Atrial type B receptor activity during head-out water immersion (HOWI) in dogs

Single-fiber atrial type B receptor activity was recorded from the cervical vagus nerves in 8 dogs during head-out water immersion (HOWI) to simulate weightlessness. The receptor activity was not affected by the water immersion, changing insignificantly from 4.4 +/- 0.9 (SEM) impulses per cardiac cycle (imp/cycle) before HOWI to 4.3 +/- 0.5 imp/cycle immediately on HOWI. At the end of 30 min of HOWI, the receptor activity was 3.9 +/- 1.2 imp/cycle (p > 0.05) while immediately after the immersion it was 4.1 +/- imp/cycle again statistically insignificant (p > 0.05). When expressed as imp/min, the activity before HOWI (803 +/- 142) rose marginally to 826 +/- 116 at the beginning of immersion (p > 0.05), but this trend was not maintained. The urine flow which was measured in 4 female dogs in a different set of experiments, did, however, increase significantly immediately on immersion from 0.89 +/- 0.21 (SEM) to 1.9 +/- 0.35 ml/min (p < 0.02). Therefore, in the dog model developed for single-fiber study during HOWI, atrial type B receptor activity is not affected, and the diuresis which was observed during HOWI could be attributed to other mechanisms.