PLASMA AND ORGAN CATECHOLAMINE LEVELS FOLLOWING STIMULATION OF THE RAT INSULAR CORTEX

The posterior insular cortex of the rat contains an area of cardiac chronotropic representation within which tachycardia sites occur rostrally to those producing bradycardia. In the current study using ketamine-anesthetized rats, the insular cortex was stimulated for 1 h using a phasic technique synchronized with the cardiac cycle. Tachycardia was associated with an increase in plasma norepinephrine concentration; epinephrine remained unchanged. This indicates a neural origin of the norepinephrine increment. The tachycardia response was completely blocked by atenolol. Plasma catecholamine levels remained unchanged during stimulation of insular bradycardia sites. Atenolol was without effect during stimulation-induced bradycardia which was completely blocked by atropine. Total cardiac norepinephrine concentration inversely correlated with change in heart rate during stimulation of tachycardia sites. No correlation between intracardiac catecholamines and heart rate variables was found for the bradycardia or control sites. These results indicate that in the ketamine-anesthetized rat, whereas insular stimulation-induced tachycardia is dependent on the sympathetic nervous system, bradycardia elecited by insular cortex stimulation is mediated by parasympathetic mechanisms. No correlation was identified between renal or skeletal muscle norepinephrine levels and any heart rate parameter. This implies that the sympathetic effects of phasic insular microstimulation may be exerted mainly on cardiac nerves, and less so in other visceral beds.