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

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 X 10(-11) and 3 X 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 X 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 microM, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)