Neural control of heart rate: a comparison between puppies and adult animals.

The heart rate responses to stimulation of the right and left cardiac autonomic nerves were evaluated in 20 puppies and 17 adult dogs. In every puppy and adult dog, the changes in heart rate were much greater with right- than with left-sided stimulation of the vagal or sympathetic nerves. At maximal stimulation frequencies, the % changes in heart rate (mean +/- S.E.M.) were: right stellate, adults +50.0 (+/- 5.1) and puppies +23.6 (+/- 2.8); left stellate, adults +20.0 (+/- 2.4) and puppies +5.6 (+/- 2.3); right vagus, adults -74.0 (+/- 5.2) and puppies -43.6 (+/- 3.3); and left vagus, adults -44.1 (+/- 2.0) and puppies -24.9 (+/- 2.3). At any given stimulation frequency, the heart rate responses were significantly greater in adult dogs than in puppies, with either stellate or vagal stimulation. In 47% of the puppies and in 11% of the adult dogs, left stellate stimulation had no detectable effect on heart rate, although there was evidence of an increase in blood pressure in each of these animals. In these animals, fibers from the left stellate ganglion did not innervate the sinoatrial node.