The effects of small cardioactive peptide B on the isolated heart and gill of Aplysia californica

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Abstract
Effects of small cardioactive peptide 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 and FMRFamide (Phe-Met-Arg-Phe-NH2) on adenylate cyclase activity were compared in particulate fractions of heart and gill tissues, respectively. Small cardioactive peptide B was found to exert dose-dependent, reversible changes in cardiac activity when perfused through the isolated heart. The EC50 values effecting changes in heart rate and force of contraction were 3 × 10−11 and 3 × 10−10 M, respectively; minimum concentrations found to effect changes in heart rate and force of contraction were normally 10−15 and 10−12 M, respectively. However, some winter hearts demonstrated threshold sensitivity to small cardioactive peptide B at concentrations as low as 10−17 M. When perfused through the isolated gill, small cardioactive peptide B was found to suppress the gill withdrawal response amplitude with a threshold concentration of 10−14 M and an EC50 value of 3 × 10−11 M. Suppression of the gill withdrawal response amplitude by small cardioactive peptide B was found to be dose dependent and reversible up to a concentration of 10−9 M. At higher concentrations, the suppression tended to persist irreversibly. Small cardioactive peptide B stimulated adenylate cyclase activity in particulate fractions of both heart and gill tissues with an EC50 of 0.1 and 1.0 μM, respectively. In the heart, adenylate cyclase activity increased by 32-fold at a concentration of 5 × 10−6 M small cardioactive peptide B. FMRFamide stimulated comparable adenylate cyclase activity in the gill only. The evidence presented suggests that small cardioactive peptide B plays an important role in the regulation of important homeostatic functions in Aplysia.