Methodology and instrumentation for the in vitro sinus node recovery time determination

A new strategy for direct measurement of sinus atrial conduction time by sinus node threshold determination in a rat-isolated atrium

In this work we have searched for experimental evidence that would corroborate the idea that nodal threshold (NT) is the minimum electric field strength able to promote direct sinus node reset that, in turn, can be verified by a decrease in corrected sinus node recovery time (CSNRT). We have performed direct measurements of sinus node conduction time (SACT) and estimation of atrium sinus conduction time (ASCT). Stimulating pulses (pulse and train methods) generated a uniform electric field, with strength ranging from 1.5-fold to 5-fold the atrial threshold (AT), in the center of the perfusion chamber where isolated right atria were placed. The AT and NT were 67.4 and 192.7 mV cm(-1), respectively. The CSNRT values, obtained with stimulation strength below or above NT, were, respectively (ms; mean +/- standard error of mean) 38.1 +/- 0.42 and 25.2 +/- 0.30. In addition, we verified that NT corresponds to approximately 3-fold the AT; SACT was 1.34-fold ASCT and the overdrive suppression decreases with the increment of stimulation pulse strength. Therefore, by using a new and accurate approach for SNRT determination, we have provided additional experimental evidence for the development of alternative sinus node evaluation methods.

Role of acetylcholine in electrical stimulation-induced arrhythmia in rat isolated atria

In this study, we used the spontaneously beating, isolated rat right atrium as an in vitro model to study arrhythmogenic effects of electrical stimulation. A tetrapolar platinum electrode was used for stimulation and recording of atrial electrical activity at 36.5 degrees C (spontaneous rate, 4.9+/-0.3 Hz). A flutter-like pattern of arrhythmia was reproducibly induced by application of stimulus trains (250 pulses, 66.7 Hz). Arrhythmia was characterized by regular and very short cycle length (40-70 ms), each episode lasting from 3 s to >5 min. In control conditions, application of one to five pulse trains was sufficient to induce arrhythmia. However, atropine (but not propranolol) completely blocked arrhythmia induction (10-15 consecutive trains were ineffective). The ability of electrical stimulation to evoke arrhythmia was restored after atropine washout. A milder stimulation protocol (30 pulses, 50 Hz), which was unable to evoke arrhythmia in control conditions, was fully effective in the presence of 1 microM acetylcholine (ACh). Furthermore, a similar flutter-like pattern could be induced in isolated left atria in the presence of ACh. Our results point out an arrhythmogenic effect of neurally released ACh in the isolated right atrium on atrial electrical stimulation.

Effect of ryanodine on sinus node recovery time determined in vitro

Evidence has indicated that the sarcoplasmic reticulum (SR) might be involved in the generation of spontaneous electrical activity in atrial pacemaker cells. We report the effect of disabling the SR with ryanodine (0.1 microM) on the sinus node recovery time (SNRT) measured in isolated right atria from 4-6-month-old male Wistar rats. Electrogram and isometric force were recorded at 36.5 degree C. Two methods for sinus node resetting were used: a) pulse: a single stimulus pulse interpolated at coupling intervals of 50, 65 or 80% of the regular spontaneous cycle length (RCL), and b) train: a 2-min train of pulses at intervals of 50, 65 or 80% of RCL. Corrected SNRT (cSNRT) was calculated as the difference between SNRT (first spontaneous cycle length after stimulation interruption) and RCL. Ryanodine only slightly increased RCL (<10%), but decreased developed force by 90%. When the pulse method was used, cSNRT ( approximately 40 ms), which represents intranodal/atrial conduction time, was independent of the coupling interval and unaffected by ryanodine. However, cSNRT obtained by the train method was significantly higher for shorter intervals between pulses, indicating the occurrence of overdrive suppression. In this case, ryanodine prolonged cSNRT in a rate-dependent fashion, with a greater effect at shorter intervals. These results indicate that: a) a functional SR, albeit important for force development, does not seem to play a major role in atrial automaticity in the rat; b) disruption of cell Ca2+ homeostasis by inhibition of SR function does not appear to affect conduction; however, it enhances overdrive-induced depression of sinusal automaticity.