|
Studies using isolated sinoatrial node (SAN) cells indicate that rhythmic spontaneous sarcoplasmic reticulum Ca release
(Ca clock) plays an important role in SAN automaticity. However, it is difficult to translate these findings into intact
SAN because the SAN is embedded in the right atrium (RA). Cross contamination of the optical signals between SAN
and RA prevented the definitive testing of Ca clock hypothesis in intact SAN. We use a novel approach to selectively
map intact SAN to examine the Ca clock function in intact RA. We simultaneously mapped intracellular Ca (Cai) and
membrane potential (Vm) in 7 isolated, Langendorff perfused normal canine RA. Electrical conduction from the SAN to
RA was inhibited with high potassium (10 mmol/L) Tyrode's solution, allowing selective optical mapping of Vm and
Cai of the SAN. Isoproterenol (ISO, 0.03 μmol/L) decreased cycle length of the sinus beats from 586±17 ms at baseline
to 366±32 ms, and shifted the leading pacemaker site from the middle or inferior SAN to the superior SAN in all RAs.
The Cai upstroke preceded the Vm in the leading pacemaker site by up to 18±2 ms. ISO-induced changes to SAN were
inhibited by ryanodine (3 μmol/L), but not ZD7288 (3 μmol/L), a selective If blocker. We conclude that a high
extracellular potassium concentration results in intermittent SAN-RA conduction block, allowing selective optical
mapping of the intact SAN. Acceleration of Ca cycling in the superior SAN underlies the mechanism of sinus
tachycardia during sympathetic stimulation.
|
