The conduction velocity-potassium relationship in the heart is modulated by sodium and calcium

The relationship between cardiac conduction velocity (CV) and extracellular potassium (K⁺) is biphasic, with modest hyperkalemia increasing CV and severe hyperkalemia slowing CV. Recent studies from our group suggest that elevating extracellular sodium (Na⁺) and calcium (Ca²⁺) can enhance CV by an extracellular pathway parallel to gap junctional coupling (GJC) called ephaptic coupling that can occur in the gap junction adjacent perinexus. However, it remains unknown whether these same interventions modulate CV as a function of K⁺. We hypothesize that Na⁺, Ca²⁺, and GJC can attenuate conduction slowing consequent to severe hyperkalemia. Elevating Ca²⁺ from 1.25 to 2.00 mM significantly narrowed perinexal width measured by transmission electron microscopy. Optically mapped, Langendorff-perfused guinea pig hearts perfused with increasing K⁺ revealed the expected biphasic CV-K⁺ relationship during perfusion with different Na⁺ and Ca²⁺ concentrations. Neither elevating Na⁺ nor Ca²⁺ alone consistently modulated the positive slope of CV-K⁺ or conduction slowing at 10-mM K⁺; however, combined Na⁺ and Ca²⁺ elevation significantly mitigated conduction slowing at 10-mM K⁺. Pharmacologic GJC inhibition with 30-μM carbenoxolone slowed CV without changing the shape of CV-K⁺ curves. A computational model of CV predicted that elevating Na⁺ and narrowing clefts between myocytes, as occur with perinexal narrowing, reduces the positive and negative slopes of the CV-K⁺ relationship but do not support a primary role of GJC or sodium channel conductance. These data demonstrate that combinatorial effects of Na⁺ and Ca²⁺ differentially modulate conduction during hyperkalemia, and enhancing determinants of ephaptic coupling may attenuate conduction changes in a variety of physiologic conditions.

Modulating cardiac conduction during metabolic ischemia with perfusate sodium and calcium in guinea pig hearts

We previously demonstrated that altering extracellular sodium (Na_o) and calcium (Ca_o) can modulate a form of electrical communication between cardiomyocytes termed "ephaptic coupling" (EpC), especially during loss of gap junction coupling. We hypothesized that altering Na_o and Ca_o modulates conduction velocity (CV) and arrhythmic burden during ischemia. Electrophysiology was quantified by optically mapping Langendorff-perfused guinea pig ventricles with modified Na_o (147 or 155 mM) and Ca_o (1.25 or 2.0 mM) during 30 min of simulated metabolic ischemia (pH 6.5, anoxia, aglycemia). Gap junction-adjacent perinexal width ( W_P), a candidate cardiac ephapse, and connexin (Cx)43 protein expression and Cx43 phosphorylation at S368 were quantified by transmission electron microscopy and Western immunoblot analysis, respectively. Metabolic ischemia slowed CV in hearts perfused with 147 mM Na_o and 2.0 mM Ca_o; however, theoretically increasing EpC with 155 mM Na_o was arrhythmogenic, and CV could not be measured. Reducing Ca_o to 1.25 mM expanded W_P, as expected during ischemia, consistent with reduced EpC, but attenuated CV slowing while delaying arrhythmia onset. These results were further supported by osmotically reducing W_P with albumin, which exacerbated CV slowing and increased early arrhythmias during ischemia, whereas mannitol expanded W_P, permitted conduction, and delayed the onset of arrhythmias. Cx43 expression patterns during the various interventions insufficiently correlated with observed CV changes and arrhythmic burden. In conclusion, decreasing perfusate calcium during metabolic ischemia enhances perinexal expansion, attenuates conduction slowing, and delays arrhythmias. Thus, perinexal expansion may be cardioprotective during metabolic ischemia. NEW & NOTEWORTHY This study demonstrates, for the first time, that modulating perfusate ion composition can alter cardiac electrophysiology during simulated metabolic ischemia.

Design and validation of a tissue bath 3-D printed with PLA for optically mapping suspended whole heart preparations

With the sudden increase in affordable manufacturing technologies, the relationship between experimentalists and the designing process for laboratory equipment is rapidly changing. While experimentalists are still dependent on engineers and manufacturers for precision electrical, mechanical, and optical equipment, it has become a realistic option for in house manufacturing of other laboratory equipment with less precise design requirements. This is possible due to decreasing costs and increasing functionality of desktop three-dimensional (3-D) printers and 3-D design software. With traditional manufacturing methods, iterative design processes are expensive and time consuming, and making more than one copy of a custom piece of equipment is prohibitive. Here, we provide an overview to design a tissue bath and stabilizer for a customizable, suspended, whole heart optical mapping apparatus that can be produced significantly faster and less expensive than conventional manufacturing techniques. This was accomplished through a series of design steps to prevent fluid leakage in the areas where the optical imaging glass was attached to the 3-D printed bath. A combination of an acetone dip along with adhesive was found to create a water tight bath. Optical mapping was used to quantify cardiac conduction velocity and action potential duration to compare 3-D printed baths to a bath that was designed and manufactured in a machine shop. Importantly, the manufacturing method did not significantly affect conduction, action potential duration, or contraction, suggesting that 3-D printed baths are equally effective for optical mapping experiments.NEW & NOTEWORTHY This article details three-dimensional printable equipment for use in suspended whole heart optical mapping experiments. This equipment is less expensive than conventional manufactured equipment as well as easily customizable to the experimentalist. The baths can be waterproofed using only a three-dimensional printer, acetone, a glass microscope slide, c-clamps, and adhesive.

Heart Rate and Extracellular Sodium and Potassium Modulation of Gap Junction Mediated Conduction in Guinea Pigs

Background: Recent studies suggested that cardiac conduction in murine hearts with narrow perinexi and 50% reduced connexin43 (Cx43) expression is more sensitive to relatively physiological changes of extracellular potassium ([K⁺]_o) and sodium ([Na⁺]_o).

Purpose: Determine whether similar [K⁺]_o and [Na⁺]_o changes alter conduction velocity (CV) sensitivity to pharmacologic gap junction (GJ) uncoupling in guinea pigs.

Methods: [K⁺]_o and [Na⁺]_o were varied in Langendorff perfused guinea pig ventricles (Solution A: [K⁺]_o = 4.56 and [Na⁺]_o = 153.3 mM. Solution B: [K⁺]_o = 6.95 and [Na⁺]_o = 145.5 mM). Gap junctions were inhibited with carbenoxolone (CBX) (15 and 30 μM). Epicardial CV was quantified by optical mapping. Perinexal width was measured with transmission electron microscopy. Total and phosphorylated Cx43 were evaluated by western blotting.

Results: Solution composition did not alter CV under control conditions or with 15μM CBX. Decreasing the basic cycle length (BCL) of pacing from 300 to 160 ms decreased CV uniformly with both solutions. At 30 μM CBX, a change in solution did not alter CV either longitudinally or transversely at BCL = 300 ms. However, reducing BCL to 160 ms caused CV to decrease more in hearts perfused with Solution B than A. Solution composition did not alter perinexal width, nor did it change total or phosphorylated serine 368 Cx43 expression. These data suggest that the solution dependent CV changes were independent of altered perinexal width or GJ coupling. Action potential duration was always shorter in hearts perfused with Solution B than A, independent of pacing rate and/or CBX concentration.

Conclusions: Increased heart rate and GJ uncoupling can unmask small CV differences caused by changing [K⁺]_o and [Na⁺]_o. These data suggest that modulating extracellular ionic composition may be a novel anti-arrhythmic target in diseases with abnormal GJ coupling, particularly when heart rate cannot be controlled.