Amine modulation of the neurogenic Limulus heart

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Abstract
(1) The biogenic amines octopamine (OCT), dopamine (DA), epinephrine (E), and norepinephrine (NE) cause dose‐dependent increases in both the rate and amplitude of contractions of the isolated Limulus heart‐cardiac ganglion. Their relative ability to produce this excitation is OCT > DA ∼ E > NE. (2) The excitatory effects of all these amines are antagonized by the α‐adrenergic blocker phentolamine and the dopaminergic antagonist haloperidol. The β‐adrenergic antagonist dichloroisoproterenol slightly reduces amine excitation, but is also a partial agonist. The β‐adrenergic antagonist propanolol, the α‐blocker phenoxybenzamine, and the serotonin antagonist metergoline are ineffective. (3) In addition to their excitatory effects, DA and, to a lesser extent, NE initially reduce contraction rate and amplitude. (4) The transient inhibition is eliminated selectively by metergoline and is unaffected by the other antagonists. (5) The amines all increase the frequency of cardiac ganglion electrical bursting activity, whether ganglia are isolated or attached to cardiac muscle. Dopamine and NE also transiently inhibit the cardiac ganglion. (6) The amines do not alter myocardial resting tension, contractility, or membrane potential. (7) These amines appear to exert their modulatory effects on Limulus heart by altering the properties of the neurons which comprise its cardiac ganglion.