A model of the muscarinic receptor-induced changes in K(+)-current and action potentials in the bullfrog atrial cell

Negative inotropic and chronotropic effects of oxytocin

We have previously shown that oxytocin receptors are present in the heart and that perfusion of isolated rat hearts with oxytocin results in decreased cardiac flow rate and bradycardia. The mechanisms involved in the negative inotropic and chronotropic effects of oxytocin were investigated in isolated dog right atria in the absence of central mechanisms. Perfusion of atria through the sinus node artery with 10(-6) mol/L oxytocin over 5 minutes (8 mL/min) significantly decreased both beating rate (-14.7+/-4.9% of basal levels, n=5, P<0.004) and force of contraction (-52.4+/-9.1% of basal levels, n=5, P<0.001). Co-perfusion with 10(-6) mol/L oxytocin receptor antagonist (n=3) completely inhibited the effects of oxytocin on frequency (P<0.04) and force of contraction (P<0.004), indicating receptor specificity. The effects of oxytocin were also totally inhibited by co-perfusion with 5x10(-8) mol/L tetrodotoxin (P<0.02) or 10(-6) mol/L atropine (P<0.03) but not by 10(-6) mol/L hexamethonium, which implies that these effects are neurally mediated, primarily by intrinsic parasympathetic postganglionic neurons. Co-perfusion with 10(-6) mol/L NO synthase inhibitor (L-NAME) significantly inhibited oxytocin effects on both beating rate (-1.85+/-1.27% versus -14.7+/-4.9% in oxytocin alone, P<0.05) and force of contraction (-24.9+/-4.4% versus -52.4+/-9.1% in oxytocin alone, n=4, P<0.04). The effect of oxytocin on contractility was further inhibited by L-NAME at 10(-4) mol/L (-8.1+/-1.8%, P<0.01). These studies imply that the negative inotropic and chronotropic effects of oxytocin are mediated by cardiac oxytocin receptors and that intrinsic cardiac cholinergic neurons and NO are involved in these actions.

Simulations of passive properties and action potential conduction in an idealized bullfrog atrial trabeculum

This study investigates the properties of a distributed parameter model of an idealized trabeculum of cardiac muscle surrounded by a resistive-capacitive trabecular sheath. A mathematical approach is developed that permits the direct solution for the absolute potential in each medium [i.e., the intracellular (Vi), interstitial (Ve), and external (Vo) potentials), as opposed to obtaining solutions for the transmembrane potential V (where V identical to Vi-Ve). The mathematical description of the underlying individual cell is based upon quantitative whole-cell voltage-clamp measurements in bullfrog atrial myocytes. "Reduced" or "simplified" cell membrane models that lack the complete complement of transmembrane currents are compared with regard to their accuracy in representing the root, upstroke, and plateau regions of the propagated action potential in the complete model. The results show that a reduced cell membrane model must contain the sodium current INa, calcium current ICa, and background-rectifying K+ current IK1. A cell membrane model that contains a linear background K+ current IL instead of IK1 results in much poorer approximation to the upstroke, plateau, and conduction velocities of an action potential. The effects of varying the resistive-capacitive parameters of the trabecular sheath on both the passive properties (the time and space constants and the input resistance) and conduction parameters (time and space constants of the foot and conduction velocity of the action potential) of the trabeculum are also investigated. These simulations show that electrical activity within the trabeculum is much more sensitive to variations in the resistive component than in the capacitive component of the sheath. The trabecular sheath reduces the extracellular resistance seen by the cell by shunting current away from highly resistive interstitial medium into the volume conductor medium, which is of low resistance, and thereby increases conduction velocity. Finally, the addition of the cholinergic neurotransmitter acetylcholine to the extracellular medium reduces both the space constant of the trabeculum and the conduction velocity of propagated electrical activity.

Cardiac electrophysiological experiments in numero, Part III: Simulation of arrhythmias and pacing

This paper is the third and final part of a series of articles reviewing mathematical and computer models of the electrophysiological processes. This section reviews the arrhythmia simulation and discusses models of arrhythmogenic processes, fibrillation and defibrillation, and of heart-pacemaker interaction. The models of arrhythmogenesis are classified into three main sections: models of reentry and vortex reentry, models of myocardial electrotonic interactions, and models of macroreentrant supraventricular tachycardias. This final part of the review discusses the future potential of mathematical and computer models of different cardiac processes.

A model of the phase-sensitivity of the pacemaking cell in the bullfrog heart

In this study, mathematical models of the bullfrog sinus venosus (SV) pacemaker cell (Rasmusson et al., 1990, Am. J. Physiol. 259, H352-H369) and the ACh-sensitive K+ channel (Shumaker et al., 1990, Biophys. J. 57, 567-576) are combined to simulate the response of the SV myocyte to brief hyperpolarizing currents or acetylcholine (ACh) pulses. These simulations provide an ionic basis for the interpretation of the response of this pacemaker cell to either single perturbation or periodic stimuli. The model predicts that the effects of ACh stimulation on the pacemaker cycle length are dependent both on the phase and temporal characteristics of the [ACh] waveform. For example, the simulations show that (1) although ACh normally has an inhibitory effect on the pacemaker model, for cases where the rise time and duration of the [ACh] waveform are sufficiently brief, ACh can paradoxically accelerate the beat in which a single stimulus is given; (2) the SV pacemaker normally exhibits type 1 (odd) phase-resetting in response to ACh delivery, however type 0 (even) phase-resetting behavior may be exhibited when the [ACh] waveform is large enough and has a very fast rise time; and (3) the SV pacemaker may become phase-locked to a repetitive ACh stimulus applied with either a constant period or coupling interval. In the latter case, this entrainment phenomenon has implications for the control of the cardiac pacemaker by a neural oscillator (e.g. located in the medullary cardiovascular control center) which provides input to the pacemaker cell via the vagus nerve. In these regions of capture, repetitive ACh stimulation produces a well-known paradoxical accelerative effect on the SV pacemaker cell, similar to that seen in a variety of other species.