Correlation of the localization of La+++ with its effects on Ca++ exchange in cultured rat heart cells is examined with the use of a recently developed technique. 75% of cellular Ca++ is exchangeable and is completely accounted for by two kinetically defined phases. The rapidly exchangeable phase has a t ½ = 1.15 min and accounts for 1 1 mmoles Ca++/kg wet cells or 43% of the exchangeable Ca++ (cells perfused with [Ca++]o = 1 mM) Phase 2 has a t ½ = 19.2 min and accounts for 1.5 mmoles Ca++/kg wet cells or 57% of the exchangeable Ca++. 0.5 mM [La+++]o displaces 0 52 mmoles Ca++/kg wet cells—all from phase 1—and almost completely abolishes subsequent Ca++ influx and efflux The presence of La+++ in the washout converts the washout pattern to a single phase system with a t ½ = 124 min. The effects upon Ca++ exchange are coincident with abolition of contractile tension but regenerative depolarization of the tissue is maintained Electron microscope localization of the La+++ places it exclusively in the external lamina or basement membrane of the cells. The study indicates that negatively charged sites in the basement membrane play a crucial role in the E-C coupling process in heart muscle

/pdf/lanthanum-in-heart-cell-culture-effect-on-calcium-exchange-47dteov1uq.pdf

LANTHANUM IN HEART CELL CULTURE : Effect on Calcium Exchange Correlated with Its Localization

Excitation-contraction coupling in cardiac muscle.

Currents flowing through single dihydropyridine-sensitive Ca2+ channels were recorded from cell-attached patches on C2 myotubes. In the presence of dihydropyridine agonist to prolong the duration of single-channel openings, adding micromolar concentrations of lanthanum (La), cerium (Ce), neodymium (Nd), gadolinium (Gd), dysprosium (Dy), or ytterbium (Yb) to patch electrodes containing 110 mM BaCl2 caused the unitary Ba2+ currents to fluctuate between fully open and shut states. The kinetics of channel blockade followed the predictions of a simple open channel block model in which the fluctuations of the single-channel current arose from the entry and exit of blocking ions from the pore. Entry rates for all the lanthanides tested were relatively insensitive to membrane potential, however, exit rates depended strongly on membrane potential increasing approximately e-fold per 23 mV with hyperpolarization. Individual lanthanide ions differed in both the absolute rates of ion entry and exit: entry rates decreased as cationic radius decreased; exit rates also decreased with cationic radius during the first part of the lanthanide series but then showed little change during the latter part of the series. Overall, the results support the idea that smaller ions enter the channel more slowly, presumably because they dehydrate more slowly; smaller ions also bind more tightly to a site within the channel pore, but lanthanide residence time within the channel approaches a maximum for the smaller cations with radii less than or equal to that of Ca2+.

/pdf/blockade-of-current-through-single-calcium-channels-by-3zkxnd5zn3.pdf

Blockade of current through single calcium channels by trivalent lanthanide cations. Effect of ionic radius on the rates of ion entry and exit.

Transient diastolic dilatation of the isolated canine left ventricle predictably elicits arrhythmias. To test the hypothesis that such arrhythmias may be mediated by sarcolemmal stretch-activated channels, we attempted to inhibit stretch-induced arrhythmias with gadolinium (Gd3+), a potent stretch-activated channel blocker. In experiments with six isolated canine hearts, left ventricular volume was increased for 50 msec during early diastole and then returned to initial volume by a computerized servopump. The stretch volume was adjusted to yield a probability of eliciting a stretch-induced arrhythmia of 95 +/- 2% before treatment with Gd3+. When Gd3+ (1-10 microM) was administered, dose-dependent suppression of stretch-induced arrhythmias was observed. The probability of a stretch-induced arrhythmia was reduced to 13 +/- 10% (p less than 0.05) with 10 microM Gd3+. Washout of Gd3+ completely reversed this effect. Since Gd3+ is known to be a calcium channel antagonist, we compared the effect of Gd3+ on stretch-induced arrhythmias with that of verapamil and nifedipine. These calcium channel blockers produced no demonstrable inhibition of stretch-induced arrhythmias when administered at concentrations (1 microM) that substantially depressed left ventricular pressure development. Thus, our results indirectly implicate stretch-activated channels in the genesis of stretch-induced arrhythmias and provide preliminary evidence for a potential new mode of antiarrhythmic drug action--blockade of stretch-activated channels.

Dose-dependent inhibition of stretch-induced arrhythmias by gadolinium in isolated canine ventricles. Evidence for a unique mode of antiarrhythmic action.

Calcium antagonists and calmodulin inhibitors block cytokinin-induced bud formation in Funaria☆

Arterially cannulated rabbit interventricular septal tissue was exposed to 5–40 µM La in 2.5 mM Ca perfusate. Immediately following perfusion with La two concurrent events were consistently observed: (a) a rapid decline of active tension to a lesser steady-state value, and (b) an abrupt, release of short duration of tissue-bound Ca. The magnitude of both events was directly related to the [La]o. If the duration of exposure to La was brief, contractility returned toward normal upon return to the La-free perfusate. Elevation of [Ca]o during exposure to La counteracted its effect and induced a concurrent displacement of tissue-bound La. Cellular action potentials recorded during brief perfusion with La demonstrated that essentially normal regenerative depolarization was maintained. Analysis of the quantities of 45Ca released following exposure to 10 µM La indicated that this La-susceptible Ca was being displaced from a homogeneous pool—the one previously shown by Langer to represent contractile dependent Ca. These data led to the following conclusions: During perfusion with 2.5 mM Ca contractile dependent Ca was derived primarily from "superficially" located sites. La effected the release of contractile dependent Ca by modifying the normal permselectivity of this "superficial" membrane for activator Ca. These and other data infer that contractile dependent Ca is derived primarily from superficially located sites.

https://rupress.org/jgp/article-pdf/56/2/191/1244895/191.pdf

Specific Uncoupling of Excitation and Contraction in Mammalian Cardiac Tissue by Lanthanum Kinetic studies

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Specific Uncoupling of Excitation and Contraction in Mammalian Cardiac Tissue by Lanthanum Kinetic studies