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C M Baumgarten

Publications and source records attributed to C M Baumgarten.

At least 37 records · Page 2Linked to original sources
PubMed1996

Osmotic gradient-induced water permeation across the sarcolemma of rabbit ventricular myocytes.

The mechanism of water permeation across the sarcolemma was characterized by examining the kinetics and temperature dependence of osmotic swelling and shrinkage of rabbit ventricular myocytes. The magnitude of swelling and the kinetics of swelling and shrinkage were temperature dependent, but the magnitude of shrinkage was very similar at 6 degrees, 22 degrees, and 37 degrees C. Membrane hydraulic conductivity, Lp, was approximately 1.2 x 10(-10) liter.N-1.s-1 at 22 degrees C, corresponding to an osmotic permeability coefficient, Pf, of 16 microns.s-1, and was independent of the direction of water flux, the magnitude of the imposed osmotic gradient (35-165 mosm/liter), and the initial cell volume. This value of Lp represents an upper limit because the membrane was assumed to be a smooth surface. Based on capacitive membrane area, Lp was 0.7 to 0.9 x 10(-10) liter.N-1.s-1. Nevertheless, estimates of Lp in ventricle are 15 to 25 times lower than those in human erythrocytes and are in the range of values reported for protein-free lipid bilayers and biological membranes without functioning water channels (aquaporin). Evaluation of the effect of unstirred layers showed that in the worst case they decrease Lp by < or = 2.3%. Analysis of the temperature dependence of Lp indicated that its apparent Arrhenius activation energy, Ea', was 11.7 +/- 0.9 kcal/mol between 6 degrees and 22 degrees C and 9.2 +/- 0.9 kcal/mol between 22 degrees and 37 degrees C. These values are significantly greater than that typically found for water flow through water-filled pores, approximately 4 kcal/mol, and are in the range reported for artificial and natural membranes without functioning water channels. Taken together, these data strongly argue that the vast majority of osmotic water flux in ventricular myocytes penetrates the lipid bilayer itself rather than passing through water-filled pores.

Animals↗
PubMed1996

Atrial natriuretic peptide and cardiac electrophysiology: autonomic and direct effects.

Atrial natriuretic peptide (ANP) has varied effects on cardiac electrophysiologic parameters including heart rate, intraatrial conduction time, and refractory period. ANP's vagoexcitatory and sympathoinhibitory actions as well as its direct actions on cardiac ion currents may be responsible for some of these effects. This review discusses the role of ANP in cardiac electrophysiology, its interactions with the autonomic nervous system and baroreceptor reflex, and its effects on cardiac ion currents.

Animals↗
PubMed1995

Stretch-activated channel blockers modulate cell volume in cardiac ventricular myocytes.

Stretch-activated channels (SAC) are postulated to regulate cell volume. While this hypothesis is appealing, direct evidence is lacking. Using digital video microscopy, we found that pharmacological blockade of SACs alters the cell volume of isolated rabbit ventricular myocytes during hypoosmotic stress. Under control conditions, relative cell volume increased from 1.0 to 1.311 +/- 0.019 after 10 min in 195 mosmol/l solution. The cation SAC blocker gadolinium (Gd3+; 10 microM) reduced the amount of swelling in hypoosmotic solution by 24% and induced a regulatory volume decrease otherwise not observed. In contrast, the anion SAC blocker 9-anthracene carboxylic acid (9-AC; 1 mM) increased swelling by 44% under the same conditions. Based on the direction of SAC currents, Gd3+ and 9-AC are expected to have opposite effects on cell volume. Furthermore, Gd3+ and 9-AC changed cell volume by only approximately 2% in isosmotic solutions when SACs are expected to be closed. This supports the idea that Gd3+ and 9-AC affect stretch-activated transport processes. In contrast, omitting bath Ca2+ did not alter cell volume under iso- or hypoosmotic conditions suggesting stretch-activated Ca2+ influx is not important in setting cell volume. Not all channels can affect cell volume. Opening ATP-sensitive K+ channels with aprikalim (100 microM) or blocking them with glibenclamide (1 microM) did not alter cell volume under isosmotic or hypoosmotic conditions. These data support the idea that SACs are involved in cardiac cell volume regulation.

Animals↗
PubMed1995

Unitary conductance of Na+ channel isoforms in cardiac and NB2a neuroblastoma cells.

Unitary conductances of native Na+ channel isoforms (gamma Na) have been determined under a variety of conditions, making comparisons of gamma Na difficult. To allow direct comparison, we measured gamma Na in cell-attached patches on NB2a neuroblastoma cells and rabbit ventricular myocytes under identical conditions [pipette solution (in mM): 280 Na+ and 2 Ca2+, pH 7.4; 10 degrees C]. gamma Na of NB2a channels, 22.9 +/- 0.9 pS, was 21% greater than that of cardiac channels, 18.9 +/- 0.9 pS. In contrast, respective extrapolated reversal potentials, +62.4 +/- 4.6 and +57.9 +/- 5.1 mV, were not significantly different. Several kinetic differences between the channel types were also noted. Negative to -20 mV, mean open time (MOT) of the NB2a isoform was significantly less than that of cardiac channels, and, near threshold, latency to channel opening decayed more rapidly in NB2a. On the basis of analysis of MOT between -60 and 0 mV, the rate constants at 0 mV for the open-to-closed (O-->C) and open-to-inactivated (O-->I) transitions were 0.42 +/- 0.11 and 0.47 +/- 0.11 ms-1 in NB2a and 0.10 +/- 0.06 and 1.19 +/- 0.07 ms-1 in myocytes. The slope factors were -38.9 +/- 8.7 and +10.7 +/- 6.1 mV in NB2a and -27.3 +/- 7.1 and +23.7 +/- 4.9 mV in myocytes. Transition rate constants were significantly different in NB2a and cardiac cells, but voltage dependence was not.

Animals↗
PubMed1995

cGMP and atrial natriuretic factor regulate cell volume of rabbit atrial myocytes.

Atrial natriuretic factor (ANF) reduces the volume of atrial myocytes by inhibiting Na+/K+/2Cl- cotransport. We determined the role of cGMP and cAMP in ANF-induced shrinkage by using digital video microscopy to measure cell volume; volumes are reported relative to control. ANF (1 mumol/L) reversibly reduced atrial cell volume from 1.0 to 0.915 +/- 0.005 (mean +/- SEM). This effect was mimicked by 10 mumol/L 8-bromo-cGMP (8-Br-cGMP), which decreased myocyte volume to 0.894 +/- 0.007 with an ED50 of 0.99 +/- 0.05 mumol/L. In contrast, 100 mumol/L 8-bromo-cAMP (8-Br-cAMP) did not affect volume, and activating the cAMP pathway with 100 mumol/L 8-Br-cAMP did not alter the volume decrease caused by 8-Br-cGMP or ANF. Inhibition of Na+/K+/2Cl- cotransport with bumetanide (1 mumol/L) also reduced cell volume and prevented further shrinkage on subsequent exposure to 8-Br-cGMP. Similarly, 8-Br-cGMP (10 mumol/L) prevented further shrinkage by ANF. Block of Na(+)-H+ exchange, a participant in volume regulation in other cells, did not alter the response to 8-Br-cGMP. More evidence implicating cGMP was obtained by altering its metabolism. LY83583 (10 mumol/L), a guanylate cyclase inhibitor, blocked ANF-induced cell shrinkage. Zaprinast (100 mumol/L), a cGMP-specific phosphodiesterase inhibitor, markedly potentiated the effect of a threshold concentration of ANF (0.01 mumol/L). The actions of ANF, LY83583, and zaprinast on cGMP levels were verified by radioimmunoassay. These data strongly support the idea that the cGMP cascade is the intracellular signaling pathway responsible for ANF-induced atrial cell shrinkage.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

Modification of cardiac sodium channels by carboxyl reagents. Trimethyloxonium and water-soluble carbodiimide.

In TTX-sensitive nerve and skeletal muscle Na+ channels, selective modification of external carboxyl groups with trimethyloxonium (TMO) or water-soluble carbodiimide (WSC) prevents voltage-dependent Ca2+ block, reduces unitary conductance, and decreases guanidinium toxin affinity. In the case of TMO, it has been suggested that all three effects result from modification of a single carboxyl group, which causes a positive shift in the channel's surface potential. We studied the effect of these reagents on Ca2+ block of adult rabbit ventricular Na+ channels in cell-attached patches. In unmodified channels, unitary conductance (gamma Na) was 18.6 +/- 0.9 pS with 280 mM Na+ and 2 mM Ca2+ in the pipette and was reduced to 5.2 +/- 0.8 pS by 10 mM Ca2+. In contrast to TTX-sensitive Na+ channels, Ca2+ block of cardiac Na+ channels was not prevented by TMO; after TMO pretreatment, gamma Na was 6.1 +/- 1.0 pS in 10 mM Ca2+. Nevertheless, TMO altered cardiac Na+ channel properties. In 2 mM Ca2+, TMO-treated patches exhibited up to three discrete gamma Na levels: 15.3 +/- 1.7, 11.3 +/- 1.5, and 9.8 +/- 1.8 pS. Patch-to-patch variation in which levels were present and the absence of transitions between levels suggests that at least two sites were modified by TMO. An abbreviation of mean open time (MOT) accompanied each decrease in gamma Na. The effects on channel gating of elevating external Ca2+ differed from those of TMO pretreatment. Increasing pipette Ca2+ from 2 to 10 mM prolonged the MOT at potentials positive to approximately -35 mV by decreasing the open to inactivated (O-->I) transition rate constant. On the other hand, even in 10 mM Ca2+ TMO accelerated the O-->I transition rate constant without a change in its voltage dependence. Ensemble averages after TMO showed a shortening of the time to peak current and an acceleration of the rate of current decay. Channel modification with WSC resulted in analogous effects to those of TMO in failing to show relief from block by 10 mM Ca2+. Further, WSC caused a decrease in gamma Na and an abbreviation of MOT at all potentials tested. We conclude that a change in surface potential caused by a single carboxyl modification is inadequate to explain the effects of TMO and WSC in heart. Failure of TMO and WSC to prevent Ca2+ block of the cardiac Na+ channel is a new distinction among isoforms in the Na+ channel multigene family.

Animals↗
PubMed1993

Bursting of cardiac sodium channels after acute exposure to 3,5,3'-triiodo-L-thyronine.

Physiological concentrations of 3,5,3'-triiodo-L-thyronine (T3) acutely increased burst-mode gating of Na+ channels in rabbit ventricular myocytes. Bursting was measured as the ratio of long events to the total number of events multiplied by 100 (%LE); a long event was defined as a set of openings or a single opening with a total duration greater than or equal to five times the control mean open time (MOT) for cell-attached patches. In the cell-attached configuration, adding either 5 or 50 nM T3 to the pipette increased the %LE. %LE had a biphasic voltage dependence and peaked at -50 mV, although the largest percentage change from control occurred between -30 and -40 mV. Neither unitary conductance nor the overall MOT was altered by T3-induced bursting. However, the MOT of openings within bursts increased, implying a kinetically distinct mode of channel gating during bursts. Long events sometimes were grouped into runs, but the more usual pattern suggested that modal shifts occurred in approximately 1 second. Similar behavior was observed with triiodothyroacetic acid, a T3 analogue that does not elicit protein synthesis. To investigate involvement of soluble second messengers, cell-attached recordings were made with and without T3 in the bath. Placed outside the pipette, 50 and 100 nM T3 failed to alter MOT, unitary current, or %LE. Na+ channel gating also was unaffected by patch excision and by exposing the cytoplasmic face of inside-out patches to 50 nM T3. Nevertheless, excision to the inside-out configuration with 5 nM T3 in the pipette dramatically increased the %LE and lengthened MOT. These results suggest that T3 induced Na+ channel bursting by an extranuclear mechanism that requires proximity of T3 to the extracellular face of the Na+ channel. Furthermore, T3 was not membrane permeant on the time scale of these experiments. Na+ channel bursting may contribute to the propensity for arrhythmias in hyperthyroidism and to the positive inotropic effect of acute T3 administration in the stunned and ischemic myocardium.

Animals↗
PubMed1992

Modulation of rabbit ventricular cell volume and Na+/K+/2Cl- cotransport by cGMP and atrial natriuretic factor.

Previously we showed that atrial natriuretic factor (ANF) decreases cardiac cell volume by inhibiting ion uptake by Na+/K+/2Cl- cotransport. Digital video microscopy was used to study the role of guanosine 3',5'-monophosphate (cGMP) in this process in rabbit ventricular myocytes. Each cell served as its own control, and relative cell volumes (volume(test)/volume(control)) were determined. Exposure to 10 microM 8-bromo-cGMP (8-Br-cGMP) reversibly decreased cell volume to 0.892 +/- 0.007; the ED50 was 0.77 +/- 0.33 microM. Activating guanylate cyclase with 100 microM sodium nitroprusside also decreased cell volume to 0.889 +/- 0.009. In contrast, 8-bromo-adenosine 3',5'-monophosphate (8-Br-AMP; 0.01-100 microM) neither altered cell volume directly nor modified the response to 8-Br-cGMP. The idea that cGMP decreases cell volume by inhibiting Na+/K+/2Cl- cotransport was tested by blocking the cotransporter with 10 microM bumetanide (BUM) and removing the transported ions. After BUM treatment, 10 microM 8-Br-cGMP failed to decrease cell volume. Replacement of Na+ with N-methyl-D-glucamine or Cl- with methanesulfonate also prevented 8-Br-cGMP from shrinking cells. The data suggest that 8-Br-cGMP, like ANF, decreases ventricular cell volume by inhibiting Na+/K+/2Cl-cotransport. Evidence that ANF modulates cell volume via cGMP was also obtained. Pretreatment with 10 microM 8-Br-cGMP prevented the effect of 1 microM ANF on cell volume, and ANF suppressed 8-Br-cGMP-induced cell shrinkage. Inhibiting guanylate cyclase with the quinolinedione LY83583 (10 microM) diminished ANF-induced cell shrinkage, and inhibiting cGMP-specific phosphodiesterase with M&B22948 (Zaprinast; 100 microM) amplified the volume decrease caused by a low dose of ANF (0.01 microM) approximately fivefold. In contrast, neither 100 microM 8-Br-cAMP nor 50 microM forskolin affected the response to ANF. The effects of ANF, LY83583, and M&B29948 on cGMP levels in isolated ventricular myocytes were confirmed by 125I-cGMP radioimmunoassay. These data argue that ANF shrinks cardiac cells by increasing intracellular cGMP, thereby inhibiting Na+/K+/2Cl- cotransport. Basal cGMP levels also appear to modulate cell volume.

3',5'-Cyclic-GMP Phosphodiesterases↗
PubMed1991

External site for local anesthetic block of cardiac Na+ channels.

We report patch clamp studies of single Na+ channels from cardiac ventricular and Purkinje cells that support the hypothesis that local anesthetics can act from the outside of the membrane, and that demonstrate some aspects of their mechanism of action. Inclusion of lidocaine (0.1 mM) or QX-314 (0.5 mM), a membrane-impermeant, quaternary ammonium derivative of lidocaine, in the pipette solution for on-cell single channel recording demonstrated four important findings. (1) The open probability of the channel is reduced by drug in a use-dependent way. (2) Late openings are preferentially reduced. (3) Mean open time is shortened. (4) Hyperpolarization enhances recovery of the drug-bound channels. These findings are consistent with a hyperpolarizing shift of the transition rates for drug-bound channel. Further, we postulate that there is a drug-bound channel conformation which conducts current. At least some of the properties of local anesthetic interaction with the cardiac Na+ channels may be the result of kinetic effects mediated by binding to an external site.

Anesthetics, Local↗
PubMed1991

Prevention of myocardial intracellular edema induced by St. Thomas' Hospital cardioplegic solution.

During cardiac surgery, the heart is infused with cold crystalloid cardioplegic solutions such as St. Thomas' Hospital (StT) solution, which contains high concentrations of K+ and Mg2+. The high K+ and Mg2+ block impulse conduction and inhibit Ca2+ influx, thereby arresting the heart and reducing cardiac oxygen consumption. Nevertheless, myocardial edema and post-operative abnormalities have been noted after cardioplegia and attributed to ischemia and reflow or to hypothermia. We found, however, that cold StT (9 degrees C) was hypotonic and induced cell swelling in the absence of ischemic injury. Cell swelling in cold StT was not due to hypothermia alone, but rather was caused by KCl influx and was prevented by partially replacing Cl- with an impermeant anion. After exposure to cold StT, cells transiently shrank to less than control volume on rewarming in physiological saline (Tyrode's solution, 37 degrees C). The transient shrinkage was blocked by ouabain suggesting that Na+ loading of depolarized hypothermic cells and Na(+)-K+ pump activation on rewarming were responsible. Hypothermic ventricular cells seem to follow Donnan equilibrium, and the product of [K+] x [Cl-] in cardioplegic solutions affects cell volume in the absence of ischemic injury.

Animals↗
PubMed1991

Regulation of cellular volume in rabbit ventricular myocytes: bumetanide, chlorothiazide, and ouabain.

Video microscopy was used to study the regulation of cell volume in isolated rabbit ventricular myocytes. Myocytes rapidly (less than or equal to 2 min) swelled and shrank in hyposmotic and hyperosmotic solutions, respectively, and this initial volume response was maintained without a regulatory volume decrease or increase for 20 min. Relative cell volumes (normalized to isosmotic solution, 1T) were as follows: 1.41 +/- 0.01 in 0.6T, 1.20 +/- 0.04 in 0.8T, 0.71 +/- 0.04 in 1.8T, and 0.57 +/- 0.03 in 2.6T. These volume changes were significantly less than expected if all of the measured volume was osmotically active water. Changes in width and thickness were significantly greater than changes in cell length. The idea that cotransport contributes to cell volume regulation was tested by inhibiting Na(+)-K(+)-2Cl- cotransport with bumetanide (BUM) and Na(+)-Cl- cotransport with chlorothiazide (CTZ). Under isotonic conditions, a 10-min exposure to BUM (1 microM), CTZ (100 microM), or BUM (10 microM) plus CTZ (100 microM) decreased relative cell volume to 0.87 +/- 0.01, 0.86 +/- 0.02, and 0.82 +/- 0.04, respectively. BUM plus CTZ also modified the response to osmotic stress. Swelling in 2.6T medium was 76% greater and shrinkage in 0.6T medium was 29% less than in the absence of diuretics. In contrast to the rapid effects of diuretics, inhibition of the Na(+)-K+ pump with 10 microM ouabain for 20 min did not affect cell volume in 1T solution. Nevertheless, ouabain decreased swelling in 0.6T medium by 52% and increased shrinkage in 1.8T medium by 34%. These data suggest that under isotonic conditions Na(+)-K(+)-2Cl- and Na(+)-Cl- cotransport are critical in establishing cell volume, but osmoregulation can compensate for Na(+)-K+ pump inhibition for at least 20 min. Under anisotonic conditions, the Na(+)-K+ pump and Na(+)-K(+)-2Cl- and/or Na(+)-Cl- cotransport are important in myocyte volume regulation.

Animals↗
PubMed1991

Atrial natriuretic factor decreases cell volume of rabbit atrial and ventricular myocytes.

The effect of atrial natriuretic factor (ANF) on cell volume was studied using video microscopy of rabbit atrial and ventricular myocytes. Each cell served as its own control, and relative cell volumes were determined. ANF (1 microM) significantly decreased relative cell volume to 0.929 +/- 0.006 (n = 7) in atrial and 0.930 +/- 0.013 (n = 5) in ventricular myocytes (normalized to volume without ANF). Reduction of volume was detectable at greater than or equal to 0.01 microM ANF, and the ED50 was 0.072 +/- 0.007 microM (n = 15). The effect of ANF also was examined under hypotonic (0.55T, 168 mosmol/l) and hypertonic (1.82T, 560 mosmol/l) conditions; osmolarity was adjusted using mannitol with NaCl fixed at 65 mM. In 0.55T, 1 microM ANF decreased cell volume to 0.941 +/- 0.014 (n = 5) in atrial and 0.942 +/- 0.017 (n = 7) in ventricular cells (normalized to 0.55T without ANF). In contrast, 1 microM ANF had no effect on atrial (n = 13) or ventricular (n = 11) cell volume in 1.82T. The hypothesis that ANF decreases cell volume by inhibiting Na(+)-K(+)-2Cl- cotransport was tested by blocking the cotransporter with bumetanide (10 microM). After inhibition of Na(+)-K(+)-2Cl- cotransport, 1 microM ANF failed to reduce cell volume in either atrial (n = 6) or ventricular myocytes (n = 6). Block of ANF-induced cell shrinkage by bumetanide was not due to changes in cell volume, since similar results were obtained using atrial (n = 7) and ventricular (n = 7) cells swollen in hypotonic (0.80T, 244 mosmol/l) solution. Replacement of Na+ with N-methyl-D-glucamine or Cl- with methanesulfonate abolished the ability of both ANF and bumetanide to decrease volume of atrial and ventricular cells in 1T and 0.8T solution. These data suggest that ANF can decrease the volume of atrial and ventricular cells under isotonic and hypotonic conditions by a mechanism that may involve Na(+)-K(+)-2Cl- cotransport. An ANF-induced decrease in cell volume may act as negative feedback and inhibit the stretch-induced release of ANF from atrial and ventricular cells. Furthermore, it may contribute to cell volume maintenance in myocytes in the setting of congestive heart failure or myocardial hypoxia when ANF release is elevated.

Animals↗
PubMed1990

Reversal of low voltage and infarction pattern on the surface electrocardiogram after renal hemodialysis for pulmonary edema.

A case is presented in which markedly low surface electrocardiographic (ECG) voltage and an infarction pattern are rapidly reversed with renal hemodialysis for pulmonary edema. A patient presenting with QRS voltages below 0.1 mV in all the limb and augmented limb leads and a waveform pattern suggestive of an anterior and inferior myocardial infarction experienced a dramatic increase in voltage and a reversal of the infarction pattern after dialysis. A hypothesis is proposed in which alterations in chest wall impedance and in electrolytes are involved in the ECG changes resulting from dialysis. This case illustrates one source of diagnostic error, and that dialysis may result in large, rapid changes in the ECG. Diagnostic errors may be avoided by obtaining serial ECGs in patients undergoing dialysis.

Diagnostic Errors↗
PubMed1989

Current fluctuations and oscillations in smooth muscle cells from hog carotid artery. Role of the sarcoplasmic reticulum.

Electrical activity of enzymatically isolated, smooth muscle cells from hog carotid arteries was recorded under current clamp and voltage clamp. Under the experimental conditions, membrane potential usually was not stable, and spontaneous hyperpolarizing transients of approximately 100-msec duration were recorded. The amplitude of the transients was markedly voltage dependent and ranged from about 20 mV at a membrane potential of 0 mV to undetectable at membrane potentials negative to -60 mV. Under voltage clamp, transient outward currents displayed a similar voltage dependency. These fluctuations reflect a K+ current; they were abolished by 10 mM tetraethylammonium chloride, a K+ channel blocker, and the current fluctuations reversed direction in high extracellular K+ concentration. Modulators of intracellular Ca2+ concentration also affected electrical activity. Lowering intracellular Ca2+ concentration by addition of 10 mM EGTA to the pipette solution or suppressing sarcoplasmic reticulum function by superfusion with caffeine (10 mM), ryanodine (1 microM), or histamine (3-10 microM) blocked the rapid voltage and current spikes. However, caffeine and histamine induced a much slower hump of outward current before blocking the rapid spikes. This slower transient outward current could be elicited only once after external Ca2+ was removed and is consistent with an activation of K+ channels by Ca2+ released from internal stores. In contrast, removal of external Ca2+ alone failed to abolish the rapid spikes. These results suggest that 1) a Ca2+-dependent K+ conductance can markedly affect the electrical behavior of arterial smooth muscle cells and 2) internal Ca2+ stores, probably the sarcoplasmic reticulum, can support rapid and frequent releases of Ca2+. Exposure to a low concentration of histamine (3 microM) caused synchronization of the irregular, rapid fluctuations giving rise to slow, periodic oscillations of Ca2+-activated K+ conductance with a frequency of 0.1-0.3 Hz. These regular oscillations are reminiscent of periodic Ca2+-induced Ca2+ release, were inhibited by 10 mM caffeine, and point to a modulation of sarcoplasmic reticulum Ca2+ release by histamine.

Animals↗
PubMed1987

Intra- and extracellular potassium activities and the potassium equilibrium potential in partially depolarized human atrial cells.

Under tissue bath conditions, isolated specimens of human right atrium are characterized by the presence of large numbers of partially depolarized cells. The basis for the depolarization is still not understood. To determine if reduced intracellular potassium activity (aKi) is responsible for the low levels of maximum diastolic potential (MDP), aKi was directly measured with potassium ion-selective micro-electrodes (KISE). The effects of varying bath potassium concentration ([K+]0) on aiK and on the relationship between MDP and the potassium equilibrium potential (EK) also were determined. In 4 mM [K+]0, all specimens studied were partially depolarized (MDP = -43.9 +/- 1.3 mV [mean +/- S.E.]) and beat spontaneously. aKi was 98.1 +/- 1.5 mM, and EK was -93.3 +/- 0.4 mV. Changes in [K+]0 between 2.5 and 40 mM did not change aKi significantly. MDP was relatively insensitive to [K+]0 and was always far positive to EK over the entire range of [K+]0 studied. The ability of the KISE to reliably measure aKi in the face of the spontaneous diastolic depolarization was assessed by using acetylcholine or cooling to depress automaticity. These measures did not significantly alter the calculated aKi. The results indicate that: aKi in human atrial specimens is similar to that in atria from other mammalian species, and the low level of MDP exhibited by human atrial cells subject to conventional tissue bath conditions does not primarily result from low levels of aKi.

Body Fluids↗
PubMed1986

K+, Na+, and Cl- activities in ventricular myocytes isolated from rabbit heart.

Intracellular K+, Na+, and Cl- activities (aiK, aiNa and aiCl) were measured in ventricular myocytes enzymatically isolated from adult rabbit heart. The activities in normal Tyrode solution containing 2.5 mM Ca2+ were the following (in mM): aiK = 100.0 +/- 3.5 (n = 9); aiNa = 8.4 +/- 1.5 (n = 6); and aiCl = 17.9 +/- 1.5 (n = 11) (mean +/- SE). Membrane potential was -81.6 +/- 0.7 mV (n = 26). These values were determined after correction for changes of junction and tip potential at the reference electrode, estimated to be 4.9 +/- 0.6 mV (n = 7) for 0.15 M KCl-filled electrodes; and intracellular interference detected by the Cl- ion-selective electrode, 11.2 +/- 0.6 mM (n = 4). Extended-tip shunting was avoided by fabricating Na+ ion-selective microelectrodes from aluminosilicate rather than borosilicate glass. These results show that isolated cardiac cells can maintain normal intracellular ion activities. Diffusion of electrolyte from the reference electrode can rapidly alter the intracellular milieu, however. After 10 min of impalement with 0.15 M KCl-filled microelectrodes (resistance approximately equal to 25 M omega), aiK increased by 8.7 +/- 2.0 mM and aiCl by 10.3 +/- 3.1 mM. In contrast, aiNa did not significantly change during the double impalement.

Animals↗
PubMed1986

Isoproterenol directly stimulates the Na+-K+ pump in isolated cardiac myocytes.

It has been suggested that catecholamines directly stimulate Na+-K+ pump activity in heart; however, these studies on multicellular preparations are confounded by possible alterations of extracellular K+ concentrations ([K+]o). We reinvestigated this problem by studying the effect of the beta-agonist isoproterenol (Iso) on intracellular Na+ activity (aiNa) in ventricular myocytes enzymatically isolated from rabbit heart. In 5 mM [K+]o, 0.1 microM Iso caused a 24.6 +/- 2.0% decrease of aiNa. Exposure to 1 microM Iso only caused a small additional decrease (27.8 +/- 2.4%), while a diminution of aiNa could already be noticed with only 10 nM Iso (12.8 +/- 1.9% diminution). Myocytes superfused with 15 mM [K+]o also exhibited a significant decrease of aiNa (22.9 +/- 3.6%) when exposed to 0.1 microM Iso. These data argue that accumulation of external K+ does not account for the effect of Iso on steady-state aiNa as postulated by Gadsby (Nature Lond. 306: 691-693, 1983). Furthermore, aiNa in myocytes superfused with 1.5 mM [K+]o decreased by only 8.7% on addition of 0.1 microM Iso. The latter observation suggests that the beta-agonist effect on aiNa regulation is directly or indirectly dependent on membrane potential and/or aiNa. Finally, kinetic analysis of the effect of 1 microM Iso on the decrease in aiNa on changing [K+]o from 1.5 to 5 mM demonstrated that the time course of aiNa recovery was accelerated by a factor of 1.9. This readily suggests that active Na+ transport is directly stimulated by Iso. The much greater relative effect of Iso on the time constant than on steady-state aiNa further indicates that Iso may also increase passive Na+ influx.

Action Potentials↗