The sinus gland (a neurohaemal organ) and its nerve have been isolated from the eyestalk of the crabs Cardisoma carnifex and Portunus sanguinolentus for studies correlating electrical responses recorded extra-cellularly from the sinus gland with hormone release. The appearance of erythrophore concentrating hormone (ECH) in the perfusate was followed by bioassay on leg segments of Ocypode pallidula.
Electrical stimulation of the sinus gland nerve (175 pulses in trains of 5 at 5/s, every 10 s) results in significant amounts of hormone appearing in the perfusate, provided that a propagated compound action potential is recorded from the sinus gland. Release is normally below assayable levels in equivalent unstimulated control periods. A single preparation will release in excess of the equivalent of 20 pg of synthetic ECH in response to the standard routine of 175 stimuli. Many such secretory responses can be obtained over periods extending to as long as 30 h.
Addition of tetrodotoxin (3 × 10-7 M) rapidly abolishes propagated electrical responses and secretion; the effects are reversible.
Perfusion with saline having 30% (7.5 mM) of normal (25 mM) calcium reduces hormone release to about 10%, while the electrical response is often augmented. In 10% normal calcium, release is further decreased, and in 1% is indistinguishable from unstimulated release; electrical responses are reduced. Reduced calcium salines also induce spontaneous unit potentials, which persist in 30% calcium saline but subside after 10 min or longer exposure to salines of lower calcium concentration. All the effects of reduced calcium salines are reversible.
Inhibition of secretion without reduction of electrical responses in 30% calcium saline provides evidence for a direct role of calcium in excitationsecretion coupling. In salines that are more calcium deficient, failure of terminal electrical responses may also contribute to reduction of secretion.
In the crab sinus gland, as in other neurosecretory systems, propagation of action potentials to the terminals causes hormone release for which the presence of external calcium is essential.