[Transmembrane inward currents during excitation of the heart (author's transl)]

Thermal modulation of epicardial Ca2+ dynamics uncovers molecular mechanisms of Ca2+ alternans

Ca²⁺ alternans (Ca-Alts) are alternating beat-to-beat changes in the amplitude of Ca²⁺ transients that frequently occur during tachycardia, ischemia, or hypothermia that can lead to sudden cardiac death. Ca-Alts appear to result from a variation in the amount of Ca²⁺ released from the sarcoplasmic reticulum (SR) between two consecutive heartbeats. This variable Ca²⁺ release has been attributed to the alternation of the action potential duration, delay in the recovery from inactivation of RYR Ca²⁺ release channel (RYR2), or an incomplete Ca²⁺ refilling of the SR. In all three cases, the RYR2 mobilizes less Ca²⁺ from the SR in an alternating manner, thereby generating an alternating profile of the Ca²⁺ transients. We used a new experimental approach, fluorescence local field optical mapping (FLOM), to record at the epicardial layer of an intact heart with subcellular resolution. In conjunction with a local cold finger, a series of images were recorded within an area where the local cooling induced a temperature gradient. Ca-Alts were larger in colder regions and occurred without changes in action potential duration. Analysis of the change in the enthalpy and Q₁₀ of several kinetic processes defining intracellular Ca²⁺ dynamics indicated that the effects of temperature change on the relaxation of intracellular Ca²⁺ transients involved both passive and active mechanisms. The steep temperature dependency of Ca-Alts during tachycardia suggests Ca-Alts are generated by insufficient SERCA-mediated Ca²⁺ uptake into the SR. We found that Ca-Alts are heavily dependent on intra-SR Ca²⁺ and can be promoted through partial pharmacologic inhibition of SERCA2a. Finally, the FLOM experimental approach has the potential to help us understand how arrhythmogenesis correlates with the spatial distribution of metabolically impaired myocytes along the myocardium.

On the mechanism of action of calcium antagonists

A short review is given of possible mechanisms of action of the organic "calcium antagonists". Calcium antagonists comprise a chemically heterogenous group of drugs, and the term may be used to denote agents that inhibit Ca2+-dependent processes or regulatory mechanisms without acting at other sites. Such drugs may be subdivided into those that decrease the availability of Ca2+ to the myoplasm, and those that decrease the cellular effects of Ca2+ without lowering the intracellular Ca2+ concentration. Accordingly, calcium channel blockers, such as verapamil, nifedipine, and diltiazem, form a subgroup of calcium availability inhibitors, as they block influx of extracellular calcium through ion selective channels in the membrane both in cardiac and smooth muscle. However, it cannot be excluded that some of these drugs, particularly in smooth muscle, may have additional sites of action, which must be taken into consideration when they are used as investigational tools.

[The functional dissociation of conduction within the human AV-node (author's transl)]

The functional dissociation (FD) of conduction within the AV-node is characterized by sudden prolongations and/or shortenings of the AH-time during stimulation. Examples for FD are presented during regular atrial stimulation and atrial extrastimulus technique. The appearance of FD is no proof for functional impairment of the AV-node. The blockade of the parasympathetic nervous system abolishes FD and leads to the well known continuous and regular adaptation of the AH-time with the various kinds of stimulation examined. It is recommended to replace the term "pathways" by the more comprehensive concept of functional dissociation with the AV-node.

[The adaptation of AV-nodal conduction time on gliding increase and decrease of atrial frequency before and after autonomic blockade (author's transl)]

In 19 patients with healthy AV-nodes the adaptation of the intranodal conduction time (A-H time) to gliding increase and decrease in atrial frequency and to the blockade of the autonomic nervous system was investigated using His bundle electrograms. The measurements were performed during right atrial stimulation with three frequencies, each with a duration of one minute, before and after blockade of the parasympathetic (8 pat.; 1 mg atropine i.v.) and the sympathetic (11 pat.; 0.4 mg Visken i.v.) nervous system. Gliding increase and decrease in atrial frequency results in a staircase pattern of A-H-adaptation in 18 of the patients. The height of the steps was identical in both phases of stimulation in each individual patient. One patient showed functional dissociation of intranodal conduction which was different during increase and decrease of atrial frequency. With parasympathetic blockade the staircase behavior of the A-H time basically remained unchanged with the exception of shorter A-H intervals resulting in lower steps. Atropine abolished the functional dissociation of intranodal conduction; thus the drug might prevent reentrytachycardias due to functional dissociation in the AV-node. Sympathetic blockade lengthens the intranodal conduction time; thus shifting the staircase pattern of the A-H time to higher levels. The results are discussed with respect to the electrophysiological characteristics of AV nodal cells as slow response fibers, and to the changes caused by atrial stimulation, acetylcholine and adrenaline.

Alternans in human atrial monophasic action potenial

This report describes the case of a patient with a supraventricular tachycardia with a ventricular rate of 109/min and no visible P waves in the electrocardiogram. The recording of the monophasic action potential of the right atrium disclosed an atrial tachycardia with a rate of 218/min. There was an alternans of the phase 2 of repolarisation in the action potential. This report emphasises the fact that the phenomenon of alternans occurs in man, as in the experimental animal, at a cellular level.

[Influence of cycle length shortening, atropine and beta-receptor blockage on sinus node recovery time (SRT) in patients with healthy sinus node (author's transl)]

Sinus node recovery time (SRT) was measured in 30 patients with healthy sinus nodes to examine the influence of the increase in atrial frequency, atropine and beta-receptor blockade. The measurements were performed following pacing with 3 atrial frequencies before and after administration of 1 mg atropine i.v. (13 patients) and 0.4 mg prindolol (Visken) i.v. (17 patients). Total group; Increase of frequency alone caused prolongation of the SRT in 17 patients and shortening in 13 patients. Atropine group: Blockade of the parasympathicus alone induced a highly significant reduction in the SRT. Simultaneous increase in frequency and blockade of the parasympathicus led to greater SRT-shortening during low frequency than with high frequency. This interrelationship of frequency and blockade of the parasympathicus influencing the SRT is statistically significant. Visken-group: Blockade of the sympathicus causes a highly significant prolongation of the SRT. Simultaneous increase in frequency and blockade of the sympathicus led to greater SRT-prolongation during low frequency than with high frequency. This interrelationship of frequency and blockade of the sympathicus influencing the SRT has slight statistical significance. The results are discussed in respect to the electrophysiological influences of atrial cycle length shortening, acetylcholine and catecholamines on the sinus node, the perinodal atrial fibres and the atrial working myocardium.

Elementary events in excitation-contraction coupling of the mammalian myocardium

Excitation-contraction coupling of the mammalian myocardium is widely assumed to comprise the following chain of events: (1) Influx of Ca++ into the cell during the plateau phase of the action potential as well as Ca++ release from the subsarcolemmal cisternae of sarcoplasmic reticulum. (2) Activation of the contractile proteins by interaction of Ca++ with the troponin-tropomyosin system. (3) Relaxation due to reuptake of Ca++ by the sarcotubular system as well as by pumping of Ca++ out of the cell through the surface membrane and the transverse tubular system. (4) Movement of Ca++ in the longitudinal tubules back to the subsarcolemmal cisternae. A short comment is included about the effects of hypertrophy on the excitatory processes as well as on cellular structural constituents directly involved in excitation-contraction coupling.

Effects of the local anesthetics brufacain and lidocaine on transmembrane action potentials, refractory period, and reactivation of the sodium system in guinea pig heart muscle

Lidocaine (3-10(-5) and 10(-4) M) and equimolar concentrations of brufacain dose-dependently reduced the depolarization velocity (dV/dtmax) and the force of contraction of guinea pig atria and papillary muscles. Repolarization time was increased in atria but reduced in papillary muscles. The maximum effects on action potential parameters were identical but developed with brufacain more slowly than with lidocaine. In papillary muscles lidocaine (3-10(-5) M) prolonged the functional refractory period. No such effect but even a shortening was observed with brufacain. As brufacain reduced the duration of the action potential (AP) more than the functional refractory period, the "relative refractory period" (in percent of AP duration) was slightly prolonged. The half-time of the recovery of the Na+ system was increased three to five times the control value after lidocaine but only doubled after brufacain.