Cholinergic modulation of activation sequence in the atrial myocardium of non-mammalian vertebrates

Seasonal changes of cholinergic response in the atrium of Arctic navaga cod (Eleginus navaga)

Fishes of north-temperate latitudes exhibit marked seasonal changes in electrical excitability of the heart partly as an outcome of temperature-dependent changes in the density of major K⁺ ion currents: delayed rectifiers (I_Kr, I_Ks) and background inward rectifier (I_K1). In the arctic teleost, navaga cod (Eleginus navaga), I_Kr and I_K1 are strongly up-regulated in winter. The current study tests the hypothesis that the ligand-gated K⁺ current, the acetylcholine-activated inward rectifier, I_KACh, is also modified by seasonal acclimatization in atrial myocytes of navaga. In sinoatrial preparations of the summer-acclimatized (SA) navaga, 10^-6 M carbamylcholine chloride (CCh) caused slowing of heart rate, shortening of atrial action potential (AP) duration and a drastic reduction of AP amplitude, eventually resulting in inexcitability. In winter-acclimatized (WA) atria CCh slowed HR and reduced AP duration, but reduction of AP amplitude was modest and never resulted in inexcitability. The difference in cholinergic response between SA and WA navaga is explained by seasonal changes in I_KACh density. The peak density of I_KACh, induced by 10^-5 M CCh, at the common experimental temperature (+6 °C) was 0.97 ± 0.28 pA/pF in SA navaga but only 0.183 ± 0.013 pA/pF in WA navaga (a 5.3-fold difference, P < 0.05). At acclimatization temperatures of the fish I_KACh density was 2.8 ± 0.50 (at +12 °C) and 0.11 ± 0.06 pA/pF (at +3 °C) (a 26-fold difference, P < 0.05) for SA and WA navaga, respectively. Thus, acclimatization to summer induces a drastic up-regulation of the atrial I_KACh, which effectively shortens atrial AP. The reverse temperature compensation of the atrial I_KACh may be advantageous in summer under variable water temperatures and oxygen concentrations by reducing workload of the heart.

Electrical excitability of the heart in a Chondrostei fish, the Siberian sturgeon (Acipenser baerii)

Sturgeon (family Acipenseridae) are regarded as living fossils due to their ancient origin and exceptionally slow evolution. To extend our knowledge of fish cardiac excitability to a Chondrostei fish, we examined electrophysiological phenotype of the Siberian sturgeon ( Acipenser baerii) heart with recordings of epicardial ECG, intracellular action potentials (APs), and sarcolemmal ion currents. Epicardial ECG of A. baerii had the typical waveform of the vertebrate ECG with Q-T interval (average duration of ventricular AP) of 650 ± 30 ms and an intrinsic heart rate of 45.5 ± 5 beats min−1 at 20°C. Similar to other fish species, atrial AP was shorter in duration (402 ± 33 ms) than ventricular AP (585 ± 40) ( P < 0.05) at 20°C. Densities of atrial and ventricular Na+ currents were similar (−47.6 ± 4.5 and −53.2 ± 5.1 pA/pF, respectively) and close to the typical values of teleost hearts. Two major K+ currents, the inward rectifier K+ current ( IK1), and the delayed rectifier K+ current ( IKr) were found under basal conditions in sturgeon cardiomyocytes. The atrial IKr (3.3 ± 0.2 pA/pF) was about twice as large as the ventricular IKr (1.3 ± 0.4 pA/pF) ( P < 0.05) conforming to the typical pattern of teleost cardiac IKr. Divergent from other fishes, the ventricular IK1 was remarkably small (−2.5 ± 0.07 pA/pF) and not different from that of the atrial myocytes (−1.9 ± 0.06 pA/pF) ( P > 0.05). Two ligand-gated K+ currents were also found: ACh-activated inward rectifier ( IKACh) was present only in atrial cells, while ATP-sensitive K+ current ( IKATP) was activated by a mitochondrial blocker, CCCP, in both atrial and ventricular cells. The most striking difference to other fishes appeared in Ca2+ currents ( ICa). In atrial myocytes, ICa was predominated by nickel-sensitive and nifedipine-resistant T-type ICa, while ventricular myocytes had mainly nifedipine-sensitive and nickel-resistant L-type ICa. ICaT/ ICaL ratio of the sturgeon atrial myocytes (2.42) is the highest value ever measured for a vertebrate species. In ventricular myocytes, ICaT/ ICaL ratio was 0.09. With the exception of the large atrial ICaT and small ventricular IK1, electrical excitability of A. baerii heart is similar to that of teleost hearts.

A new potassium ion current induced by stimulation of M2 cholinoreceptors in fish atrial myocytes

A novel potassium ion current induced by muscarinic stimulation (IKACh2) is characterized in atrial cardiomyocytes of teleost fishes (crucian carp, Carassius carassius; rainbow trout, Oncorhynchus mykiss) by means of the whole-cell patch-clamp technique. The current is elicited in atrial, but not ventricular, cells by application of carbamylcholine (CCh) in moderate to high concentrations (10(-7)-10(-4) mol l(-1)). It can be distinguished from the classic IKACh, activated by the βγ-subunit of the Gi-protein, because of its low sensitivity to Ba(2+) ions and distinct current-voltage relationship with a very small inward current component. Ni(2+) ions (5 mmol l(-1)) and KB-R7943 (10(-5) mol l(-1)), non-selective blockers of the sodium-calcium exchange current (INCX), strongly reduced and completely abolished, respectively, the IKACh2. Therefore, IKACh2 was initially regarded as a CCh-induced outward component of the INCX. However, the current is not affected by either exclusion of intracellular Na(+) or extracellular Ca(2+), but is completely abolished by intracellular perfusion with K(+)-free solution. Atropine (10(-6) mol l(-1)), a non-selective muscarinic blocker, completely eliminated the IKACh2. A selective antagonist of M2 cholinoreceptors, AF-DX 116 (2×10(-7) mol l(-1)) and an M3 antagonist, 4-DAMP (10(-9) mol l(-1)), decreased IKACh2 by 84.4% and 16.6%, respectively. Pertussis toxin, which irreversibly inhibits Gi-protein coupled to M2 receptors, reduced the current by 95%, when applied into the pipette solution. It is concluded that IKACh2, induced by stimulation of M2 cholinoceptors and subsequent Gi-protein activation, represents a new molecular target for the cardiac parasympathetic innervation.

The design and use of an optical mapping system for the study of intracardiac electrical signaling

Fluorescent optical mapping of electrically active cardiac tissues provides a unique method to examine the excitation wave dynamics of underlying action potentials. Such mapping can be viewed as a bridge between cellular level and organ systems physiology, e.g., by facilitating the development of advanced theoretical concepts of arrhythmia. We present the design and use of a high-speed, high-resolution optical mapping system composed entirely of "off the shelf" components. The electrical design integrates a 256 element photodiode array with a 16 bit data acquisition system. Proper grounding and shielding at various stages of the design reduce electromagnetic interference. Our mechanical design provides flexibility in terms of mounting positions and applications (use for whole heart or tissue preparations), while maintaining precise alignment between all optical components. The system software incorporates a user friendly graphical user interface, e.g., spatially recorded action potentials can be represented as intensity graphs or in strip chart format. Thus, this system is capable of displaying cardiac action potentials with high spatiotemporal resolution. Results from cardiac action potential mapping with intact mouse hearts are provided. It should be noted that this system could be readily configured to study isolated myocardial biopsies (e.g., isolated ventricular trabeculae). We describe the details of a versatile, user-friendly system that could be employed for a magnitude of study protocols.