Extracellular calcium dependence of contracture and modulation by serotonin in buccal muscle E1 of Aplysia.

Serotonin [5-hydroxytryptamine (5-HT)] enhances acetyl choline (ACh)-elicited contractures of Aplysia buccal muscles E1 and I5 . The possible role of external calcium in regulating the magnitude of ACh contracture in the presence and absence of 5-HT was investigated. Superfusion of E1 with zero calcium medium caused ACh contractures to fail within one to two minutes. Recovery of ACh contracture upon restoring normal medium occurred within two to four minutes. In the absence of 5-HT, ACh contracture decreased proportionally to external [Ca++] in the concentration range of 0-10 mM; however, the amount of enhancement of ACh contracture following 5-HT treatment did not decrease with external [Ca++] as long as [Ca++] was above a threshold concentration that varied from preparation to preparation. For most preparations, the enhancement of ACh contracture by 5-HT was dependent on the presence of external calcium during 5-HT treatment. Calcium influx into muscles E1 and I5 increased approximately two and a half fold in the presence of 10(-6) M 5-HT. A model in which 5-HT brings about calcium "loading" of an ACh releasable intracellular storage site is discussed.