D-600 blocks spontaneous discharge, excitability and contraction of cultured embryonic chick heart cells

An ionic model for rhythmic activity in small clusters of embryonic chick ventricular cells

We recorded transmembrane potential in whole cell recording mode from small clusters (2-4 cells) of spontaneously beating 7-day embryonic chick ventricular cells after 1-3 days in culture and investigated effects of the blockers D-600, diltiazem, almokalant, and Ba2+. Electrical activity in small clusters is very different from that in reaggregates of several hundred embryonic chick ventricular cells, e.g., TTX-sensitive fast upstrokes in reaggregates vs. TTX-insensitive slow upstrokes in small clusters (maximum upstroke velocity approximately 100 V/s vs. approximately 10 V/s). On the basis of our voltage- and current-clamp results and data from the literature, we formulated a Hodgkin-Huxley-type ionic model for the electrical activity in these small clusters. The model contains a Ca2+ current (ICa), three K+ currents (IKs, IKr, and IK1), a background current, and a seal-leak current. ICa generates the slow upstroke, whereas IKs, IKr, and IK1 contribute to repolarization. All the currents contribute to spontaneous diastolic depolarization, e.g., removal of the seal-leak current increases the interbeat interval from 392 to 535 ms. The model replicates the spontaneous activity in the clusters as well as the experimental results of application of blockers. Bifurcation analysis and simulations with the model predict that annihilation and single-pulse triggering should occur with partial block of ICa. Embryonic chick ventricular cells have been used as an experimental model to investigate various aspects of spontaneous beating of cardiac cells, e.g., mutual synchronization, regularity of beating, and spontaneous initiation and termination of reentrant rhythms; our model allows investigation of these topics through numerical simulation.

Verapamil regulation of Na-K pump levels in rat skeletal myotubes: role of spontaneous activity and Na channels

The effects of low and high doses of verapamil on expression of Na-K pump and Na channels in cultured rat skeletal muscle were studied and compared. Myotubes were treated for 3 days with either 20 or 100 microM verapamil, and measurements were made of transmembrane resting potential, spontaneous action potential frequency, and specific binding of [3H]ouabain and [3H]saxitoxin. Low verapamil upregulated both Na-K pumps and Na channels, whereas high verapamil down regulated the former and upregulated the latter. Changes in channel levels preceded those in pump levels. Spontaneous contractile activity could be observed during treatment with low but not high verapamil. Simultaneous treatment with verapamil and elevated external K+ reversed the effect on high verapamil-induced changes in pump levels, and potentiated the effects of both concentrations of verapamil on Na channel levels. Scatchard analysis showed that verapamil caused changes in Bmax without altering Kd. The verapamil-induced changes in both Na-K pumps and Na channels were blocked by inhibition of protein and RNA synthesis. We conclude that the differences in effects on Na-K pumps obtained with the two doses are due to the different effects on spontaneous activity and associated changes in intracellular Na concentration.

Simultaneous measurements of action potentials and contractions in single cultured adult and embryonic heart muscle cells

Using both a microphotometrical technique and conventional microelectrodes, excitation and contractions have been measured simultaneously in spontaneously active and electrically driven isolated cultured heart muscle cells of embryonic chicks as well as adult rats and guinea-pigs. This experimental arrangement makes it possible to measure continuously and with high accuracy, excitation and E.C. coupling at the single cell level. The contraction time course of spontaneously active embryonic cells and electrically driven adult cells resembles contractions of macroscopic heart muscle preparations against various preloads; mostly a nearly isometric contraction form is found. The effects of various inotropic factors on excitation and contraction (beat frequency, Ca, epinephrine) were studied and it could be shown that the embryonic cells show different features of E.C. coupling. The activity of adult cells resembles very strongly that characteristically found in macroscopic preparations. This proved the feasibility of the method in studies of cardiac E.C. coupling at the single cell level.

Temperature dependence of verapamil action

Ca2+-tolerant ventricular myocytes from adult rats were electrically stimulated. The maximal contraction frequency (fm) was determined at different temperatures. In drug-free Tyrode solution, fm follows the Arrhenius equation from 7 to 39.5 degrees C. However, verapamil introduces a discontinuity around 27 degrees C into the Arrhenius plot of fm. Above this transition temperature the calcium antagonist lowers fm more pronouncedly than below. Below, a tenfold higher concentration is needed for the same relative effect as at 37 degrees C. It is argued that this finding might be important in cardiac surgery when calcium antagonists are used for cardioplegia at deep hypothermia.