Methods
The study was approved by the Human Research Ethics Committee of the Silesian Academy of Medicine. (I) Ten healthy volunteers (3F and 7 M, average age 29,0, range 18-36 years) were included in the first stage. Every subject underwent in random order two examinations on separate days. After ingestion of an acaloric liquid test meal (300 ml tap water) sham or active exposure to TENS (biphasic rectangular impulses with stochastically varied frequency, range of 3.6 to 8.9 Hz, 6.25 Hz median frequency) was applied to the wrist of a non-dominant hand. The intensity of the TENS current was adjusted so that rhythmic flexion of the fingers was observed without producing pain. (II) The study group in the second stage comprised 11 healthy volunteers (6F and 5 M, average age 31.6, range 19-37 years). Every subject was examined in random order on two separate days. After ingestion of a caloric liquid test meal (300 ml, 365 kcal total energy content), active or sham TENS was applied as described above. In either group the gastric evacuation (GE) and gallbladder emptying (GBE) was measured ultrasonographically during 45 min of TENS exposure and then for another 75 min post-TENS period. In either group intensity of the perceived subjective discomfort during active/sham TENS stimulation was assessed by means of a 10-score visual analogue scale. Data are means±SE.

Results
(I) The mean score of the perceived discomfort amounted to 1.1±0.1 and 5.3±0.3 on the sham and active TENS day, respectively. TENS did not affect the gastric half emptying time of the acaloric meal, T1/2: 24.6±2.9 min (sham TENS) vs 21.7±3.0 min (active TENS). On the other hand, TENS significantly increased the acaloric meal-induced gallbladder contraction – the maximum gallbladder ejection fraction amounted to 24.7±2.1% (sham TENS) vs 31.0±2.7% (active TENS), p<0.05. (II) The mean score of the perceived discomfort amounted to 1.0±0.0 and 5.4±0.4 on the sham and active TENS day, respectively. TENS did not significantly change the gastric half emptying time of the caloric meal, T1/2: 38.2±4.2 min (sham TENS) vs 40.6±4.0 min (active TENS). Neither was observed any significant effect of TENS on the caloric meal induced gallbladder contraction – the maximum gallbladder ejection fraction amounted to 78.5±3.1% (sham TENS) vs 78.0±3.1% (active TENS).

Conclusions
Nonpainful low-frequency TENS administered to the wrist area elicits a significant facilitation of the GBE evoked by ingestion of an acaloric liquid meal, whereas no such effect is observed in the case of a caloric meal stimulated gallbladder contraction.