Search PubMed⌕ Search

Biomedical subjects

K W Spitzer

Publications and source records attributed to K W Spitzer.

At least 55 records · Page 3Linked to original sources
PubMed1990

The relationship between charge movements associated with ICa and INa-Ca in cardiac myocytes.

Ventricular myocytes exhibit a nifedipine-sensitive inward calcium current (ICa) and contracture when they are voltage clamped from -40 to 0 millivolt in the presence of caffeine and in the absence of extracellular sodium. However, upon repolarization they fail to relax because neither the sarcoplasmic reticulum nor the sodium-calcium exchange can reduce intracellular calcium. Sudden application of extracellular sodium during the contracture (but after repolarization) causes immediate relaxation and activates a transient inward sodium-calcium exchange current (INa-Ca), whose peak slightly precedes mechanical relaxation. The total charge carried by the nifedipine-sensitive ICa is twice the total charge carried by the transient inward INa-Ca. Assuming an exchange stoichiometry of three sodium to one calcium, these results indicate that all the calcium entering the cell during the initial depolarization is extruded by the sodium-calcium exchange.

Animals↗
PubMed1990

Effects of intracellular acidosis on [Ca2+]i transients, transsarcolemmal Ca2+ fluxes, and contraction in ventricular myocytes.

We examined the effects of intracellular acidosis produced by washout of NH4Cl on [Ca2+]i transients (indo-1 fluorescence), cell contraction (video motion detector), and 45Ca and 24Na fluxes in cultured chick embryo ventricular myocytes. Exposure of cells to 10 mM NH4Cl produced intracellular alkalosis (pH 7.6), and subsequent washout resulted in a transient acidosis (pH 6.5). Exposure to 10 mM NH4Cl slightly decreased [Ca2+]i transients but increased the amplitude of cell contraction. Subsequent washout of NH4Cl initially increased diastolic [Ca2+]i and decreased the peak positive and negative d[Ca2+]i/dt, while the amplitude of cell contraction was markedly decreased. Subsequently, peak systolic [Ca2+]i increased with partial recovery of contraction. A similar increase in [Ca2+]i and decrease in contraction after washout of NH4Cl was observed in single paced adult guinea pig ventricular cells. Acidosis decreased 45Ca uptake by sarcoplasmic reticulum vesicles isolated from chick embryo ventricle. However, the [Ca2+]i increase caused by intracellular acidosis was also observed in the presence of 10 mM caffeine, suggesting that altered sarcoplasmic reticulum handling of calcium is not the only mechanism involved. Intracellular acidosis only slightly increased total 24Na uptake under these conditions, an effect resulting from the combination of a stimulation of amiloride-sensitive sodium influx (Na(+)-H+ exchange) and inhibition of sodium influx via Na(+)-Ca2+ exchange, manifested by a significant decrease in 45Ca efflux. Further support for a lack of involvement of an increased [Na+]i in the observed increase in [Ca2+]i during acidosis was low-sodium, nominal 0-calcium extracellular solution, an experimental condition that minimizes the possible effects of Na(+)-H+ exchange and Na(+)-Ca2+ exchange. We conclude that the [Ca2+]i increase caused by intracellular acidosis in cultured ventricular cells is primarily due to changes in [Ca2+]i buffering and [Ca2+]i extrusion, rather than to an increase in transsarcolemmal calcium influx. Intracellular acidosis also markedly decreases the sensitivity of the contractile elements to [Ca2+]i in cultured chick embryonic and adult guinea pig ventricular myocytes.

Acidosis↗
PubMed1989

Intracellular Ca2+ transients during rapid cooling contractures in guinea-pig ventricular myocytes.

1. We measured intracellular Ca2+ transients during rapid cooling contractures (RCCs) in guinea-pig ventricular myocytes using the fluorescent Ca2+ indicator, Indo-1. 2. Rapid cooling of myocytes from 22 to 0-1 degrees C induced a rapid increase in [Ca2+]i which preceded the peak of the contraction and was sometimes large enough to saturate Indo-1. This indicates that [Ca2+]i may reach greater than 10 microM during an RCC. 3. The [Ca2+]i during the RCC slowly declined from its peak value and most of this decline in [Ca2+]i can be attributed to slow reaccumulation of Ca2+ by the sarcoplasmic reticulum (SR) in the cold. RCCs induced in the absence of Cao2+, were not different from control, supporting previous conclusions that RCCs depend exclusively on intracellular Ca2+ stores. 4. RCCs are depressed by long rest periods (rest decay) or by exposure to ryanodine or caffeine, which supports conclusions that RCCs are due to Ca2+ release from the SR. The rest decay of RCCs can be almost completely prevented by applying Nao(+)-free solution during the rest period. This implies that the loss of SR Ca2+ during rest depends on the sarcolemmal Na(+)-Ca2+ exchange (and not the sarcolemmal Ca2(+)-ATPase pump). 5. Rapid rewarming during an RCC normally leads to an additional transient contraction (or rewarming spike), without any increase in [Ca2+]i. Thus, the rewarming spike might be attributable to an increase in myofilament Ca2+ sensitivity induced by rewarming. 6. A second RCC is used to assess the fraction of Ca2+ which is re-sequestered by the SR during relaxation from the first RCC. In control solution progressive RCCs decline in amplitude, but in Na(+)-free, Ca2(+)-free solution they are of constant amplitude. We conclude that the SR Ca2+ pump and Na(+)-Ca2+ exchange are responsible for relaxation and that the latter may account for 20-50% of relaxation. 7. These results support the use of RCCs as a useful means of assessing SR Ca2+ content in intact cardiac muscle cells.

Animals↗
PubMed1989

A simple device for rapidly exchanging solution surrounding a single cardiac cell.

This study describes the design and various physiological applications of a simple device to rapidly change the solution surrounding a single intact cardiac cell. It consists of a short length of double-barreled glass tubing (theta-tubing) attached to a miniature solenoid. A cell is positioned in one of two parallel streams of solution that simultaneously flow from each barrel. Rapid solution switching is achieved by activation of the solenoid that directs the adjacent stream over the cell, changing the bulk solution within 7 ms. Approximately 150 ms were required to change the solution at the membrane surface of guinea pig ventricular cells, judging from potassium-induced changes in resting membrane potential. This delayed response was probably due to, in part, restricted diffusion in the transverse tubular system. The switching speed of this device makes it possible to change extracellular solutions during action potentials and voltage-clamp pulses.

Action Potentials↗
PubMed1988

Relaxation of isolated ventricular cardiomyocytes by a voltage-dependent process.

Cell contraction and relaxation were measured in single voltage-clamped guinea pig cardiomyocytes to investigate the contribution of sarcolemmal Na+-Ca2+ exchange to mechanical relaxation. Cells clamped from -80 to 0 millivolts displayed initial phasic and subsequent tonic contractions; caffeine reduced or abolished the phasic and enlarged the tonic contraction. The rate of relaxation from tonic contractions was steeply voltage-dependent and was significantly slowed in the absence of a sarcolemmal Na+ gradient. Tonic contractions elicited in the absence of a Na+ gradient promptly relaxed when external Na+ was applied, reflecting activation of Na+-Ca2+ exchange. It appears that a voltage-dependent Na+-Ca2+ exchange can rapidly mechanically relax mammalian heart muscle.

Animals↗
PubMed1988

Nonuniform epicardial activation and repolarization properties of in vivo canine pulmonary conus.

The relation between nonuniform epicardial activation and ventricular repolarization properties was studied in 14 pentobarbital anesthetized dogs and with a computer model. In 11 dogs, isochrone maps of epicardial activation sequence were constructed from electrograms recorded from the pulmonary conus with 64 electrodes on an 8 X 8 grid with 2-mm electrode separation. The heart was paced from multiple sites on the periphery of the array. Uniformity of epicardial activation was estimated from activation times at test sites and their eight neighboring sites. Acceleration shortened and deceleration prolonged refractory periods. The locations of acceleration and deceleration sites of activation differed during drives from various sites, and differences in uniformity of activation during pairs of drives were correlated to differences in refractory periods (r = 0.76, range 0.59-0.93). In three additional experiments, transmural activation sequence maps were constructed from electrograms recorded from needle-mounted electrodes placed upstream and downstream to epicardial activation delays. Activation proceeded from epicardium to endocardium upstream to the delays and from endocardium to epicardium downstream to the delays. A computer simulation of two-dimensional action potential propagation based on the Beeler-Reuter myocardial membrane model provided insights to the mechanism for the results of the animal experiments. The two-dimensional sheet modeled the transmural anisotropic histology of the canine pulmonary conus and corresponded to previous reports and histology of specimens from five experiments. Simulated activation patterns were similar to those found in the experimental animals. In addition, action potentials were electronically prolonged at sites of deceleration and shortened at sites of acceleration, results comparable to the animal experiments. Our findings demonstrate that the location of areas of nonuniform epicardial activation is dependent on drive site and that nonuniform activation electronically modulates repolarization properties. Therefore it seems likely that the site of origin of ectopic ventricular complexes, especially in ischemic myocardium where activation is nonuniform, could be an important determinant of whether ectopic activity initiates sustained tachyarrhythmias.

Action Potentials↗
PubMed1985

Effects of activation sequence on ventricular refractory periods of ischemic canine myocardium.

Refractory periods were measured in pentobarbital-anesthetized dogs during control periods and one to one and a half hours after distal left anterior descending coronary artery occlusion. The refractory period test site was on the anterior surface of the left ventricle in the distribution of the artery to be occluded. Measurements were made during drive of the refractory period test site, drive of a distant site on the pulmonary conus and during fusion drive in which drive of the test site was delayed with respect to drive of the pulmonary conus. Refractory periods were longer during test site drive than during pulmonary conus or fusion drives in both the control periods and following coronary occlusion. However, the effects of driving mode on refractory periods were greater following coronary occlusion than in the control periods. The findings are likely secondary to different magnitudes of change in electrotonic interactions associated with changes in activation sequence in ischemic and nonischemic myocardium. The greater dependence of repolarization properties in ischemic than nonischemic tissue suggests that inhomogeneity of these properties could be modified considerably by the site of origin of ectopic ventricular complexes.

Action Potentials↗
PubMed1980

Intracellular chloride activity in quiescent cat papillary muscle.

Intracellular chloride activity (alpha iCl) was measured in quiescent cat papillary muscles with chloride liquid ion-exchanger microelectrodes. Muscles were superfused in bicarbonate-buffered solutions containing a control concentration of chloride (146.1 mM) and either 2.7, 4.4, or 5.4 mM potassium. The resulting mean membrane potentials were -96.2, -86.8, and -80.6 mV, respectively. In contrast, alpha iCl and the chloride equilibrium potential were unchanged by these alterations in external potassium and remained stable at 11 mM and -60 mV for up to 10 h following removal of the tissue from the animal. This value of alpha iCl is approximately 2-4 times greater than that predicted assuming chloride is passively distributed. The upper limit of intracellular interference was estimated from low external chloride superfusion and found to be no greater than 2-3 mM. Prolonged exposure to bicarbonate-free solutions buffered with N-2-hydroxyethyl piperazine-N'-2-ethanesulfonic acid (HEPES) did not reduce the apparent value of alpha iCl, suggesting that intracellular interference from intracellular bicarbonate does not significantly influence the measurement of alpha iCl. These results suggest that chloride can move into cat papillary muscle against an electrochemical gradient.

Animals↗
PubMed1979

The effects of acidosis and bicarbonate on action potential repolarization in canine cardiac Purkinje fibers.

Studies were performed on canine cardiac Purkinje fibers to evaluate the effects of acidosis and bicarbonate (HCO3) on action potential repolarization. Extracellular pH (pHe) was reduced from 7.4 to 6.8 by increasing carbon dioxide (CO2) concentration from 4 to 15% in a HCO3-buffered solution or by NaOH titration in a Hepes-buffered solution. Both types of acidosis produced a slowing of the rate of terminal repolarization (i.e., period of repolarization starting at about -60 mV and ending at the maximum diastolic potential) with an attendant increase in action potential duration of 10--20 ms. This was accompanied by a reduction in the maximum diastolic potential of 2--8 mV. In contrast, if the same pH change was made by keeping CO2 concentration constant and lowering extracellular HCO3 from 23.7 to 6.0 mM, in addition to the slowing of terminal repolarization, the plateau was markedly prolonged resulting in an additional 50- to 80-ms increase in action potential duration. If pHe was held constant at 7.4 and HCO3 reduced from 23.7 mM to 0 (Hepes-buffered solution), the changes in repolarization were nearly identical to those seen in 6.0 mM HCO3 except that terminal repolarization was unchanged. This response was unaltered by doubling the concentration of Hepes. Reducing HCO3 to 12.0 mM produced changes in repolarization of about one-half the magnitude of those in 6.0 mM HCO3. These findings suggest that in Purkinje fibers, HCO3 either acts as a current that slows repolarization or modulates the ionic currents responsible for repolarization.

Action Potentials↗
PubMed1975

Manganese amd electrogenic phenomena in canine Purkinje fibers.

Studies were performed on canine cardiac Purkinje fibers to evaluate the effects of manganese on membrane electrogenesis. The results indicate that manganese has a calciumlike effect on the excitatory sodium current and inhibitory effects on potassium conductance and slow inward current. The calciumlike effect of manganese on sodium current was reflected through a leftward (toward less negative potentials) and downward shift in the curve relating maximum upstroke velocity to membrane potential. The inhibitory action of manganese on potassium conductance was suggested by the following observations. (1) Manganese caused an initial increase in action potential duration largely due to a lengthening of the plateau and decreases in the rates of phase 3 and terminal repolarization. (2) Manganese increased the rate of diastolic depolarization. (3) Manganes blocked the initial fall in maximum diastolic potential accompanying rapid stimulation. (4) Manganese in high concentrations caused generalized depolarization which was reversed by rapid stimulation and by increased extracellular potassium concentrations. The action of manganese to block slow inward current was indicated by the eventual shortening of the plateau and by the elimination of responses initiated from low levels of membrane potential (less than minus 55 mv). In addition to these effects, manganese also reduced membrane excitability, eliminated arrhythmic beats occurring during low-frequency electrical stimulation, and caused membrane hyperpolarization which was blocked by tetrodotoxin.

Action Potentials↗