Amino acids modify activity of canine intrinsic cardiac neurons involved in cardiac regulation

The effects of amino acids on intrinsic cardiac neuronal activity identified in 10 anesthetized dogs were investigated. Local injection of small volumes (1-10 microliters) of the excitatory amino acids, glutamate (100 mM) and aspartate (10 mM), and the inhibitory amino acids, gamma-aminobutyric acid (GABA; 10 mM) and glycine (10 mM), modified the activity of 39 of 50 identified neurons. Spontaneous activity of eight neurons was modified by both excitatory and inhibitory amino acids. Cardiodynamic responses accompanied neuronal activity modification in 15 instances. After acute decentralization, repeat doses of amino acids altered the activity of 21 neurons and elicited cardiovascular responses in 7 instances. Neuronal and cardiovascular responses were elicited after atropine administration. Neuronal but not cardiac responses were elicited after subsequent timolol administration. In other animals, GABA but not other amino acids elicited neuronal and cardiodynamic responses after hexamethonium administration in decentralized preparations, indicating that non-nicotinic synapses were involved in some GABA-induced responses. These results demonstrate that excitatory and inhibitory amino acids can modify intrinsic cardiac neuronal activity such that, as a consequence, cardiac variables can be modified.