MET-ASN-TYR-LEU-ALA-PHE-PRO-ARG-MET-NH2

EC50: 0.1 μM (adenylate cyclase, in heart tissues), 1.0 μM (adenylate cyclase, in gill tissues)

Small cardioactive peptide B (SCP _B ) is a neurally active peptide endogenous to Aplysia . Small cardioactive peptide B (SCP _B ) possesses cardioexcitatory effects in Aplysia and reported a threshold concentration of 0.01 nM for both native and synthetic Small cardioactive peptide B (SCP _B ) stimulated effects on the isolated heart. Effects of Small Cardioactive Peptide B (SCP _B ) on the physiology of the isolated heart and gill preparations from the mollusc Aplysia californica were examined. In addition, the effects of Small Cardioactive Peptide B (SCP _B ) and FMRFamide (Phe-Met-Arg-Phe-NH2) on adenylate cyclase activity are compared in particulate fractions of heart and gill tissues, respectively. Small Cardioactive Peptide B (SCP _B ) is found to exert dose-dependent, reversible changes in cardiac activity when perfused through the isolated heart. The EC ₅₀ values effecting changes in heart rate and force of contraction are 0.03 and 0.3 nM, respectively; minimum concentrations find to effect changes in heart rate and force of contraction are normally 0.001 and 1 pM, respectively. When perfused through the isolated gill, Small Cardioactive Peptide B (SCP _B ) is found to suppress the gill withdrawal response amplitude with a threshold concentration of 0.01 pM and an EC ₅₀ value of 0.03 nM. Suppression of the gill withdrawal response amplitude by Small Cardioactive Peptide B (SCP _B )is found to be dose dependent and reversible up to a concentration of 1nM. At higher concentrations, the suppression tended to persist irreversibly. Small Cardioactive Peptide B (SCP _B ) stimulates adenylate cyclase activity in particulate fractions of both heart and gill tissues with an EC ₅₀ of 0.1 and 1.0μM, respectively.