Heart Wall - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 320

IKr

IKs

IKto

IK1 IKb

INaf

CaTN

INav

INab

ICaSERCA

ICaRR

INaK

CaCsq

CaCam

INaCa

ICab

ICaL

ICap

Fig. 6.5 Main ionic currents associated with the action potential and calcium buffers in the
ventricular cardiomyocyte (Sources: [ 612 - 617 ]; IcaL: L-type Ca 2 + current; ICap: sarcolemmal
Ca 2 + pump current; ICab: background Ca 2 + current; ICaRR: Ca 2 + ryanodine release current;
ICaSERCA: sarcoplasmic reticulum Ca 2 + pump (uptake) current; INaCa: Na + -Ca 2 + exchanger
current; INaK: Na + -K + pump current; INaf: fast Na + current; INab: background Na + current;
IKto: transient outward K + current; IKr, IKs: rapid and slow K + delayed rectifier current; IK1:
inward rectifier K + current). The 3 main transient buffers are the complexes of Ca 2 + with
calsequestrin (Ca-Csq) in the sarcomplasmic reticulum, troponin (Ca-TN) in the sarcomere, and
calmodulin (Ca-Cam) in the sarcoplasma. The ventricular cell model can also include accumu-
lation and depletion of calcium ions in tiny spaces between the sarcolemma and sarcoplasmic
reticulum.

The excitation-contraction process is triggered by calcium and involves
especially the complexes (couplons or dyads) formed by sarcolemmal Ca V 1
channels and ryanodine receptors of the junctional region of the sarcoplasmic
reticulum. Reverse-mode Na + -Ca 2 + exchangers and Ca V 3 channels also trigger
the Ca 2 + release, but with less efficiency [ 618 ].

6.4.2.1 Electrophysiological Compartmentation and Gap Junctions

Gap junctions are involved in synchronization of propagation of electrochemical im-
pulses and cardiomyocyte repolarization. Heterogeneous expression of the principal
ventricular connexin-43 creates a dispersion of action-potential duration across the
ventricular wall. An electrophysiological regionalization of heart exists according
to the anteroposterior axis, at least in canine hearts [ 619 ].
Transmural dispersion of action potential duration is significantly greater in
anterior than posterior left ventricle [ 619 ]. Conduction velocity distribution is also
markedly wider across the anterior than posterior left ventricle. Expression of Cx43
varies more strongly in the anterior than posterior left ventricle. Local underexpres-
sion of Cx43 is correlated to transmural electrophysiological heterogeneity on the
anterior left ventricle.