell-attached patch-clamp mode, the single-channel conductance of the transient type calcium current was 8.5 pS, by using 100 mM-BaCl2 in the pipette, whereas that of the long-lasting type was 16.0 pS, under the same conditions. Each of these values was similar to those found in other cells, respectively. 6. In the whole-cell clamp mode, the transient type current began to inactivate at -70 mV and was fully inactivated at -40 mV. The steady-state inactivation curve of the transient type current was approximately 50 mV negative to that of the long-lasting type. The overlap of the membrane potential between the activation and inactivation curves was small. The time constant of the inactivation shortened from 20 to 5 ms as the potential became progressively positive over the range from -80 to +30 mV. 7. Isoprenaline (1 microM) increased the amplitude of the long-lasting type Ca2+ current, but was not effective on the transient type, suggesting that the long-lasting type calcium current may be responsible for the positive chronotropic effect of isoprenaline. 8. While recording spontaneous electrical activity of the cell, application of 40 microM-nickel induced bradycardia and this effect was enhanced when the membrane was constantly hyperpolarized.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane currents in cardiac pacemaker tissue

The present work is a brief survey of the mechanism of the cardiac pacemaker in sinoatrial node cells. Information on the pacemaker mechanism in cardiac tissue has been greatly enhanced by the development of the single cell isolation technique and the patch clamp technique. These methods circumvent to a large extent the difficulties involved in voltage clamping multicellular preparations. The calcium current (ICa), delayed rectifier potassium current (IK), transient outward current (Ito;IA), and the hyperpolarization activated inward current (Ih or If) were found both in whole cell preparations and in single channel analysis. The physiological significance of these currents, together with the exchange current systems for the pacemaker depolarization are discussed.

Intracellular Ca2+ and protein kinase C modulate K+ current in guinea pig heart cells

Effects of protein kinase C (PKC) and intracellular calcium ion (Cai2+) on the delayed rectifier K+ current (IK) were investigated in the single ventricular cells of guinea pig by use of an internal-dialysis method and a whole cell voltage-clamp technique. 12-O-tetradecanoylphorbol-13-acetate (TPA, 10(-9) M), an activator of PKC, increased the amplitude of IK in the presence of Cai2+ higher than 10(-10) M. This effect of TPA was mimicked by a synthetic diacylglycerol, 1-oleoyl-2-acetylglycerol (OAG), 50 micrograms/ml, 125 microM, and was blocked by 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (10 microM). The above findings suggest that IK channels were phosphorylated by PKC and thereby the amplitude of IK was increased. IK was also increased by elevating the concentration of Cai2+ in the absence of TPA. It is thus indicated that IK channels are modulated by Cai2+ not only through activation of PKC but also directly. Our observation may provide a possible mechanism by which Cai2+ mediates the link between the Ca2+ transients during contraction and the action potential duration.

Rectification of muscarinic K+ current by magnesium ion in guinea pig atrial cells

Rectifying properties of the acetylcholine (ACh)-sensitive K+ channels were studied using a patch-clamp method in single atrial cells prepared enzymatically from adult guinea pig hearts. In the presence of micromolar concentration of nonhydrolyzable guanosine 5'-triphosphate (GTP) analogue 5'-guanylylimidodiphosphate (GppNHp) and the absence of Mg2+ at the inner surface of patch membrane [( Mg2+]i), the channel activity recovered in inside-out patch condition. The single channel conductance became ohmic between -80 and +80 mV (symmetrical 150 mM K+ solutions). The rapid relaxation of outward single channel currents was disclosed on a depolarization. [Mg2+]i blocked the outward current through the channel dose- and voltage-dependently and also induced a dose-dependent increase in the channel activation. The apparent paradoxical role of [Mg2+]i is important for the cholinergic control in the heart; voltage-dependent Mg block ensures a low K+ conductance of cell membrane at the plateau of action potentials during the exposure to ACh, thereby slowing the heart rate without unfavorable shortening of the action potentials.

Voltage-dependent magnesium block of adenosine-triphosphate-sensitive potassium channel in guinea-pig ventricular cells

1. The adenosine-5'-triphosphate (ATP)-sensitive K+ channel of guinea-pig ventricular cells was examined in the presence and absence of internal Mg2+ or Na+ using an open cell-attached configuration of the patch-clamp technique. 2. Millimolar concentrations of internal Mg2+ ([Mg2+]i) produced marked fluctuations in the outward current, and the amplitude of the open-channel current was reduced with increasing [Mg2+]i. Millimolar Na+ applied internally also decreased the mean amplitude of the outward current, but the increase in current noise was not obvious. These effects became larger when the membrane potential was shifted to be more positive from the K+ equilibrium potential (EK). At potentials negative to EK the inward current was affected by neither internal Mg2+ nor Na+. 3. The external application of Na+, Mg2+ or Ca2+, however, failed to affect the single-channel current. 4. After removal of both internal Mg2+ and Na+, the mean open-channel current-voltage relationship became virtually linear. Referring to these unblocked values, relative amplitudes were determined at different levels of [Mg2+]i or [Na+]i. The dose-response relations gave a Hill coefficient of approximately 1 for Mg2+ block and approximately 2 for Na+ block. The half-maximum concentrations (Kh) for both Mg2+ and Na+ block were shifted to lower values with increasing positive potentials. 5. The power-density spectrum of the open-channel current noise induced by internal Mg2+ showed a Lorentzian function with a corner frequency above 1 kHz, suggesting that the current noise is due to rapid fluctuations of open-channel current between blocked and unblocked states. The corner frequencies gave Mg2+ block and unblock rate constants which were of the order of 10(7) M-1 s-1 and 10(4) s-1, respectively. 6. With increasing external K+ concentration ([K+]o) from 0 to 140 mM the current fluctuations became less prominent, and Kh for Mg2+ block was shifted to higher values. Raising [K+]o enhanced the unblock rate derived from the noise analysis while the block rate was not significantly altered. 7. The above findings could be explained by assuming a binding site for one Mg2+ or two Na+ located 30-35% of the electrical drop across the membrane from the inner mouth of the channel, thereby resulting in the ionic block of K+ passage. An apparent inward rectification observed in the single-channel current-voltage relation is attributable to the blockade of the channel by intracellular Mg2+ and/or Na+.

Ohmic conductance through the inwardly rectifying K channel and blocking by internal Mg2+

The inwardly rectifying K channel provides the resting K conductance in a variety of cells. This channel acts as a valve or diode, permitting entry of K+ under hyperpolarization, but not its exit under depolarization. This behaviour, termed inward rectification, permits long depolarizing responses which are of physiological significance for the pumping function of the heart and for fertilization of egg cells. Little is known about the outward currents through the inwardly rectifying K channel, despite their great physiological importance, and the mechanism of inward rectification itself is unknown. We have used improved patch clamp techniques to control the intracellular media, and have recorded the outward whole-cell and single-channel currents. We report here that the channel conductance is ohmic and that the well-known inward rectification of the resting K conductance is caused by rapid closure of the channel accompanied by a voltage-dependent block by intracellular Mg2+ ions at physiological concentrations.

Cell-to-cell diffusion of fluorescent dyes in paired ventricular cells

The intracellular and cell-to-cell diffusion of fluorescent dyes of various molecular sizes were studied in enzymatically isolated paired ventricular cells of the guinea pig heart. Fluorescein sodium (mol wt 332), 6-carboxyfluorescein (mol wt 376), Lucifer yellow CH (mol wt 457), lissamine rhodamine B-200 (mol wt 559), and tetraglycine-conjugated lissamine rhodamine B-200 (mol wt 859) were all diffused into the single ventricular cell through the patch-clamp pipette. All these dyes were able to diffuse through the gap junction of the paired cells. The diffusion coefficient of 6-carboxyfluorescein in the cytoplasm was 5.8 X 10(-6) cm2/s, Lucifer yellow CH was 3.0 X 10(-6) cm2/s, and lissamine rhodamine B-200 was 8.6 X 10(-7) cm2/s. Permeability of the gap-junctional membrane was 6.8 X 10(-4) cm/s for 6-carboxyfluorescein, 2.8 X 10(-4) cm/s for Lucifer yellow CH and 7.4 X 10(-5) cm/s for lissamine rhodamine B-200. Thus lissamine rhodamine B-200 diffused in the cytoplasm and through the gap junction approximately 10 times slower than 6-carboxyfluorescein. Tetraglycine-conjugated lissamine rhodamine B-200 (mol wt 859) passed through the gap junction, suggesting that the critical pore size of the ventricular gap junction may be somewhat larger than that of the conjugated tetraglycine rhodamine.

Effects of pH on the Na-Ca exchange current in single ventricular cells of the guinea pig

Using isolated guinea pig single ventricular cell, the sodium-calcium exchange current was recorded. Both internal and external acidification to pH 6 abolished most of the sodium-calcium exchange current and alkalinization to pH 8 decreased the exchange current. Thus, pH change modulates Na-Ca exchange current both from inside and outside of the cell.

Intra- and extracellular actions of proton on the calcium current of isolated guinea pig ventricular cells

External or internal application of proton on the calcium current was investigated using the whole-cell patch clamp technique in single ventricular cells of the guinea pig. The effect of acid pHo on the calcium current depended on the strength of the buffer in the internal solution. In the presence of 5 mM HEPES in the pipette, external acidification depressed calcium current with the half maximum inhibition of pHo 5.5. When the intracellular pH was buffered strongly to pH 7.2 by 50 mM HEPES within the pipette solution, half maximum inhibition of the calcium current was shifted to the acidic side markedly. When acid was applied in the patch electrode, calcium current was hardly affected by the pH values down to 6.0 in the pipette solution but was depressed by approximately half on further acidification to pH 4.5. When the Na+-H+ exchange system was blocked by superfusing the cell with either amiloride or Na+-deficient solutions (Tris substituted), calcium current was decreased by half at a pipette pH of around 6.5 and was completely and irreversibly blocked at pHpip 6.0. From the above results, we concluded that the calcium current of the ventricular cell is much more sensitive to intracellular protons than extracellular ones.

Simulation analysis of excitation conduction in the heart: propagation of excitation in different tissues

The normal excitation and conduction in the heart are maintained by the coordination between the dynamics of ionic conductance of each cell and the electrical coupling between cells. To examine functional roles of these two factors, we proposed a spatially-discrete model of conduction of excitation in which the individual cells were assumed isopotential. This approximation was reasoned by comparing the apparent space constant with the measured junctional resistance between myocardial cells. We used the four reconstruction models previously reported for five kinds of myocardial cells. Coupling coefficients between adjacent cells were determined quantitatively from the apparent space constants. We first investigated to what extent the pacemaker activity of the sinoatrial node depends on the number and the coupling coefficient of its cells, by using a one-dimensional model system composed of the sinoatrial node cells and the atrial cells. Extensive computer simulation revealed the following two conditions for the pacemaker activity of the sinoatrial node. The number of the sinoatrial node cells and their coupling coefficients must be large enough to provide the atrium with the sufficient electric current flow. The number of the sinoatrial node cells must be large so that the period of the compound system is close to the intrinsic period of the sinoatrial node cell. In this simulation the same sinoatrial node cells produced action potentials of different shapes depending on where they were located in the sinoatrial node. Therefore it seems premature to classify the myocardial cells only from their waveforms obtained by electrical recordings in the compound tissue. Second, we investigated the very slow conduction in the atrioventricular node compared to, for example, the ventricle. This was assumed to be due to the inherent property of the membrane dynamics of the atrioventricular node cell, or to the small value of the coupling coefficient (weak intercellular coupling), or to the electrical load imposed on the atrioventricular node by the Purkinje fibers, because the relatively small atrioventricular node must provide the Purkinje fibers with sufficient electric current flow. Relative contributions of these three factors to the slow conduction were evaluated using the model system composed of only the atrioventricular cells or that composed of the atrioventricular and Purkinje cells. We found that the weak coupling has the strongest effect. In the model system composed of the atrioventricular cells, the propagation failure was not observed even for very small values of the coupling coefficient.(ABSTRACT TRUNCATED AT 400 WORDS)

Observations on the fine structure of nodal, Purkinje and working myocardial cells isolated from rabbit hearts

Sinoatrial and atrioventricular nodal cells as well as Purkinje and working myocardial cells isolated enzymatically from rabbit hearts were examined by transmission electron microscopy. The cardiac cell fine structure remained well organized in the storage solution after isolation. In normal Tyrode solution, the nodal cells changed from spindle shaped to spherical, whereas the shapes of Purkinje and working myocardial cells remained unchanged. The nodal cell fine structure became disorganized with respect to its sarcomeric arrangement in normal Tyrode solution. This was assumed to result from a combination of several factors seen in the nodal cells: the loss of the normal anchoring of the myofibrils at the previous intercalated discs, breakdown of the Z bands and the alteration of the integrity of the intermediate filaments. Also, extensive restoration of plasma membrane damaged by of the intermediate filaments. Also, extensive restoration of plasma membrane damaged by the isolation procedure was observed in this solution. This might correspond to the retention of the normal physiological properties of the plasma membrane of the nodal cells despite gross morphological changes undergone in normal Tyrode solution.

Na-Ca exchange current in mammalian heart cells

Electrogenic Na-Ca exchange has been known to act in the cardiac sarcolemma as a major mechanism for extruding Ca ions. Ionic flux measurements in cardiac vesicles have recently suggested that the exchange ratio is probably 3 Na:1 Ca, although a membrane current generated by such a process has not been isolated. Using the intracellular perfusion technique combined with the whole-cell voltage clamp, we were able to load Na+ inside and Ca2+ outside the single ventricular cells of the guinea pig and have succeeded in recording an outward Na-Ca exchange current while blocking most other membrane currents. The current is voltage-dependent, blocked by La3+ and does not develop in the absence of intracellular free Ca2+. This report presents the first direct measurement of the cardiac Na-Ca exchange current, and should facilitate the study of Ca2+ fluxes during cardiac activity, together with various electrical changes attributable to the Na-Ca exchange and the testing of proposed models.

Effects of intracellular acidification on membrane currents in ventricular cells of the guinea pig

The membrane currents of single ventricular cells were measured under whole cell voltage clamp using a giga-sealed patch electrode, and the effects of intracellular acidification were examined by perfusing the electrode pipette with different pH solutions. The plateau of the action potential was shortened when the pH of the pipette solution was lowered from the control of 7.2 to 6, and finally to 5. The pH 6 pipette solution evoked a time-independent outward current at positive potentials and increased the slope conductance near the resting potential. These changes were suppressed by removal of both intra- and extracellular potassium ion, indicating that these currents were carried by potassium ions, but not by protons. Increasing the calcium concentration in the pipette from pCa 8 to pCa 6 induced a time-dependent outward current which had a reversal potential of about -13 mV. This result clearly differed from the changes induced by the acidic pipette solution, suggesting that the calcium-mediated conductance was not involved in the genesis of the acidic effects. The calcium current was not significantly affected by perfusion at pH 6, but was decreased by the more acidic (pH 5) solution. When the calcium current was recorded in sodium- and potassium-free external solution but with a cesium-rich internal solution, however, the calcium current was suppressed even with a weak acidic (pH 6.8) pipette solution. This effect was attributed not to an increased sensitivity of the calcium channel to protons, but to a more extensive intracellular acidification, which might have been caused by a depressed extrusion of proton via a sodium-hydrogen exchange mechanism on the surface membrane.

Transient outward current carried by potassium and sodium in quiescent atrioventricular node cells of rabbits

Single atrioventricular node cells were dispersed by treating the rabbit heart with collagenase. In Tyrode's solution, the cells became rounded, and about 20% of them showed spontaneous activity, whereas the rest remained quiescent. When those quiescent cells were whole-cell clamped, depolarizing clamp pulses from the holding potential of -83 mV induced an outward current which decayed quickly, with a time course similar to that of the transient outward current in the Purkinje fiber. The amplitude of the current became larger when progressively more positive clamp pulses were given from a very negative holding potential. The inactivation time course of this current consisted of two exponential components. Single-channel current recordings from those cells revealed a class of channels that activated more frequently during the initial part of depolarizing pulses. Summation of those unitary currents reproduced activation and inactivation time courses of the macroscopic current well, suggesting that this channel corresponds to the transient outward current. The current-voltage relationship of the channel was linear with the slope conductance of 19.9 +/- 1.8 pS (n = 7), and the reversal potential was near the resting potential of the atrioventricular node cell with 5.4 mM potassium chloride and 134.6 mM sodium chloride in the pipette. The channel was passing mainly potassium ions, but sodium ions also seemed to carry a fraction of the current. The possible role of the transient outward current in the quiescent node cell is discussed.

Membrane currents and their modification by acetylcholine in isolated single atrial cells of the guinea-pig

The ionic currents in isolated single atrial cells of the guinea-pig heart were analysed by the patch-clamp technique applied to whole-cell recordings and the effects of acetylcholine (ACh) on the membrane potential and currents were studied. Resting and action potentials of single isolated cells were normal. Upstroke velocity was sensitive to tetrodotoxin. Action potential duration was slightly shorter than in multicellular preparations. Voltage-clamp experiments demonstrated the presence of a Ca2+ current (iCa), and inward rectifying and outward rectifying K+ currents. The Ca2+ current was abolished by 2 mM-Co2+ or 10(-6) M-D600, and the K+ currents were greatly reduced by intracellular application of Cs+ using the patch electrode and simultaneously superfusing the cell with 5 mM-Cs+ Tyrode solution. Acetylcholine shortened the action potential duration in a dose-dependent manner. The threshold dose of ACh was about 10(-9) M and the maximal effect was obtained with 10(-6) M. The resting membrane potential was hyperpolarized by 1-3 mV. ACh (10(-8)-10(-6) M) increased the K+ currents both on depolarization and on hyperpolarization in a dose-dependent manner in the presence of 10(-6) M-D600. The ACh-induced K+ outward current revealed a progressive deactivation ('relaxation'), with a time constant of 111 +/- 16 ms at around 0 mV. When the K+ outward currents were minimized with Cs+, the reduction of iCa by 10(-8) M-ACh was insignificant, and became 17 +/- 2.5, 26 +/- 3.5 and 26 +/- 5% of the control value in 10(-7), 10(-6) and 10(-5) M-ACh, respectively. The inactivation time course of iCa recorded from the Cs+-loaded cells was not affected by 10(-7) M-ACh. These results suggest that ACh activates predominantly a K+ conductance in the guinea-pig atrium. The superimposition of a relaxing K+ outward current on iCa may lead to over-estimation of the decrease in iCa.

Action potential and membrane currents of single pacemaker cells of the rabbit heart

Single, viable pacemaker cells were isolated from sinoatrial (S-A) and atrioventricular (A-V) nodes by treating with collagenase. In normal Tyrode solution containing 1.8 mM Ca2+, these pacemaker cells had a round configuration and contracted rhythmically at a frequency of about 150-260/min. The amplitude, duration, and maximum rate of rise of the spontaneous action potentials recorded using patch clamp electrodes were similar to those obtained from multicellular preparations. Amplitudes of the recorded membrane current were normalized with reference to the surface area of the cell by assuming the cell shape as a plane oblate spheroid. The membrane resistance of the isolated nodal cells was 14.9 +/- 4.0 k omega . cm2 (n = 12) at about -35 mV and the membrane capacitance was 1.30 +/- 0.24 microF/cm2 (n = 18). The inactivation time course of the slow inward current, isi, was fitted with a sum of two exponentials with time constants of 6.7 +/- 0.6 ms and 46.6 +/- 15.3 ms (n = 4) at +10 mV. The amplitude of isi peaked at 0 approximately +10 mV in the current-voltage relationship and was 18.2 +/- 8.4 microA/cm2. The potassium current, iK, was activated in the voltage range positive to -50 mV and was saturated at about +20 mV. The amplitude of the fully-activated iK at -40 mV was 3.3 +/- 1.4 microA/cm2 (n = 10) and showed an inward-going rectification. The activation of the hyperpolarization-activated current was observed at potentials negative to -70 mV in seven of 14 experiments. The current density and membrane capacitance calculated could be overestimated and the membrane resistance underestimated, because of the presence of caveolae on the cell surface. However, these data give the nearest possible estimates of the electrical constants in the nodal cells, which cannot be measured accurately in the conventional multicellular preparations.

Intracellular Na+ activates a K+ channel in mammalian cardiac cells

In a wide variety of cells, various intracellular agents, such as Ca2+, ATP and cyclic nucleotides, regulate ionic conductances of the membrane. In cardiac cells, the intracellular Na+ concentration [( Na+]i) frequently increases when a disturbance occurs in the electrogenic Na-K pump activity or the Na-Ca exchange mechanism. We have investigated a possible role of [Na+]i in controlling ion channels by using a patch-clamp method, and have found a K+ channel that is gated by [Na+]i greater than 20 mM, but not by the intracellular Ca2+ concentration (approximately 10(-4) M). We report here that the channel has a unitary conductance of 207 +/- 19 pS (n = 16) with K+ concentrations of 150 mM outside and 49 mM inside, and shows no detectable voltage-dependent kinetics. The Na+-activated K+ channel represents a novel class of ionic channel.

Electrophysiology of single cardiac cells

Whatever techniques of isolation one may use, it is certainly true that the isolated single heart cells are very useful in various physiological experiments. For electrophysiology, the application of the single cell serves to test findings previously obtained in the multicellular preparation. Therefore, summary and comparison of the data will become necessary in the near future. Furthermore, the application of the single cell opens new areas for electrophysiology of the heart, because when using only the single cell preparation, one can dialyse intracellular millieu and can also measure single channel analyses.

Effects of various intracellular Ca ion concentrations on the calcium current of guinea-pig single ventricular cells

The effects of changing the intracellular Ca concentration ([Ca]i) on the calcium current (iCa) were studied in isolated single ventricular cells of the guinea-pig. [Ca]i was varied by an intracellular perfusion technique using a suction pipette. iCa measured from internally perfused cells at a pCa lower than 9.0 was dependent on the extracellular Ca concentration ([Ca]o). Increasing [Ca]o from 1.8 to 5.4 mM increased the amplitude of iCa, and reduction of [Ca]o from 1.8 to 0.01 mM decreased the amplitude. The inactivation time course of iCa became faster as [Ca]o was increased and slower as [Ca]o was reduced. By decreasing the pCa of the internal perfusate from 9.0 to 6.8, the amplitude of iCa was decreased markedly, but no significant change was observed in its time course. Analysis of the I-V curve led to the conclusion that a change in the driving force was not a major factor in the reduction of iCa. The "steady state inactivation" of iCa was measured by a double-pulse method. The amplitude of iCa elicited by the test pulse was smaller at pCa 7.4 than at pCa 9.0 at potentials of between -27 and +33 mV. By normalizing the iCa amplitude, however, the "steady state inactivation" curve in the control and at high [Ca]i overlapped. Similar results were obtained in Sr-Tyrode solution. The degree of "steady state inactivation" of iCa at the potentials positive to +10 mV was larger in Ca-Tyrode than in Sr-Tyrode solution. It is proposed that the reduction in amplitude of iCa at higher [Ca]i is caused by a reduction of the maximum conductance of iCa (gCa) and that Ca ions passing through iCa channels have a remarkable effect on its inactivation.

Resting K conductances in pacemaker and non-pacemaker heart cells of the rabbit

Currents through the inward rectifier K channel (iK X rec) and the ACh-operated K channel (iK X ACh) were recorded in isolated heart cells of rabbit using the patch clamp technique with electrodes having 0.5-1 micron tip inner diameter. The maximum number of overlaps of open iK X rec channels per patch was measured over 347 experiments. An average of 2.3 was found in ventricular cells and 0.03-0.06 in sinoatrial (S-A) and atrioventricular (A-V) node cells. The estimated total number of the iK X rec channels for each ventricular cell was great enough to supply the resting K conductance of the cell. The iK X ACh channel was present in S-A and A-V node cells, but was never observed in the ventricular cells. The resting conductance of the nodal cells measured with whole cell clamp recordings was about 16 times smaller than that of the ventricular cells, and was hardly decreased at all by the removal of K+ from the bath solution. Thus, the lower membrane potential of the nodal cells compared with that of the ventricular cells was attributed to the smaller K conductance of the resting membrane, which is due to the very low density of the iK X rec channel. On the other hand, the iK X ACh channel, when activated by neural regulation, may play a major role in generating the resting K conductance of the nodal cells.

Modification of the cardiac action potential by intracellular injection of adenosine triphosphate and related substances in guinea pig single ventricular cells

Effects of varying the intracellular adenosine triphosphate level on both the action potential and the membrane current were studied in single ventricular cells isolated from the guinea pig heart, using collagenase. Intracellular injection of adenosine triphosphate elevated the plateau potential level and prolonged the action potential duration. Similar results were obtained by injecting adenosine diphosphate, adenosine monophosphate, or creatine phosphate, i.e., substances considered to increase the intracellular concentration of adenosine triphosphate. In contrast, the action potential was depressed by procedures which could reduce the intracellular adenosine triphosphate level, such as an injection of creatine, superfusion of glucose-free Tyrode's solution containing 5.4 mM cyanide ion, or an injection of adenosine monophosphate into the cyanide-superfused cell. When the membrane current was recorded under the voltage clamp, it was found that the injection of adenosine triphosphate increased the amplitude of the slow inward current, whereas the superfusion of cyanide ion did not significantly decrease the slow inward current, although the action potential became considerably shorter. It was also found that the adenosine monophosphate injection decreased the amplitude of the net outward membrane current at the plateau level and increased it at around -40 mV, and thus intensified the N-shape of the isochronal 0.3-second current-voltage curve. The cyanide ion superfusion produced the opposite effect; in response to depolarizing clamp pulses more positive to the plateau level, the membrane current increased significantly with cyanide ion, but increased only slightly with adenosine triphosphate. These results suggest that intracellular adenosine triphosphate modifies the membrane currents at the plateau potential range, thus altering the action potential duration.

Does the "pacemaker current" generate the diastolic depolarization in the rabbit SA node cells?

Small preparations of spontaneously beating rabbit sino-atrial node (SA node) were voltage clamped with the two-microelectrode technique. The effects of 0.25-5 mM Cs+ on the spontaneous pacing rate and the time-dependent inward "pacemaker" current, ih, were studied. In the presence of 2 mM Cs+, the spontaneous pacing rate decreased only slightly even though ih was strongly depressed at potentials negative to -60 mV Cs+ had little or no effect on other time-dependent currents observed with clamp pulses less negative than -50 mV. Since no voltage-dependence to the Cs+ effect on ih could be measured (between -90 mV and -20 mV), it was considered unlikely that the lack of Cs+ effect on the rate of diastolic depolarization results from a voltage-dependent effect of Cs+ on the ih channel. Adrenaline produced a marked positive chronotropic effect in Cs+-treated SA node cells. This effect was accompanied by marked enhancement of the slow inward current (isi) with no change in the Cs+-blocked ih current. These results are consistent with the idea that ih plays a minor role in generation of pacemaker depolarization, and suggest a more prominent role of isi in the generation of diastolic depolarization in SA nodal cells.

Modulation by intracellular ATP and cyclic AMP of the slow inward current in isolated single ventricular cells of the guinea-pig

Effects of ATP and of cyclic AMP on membrane current systems were investigated in isolated single ventricular cells from guinea-pig hearts by applying the suction electrode method. The intracellular milieu was dialysed with various solutions which were perfused continuously through the suction pipette. The presence of ATP, cyclic AMP and EGTA in the perfusion solution kept the plateau phase of the action potential almost intact for as long as 30 min. With depolarizing voltage-clamp pulses from holding potentials between -30 and -40 mV, the slow inward current (isi) was activated at potentials positive to -20 mV. The inactivation time course of isi was fitted by two exponential components in the potential range between -10 mV and +30 mV. By increasing ATP from 2 to 9.5 mM in the solution, the amplitude of isi was increased and the slow component of inactivation was accelerated. The steady-state current-voltage relationship (I-V curve), exhibited a negative slope that became steeper after increasing the ATP concentration. The current was shifted towards the outward direction between -40 mV and -10 mV and became more inward between -10 mV and +40 mV. Increase of the cyclic AMP concentration from 30 to 60 microM also enhanced the amplitude of isi, but the negative slope in the steady-state I-V curve was unaffected. Assuming that the concentration of free Ca2+ in the cell was well buffered at a low level by the EGTA-Ca buffer solution in the pipette, it was concluded that [ATP]i and [cyclic AMP]i exert a direct influence on membrane current systems of the ventricular cell.

Transient depolarization and spontaneous voltage fluctuations in isolated single cells from guinea pig ventricles. Calcium-mediated membrane potential fluctuations

Under the influence of cardiotonic steroids, single ventricular cells exhibit transient depolarization after a train of driven action potentials or, in voltage clamp experiments, transient inward current after a depolarizing clamp pulse. Transient depolarization or transient inward current was abolished by an intracellular injection of ethyleneglycol-bis (beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) or by superfusion of 5 mM caffeine. Transient depolarization was elicited even in the control Tyrode's solution by an intracellular injection of CaCl2 or augmented by an injection of adenosine 3',5'-cyclic monophosphoric acid (cAMP). Along with transient depolarization or transient inward current, digitalis intoxication promoted spontaneous oscillatory fluctuations in membrane potential or in membrane current. Their power spectra showed peaks at frequencies ranging from 2 to 7 Hz, which coincided well with the frequency of repetitive transient depolarization or transient inward current. The fluctuations were eliminated by intracellular injections of EGTA and decreased in amplitude by 5 mM caffeine with a shift toward higher frequencies. Depolarization of the membrane caused a shift of the spectrum peak toward higher frequencies. These results suggest that an oscillatory release of Ca from intracellular storage sites is the common basis underlying both the transient events (depolarization or inward current) and the spontaneous miniature fluctuations in membrane potential or current.

Membrane currents in the rabbit atrioventricular node cell

The rabbit A-V node was dissected into pieces (0.2 x 0.2 x 0.2 mm) smaller than its space constant of 692 +/- 96 micrometers (n = 5). These small specimens showed spontaneous action potentials whose configurations were similar to those of large specimens before dissection. The membrane time constant was 21.5 +/- 1.5 ms (n = 5). Voltage clamp experiments were performed on the above specimens using the two-microelectrode technique. On depolarization from the holding potential of -40 mV to various potential levels a transient inward current and delayed outward current were recorded. On repolarization an outward current tail was observed. The transient inward current was blocked by application of D 600 (2 x 10(-7) g/ml) but was insensitive to TTX (1 x 10(-7) g/ml). The inward current was decreased by superfusion with Na- or Ca-free Tyrode solution. Thus, this current was classified as the slow inward current (is). When the K concentration in the Tyrode solution was varied, the reversal potential of the outward current tail changed as expected for a K electrode, indicating that the outward current was carried by K ions. On hyperpolarization slow activation of inward current was recorded. The reversal potential of this current was between -20 and -30 mV, which was analogous to hyperpolarization activated current, ih, in the S-A node. A contribution of sodium current (iNa) to the action potential was obviously demonstrated from an inhibitory effect of TTX on the upstroke of the anodal break excitation. The ionic selectivity of each current system is compared with analogous current systems in other cardiac tissues and a possible mechanism for the slow conduction in the A-V node is discussed.

Electrogenic Na pump evidenced by injecting various Na salts into the isolated A-V node cells of rabbit heart

Electrogenicity of the Na pump was confirmed by injecting Na salts into a small cluster of A-V node cells. Injection of Na glutamate or Na acetate induced marked hyperpolarization, accompanying with cessation of spontaneous activity. The hyperpolarization exceeded EK in 27 mM K Tyrode solution and was inhibited by 10(-5) M strophanthidin. Injection of NaCl or NaI depolarized the membrane. These data showed that inward-going current carried by the injected anion antagonized the outward-going pump current and thus determined the net effect of the injection.

Electrogenic sodium pump in rabbit atrio-ventricular node cell

Electrogenicity of the Na pump was demonstrated in rabbit A-V node cells by analyzing the K-induced hyperpolarization occurring after a short period of K-free perfusion. The transient hyperpolarization was abolished completely by strophanthidin (10(-5)M). The membrane slope conductance remained unchanged during the transient hyperpolarization. On perfusion of 50 mM K and K-free incubation the transient hyperpolarization reached --69 mV which was more negative than the expected EK (about --28 mV). The order of potencies of monovalent cations to activate the K site of the Na pump was Tl greater than Rb equal to K greater than NH4 equal to Cs greater than Li, which was similar to the sequence reported in the literature. Michaelis-Menten type activation of the K site of the Na pump was suggested from the relationship between the decay rate constant of the K-induced outward current transient and [K]o. These findings obviously indicate that the Na pump in the A-V node cells shares common characteristics with those of other excitable tissues. Direct contribution of the pump activity to the membrane potential under physiological conditions was suggested by a significant depolarization occurring immediately after application of strophanthidin.

Kinetics and rectification of the slow inward current in the rabbit sinoatrial node cell

The slow inward current (is) in the rabbit sinoatrial node cell was studied by the conventional two-microelectrode voltage clamp technique. When is was measured as the difference between two records obtained before and after blocking is with D 600, the fully activated current (is)-voltage relation was non-linear; the conductance decreased in the negative potential range resulting in an almost constant amplitude of is negative to -10 mV. The degree of steady-state activation was about 1 at -5mV and 0 at -65mV. The recovery time course of is during repolarization was measured by varying the interval between two sequential depolorizing pulses with various holding potentials. The time constant of the exponential recovery time course was about 120 msec at -40 mV and decreased to about 40 msec at -70 mV. The steady-state conductance of is, calculated from the activation and inactivation curves, produced a large hump in the steady-state current voltage relation between -60 and -20 mV, which was not observed in the experiment. When the above kinetics were incorporated, the S-A node model failed to discharge the spontaneous activity. The activation and inactivation curves which can simulate the experimental I-V curve and the action potential were proposed.

Spontaneously active cells isolated from the sino-atrial and atrio-ventricular nodes of the rabbit heart

Single cells or cell clusters composed of 3-10 cells were isolated from the S-A and A-V nodes of the rabbit heart by coronary perfusion of collagenase dissolved in Ca-free Tyrode solution (0.04%, for 1 hr). For comparison, atrial and ventricular cells were also isolated from the same heart. Shapes of the isolated nodal cell were either rod or round and nodal cells were slightly smaller than ventricular cells. Spontaneous activity was observed in both rod and round nodal cells. The action potentials had the configurations similar to those recorded from larger conventional preparations. The membrane current recorded from the small nodal cell clusters (5-10 cells) by the two-microelectrode voltage clamp technique showed a time course similar to that of previous recordings from conventional preparations, but the amplitude of the currents was 5-10 times smaller. The isolated cells showed normal sensitivities to both acetylcholine and epinephrine. Findings given in this study indicate that the isolated cells maintain the typical membrane characteristics of the nodal cells and that they are suitable for electrophysiological studies of the cardiac pacemaker cell.

Effect of procaine amide on the membrane currents of the sino-atrial node cells of rabbits

Using small rabbit sino-atrial node preparations, the effects of procaine amide in concentrations from 0.01 to 2 mg/ml on the membrane potentials currents were studied by both current-clamp and voltage-clamp experiments. Procaine amide in concentrations over 0.1 mg/ml reduced the peak of the action potential, maximum diastolic potential and the maximum rate of depolarization. The action potential duration was prolonged, the resting potential was decreased and the heart rate was reduced. In the voltage-clamp experiments, procaine amide (0.1 mg/ml) reduced the slow inward current (is), the outward current (iK) and the inward current activated by hyperpolarization (ih). The major effect, however, was the reduction of the outward current. Sine the degree of the steady-state activation of iK and its time constant were unchanged, the observed reduction of iK could have been caused by a reduction of iK.

Potassium current during the pacemaker depolarization in rabbit sinoatrial node cell

Voltage clamp experiments were carried out on the rabbit sinoatrial (S-A) node. The delayed outward current in the voltage range between --60 mV and --22 mV almost disappeared in the presence of 5 mM Ba2+. The slow inward current and the hyperpolarization-activated current remained unaffected. In the absence of the time-dependent potassium current the S-A node cell generated spontaneous action potentials, provided that the membrane was hyperpolarized by constant outward current. Therefore it seems unlikely that the potassium current plays an essential role in generating the pacemaker potential in the S-A node. The time course of the potassium current (iK) during the cardiac cycle was calculated using equations simulating the kinetics of iK. According to this computation, the change of iK in the S-A node is small during pacemaker depolarization. It is proposed that the gradual decay of potassium conductance is less important for the development of the pacemaker potential than the contribution of the slow inward current.

Slow inward current and its role mediating the chronotropic effect of epinephrine in the rabbit sinoatrial node

The ionic mechanism underlying the chronotropic effect of epinephrine on the rabbit sinoatrial (S-A) node has been studied. Epinephrine (5.5 X 10(-6) M) increased the spontaneous rate from 206 +/- 25 min-1 to 242 +/- 39 min-1. The effect of epinephrine was reproducible on repetitive applications. Voltage clamp experiments using the two microelectrode technique revealed the following changes in the membrane current: epinephrine (5.5 X 10(-7) M) increased the limiting conductance for the slow inward current (is) by approximately 30% and the potassium current (ik) by about 10%, keeping the kinetics of is and ik constant. From the holding potential of -70 mV the activation of is was observed on step depolarization positive to -60 or -55 mV in both control and epinephrine solution. The hyperpolarization-activated current (ih) was also increased by about 20% at -70 mV, and its time course was slightly accelerated. Participation of is for the chronotropic effect of epinephrine was strongly suggested by the findings that is was partially available positive to -60 mV and that epinephrine could not increase the slope of diastolic depolarization when is was blocked by D 600.