Stimulation of single isolated adult ventricular myocytes within a low volume using a planar microelectrode array

Abstract

Microchannels (40- microm wide, 10- microm high, 10-mm long, 70- microm pitch) were patterned in the silicone elastomer, polydimethylsiloxane on a microscope coverslip base. Integrated within each microchamber were individually addressable stimulation electrodes (40- microm wide, 20- microm long, 100-nm thick) and a common central pseudo-reference electrode (60- microm wide, 500- microm long, 100-nm thick). Isolated rabbit ventricular myocytes were introduced into the chamber by micropipetting and subsequently capped with a layer of mineral oil, thus creating limited volumes of saline around individual myocytes that could be varied from 5 nL to 100 pL. Excitation contraction coupling was studied by monitoring myocyte shortening and intracellular Ca(2+) transients (using Fluo-3 fluorescence). The amplitude of stimulated myocyte shortening and Ca(2+) transients remained constant for 90 min in the larger volume (5 nL) configuration, although the shortening (but not the Ca(2+) transient) amplitude gradually decreased to 20% of control within 60 min in the low volume (100 pL) arrangement. These studies indicate a lower limit for the extracellular volume required to stimulate isolated adult cardiac myocytes. Whereas this arrangement could be used to create a screening assay for drugs, individual microchannels (100 pL) can also be used to study the effects of limited extracellular volume on the contractility of single cardiac myocytes.

FIGURE 1

Schematic diagram outlining the fabrication processes of the array of the electrodes (A) and the array of the microchannels (B). The height of the channels was defined by the thickness of the patterned photoresist film used as a master for replica molding of PDMS (B, iii). PDMS was cast against the master and the cured polymer was thoroughly washed in acetone to remove all residual resist and to clear the surface of the microelectrodes (B, v). Without the support of the photoresist, the thin film of PDMS covering the microchannels collapsed and was easily washed off. The PDMS film covering the bulk electrodes served as an insulating layer to avoid short circuits of the leads through buffer spill.

FIGURE 2

(A) Schematic diagram illustrating the different layers of the microfabricated array: glass coverslip (1), gold electrodes (2), PDMS microchannel (3), and mineral oil (4). (B) A micrograph of an array of adult ventricular myocytes aligned to the microelectrodes by means of the PDMS microchannels. The field of view shown could be illuminated with a light source for fluorophore excitation and imaged with a fast low light camera through a high NA (0.9) 20× lens. This format thus has the potential to record Ca²⁺ transients from all the cells in the array simultaneously.

FIGURE 3

Visualization of the picoliter volume of a microchamber holding a single ventricular myocyte. The extracellular dye (Ca²⁺ bound Fluo-3) was excited with a two-photon laser and series of optical sections along the z-axis were recorded. (A) Transmitted light picture (i), fluorescent picture (ii), and vertical cross-section (iii) through the long axis of the cell after reconstructing the z-stack to generate a three-dimensional view (z-spacing is 0.5 μm; scale bar is 20 μm). (B) Micrograph of a cardiomyocyte in microchannel oriented parallel to the electric field between the microelectrodes; distances indicated as μm. (B′) illustration of an orthogonal cross-section (white line in Fig. 3 B). (C) Sketch illustrating a longitudinal cross-section (black line in Fig. 3 B) through the center of a microchannel. Unlimited diffusion in bath comprising the entire length of the microchannel with the ends of the channel extending ∼5 mm beyond each electrode (breaks indicated), thus comprising a volume of >5 nL. (D) Limited diffusion in bath comprising solely the area between the electrodes (40-μm wide, 250-μm long, and 10-μm high, ∼100 pL volume). Coverslip with microelectrodes (1), bath with cardiomyocyte (2), mineral oil (3), and microchannel in PDMS (4).

FIGURE 4

(A) Time course of the pH change in the 100-pL bath during a 10-min pulse train at field strength above the threshold for electrical stimulation of adult ventricular myocytes. The pH-sensitive fluorochrome BCECF was added to the saline buffered with 1 mM HEPES and excited every 200 ms for 5 ms to avoid bleaching. The change in emission was related to the initial emission and plotted against time. Pulses of 1-V amplitude applied to the stimulating electrode which was separated from the reference electrode by a 200-μm gap resulted in 50-V/cm field strength. Due to the limited diffusion in the pL volume, the pH did not recover from the drop caused by electrolysis at higher field strength (>75 V/cm). (B) The average field strength for suprathreshold stimulation was calculated to 27 V/cm ± 10 V/cm (n = 6). The shaded area indicates the range of field strengths which could possibly be used to continually pace myocytes of different length without impairment through electrolysis/electroporation.

FIGURE 5

All-or-none excitatory response to biphasic stimuli. The top trace shows the train of current spikes recorded on one of the stimulating electrodes. The bottom trace shows the concurrent intracellular Ca²⁺ transients occurring only after suprathreshold stimulation. The amplitude of the stimulus was gradually increased until the first Ca²⁺ transient was triggered. The inset shows the last current spike before the onset of excitation on an expanded timescale together with the correspondent voltage and charge profile. The Ca²⁺ transients were recorded with the Ca²⁺-sensitive dye Fluo-3.

FIGURE 6

Uniform rise of [Ca²⁺]_i upon electrical stimulation with microelectrodes. The confocal scan line was aligned to the longitudinal axis of a Fluo-3 loaded cell (insert) paced with biphasic stimuli. The acquired line scan image shows the uniform rise in [Ca²⁺]_i immediately after the excitation. A pulse triggering an LED flash was coupled 25 ms in advance to the stimulating pulse to mark the event on the confocal record. Note the cell shortening in response to the [Ca²⁺]_i rise and relaxation after the Ca²⁺ clearance. The fluorescence profile in a 10-μm-wide band (indicated) is plotted against time (thick line) together with the voltage profile of the stimulator (thin line).

FIGURE 7

The effect of continual pacing on contractility of single cardiomyocytes. Myocyte contractility was measured as the change of the sarcomere length from cardiomyocytes stimulated at 1 Hz (19–21°C). (A) Superimposed records of relative sarcomere length from an isolated rabbit cardiomyocyte stimulated in a 5-nL microchannel; single transients from a continuous record are shown. The values on the right-hand side represents period from development of steady-state shortening. (B) Relative mean sarcomere length records from an isolated rabbit cardiomyocyte stimulated in a restricted extracellular volume (∼100 pL) at 1 Hz (19–21°C). Contractility decayed within a 60-min period of continuous stimulation but partial recovery was observed after renewal of the buffer.

FIGURE 8

[Ca²⁺] transients during continual pacing. Fluo-3 loaded cardiomyocytes were electrically stimulated in the restricted extracellular space of ∼100 pL at alternating frequencies of 0.5 Hz (white bar) and 1.0 Hz (black bar) for periods of 5 (^*), 10 (^**), or (20 (^***) min either with or without FCCP, a mitochondrial uncoupler. The fluorescence was recorded at 50 Hz with an intensified charge-coupled device camera. The amplitude of the Ca²⁺ transient of the control attenuated after 70 min of continual pacing in the picoliter volume in the absence of FCCP (Fig. 8 A). No cell shortening was detectable after 70 min of continual stimulation in the restrained extracellular space (see also Fig. 7 B). In the presence of 1 μM FCCP, the cell no longer responded to the electrical stimulation at the higher frequency after 50-min pacing (Fig. 8 B, arrow), and 10 min later the Ca²⁺ transients evoked at 0.5 Hz ceased almost completely (Fig. 8 B, arrowhead). Immediately after the renewal of buffer, the cell started Ca²⁺ waves at low frequency in the absence of electrical stimulation. The cell partially recovered from its unresponsiveness to the electrical stimulation.