ABSTRACT
In Homarus americanus, the resistance to fluid flow through each of the arteries leaving the heart, including the complete hemocoelic return pathways, can be controlled. Each of the five arterial types (anterior median, paired anterior lateral, paired hepatic, sternal and dorsal abdominal) exhibits a unique spectrum of responses to a battery of neurotransmitters (acetylcholine, glutamic acid, γ-aminobutyric acid) and neurohormones (dopamine, octopamine, 5-hydroxytryptamine, crustacean cardioactive peptide, FLRFamide-related peptides F1 and F2, and proctolin). Acetylcholine causes increases in resistance in all arteries except the anterior median artery; in the dorsal abdominal artery, this increase is antagonized by γ-aminobutyric acid. All neurohormones that are effective in a particular artery cause increases in resistance to flow. The sites of action of these compounds in the dorsal abdominal artery are valves located at major branch points; the sites of control in the other arteries are not known. It is concluded that the control of arterial resistance is a mechanism which the animal can exploit to produce different flow patterns among the various arteries.
