Biomedical Engineering Reference
In-Depth Information
One might expect that if coupling is too tight, nodal cells will be clamped by
surrounding myocardial cells to their resting membrane potential, thus suppressing
spontaneous activity. This is an issue we have not addressed, as we found that in our
initial experiments, the ratio between current developed, cell-cell coupling and
propagation of impulses was such that pacemaker expression was readily and
consistently apparent. This may suggest that our experiments exist on a saturated part
of a dose-response curve between cells delivered, current expressed and gap junctions
formed. Future experiments will test this hypothesis.
The next issue we considered was the autonomic responsiveness of the hMSCs [8].
As shown in Fig. 10, panels a-e, the addition of isoproterenol to hMSCs loaded with
HCN2 resulted in a shift in activation such that increased current flowed at more
positive potentials. The result, as would be expected for native HCN2, should be an
increased pacemaker rate. In Fig. 10, panels e-h show the response of
I
f
expressed by
hMSCs to acetylcholine. Acetylcholine alone had no effect on current, but in the
presence of isoproterenol, antagonized the beta-adrenergic effect of the latter. This is
entirely consistent with the physiologic phenomenon of accentuated antagonism.
We then injected hMSCs loaded with HCN2 into the hearts of dogs in which vagal
stimulation was used to terminate sinoatrial pacemaker function and/or atrioventricular
conduction [11]. This was done effectively and demonstrated spontaneous pacemaker
function that was pace-mapped to the site of injection (Fig. 11). Moreover, tissues
removed from the site showed gap junctional formation between myocyte and hMSC
elements [11]. Finally, the stem cells stained positively for vimentin, indicating that
they were mesenchymal and positively for human CD44 antigen, indicating that they
were of human origin (Fig. 12) [11].
Fig. 11.
Human mesenchymal stem cells (hMSC)-based pacemaker function in canine heart in
situ.
Top to bottom
ECG leads I, II, III, AVR, AVL, and AVF.
Left
Last two beats in sinus
rhythm and onset of vagal stimulation (
arrow
) causing sinus arrest in a dog studied 7 days after
implanting mHCN2- transfected hMSCs in LV anterior wall epicardium.
Middle
During
continued vagal stimulation, an idioventricular escape focus emerges, having a regular rhythm.
Right
On cessation of vagal stimulation (
arrow
), there is a postvagal sinus tachycardia
(reprinted by permission from reference [8]).
