Page 93 - YORAM RUDY BOOK FINAL

P. 93
Conceptually, there is no difference between

the regional increase of coupling discussed above,
fiber branching, or tissue expansion; they all
introduce an increase of electrical load and a source-
sink mismatch. Propagation across an expansion

(branching) site is shown in Figure 3.13. Clearly, the
behavior is strikingly similar to that in Figure 3.11,
where the load increase at the transition was due A
to increased gap junction coupling. In fact, as was

shown theoretically and experimentally, 218,219,220
reduced coupling can compensate for increased
loading due to tissue expansion and restore
successful propagation. It should also be noted that

the concepts of source, sink, loading and source-sink
mismatch are not limited to structural properties, but
apply to functional asymmetries as well. For example,
a rotating wave front in a reentry circuit assumes a B

large curvature around pivot points and provides
depolarizing current to a large volume of tissue
(“fanning-out” effect), a situation of small source to
large sink mismatch. A similar situation arises at the

tip region of a spiral wave. In concept, these situations
are the same as the increased loading due to greater
intercellular coupling or tissue expansion presented in
Figures 3.11 and 3.13.
C

The reliance of propagation in inhomogeneous
tissue on I when long conduction delays are
Ca,L
present, was demonstrated experimentally. I
221
Ca,L
suppression by nifedipine (an I blocker) leads to
Ca,L
conduction block under such circumstances.
Conversely, I enhancement by BAY K 8644 or
Ca,L
isoproterenol (I agonists) facilitates conduction
Ca,L
across structural inhomogeneities. An example of
such experiment is shown in Figure 3.14 , where Figure 3.13. Propagation across an expan-
221
sion (branching) site. Top diagram: fiber
nifedipine produces unidirectional block at a site of expansion is introduced at cell 80. A. AP;
tissue expansion. numbers indicate selected cells. B. SF
along the fiber (line); bars show Q and
Na
Q at specific locations. C. Peak values of
Ca
I and I Ca,L along the fiber. ER, expansion
Na
ratio. From Wang and Rudy [218] courtesy
of The American Physiological Society.