Cardiac changes to signalled shock avoidance in dogs

Alterations in heart rate (HR), left ventricular pressure (LVP), and maximum left ventricular dp/dt (LVdp/dt max) during a signalled avoidance task were studied in eight chronically prepared dogs. Four of these animals comprised a non-shock control group. In experimental animals, HR increased during the first two days of the avoidance task but did not change significantly during the last two days, while LVP remained at the supranormal post-training levels and LVdp/dt max increased over the course of the experiment. Control animals showed no change in HR or LVP, but LVdp/dt max decreased over the four experimental days. Changes in LVdp/dt max in experimentals reflect a consistent increase in cardiac sympathetic activation. However, HR changes indicate an initial increase and a subsequent decrease in sympathetic activity. It was therefore postulated that either differential activation of sympathetic cardiac fibers occurred such that during non-stress periods and subsequent exposure to stress, sympathetic influences predominate which are reflected only in LVdp/dt max changes, or sympathetic and parasympathetic fibers differentially control cardiac function during stress and non-stress conditions.